The two main functions of sensors are navigational and tactical.

Navigational sensors are used by the astrogator to determine the spacecraft's current position, vector, and heading. They are also used by the pilot to perform the maneuvers calculated by the astrogator. Arguably a chronometer or other instrument to locate the spacecraft's current position in time is also a navigational sensor.

Tactical sensors are used to watch the the region around the spacecraft. This is mostly to monitor nearby objects (such as meteors on a collision course or enemy spacecraft). Arguably this also includes solar-storm warnings which detect deadly incoming proton events.

Navigational and Tactical sensors are generally found on all spacecraft, unless the designer is trying really hard to economize. There are some more specialized sensors only found on more specialized spacecraft:

Remote Sensing suites are used to scan and analyze the surface of a planet, moon, or asteroid. These are found on specialized spacecraft such as exploration vessels, mine prospecting ships, survey ships, customs and other hunter-type ships, and spy ships.

Combat sensors have two main types. Strategic combat sensors detect hostile spacecraft at long range, giving advanced warning of enemy attack. (remember that There Ain't No Stealth In Space). Tactical combat sensors work at close range in a battle, guiding your weapons to the enemy targets (a "firing solution"), detecting incoming enemy weapons, and analyzing the enemy for weakness.

There are two broad classes of sensors: passive and active.

Passive sensors just detect any emissions from the target, i.e., they passively look for the target. Passive sensors include telescopes and heat sensors.

Active sensors emit various frequencies and detect their reflection off the target, i.e., they actively "shine a light" on the target. Active sensors include radar and lidar/ladar.

In some SF novels, passive sensors are called "sensors" while active sensors are called "scanners."

Note that in a combat situation, using active sensors allows you to be instantly detected and targeted by hostile spacecraft. If you don't care if you are detected, or if they already know you are there, sensors can reveal valuable information about hostile spacecraft.

Crown of Infinity

IT WAS ON ITS return sweep, as it came hurtling out of the black depths, that the Master’s Patrol Vessel picked up the Ramdic radiations that could only be an attempt at communication. Stars leaped as the huge nonreflecting surfaced ship, driven by the power equivalent of a half dozen suns, raced for the source of those radiations.

Twenty miles of ultra-pressurized seamless-hulled alloyed metal, ten miles in diameter, came to a halt over the radiating object. Like a large black egg it hovered over the offending object which was buried beneath the surface of Earth.

Invisible probe rays flashed out, passing through the vehicle’s hull as if it weren’t there. The buried vessel was analyzed down to each particle of errant dust. The languages of the system’s inhabitants were plucked from the minds of the thousand collected specimens in cold vaults within the Master’s craft. Computers went through the scores of major languages and silently broke the code. Then the broadcasting vessel was vaporized, leaving only a wisp of fast dispersing white mist.

The message was totally without purpose; therefore the strange, lizard-like creature in charge reasoned that the ship had not been left as a purposeful transmitter. He smelled a trap; someone had wanted to attract him while that someone observed from a safe position.

The creature set up the computers to search the system of Sol cubic yard by cubic yard. Only minutes elapsed before they found the prey hiding in a lunar crater. The patrol vessel swooped down, instruments measuring and analyzing defensive and offensive capabilities of the ship. The information was noted and sneered at-billions of years old his species might have had a ship like it.

In microseconds every map, book, tape, pad and drawing on the ship was in the memory banks of the Master’s computer. Then the Master unleased the mind probes; they bored in, scooping up whatever thoughts and information they could detect from the poorly shielded life-force.

At last, all the mighty armament of the Crusader was loosed, but the Master’s screens barely registered the attack. The creature calmly continued his examination of the crew of the Terran ship. When the probes were finished, the creature touched a button and the Crusader followed the unmanned decoy into oblivion.

His mind automatically absorbed the findings of his computers as everything on the other ship, down to the last molecule, was analyzed and compared against Star King standard and mutation-probability charts.

The capture of a Masters’ ship was utterly impossible; yet V-101 merely sat there, a slight smile upon his face, as if he had done something of no difficulty. Bronson could not understand how it had been accomplished.

“Guess,” he insisted. “How would you capture a Masters’ ship?”

Bronson thought, puffing furiously on his pipe. But it was just impossible! Every fact that the computer held was reviewed and fitted together in every imaginable combination. All he came up with was that a Masters’ ship of any class was faster and more maneuverable, and packed more fire power than all the ships of the Star Kings combined.

Therefore, by deduction, that left an abandoned vessel—which didn’t make any sense.

“That’s the answer!” cried Bronson. “Their ships are shielded against every type of radiation known to us at present. Somehow you found a chink in their armor and duplicated the ship.”

V-101 smiled, eyes far away, as if looking at vistas such as Bronson could never know.

“It was quite simple. The technique has been around since before the United Stars was destroyed. The Masters’ ships are shielded to screen out all radiation and to absorb all detection rays, but there is one thing nothing can screen out: gravity. They can, as we do, minimize its effect, but they can’t ignore completely the basic force of the Universe.

The United Stars had gravity generators. I merely adjusted one to produce a particle gravity, extremely weak, so as to escape detection, that bends in proportion to the mass of the substance acted upon. A thousand such generators and receivers placed to form a huge sphere through which a ship passing gave all the data my computer needed to compute composition and design of that ship.

The C-S headgear rested lightly on Deal, almost an extension of his own body. He seemed to hear the distant chittering of myriad tiny insects as the cells of the headgear compared stimuli from the drifting life craft. He smiled to himself as the origin of the little craft was traced through its cellular history, down to the very origin of the mineral ores.

From Crown of Infinity by John Faucette (1968)

Navigational Sensors

Navigational sensors are used by the astrogator to determine the spacecraft's current position, vector, and heading. They are also used by the pilot to perform the maneuvers calculated by the astrogator. Arguably a chronometer or other instrument to locate the spacecraft's current position in time is also a navigational sensor.

Navigational sensors include:

  • Periscopic Sextant: used to measure angles between celestial objects. From this the spacecraft's position can be calculated.
  • Telescope
  • Coleostat
  • Star Tracker
  • Star Scanner
  • Solar Tracker
  • Sun Sensor
  • Planetary Limb Tracker
  • Planetery Limb Sensor
  • Doppler Radar
  • Inertial Tracker Platform
  • Pulsar Positioning System
Eyes and Ears

So, let’s now turn to the topic of sensors, and what exactly is in that Cilmínár Spaceworks AE-35 “standard navigational sensor suite” built into the majority of current-era starships. There are two primary groups of sensors incorporated into such a suite, referred to as “navigational” and “tactical” – even on civilian vessels – sensors, respectively, along with cross-feeds from the communications systems.


The first of these groups, the navigational sensors, are those primarily used to locate the starship itself in space and, to a lesser extent, time. Included in a standard suite are the following:

Orbital Positioning System

The Orbital Positioning System, considered the primary source of navigational data within settled space, makes use of beacons located on stargates, and orbiting in designated positions within the system, each broadcasting a unique identifying code and sequence signal. By correlating signals received from these satellites with the reference data published in astrogators’ ephemerides, or downloaded from the stargate navigation buoy on system entry, a starship’s position within the inner and the majority of the outer system can be determined precisely, although accuracy does fall off as the Shards are reached.

Star Tracker

As a backup to the Orbital Positioning System, and as the primary method of navigation in undeveloped star systems, the navigational suite includes a star tracker. This system maintains a sunlock, a continuous bearing to the local system primary, and a number of starlocks, continuous bearings to a number of well-known nearby stars, identified spectrometrically. Again, by correlating these bearings with ephemeris data, a starship’s position can be determined with considerable accuracy.

Pulsar Navigational Reference

A final backup is provided by the Pulsar Navigational Reference, which maintains continuous bearings to a number of pulsars located within the local galaxy, using the same principles as the star tracker. While unsuitable for fine navigation (due to the low available parallax of such distant reference points) it is of use in providing confirmatory gross position data.

Inertial Tracking Platform

The inertial tracking platform provides a continuous check on all other forms of navigation, a bridge during switches between beacons and starlocks, and a navigational reference for fine maneuvering; using a complex of accelerometers and gyroscopes linked to the starship’s drive systems, the ITP integrates angular velocity and linear acceleration into a continuous record of change of position and change of velocity. In the latter role, it operates alongside the timebase receiver to provide the relativistics officer with the information required to differentiate wall-clock time and empire time.

Imperial Timebase Receiver

The timebase receiver receives the continuous timebase reference signal transmitted by all stargates, based on their temporal consensus, which defines the empire time reference frame: i.e., the pseudo-absolute time frame without reference to the relativistic maneuvering of individual starships (or indeed celestial bodies); it provides an external temporal reference separate from the starship’s internal wall-clock time.

– Technarch Apt’s How-It-Works: Starships

Periscopic Sextant

The periscopic sextant is used to measure the angle between celestial objects for navigational purposes.

NASA's Apollo had two periscopes: the Space Sextant and the Scanning Telescope. The sextant had a magnification of 28x, the scanning telescope had no magnification but had a wide field of view.

A terrestrial sextant just measures one angle: between the sun or a star and the horizon, or the "altitude". A space sextant measures two angle between two stars or other objects: the radial coordinate and the angular coordinate. This is called "taking a shot" or "shooting the stars".

The sextant's main axis is called the "shaft axis", and is perpendicular to the hull. The sextant's line of sight could rotate around the shaft axis (azimuth or shaft-angle) or rotate up to 50° off axis (trunnion angle). Basically it could aim its line of sight at anything within a conical area up to 50° of the shaft axis.

When the astrogator looks through the sextant they will see the two stars, by rotating the sextant around the shaft angle and the trunnion angle the two stars are superimposed. Then the sextant will reveal the coordinate angles. The shot has been taken. In the Apollo, you press the "mark" button to make the guidance computer record the navigation angle and the time of the shot.

What is happening is that the view through the sextant is optically combining two views: from a fixed sextant scope and a movable ("indexing") sextant scope. Rotation controls are used to yaw, pitch, and roll the ship until Star Alfa (the "Landmark") is in the center of the fixed view. The astrogator then moves the movable sexant through shaft and trunnion rotation until Star Bravo overlaps Star Alfa.

For the Apollo missions, the angle between certain guide stars and the edge of Terra gave the current position and the distance from Terra. As successive shots are taken of the same landmarks and stars, the guidance computer would use something called the "Kalman Optimum Recursive Filter Formulation" to calculate the spacecraft's current position and velocity.

One task using the sextant was to correct the drift of the inertial platform. Under computer control (I use the word "computer" with some reservation, the Apollo computer was stupider than your average smart-phone) you'd tell it to align the sextant on one of the pre-programmed guide stars as Star Alfa. In the center of the fixed view is where the intertial thinks that star Alfa is. The center is helpfully marked by a reticle, since if the inertial platform has drifted Star Alfa ain't gonna be there.

You look through the movable sextant, also looking for Star Alfa, as Star Bravo so to speak. If you are incredibly lucky and there is no drift, when the movable sextant is at zero-radial and zero-angular the Star Alfa/Bravo will be bang in the center of the reticle. If you are sort of lucky the drift was small enough that the guide star is actually within 50° of the shaft axis. If you were flat out of luck the guide star would not be visible at all.

You then spin the movable sectant on its shaft angle and trunnion angle to put Star Alfa/Bravo in the center on the reticle (if the star is not visible you have to rotate the spacecraft first). Then you'd press the "apply correction" button. The computer would calculate the separation angle between the inertial and actual position, and display it. You now press the "accept" or the "reject" button depending upon whether you agreed with the angle. If you accepted it, the computed would then re-align the intertial platform as per the correction.

The author's father, Major Winchell D. Chung, (ret.) used to be the navigator/bombardier officer on a SAC B-52 aircraft. He wrote the following notes about using a periscope sextant:

B-52 Periscopic Sextant

We did have a periscopic sextant that we installed and shot the stars for navigation. This was installed in a sextant mount that had an azimuth ring so you could tell what direction that you were looking and the sextant had internal workings (that you controlled) for elevation so you could read the elevation of the heavenly body you were shooting. The ring mount was located on the top of the forward cabin along the center line of the aircraft. The sextant only revolved in one plane.

The other optical device that looked out of the B-52 was the optical bombsight located in the radar operators compartment. You placed the crosshair on the object using the tracking handle and the crosshairs would remain on the object until you moved it. They were tied into the radar crosshair so both crosshairs would be on the same object. One favorite trick to pull on a new crewmember was to place the crosshairs on something just ahead of the aircraft and let the new crew member look through the optics. The crosshairs would track the object and when you passed over the object the optical crosshairs would rotate 180 degrees to track the object as you flew away from it. You can imagine what a sensation it was when the nice stable crosshairs all of a sudden spun around. It tended to make ones stomach turn too.

The nav compartment was wide enough for two ejection seats to be side by side with enough space between them for a person to squeeze in to get into the seat. The navigator sat on the right and had a desk to work on and the radar had no desk, just a small flat surface with a tracking handle to move the crosshairs. The optical bomb sight was between his legs so by leaning forward he could look through the optics. The single tracking handle moved both the radar cross hairs and the optical crosshairs.

If you crawled up the front hatch to get into the BUF you were climbing on the navigators escape hatch. When you ejected the hatch blew a way first and then the seat fired down. My guess is that the ejection seat was about 30 inches wide.

(Author's note: this is why you keep your elbows tucked in when ejecting, unless you want your arms ripped off. The natural tendency is to pull the ejection trigger from the floor with both elbows pointed to the left and right. The correct procedure is to pull with the elbows pointed at your lap. Anything that extends past the 30 inch footprint of the seat will be sheared off by the edged of the hatch.)

In order to shoot a star first you picked an assumed position ahead of you. The time you decide that you will be at the assumed position will be the mid time of the shot and the time of the fix. Then using the coordinates of the assumed position and the time that you will be there, you calculate the azimuth and elevation of the star using the Air Almanac and the Star Tables. Then you go upstairs, put the sextant in to the azimuth ring carefully, remembering that the cabin is pressurized and the outside is not. Things tend to be attracted by the suction. If your calculations were good and your assumed position was good, you could assume the brightest star in the view finder is the star. You could then put your optical crosshair on it and keep it on it for two minutes, starting the shot 1 minute before the fix time and ending one minute after the fix time. The little knob that you use with your thumb to keep the crosshair on the star, averaged the readings automatically.

The reading that you needed to tell where you were is the elevation of the star. The aircraft does not fly at a constant altitude, it rises and falls in a regular cycle that is approximately two minutes in length. That is why you need the average reading of the elevation.

Usually the EWO (Electronic Warfare Officer) did the shooting and would call down the readings to the Nav. You normally do this with three stars about 120 degrees apart and the resulting three lines will give you your position at the time of the fix.

Major Winchell D. Chung, (ret.)

Norm the bomberguy tells me that the hand written notes on the above images are incorrect.

As a B-52 pilot, I can assure you the photos are labeled incorrectly. The cathode ray tubes in each photo are on the front panel, not the side panels as the handwritten notes indicate. The left photo is a shot of the navigator's station. The panel labeled "left side" is actually the front, and the panel labeled "front" is the right side panel. The right photo shows the radar navigator's station. The panel labeled "rt. side" is the front, and the panel labeled "front" is the left side panel.



Space Jockey

When the Skysprite locked in with Supra-New York, Pemberton went to the station’s stellar navigation room. He was pleased to find Shorty Weinstein, the computer, on duty. Jake trusted Shorty’s computations—a good thing when your ship, your passengers, and your own skin depend thereon. Pemberton had to be a better than average mathematician himself in order to be a pilot; his own limited talent made him appreciate the genius of those who computed the orbits.

"Hot Pilot Pemberton, the Scourge of the Spaceways — Hi!" Weinstein handed him a sheet of paper.

Jake looked at it, then looked amazed. "Hey, Shorty—you’ve made a mistake."

"Huh? Impossible. Mabel can’t make mistakes." Weinstein gestured at the giant astrogation computer filling the far wall.

"You made a mistake. You gave me an easy fix — ‘Vega, Antares, Regulus.’ You make things easy for the pilot and your guild’ll chuck you out." Weinstein looked sheepish but pleased.

Pemberton fed Weinstein’s tape into the robot-pilot, then turned to Kelly. "Control ready, sir."

"Blast when ready, Pilot." Kelly felt relieved when he heard himself make the irrevocable decision.

Pemberton signaled the Station to cast loose. The great ship was nudged out by an expanding pneumatic ram until she swam in space a thousand feet away, secured by a single line. He then turned the ship to its blast-off direction by causing a flywheel, mounted on gimbals at the ship’s center of gravity, to spin rapidly. The ship spun slowly in the opposite direction, by grace of Newton’s Third Law of Motion.

Guided by the tape, the robot-pilot tilted prisms of the pilot’s periscope (coelostat) so that Vega, Antares, and Regulus would shine as one image when the ship was headed right; Pemberton nursed the ship to that heading … fussily; a mistake of one minute of arc here meant two hundred miles at destination.

When the three images made a pinpoint, he stopped the flywheels and locked in the gyros. He then checked the heading of his ship by direct observation of each of the stars, just as a salt-water skipper uses a sextant, but with incomparably more accurate instruments. This told him nothing about the correctness of the course Weinstein had ordered—he had to take that as Gospel—but it assured him that the robot and its tape were behaving as planned. Satisfied, he cast off the last line.

Seven minutes to go—Pemberton flipped the switch permitting the robot-pilot to blast away when its clock told it to. He waited, hands poised over the manual controls, ready to take over if the robot failed, and felt the old, inescapable sick excitement building up inside him.

He caught a last look through the periscope. Antares seemed to have drifted. He unclutched the gyro, tilted and spun the flywheel, braking it savagely to a stop a moment later. The image was again a pinpoint. He could not have explained what he did: it was virtuosity, exact juggling, beyond textbook and classroom.

Twenty seconds … across the chronometer’s face beads of light trickled the seconds away while he tensed, ready to fire by hand, or even to disconnect and refuse the trip if his judgment told him to. A too-cautious decision might cause Lloyds’ to cancel his bond; a reckless decision could cost his license or even his life—and others.

But he was not thinking of underwriters and licenses, nor even of lives. In truth he was not thinking at all; he was feeling, feeling his ship, as if his nerve ends extended into every part of her. Five seconds … the safety disconnects clicked out. Four seconds … three seconds… two seconds… one—He was stabbing at the hand-fire button when the roar hit him.

Kelly relaxed to the pseudo-gravity of the blast and watched. Pemberton was soberly busy, scanning dials, noting time, checking his progress by radar bounced off Supra-New York.

Weinstein’s figures, robot-pilot, the ship itself, all were clicking together.

Minutes later, the critical instant neared when the robot should cut the jets. Pemberton poised a finger over the hand cut-off, while splitting his attention among radarscope, accelerometer, periscope, and chronometer. One instant they were roaring along on the jets; the next split second the ship was in free orbit, plunging silently toward the Moon. So perfectly matched were human and robot that Pemberton himself did not know which had cut the power.

From Space Jockey by Robert Heinlein (1947)

Inertia Properies

A research project led by New Mexico State University Professor Ou Ma that began in 2008 holds scientific, military and commercial promise for the growing use of satellites.

The innovative technology aims to support a growing need to develop satellite servicing capabilities that can extend the lifespan of existing satellites, support the assembly of large structures on orbit, and mitigate orbital debris. These advances can make spaceflight more efficient, sustainable and cost effective.

Ma, from the Mechanical and Aerospace Engineering Department, and his students have been developing an algorithm for identifying the inertia properties of a spacecraft while in orbit.

"The inertia properties, such as mass, location of mass center and moments of inertia, of a spacecraft can change in orbit due to fuel consumption, changes in position of hardware, payload deployments or payload capture. When this happens, the spacecraft's control system needs to know the changes to maintain proper control of the spacecraft," said Ma, who is the first recipient of the John Kaichiro Nakayama and Tome Miyaguchi Nakayama Professorship for Research Excellence.

Ma's Inertial Property Algorithm Verification (IPAV) Project identifies the inertia properties of a spacecraft based on the impulse-momentum principal by incorporating the novel use of an onboard robotic arm. A large advantage of using a robotic arm to identify inertia, over existing methods, is that it is powered by renewable solar, reducing the use of critical fuel consumption by the satellite.

Tactical Sensors

Tactical sensors are used to watch the the region around the spacecraft. This is mostly to monitor nearby objects (such as meteors on a collision course or enemy spacecraft). Arguably this also includes solar-storm warnings which detect deadly incoming proton events.

Eyes and Ears

So, let’s now turn to the topic of sensors, and what exactly is in that Cilmínár Spaceworks AE-35 “standard navigational sensor suite” built into the majority of current-era starships. There are two primary groups of sensors incorporated into such a suite, referred to as “navigational” and “tactical” – even on civilian vessels – sensors, respectively, along with cross-feeds from the communications systems.


The second of these groups, the tactical sensors, are those which concern themselves rather with the environment around the starship than with the starship’s position. Those commonly included, which is to say ignoring specialized scientific sensors, include:

All-Sky Passive EM Array

The sensor with the most general utility is assuredly the all-sky passive EM array. The latest ASPEMA designs consist of a complex array of receptor elements woven through the outer layers of much of a starship’s hull surface, operating together to function as a single large sensor. An ASPEMA’s elements are designed to maintain a consistent watch across the majority of the EM spectrum, from low-frequency RF through infrared, visible light, and up to gamma-rays. A properly configured ASPEMA gives the sensor operator a clear, moderate-resolution view of everything radiating EM within the star system, which is everything worth speaking of.

Passive EM Telescope

When higher resolution is required for identification, or profiling of a target or its emissions, the sensor suite also incorporates one or more passive EM telescopes capable of significantly higher resolution and sensitivity which can be pointed at specific targets.

Active EM Radar

When precise ranging is called for, or the emissions of the target are insufficient to permit profiling with the passive EM telescope, it is possible to “go active”. The active EM radar usually makes use of the same reception hardware as the passive EM telescope; it merely transmits a directional RF pulse and receives its reflection from the target, the time of travel providing the ranging information. The disadvantage of this technique, of course, is that the use of the active EM radar announces one’s presence to all other starships in the system, even beyond typical passive detection ranges.


The gravitometer provides an effective and highly sensitive way to measure the local degree of space-time curvature, both absolute and differential. This can be used to provide a variety of information, including current depth with the gravity well (or altitude) when near objects of known mass, bearings to high-mass objects, and detection profiles of gravity waves, including those generated when a stargate is used by another starship in-system.

Neutrino Detector

The neutrino detector chiefly provides supplementary information. Nucleonic reactions are rich sources of neutrinos; as such, other than when swamped by stellar emissions, neutrino emissions can indicate the presence of operating fusion reactors, torch drives, or other nucleonic equipment commonly found aboard starships. While providing limited additional data on its own, although aiding in the profiling of starships by their power plant, it has the advantage over other sensors that neutrinos interact very little with other matter, and as such the neutrino detector can determine the presence of a signal otherwise occluded by a lunar or planetary body.

Docking Radar

A high-frequency omnidirectional radar system designed for use at short range, the docking radar is a specialized radar system intended to provide precise location and range information while docking, operating near habitats, or otherwise in crowded orbits.

Imaging Lidar Grid

Offering a significantly higher resolution than radar, a starship lidar grid is primarily used for two purposes: first, producing a surface map of asteroids or potential landing sites, or a hull map of an unidentified vessel or hulk to look up in the database; and second, since being hit with a high-intensity lidar pulse will overwhelm most EM-based sensors and can even trigger hull thermal alarms, as a very effective way to yell “Hey, stupid!” at starships which aren’t answering standard hails.

Communications (integrated)

Fed across from the communications subsystem are two other important sources of navigational data:


The first of these is transponder data received from other starships. A transponder broadcast must include that starship’s identity, the current time, and certain important parameters (safety distance from its drives, whether it’s carrying certain hazardous cargoes, registration, and so forth). Ships currently operating under positive control include a subcode – a “squawk” – designated by space traffic control authorities for their reference, and a transponder can also signal various status codes, indicating distress situations in progress, communications failures, hijackings, and other such. The majority of transponders also transmit the ship’s own determination of its position.

IIP Interface

While an extranet feed may seem frivolous for astrogation purposes, it is an essential feature of…


The most notable common characteristics of all of these sensor systems is that they operate at the speed of light, or more slowly, and that they operate from a single point in space, which imposes a tremendous limitation on what information an astrogator may have available. The solution to this is longscan.

Using defined extranet protocols, cooperating starships broadcast their sensory gestalt to other starships in the system via the standard IIP communications relays, as do other sources of sensor data, such as habitats, stargates, and navigation satellites. Using this information, along with predictive AI and astrogator-assisted extrapolations of what each starship or other object visible has done or will do since the last update, the longscan system on each starship produces an overview of the current situation including that information which that ship could not itself sense, or which is still in transit to it, duly annotated with probability and reliability estimates for their future actions.

– Technarch Apt’s How-It-Works: Starships

The moon, now visibly larger and almost painfully beautiful, hung in the same position in the sky, such that he had to let his gaze drop as he lay in the chair in order to return its stare. This bothered him for a moment -- how were they ever to reach the moon if the moon did not draw toward the point where they were aiming?

It would not have bothered Morrie, trained as he was in a pilot's knowledge of collision bearings, interception courses, and the like. But, since it appeared to run contrary to common sense, Art worried about it until he managed to visualize the situation somewhat thus: if a car is speeding for a railroad crossing and a train is approaching from the left, so that their combined speeds will bring about a wreck, then the bearing of the locomotive from the automobile will not change, right up to the moment of the collision.

It was a simple matter of similar triangles, easy to see with a diagram but hard to keep straight in the head. The moon was speeding to their meeting place at about 2000 miles an hour, yet she would never change direction; she would simply grow and grow and grow until she filled the whole sky.

From ROCKET SHIP GALILEO by Robert Heinlein. 1947.

Gravity Gradiometer

These are used to detect object by their gravity. Typically in science fiction, they detect objects that are invisible (usually in science fiction written before the invention of radar). Dr. Robert Forward actually invented such a detector. He suggests using it to detect asteroid-mass black holes lurking in the center of asteroids.

Forward Mass Detector 1

(ed note: this is reality, not science fiction)

It is difficult to measure the mass of an object in free fall. One way is to go up to it with a calibrated rocket engine and push it. Another is to land on it with a sensitive gravity meter. There is, however, a way to measure the mass of an object at a distance without going through the hazard of a rendezvous. To do this, you need to use a mass detector or gravity gradiometer. This is a device that measures the gradient or the changes in the gravity attraction with distance. These gravity gradient forces are the tidal forces by which the Moon causes tides to rise on the Earth, even though both the Earth and the Moon are in free fall. There are a number of different ways to make a gravity gradiometer. The one that I invented uses two dumbbell shaped masses connected together at the center in the shape of an X. [See Figure 12].

When a single dumbbell is placed near a gravitating body such as an asteroid, one mass or the other on the dumbbell will be closer to the asteroid. Since the gravity field of the asteroid gets stronger with decreasing distance, the near mass of the dumbbell will be pulled harder than the far mass, causing the dumbbell to ultimately align itself with the direction to the asteroid. This natural alignment of a long object in orbit around a gravitating body is used by many Earth-pointing satellites and by the Space Shuttle during resting periods. By building my gradiometer with two crossed dumbbells at right angles to each other, one dumbbell is torqued clockwise while the other is torqued counterclockwise. The amount of differential torque between the two arms is measured by determining the change in angle between the two arms. This is a lot easier than trying to measure the angle of one arm with respect to some reference direction.

I use one more trick in the operation of the gravity gradiometer instrument that I invented. I deliberately rotate the sensor at fifteen revolutions per second. In this rotating reference frame, the tiny differential angles between the two arms turn into tiny differential vibrations, and it is a lot easier to measure vibrations than angles. My gravity gradiometer could detect the mass of my fist at thirty centimeters (one foot), me at two meters (I mass over 100 kilograms), and an asteroid-sized black hole at 1000 kilometers.

from Indistinguishable From Magic by Robert L. Forward (1995)
Forward Mass Detector 2

(ed note: this is reality, not science fiction)

Forward Mass Detector

Forward's extensive work in the field of gravitational radiation detection included the invention of the rotating cruciform gravity gradiometer or 'Forward Mass Detector', for Lunar Mascon (mass concentration) measurements. In the well-known textbook Gravitation Misner, Wheeler & Thorne point out that it can detect the curvature of spacetime produced by a fist.

The principle behind it is quite simple; getting the implementation right is tricky. Essentially, two beams are crossed over and connected with an axle through their crossing point. They are held at right angles to each other by springs. They have heavy masses at the ends of the beams, and the whole assembly spun around the common axle at high speed. The angle between the beams is measured continuously, and if it varies with a period half that of the rotation period, it means that the detector is experiencing a measurable gravitational field gradient.

From Wikipedia entry for Robert L. Forward
Forward Mass Detector 3
From Research on Gravitational Mass Sensors by R. L. Forward (1965)
Roamer of the Stars

(ed note: science fiction)

As if in answer, a loud clangor came from the Orford mass detector, on the wall back of them. Dubold shut off the rockets, while Mansell leaped to the telescope. Dubold narrowed the detector beam to a tiny spear of electron stream, and turned to the screen. The feed-back of that narrowed beam would detect a tiny mass a thousand miles out in space through the disturbance of its flowing electrons and locate it in space as accurately as if sighted by a gun. At the intersection of the cross-bar lines a tiny shadow showed like a blot of ink in its intensity. Yet Mansell could see nothing. Mansell changed the course of the ship slightly and the clangor ceased. Dubold shifted the needle beam a bit and located the mass again. A lightning calculation by Mansell told them that the mass was at least fifteen hundred miles ahead. Its detection informed them that the mass was large enough to be seen with the naked eye. Yet Mansell could see nothing.

Mansell and Dubold looked at each other doubtfully, but Shafton was not watching them. Instead, he had his eyes glued to the gravitator as if he could not believe what he saw.

"Strange, I could see nothing," Mansell admitted to Dubold, in an undertone. "Must be a phenomenon of space, something we don't understand. However, we are out of its path..."

He looked toward Shafton as he spoke. A flicker of alarm crossed his sharp-cut features as his attention riveted upon the young spaceman's face. For Shafton's eyes were staring aghast into the gravitator's dial.

"The gravitator!" Shafton cried. "It has swung over to 10-x."

"Impossible," the commander snapped,"why—why, it is unthinkable. There must be something wrong." Suddenly he switched the floor magnets off. So sensitive were the spacemen's sensations to gravitational pulls out there in almost weightless space, a slight but decided tug toward the prow could be felt. Again the commander scanned the surrounding space.

From "Roamer of the Stars" by Clyde Wilson. Thrilling Wonder Stories April 1938. Courtesy of Technovelgy
Detecting An Invisible Object

(ed note: science fiction)

John Star gulped, and his voice came faint with awe. "We're running too near—I'll change our course."

"No," Jay Kalam protested quietly. "Drive on toward it."

"Yes?" Wondering, taut with mounting dread, he obeyed.

The mass ahead tripped the gravity detectors. They had to drop below the speed of light, so that their search beams could guard them from collision. And that strange cloud grew.

"Not three hours after we had left Neptune, the telltale screens began to flash. There was nothing we could see with the tele-periscopes, but the gravity detectors betrayed an invisible object of fifty thousand tons, approaching behind us—as if it had followed us from Neptune."

"An asteroid?" Jay Kalam whispered. "You're certain?"

"I am," Bob Star said, too busy to turn. "The gravity detector shows a mass dead ahead. Millions of tons. The deflector fields wouldn't swing it an inch. But I've changed our course with the rockets—I think enough—"

From The Cometeers by Jack Williamson (1936). Courtesy of Technovelgy
Gravity Imaging

(ed note: this is science fiction. Larry Cho is at a research facility on Pluto, where they have a gravitational generator in the form of a giant ring around the moon Charon. Aliens are invading the solar system using advanced gravidic technology. Larry has to reconfigure the generator into a gravity telescope.)

The Ring was not merely an accelerator. In theory, it could be configured as a gravity-imaging system, a gravity telescope of enormous sensitivity. Such a scope could do more than collect gravity waves. It could form images out of them. No one had ever tried it. Larry decided it was time to test the theory.

He needed an imaging sequence of the Moon and vicinity. The facilities on Venus, Ganymede and Titan were all picking up strong gravity waves from the Moon, but their gear was not powerful or sensitive enough to resolve that data into a clear picture. The Lunar gravity sensors were, of course, completely swamped by the mystery gee waves. In short, none of the other gravity-sensor-equipped stations were able to form a useful image.

Nor did they have the benefit of Larry writing their imaging programs. Larry wasn’t vain—not especially so—but he knew what he was good at.

The monitor screen signaled that the reconfiguration was complete, and Larry powered up the display tank, his thoughts much more on aliens than on what he was doing.

It was as if Galileo’s mind had been on something else when he first looked through a telescope at the Moon. It never dawned on Larry that he had quite casually invented a whole new way of looking at the Universe. All he had been after was a practical way to examine the situation around Earth.

A strange place materialized in the three-dee tank. A ghostly dance of shadows gleamed up at him, black tendrils and ribbons floating in a sky field of cloud white, as if streamers of black ink were swirling through a milky sky, radiating out from a central blotch of darkness.

What the hell was he looking at? Larry glanced at the pointing instruments to check that the device was aimed and focused on the vicinity of the Moon. It was—but what was it seeing?

He was like the first person to look at an X ray, not understanding the strange, hidden, ghostly shapes and patterns revealed when the skin was transparent. Larry reminded himself that he was seeing not a solid, physical substance, but the invisible patterns of gravity waves as represented by a computer’s graphics system.

He reached for a control and adjusted the intensity of the image. The streamers faded away, and the central blotch of darkness resolved itself into two shapes: a single, pulsing point of darkness, and a spinning-wheel rim, jet black, tiny and perfect. Both shapes hovered in the tank. The point was easy to identify—it was the black hole, throbbing with gravitic potential. Even as he watched, a flash of black swept out from the hole, and a tiny dot of black moved away from it, Sunward. The only thing that would show in the tank was a gravity-wave generator. A gravity field by itself, un-manipulated, wouldn’t show at all. Which meant that that tiny dot was a gravity machine of some sort.

But what about the spinning wheel that hung in space, next to the black hole? What the hell was that?

Larry felt the hair on his neck rise. The Moon, good God, the Moon. Or no, something inside the Moon, hidden from view. Suddenly the strange shape was familiar. He checked the scale of the image, and the precise coordinates.

Shock washed over him. The Ring of Charon had a twin, a great wheel buried far below the Lunar surface, underneath the craters and the mountains of the Moon, wrapped around the Moon’s core.

From The Ring of Charon by Roger MacBride Allen (1990)


In C.J. Cherryh's Company Wars universe, ships use both radar and something called Longscan for detection and tactical information. Longscan helps cope with the lightspeed lag of radar. Its primary purpose is for spacecraft combat, but it has some civilian uses.

Remember the light-speed lag. Light moves quickly, but not at infinite speed. It takes about eight minutes to travel one astronomical unit. So if you are in orbit around Terra and you observe a spacecraft near the Sun with a telescope or radar, you are actually are seeing where the ship was eight minutes ago. By the same token, if you change course it will be eight minutes until the Sun-grazer ship will know.

Ships have two kinds of radar: the ordinary sort which operates sublight; and longscan, which is part guess and part radar.

The way it works is this:

It takes the original information of the jump range buoy and identifies every ship and object in the system, how fast they're going and in what direction.

(ed. note: a jump range buoy is a satellite parked where ships emerge from hyperspace. It gives incoming ships an instant update of the location of all known ships. If your universe does not have FTL, you can ignore it.)

It calculates a likely track and shows it on the screen as a four colored line. Red is what track the ships will take if they keep on as they bear. Yellow is what they will do if they veer as much as convenient: this is a cone-shaped projection. Blue is their position if they decide to stop.

Human operators rapidly intervene and as the computer priorities them the the fastest-moving ship data, they decide, on the basis of emotional human knowledge, what those ships are likely to do when the informational wave they have just made entering the system hits them (i.e., when the ships learn that you just popped out of hyperspace). If a warship, for instance, it may turn towards them as fast as it can. An operator is assigned for each ship under consideration while the computer handles the slow craft and the other which for various reasons do not need constant monitoring.

In the meantime two things have happened: Their ship has changed course and speed either following or not following the buoy lane assignment; and the other ships one by one pick up their presence in the system and react accordingly.

But this radar image changes constantly, so when the action begins to conform to one of the projections, the computer changes the color codes, assigning red to the most probable and so on down to blue as least. So it is part radar, part computer, and part human guesswork.

The data in the bank is the best information about the mass and engine capacity and turning ability and hostility or friendliness of each ship whose computer number is on that chart; and all ships know to be in space are in that computer memory.

Now, military craft (particularly Earth Company warships) are always making adjustments and honing their turning abilities if only by the smallest degree; this fouls up the enemy's longscan guesswork and can provide surprises. Mallory's Norway for instance, has not recently tested her adjustments to the extreme, and therefore the captain herself does not know just what Norway might do if she has to. And those refinements are only tested to the fullest, of course, when it comes to a situation where a ship either turns tighter than it is supposed to, or breaks apart — or dies in impact.

From the Company Wars universe by C.J. Cherryh
The Illusionists

In the vision tank, the fleeing disk grew and grew. During the first few minutes, it had appeared there only as a comet-tailed spark, a dozen radiant streamers of different colors fanning out behind it—not an image of the disk itself but the tank’s visual representation of any remote moving object on which the ship’s detectors were held. The shifting lengths and brightness of the streamers announced at a glance to those trained to read them the object’s distance, direction, comparative and absolute speeds and other matters of interest to a curious observer.

But as the Viper began to reduce the headstart the Bjanta had been permitted to get, at the exact rate calculated to incite it to the most intensive efforts to hold that lead, a shadowy outline of the disk’s true shape began to grow about the spark. A bare quarter million miles away finally, the disk itself appeared to be moving at a visual range of two hundred yards ahead of the ship, while the spark still flickered its varied information from the center of the image.

From The Illusionists by James Schmitz (1951)

Remote Sensing

Remote Sensing is obtaining information about an object or phenomenon without touching it. Remote sensing is found on specialized spacecraft such as exploration vessels, survey ships, customs and other hunter-type ships, and spy ships. Remote sensing is used by asteroid miners trying to figure out locate valuable lumps of ore on an asteroid or moon, survey ships assessing potential colony planets, military spacecraft trying to identify hostile contacts, or when Mr. Spock is scanning for life-signs.

Synthetic Aperture Radar

Wikipedia Article

Synthetic aperture radar (SAR) is a form of radar which is used to create images of objects, such as landscapes – these images can be either two or three dimensional representations of the object. SAR uses the motion of the radar antenna over a targeted region to provide finer spatial resolution than is possible with conventional beam-scanning radars. SAR is typically mounted on a moving platform such as an aircraft or spacecraft, and has its origins in an advanced form of side-looking airborne radar (SLAR). The distance the SAR device travels over a target in the time taken for the radar pulses to return to the antenna creates the large "synthetic" antenna aperture (the "size" of the antenna). As a rule of thumb, the larger the aperture is, the higher the image resolution will be, regardless of whether the aperture is physical (a large antenna) or 'synthetic' (a moving antenna) – this allows SAR to create high resolution images with comparatively small physical antennas.

To create a SAR image, successive pulses of radio waves are transmitted to "illuminate" a target scene, and the echo of each pulse is received and recorded. The pulses are transmitted and the echoes received using a single beam-forming antenna, with wavelengths of a meter down to several millimeters. As the SAR device on board the aircraft or spacecraft moves, the antenna location relative to the target changes with time. Signal processing of the successive recorded radar echoes allows the combining of the recordings from these multiple antenna positions – this process forms the 'synthetic antenna aperture', and allows the creation of higher resolution images than would otherwise be possible with a given physical antenna.

Current (2010) airborne systems provide resolutions of about 10 cm, ultra-wideband systems provide resolutions of a few millimeters, and experimental terahertz SAR has provided sub-millimeter resolution in the laboratory.

SAR images have wide applications in remote sensing and mapping of the surfaces of both the Earth and other planets. SAR can also be implemented as inverse SAR by observing a moving target over a substantial time with a stationary antenna.

(ed note: see Wikipedia article for more details)

From Wikipedia article on Synthetic aperture radar

Gamma Ray Spectrometer

A Gamma ray spectrometer is often used by NASA in their space probes to map chemical element and isotope regions on a planet, moon, or asteroid. Such an instrument would be incredibly useful for an asteroid miner. Note that it can only detect elements, not compounds. The spectrometer cannot, for instance, detect water (H2O). It can, however, detect suspiciously large amounts of hydrogen in the same area as oxygen which may suggest the presence of water.

Current NASA instruments can probe to a depth of about 0.1 meters, have a range of a close orbit about one body radius from the surface, and require several months to gather enough readings to make worthwhile maps of the elemental composition. Presumably this performance can be improved. Of course the gamma signal strength can be increased if the range is reduced, say, by somebody on the ground using a man-portable instrument. Trying to do detection from orbit weakens the signal due to the inverse square law.

Detecting valuable deposits of elements is done by analyzing cosmogenic gamma rays (that is, gamma rays created by cosmic rays). Galactic cosmic rays (mostly high-energy protons) from outer space bombard the upper 0.1 meter layer of the asteroid or other celestial object. When a cosmic ray proton hits an atom of the object, it splits it, creating among other things a shower of fast neutrons. The neutrons collide with other atoms (inelastic collision) and are eventually absorbed by another atom (radiative neutron capture). Both of which cause the atom involved to emit a gamma ray (γ).

The important point is that the frequency of the gamma ray depends upon what element the atom is. In other words, the frequency is the "fingerprint" of that element. If you see a gamma ray with a frequency of 6 MeV, you know it came from an oxygen atom.

All the gamma ray spectrometer does is detect gamma ray photons and notes the frequency and where on the asteroid they came from. The frequencies reveal what elements are at a given location, and the relative amounts of different frequencies reveal the relative concentrations of the various elements. For instance, if you are getting twice as many 6 MeV gamma rays as 3 MeV gamma rays from a location, it means that location has oxygen and aluminum, and there is twice as much oxygen as aluminum.

Note that since this technique depends upon cosmic rays, it doesn't work well on planets where cosmic rays are sparse. On planets with thick atmospheres (e.g., Terra and Venus) the atmosphere stops most of the cosmic rays from reaching the surface (which is good news if you are living there). I suppose on such planets one could generate gamma rays to fingerprint if you sprayed the ground with a proton particle beam weapon. But that obviously has problems.

  • Typical cosmic ray protons contain 40 million times the energy of particles accelerated by the Large Hadron Collider.
  • Particle beam weapons are power hogs. Presumably an effective cosmogenic particle accellerator would have power requirements on the order of 40 million times that of the Large Hadron Collider.
  • Particle beam weapons are, well, weapons.
  • It is hard to set the proton strength high enough to get a good gamma signal but low enough so the gamma radiation doesn't kill the user and everybody standing nearby.
  • If the proton strength is too high it will slice up the terrain, which could tend to upset people.

Gamma ray spectrometers are a scintillation counter rigged to be a spectroscope. That is, they are composed of a crystal which makes flashes of light when radiation strikes it, and a photomultiplier tube which watches the crystal and counts the flashes.

To operate as a spectrometer, the instrument has to be able to tell the frequency of each gamma ray photon. The brightness of each flash indicates the frequency of the gamma ray that caused it (the shorter the frequency of a gamma ray photon, the higher the energy, the more visible photos per gamma ray are created, which makes the flash brighter).

Lanthanum bromide (LaBr3) works poorly as a gamma detector crystal. It seems that some of the lanthanum atoms are radioactive isotopes, which means the blasted detector crystal is generating gamma rays. These internal gamma rays drown out the gamma signal from the planet or moon you are surveying.

The Lunar Prospector probe used a crude bismuth germanate (BGO) gamma detector crystal. The advantage was that the crystal did not require cryogenic cooling and was relatively inexpensive. It does require high-voltage vacuum tube photomultipliers due to the type of light flashes it emits.

The Kaguya probe used a ultra-high-res high-purity germanium (HPGe) detector crystal (the technical term is "high spectral resolution"). HPGe are considered to be the "gold standard" among gamma detector crystals. The drawback is that they require cryogenic cooling, which requires mass for the cooling equipment (remember Every Gram Counts) and limiting the gamma detector usable lifespan to the on-board supply of coolant. The crystal is also much more expensive. It also does require high-voltage vacuum tube photomultipliers.

The cutting edge, hot new advanced gamma ray detector crystal is europium-doped strontium iodide (SrI2). Pretty good spectral resolution, not too expensive, and does not require cryogentic cooling. Another advantage is that its light flashes do not require high-voltage vacuum tube photomultipliers, you can use small low powered silicon photomultipliers. This allows the construction of a orbital element scanner that is small and low powered enough to fit into a CubeSat. The drawback is that its spectral resolution is not as good as a HPGe crystal. But it is much better than a BGO crystal.

Burger Lab at Fisk University made a prototype of a CubeSat version of a SrI2 gamma-ray spectrometer built from off-the-shelf components that weighs only half a kilogram, fills 0.001 cubic meters of space and consumes about three watts of electricity yet can do the job of a full lab system that weighs 90 kilograms and fills 0.3 cubic meters of space. The crystal is only five centimeters long.

Dual-Laser Remote Sensing

A pump-probe laser sensing technique being developed at ORNL enables rapid identification of biological and chemical compounds at a distance.

The method uses a quantum cascade infrared laser to strike or “pump” a target sample and a second laser to probe the material’s response. The probe laser reads the absorption spectrum of the molecules present, and the readings can be distilled into pixels that form an image representing the molecules that constitute the sample surface.

The use of a second laser to extract information is a novel aspect of the approach, said Ali Passian of the Measurement Science and Systems Engineering Division (MSSED). “The use of a second laser provides a robust, stable readout approach independent of the pump laser settings.”

Like radar and lidar, the technique uses a return signal to convey information about the substance to be detected, but it differs in important ways, said Passian. It employs a photothermal spectroscopy configuration in which the pump and probe beams are nearly parallel. Probe beam reflectometry provides the return signal in standoff sensing, minimizing the need for expensive wavelength-dependent infrared components such as cameras, telescopes, and detectors.

The findings provide proof of principle for the technique. The work could lead to advances in standoff detectors for use in quality control, airport security, medicine, and forensics. The analysis could be extended to hyperspectral imaging, which provides topographical as well as high-resolution chemical information. “This would allow us to effectively take slices of chemical images and achieve spatial resolution down to individual pixels,” said Passian.

Monostatic Detection Of Radioactive Material

Note this technique only works in an oxygen atmosphere.

Although Barot did not build the bombs, national security experts believe terrorists continue to be interested in such devices for terror plots. Now researchers from the University of Maryland have proposed a new technique to remotely detect the radioactive materials in dirty bombs or other sources. They describe the method in a paper in the journal Physics of Plasmas.

While the explosion of a dirty bomb would likely cause more damage than the radioactive substances it spreads, the bombs could create fear and panic, contaminate property, and require potentially costly cleanup, according to the U.S. Nuclear Regulatory Commission.

Radioactive materials are routinely used at hospitals for diagnosing and treating diseases, at construction sites for inspecting welding seams, and in research facilities. Cobalt-60, for example, is used to sterilize medical equipment, produce radiation for cancer treatment, and preserve food, among many other applications. In 2013 thieves in Mexico stole a shipment of cobalt-60 pellets used in hospital radiotherapy machines, although the shipment was later recovered intact.

Cobalt-60 and many other radioactive elements emit highly energetic gamma rays when they decay. The gamma rays strip electrons from the molecules in the surrounding air, and the resulting free electrons lose energy and readily attach to oxygen molecules to create elevated levels of negatively charged oxygen ions around the radioactive materials.

It's the increased ion density that the University of Maryland researchers aim to detect with their new method. They calculate that a low-power laser aimed near the radioactive material could free electrons from the oxygen ions. A second, high-power laser could energize the electrons and start a cascading breakdown of the air. When the breakdown process reaches a certain critical point, the high-power laser light is reflected back. The more radioactive material in the vicinity, the more quickly the critical point is reached.

"We calculate we could easily detect 10 milligrams [of cobalt-60] with a laser aimed within half a meter from an unshielded source, which is a fraction of what might go into a dirty bomb" said Joshua Isaacs, first author on the paper and a graduate student working with University of Maryland physics and engineering professors Phillip Sprangle and Howard Milchberg. Lead could shield radioactive substances, but most ordinary materials like walls or glass do not stop gamma rays.

The lasers themselves could be located up to a few hundred meters away from the radioactive source, Isaacs said, as long as line-of-sight was maintained and the air was not too turbulent or polluted with aerosols. He estimated that the entire device, when built, could be transported by truck through city streets or past shipping containers in ports. It could also help police or security officials detect radiation without being too close to a potentially dangerous gamma ray emitter.

The proposed remote radiation detection method is not the first, but it has advantages over other approaches. For example, terahertz radiation has also been proposed as a way to breakdown air in the vicinity of radioactive materials, but producing terahertz radiation requires complicated and costly equipment. Another proposed method would use a high-power infrared laser to both strip electrons and break down the air, but the method requires the detector be located in the opposite direction of the laser, which would make it impractical to create a single, mobile device.

From Detecting radioactive material from a remote distance by American Institute of Physics (2016)

Terahertz Scanning

Terahertz Radiation

In physics, terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency,[1] T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of frequencies from 0.3 to 3 terahertz (THz; 1 THz = 1012 Hz). Wavelengths of radiation in the terahertz band correspondingly range from 1 mm to 0.1 mm (or 100 μm). Because terahertz radiation begins at a wavelength of one millimeter and proceeds into shorter wavelengths, it is sometimes known as the submillimeter band, and its radiation as submillimeter waves, especially in astronomy.

Terahertz radiation occupies a middle ground between microwaves and infrared light waves known as the terahertz gap, where technology for its generation and manipulation is in its infancy. It represents the region in the electromagnetic spectrum where the frequency of electromagnetic radiation becomes too high to be measured digitally via electronic counters, so must be measured by proxy using the properties of wavelength and energy. Similarly, the generation and modulation of coherent electromagnetic signals in this frequency range ceases to be possible by the conventional electronic devices used to generate radio waves and microwaves, requiring the development of new devices and techniques.

(ed note: for more details, see the full article)

From Wikipedia article Terahertz radiation
Terahertz Nondestructive Evaluation

Terahertz nondestructive evaluation pertains to devices, and techniques of analysis occurring in the terahertz domain of electromagnetic radiation. These devices and techniques evaluate the properties of a material, component or system without causing damage.

Terahertz imaging

Terahertz imaging is an emerging and significant nondestructive evaluation (NDE) technique used for dielectric (nonconducting, i.e., an insulator) materials analysis and quality control in the pharmaceutical, biomedical, security, materials characterization, and aerospace industries. It has proved to be effective in the inspection of layers in paints and coatings, detecting structural defects in ceramic and composite materials and imaging the physical structure of paintings and manuscripts. The use of THz waves for non-destructive evaluation enables inspection of multi-layered structures and can identify abnormalities from foreign material inclusions, disbond and delamination, mechanical impact damage, heat damage, and water or hydraulic fluid ingression. This new method can play a significant role in a number of industries for materials characterization applications where precision thickness mapping (to assure product dimensional tolerances within product and from product-to-product) and density mapping (to assure product quality within product and from product-to-product) are required.

Nondestructive evaluation

Sensors and instruments are employed in the 0.1 to the 10 THz range for nondestructive evaluation, which includes detection.

Terahertz Density Thickness Imager

The Terahertz Density Thickness Imager is a nondestructive inspection method that employs terahertz energy for density and thickness mapping in dielectric, ceramic, and composite materials. This non-contact, single-sided terahertz electromagnetic measurement and imaging method characterizes micro-structure and thickness variation in dielectric (insulating) materials. This method was demonstrated for the Space Shuttle external tank sprayed-on foam insulation and has been designed for use as an inspection method for current and future NASA thermal protection systems and other dielectric material inspection applications where no contact can be made with the sample due to fragility and it is impractical to use ultrasonic methods.

Rotational spectroscopy

Rotational spectroscopy uses electromagnetic radiation in the frequency range from 0.1 to 4 terahertz (THz). This range includes millimeter-range wavelengths and is particularly sensitive to chemical molecules. The resulting THz absorption produces a unique and reproducible spectral pattern that identifies the material. THz spectroscopy can detect trace amounts of explosives in less than one second. Because explosives continually emit trace amounts of vapor, it should be possible to use these methods to detect concealed explosives from a distance.

THz-wave radar

THz-wave radar can sense gas leaks, chemicals and nuclear materials. In field tests, THz-wave radar detected chemicals at the 10-ppm level from 60 meters away. This method can be used in a fenceline or aircraft mounted system that works day or night in any weather. It can locate and track chemical and radioactive plumes. THz-wave radar that can sense radioactive plumes from nuclear plants have detected plumes several kilometers away based on radiation-induced ionization effects in air.

THz tomography

THz tomography techniques are nondestructive methods that can use THz pulsed beam or millimeter-range sources to locate defaults in 3D. These techniques are terahertz compute tomography, tomosynthesis, synthetic aperture radar and time of flight. This can see details under one millimeter in objects of several tens of centimeters.

Passive/Active Imaging Techniques

Security imaging is currently being done by both active and passive methods. To be clear, active systems illuminate the subject with THz radiation whereas passive systems merely view the naturally occurring radiation from the subject.

It should be evident that passive systems are therefore inherently safe, whereas an argument can be made that any form of "irradiation" of a person is undesirable. In technical and scientific terms, however, the active illumination schemes are safe according to all current legislation and standards.

The purpose of using active illumination sources is primarily to make the signal-to-noise ratio better. This is analogous to using a flash on a standard optical light camera when the ambient lighting level is too low.

For security imaging purposes the operating frequencies are typically in the range 0.1 THz to 0.8 THz (100 GHz to 800 GHz). In this range skin is not transparent so the imaging systems can look through clothing, but not inside the body. There are privacy issues associated with such activities, especially surrounding the active systems since the active systems, with their higher quality images, can show very detailed anatomical features.

It should be noted that active systems such as the L3 Provision™ and the Smiths eqo™ are actually mm-wave imaging systems rather than Terahertz imaging systems lite Millitech™ systems. These widely deployed systems do not display images, avoiding any privacy issues. Instead they display generic "mannequin" outlines with any anomalous regions highlighted.

Since security screening is looking for anomalous images, items like false legs, false arms, colostomy bags, body-worn urinals, body-worn insulin pumps, and external breast augmentations will show up. Note that breast implants, being under the skin, will not be revealed.

Active imaging techniques can be used to perform medical imaging. Because THz radiation is biologically safe (non ionisant), it can be used in high resolution imaging to detect skin cancer.

(ed note: for more details, see the full article)

New 'T-ray' tech converts light to sound for weapons detection, medical imaging

     A device that essentially listens for light waves could help open up the last frontier of the electromagnetic spectrum—the terahertz range.
     So-called T-rays, which are light waves too long for human eyes to see, could help airport security guards find chemical and other weapons. They might let doctors image body tissues with less damage to healthy areas. And they could give astronomers new tools to study planets in other solar systems. Those are just a few possible applications.
     But because terahertz frequencies fall between the capabilities of the specialized tools presently used to detect light, engineers have yet to efficiently harness them. The U-M researchers demonstrated a unique terahertz detector and imaging system that could bridge this terahertz gap.
     "We convert the T-ray light into sound," said Jay Guo, U-M professor of electrical engineering and computer science, mechanical engineering, and macromolecular science and engineering. "Our detector is sensitive, compact and works at room temperature, and we've made it using an unconventional approach."
     The sound the detector makes is too high for human ears to hear.
     The terahertz gap is a sliver between the microwave and infrared bands of the electromagnetic spectrum—the range of light's wavelengths and frequencies. That spectrum spans from the longest, low-energy radio waves that can carry songs to our receivers to the shortest, high-energy gamma rays that are released when nuclear bombs explode and radioactive atoms decay.
     In between are the microwave frequencies that can cook food or transport cell phone signals, the infrared that enables heat vision technologies, the visible wavelengths that light and color our world, and X-rays that give doctors a window under our skin.
     The terahertz band is "scientifically rich," according to Guo and colleagues. But today's detectors either are bulky, need to be kept cold to work or can't operate in real time. That limits their usefulness for applications like weapons and chemical detection and medical imaging and diagnosis, Guo says.
     Guo and colleagues invented a special transducer that makes the light-to-sound conversion possible. A transducer turns one form of energy into another. In this case it turns terahertz light into ultrasound waves and then transmits them.
     The transducer is made of a mixture of a spongy plastic called polydimethylsiloxane, or PDMS, and carbon nanotubes. Here's how it works:
     When the terahertz light hits the transducer, the nanotubes absorb it, turning it into heat. They pass that heat on to the PDMS. The heated PDMS expands, creating an outgoing pressure wave. That's the ultrasound wave. It's more than 1,000 times too high for human ears to pick up.
     "There are many ways to detect ultrasound," Guo said. "We transformed a difficult problem into a problem that's already been solved."
     Though ultrasound detectors exist—including those used in medical imaging—the researchers made their own sensitive one in the form of a microscopic plastic ring known as a microring resonator. The structure measures only a few millimeters in size.
     They connected their system to a computer and demonstrated that they could use it to scan and produce an image of an aluminum cross.
     The response speed of the new detector is a fraction of a millionth of a second, which Guo says can enable real-time terahertz imaging in many areas.
     The system is different from other heat-based terahertz detection systems because it responds to the energy of individual terahertz light pulses, rather than a continuous stream of T-rays. Because of this, it isn't sensitive to variations in the outside temperature, Guo says.
     The study, "Efficient real-time detection of terahertz pulse radiation based on photoacoustic conversion by carbon nanotube nanocomposite," is published online in Nature Photonics. The research is funded by the National Science Foundation and the Air Force Office of Scientific Research.

NYPD deploys terahertz scanner in weapons crackdown

The New York Police Department (NYPD), in conjunction with the the US Department of Defense (DoD), is currently testing a new system that uses terahertz imaging to detect hidden weapons.

NYPD deploys terahertz scanner in weapons crackdownThe system is placed on police vehicles while officers scan crowds milling around a specific location.

"You could use it at a specific event," NYPD Commissioner Ray Kelly told CBS News. "[Alternatively], you could use it at a shooting-prone location."

Unsurprisingly, the use of such invasive technology has prompted a slew of concerned statements from privacy advocates who believe the arbitrary use of terahertz imaging violates human rights.

To be sure, the system is capable of measuring energy radiating from an individual up to 16-feet away, while detecting anomalies like a firearm.

Life Signs Sensor

In Star Trek, when the Enterprise approaches a starship or planet, one of the first things Captain Kirk does is order a scan for life signs. This will reveal if anybody lurking there. "Scanning for life-forms, Captain. We are reading life signs for nine Humans and a Klingon."

In Stargate Atlantis, the Lanteans in the Pegasus Galaxy have nifty little hand-held units called Life signs detectors. They can detect all sentient aliens within about 100 meters, as long as the user has the the Ancient Technology Activation gene and they are not trying to detect a hibernating Wraith.

This is obviously a very useful sensor to have, but how the heck does it work?

Here are some possibilities.

The Star Fleet Medical Reference Manual says life signs sensors detect Kirlian radiation from the entity's Kirlian Aura. This would be a fine explanation, were the Kirlian Aura not revealed to be a steaming pile of Phlogiston back in 1979. This is just another form of the old notion that living creatures are living due to the presence of some sort of "life force" which is as-yet undetected by science. This discredited idea is called Vitalism.

One quick and dirty way to detect life is the fact that generally living things move. Even if they are not walking around, their hearts pump blood and their lungs pump air. Plants move somewhat slowly, and microorganisms move over short distances.

A slightly less quick and dirty technique is to somehow detect the presence of DNA. Which means the sensor will oblivious to any life form with a biochemistry that doesn't use it.

Vulcans and humans (for instance) can be distinguished by their biosignatures. There will be differences in their heart rates (or whatever the acoustics are from their fluid pumping organs), heat signatures, breathing rates, exhaled gases, elements and organic molecules composing their bodies, biological chemical reactions, and the like. This would require remote sensing of sound and chemical composition. As an example, the human will have a sizable amount of iron in their bodies from the hemoglobin in their blood, while Vulcan blood is based on copper.

Laser fluorescence can detect and identify certain organic molecules on the surface of a planet (or the surface of an alien's skin).

In 2007 a company called Kai Sensors obtained a contract from the US Army to develop a unit called a LifeReader. This would use doppler radar and sophisticated computer algorithms to detect and monitor multiple subjects by their individual heart rates, even through walls. Unfortunately the Kai Sensors company appears to have vanished.

Goddard Space Flight Center scientist Sam Floyd is working on the Neutron/Gamma ray Geologic Tomography (NUGGET) instrument. A beam of neutrons is focused through a neutron lens at a specific point inside the target object. As atoms inside the object at the point absorb neutrons, they produce a characteristic gamma-ray signal for that atom's element. NUGGET detect the gamma-ray signature, thus identifying the element at the target point. By sweeping the focus through the object in a regular pattern, a three-dimensional elemental plot of the object can be created. It can also measure the relative amounts of various element pairs (if there is more copper than iron you might have detected a Vulcan).

A partnership by the Department of Homeland Security's Science and Technology Directorate and the National Aeronautics and Space Administration's Jet Propulsion Laboratory designed a device called FINDER (Finding Individuals for Disaster and Emergency Response). It uses microwaves (1150 MHz or 450 MHz, L or S band) to detect the heartbeats of victims trapped in wreckage (for instance a collapsed building after an earthquake). When a microwave beam is aimed at a pile of earthquake rubble covering a human subject or illuminated through a barrier obstructing a human subject, the microwave beam can penetrate the rubble or the barrier to reach the human subject. When the human subject is illuminated by a microwave beam, the reflected wave from the human subject will be modulated by the subject's body movements, which include the breathing and the heartbeat. It can detect heartbeats from people behind [A] 6 meters of solid concrete, [B] 9 meters of rubble, or [C] 30 meters of open space. After the April 25, 2015 earthquake in Nepal two prototype FINDERs managed to locate four men in two different locations who had been trapped under 3 meters of bricks for several days.

LifeReader would use doppler radar and sophisticated computer algorithms to detect and monitor multiple subjects by their individual heart rates, even through walls. Project was apparently discontinued.

Range-R uses a similar principle to LifeReader. It is a motion detector using radio waves that can detect the presence of people and movements as small as human breathing at a range of 15 meters or so, even through solid walls. It will penetrate most common building wall, ceiling or floor types including poured concrete, concrete block, brick, wood, stucco glass, adobe, dirt, etc. However, It will not penetrate metal or walls saturated with water. These are actually currently being used by US police, which has raised the alarm of possible Fourth Amendment abuses by law enforcement personnel.

Dan Slater is the lead technologist working on a new microphone technology called the remote acoustic sensor (RAS), which is capable of capturing sounds within extreme and often inaccessible aerospace environments. It is sensitive enough to detect the sound of microbes moving around. Things that are constantly moving are probably alive. The RAS could probably hear the sound of a Klingon's heartbeat, breathing, blood turbulence, and gastrointestinal rumbling. Probably in enough detail to distinguish the Klingon from a Vulcan or a Human.

Chen-Chia Wang et al have utilized something called a optical speckle-tolerant photo-EMF pulsed laser vibrometer (PPLV) for the detection of human heartbeats, breathing, and gross physical movement from essentially any part of a human subject's surface, even in the presence of clothing, all the while without limiting the interrogation points to specific locations like the chest and carotid areas.

Laser Speckle Bacteria Sensor

A research group in South Korea have come up with a method to detect bacteria on surfaces using laser speckle decorrelation. Basically the technique detects tiny movements. The surface is illuminated with laser light, and the speckle intensity patterns are observed over time. Anything moving, say squirming bacteria, will cause the speckle pattern to change.

The technique was developed for food safety. It can detect very low levels of harmful microorganisms on the surface of food in a non-destructive, non-contact, and rapid manner using inexpensive equipment.

While this is useful for food, it also has applications in the medical field, and in detecting life on a planet.

Eldraeverse Life Detector

“In reality, there is no such thing as a life detector. Vitalism long since having joined the scientific junk-heap, it is a regrettable fact of the universe that there is no quick, convenient, and universal ‘vital field’ that we can tap into to determine the presence of living beings.

“But there is a life detection routine in the computers of your scout ship, you ask? How does that work?

“The answer is: approximation. We know a variety of things that suggest the presence of life. The most obvious example are the signifiers of technological civilization: patterned electromagnetic emissions, the characteristic neutrino products of controlled fusion reactions, and so forth. Where there is technology, there was someone to build it – at least at some point or another, and so the probable detection of technology is also the probable detection of life.

“But there are those few common characteristics that all life does have in common. Self-replication is one, not – by and large – terribly useful for quick detection. Existing within a solvent – for a broad definition of solvent encompassing everything from nebulae to degenerate matter – is another, which can at least tell us where not to look. But of most use is the last: life is an entropy pump. It depends upon energy differentials and pumps against the natural flow, maintaining and causing inequilibria.

That gives us something to look for.

“A life detection routine hunts through the data collected by primary sensors looking for such inequilibria. Reactive gases – such as oxygen – remaining a significant component of a planetary atmosphere, implying their continuous production. Sustained low-level thermal sources, suggesting managed combustion or other energy transaction – bearing in mind that what is to be considered low-level is very different for the outer-system múrast and the star-dwelling seb!nt!at! While almost impossible to detect at any but the closest range, the electromagnetic emissions of high-order informational complexity associated with cognition are the most reliable sign – for life that is both intelligent and which makes use of electronic or electrochemical signals in its ‘nervous system’. These, and tens of thousands of other experience-learnt rules, continuously updated, are programmed into the expert system that underlies the life detection routines used by the Exploratory Service.

“It’s still no more than 80% accurate, yielding commonly both false positives and – worse yet, if missed – false negatives, and so the wise scout never trusts such a system without a close, personal investigation. But it can tell you where to place your bets.”

– A Junior Explorer’s Handbook, Vevery Publishing

Doppler Radar

You could probably get a fair approximation of a "life signs" detector for megafauna with synthetic aperture - Doppler radar. You would be looking for periodic variations in the Doppler signal with the period of a heartbeat and respiration. Versions of this are already available for first responders to search for people trapped in rubble, and it has been suggested that it could be used for SWAT or infantry storming buildings to locate where the people are behind the walls. A spacecraft in orbit would probably want an associated satellite swarm to get any kind of reasonable resolution. Candidates could be further winnowed with a thermal IR or visual search.

It won't do you much good as a generic life sensor, however — trees and algae and bacterial mats would not show up.

From Dr. Luke Campbell (2015)

     'Figure it out, Crile. You would if you weren't blind with rage for no sane reason. It's perfectly straightforward. It's a "neuronic detector". It detects nerve activity at a distance. Complex nerve activity. In short, it detects the presence of intelligence.'
     Fisher stared at her. 'You mean what doctors use in hospitals.'
     'Of course. It's a routine tool in medicine and in psychology to detect mental disorders early on — but at meter distances. I need it at astronomical distances. It's not something new. It's something old with a vastly increased range. Crile, if Marlene's alive, she'll be on the Settlement, on Rotor. Rotor will be there, somewhere, circling the star. I told you that it would not be easy to spot. If we don't find it quickly, can we be sure that it's not there — and not that we just somehow missed it, like missing an island in the ocean or an asteroid in space? Do we just continue searching for months, or years, to make sure that we haven't just missed it, that it's really not there?'
     'And the neuronic detector—'
     'Will find Rotor for us.'
     'Won't it be just as hard to detect—'
     'No, it won't. The Universe is overrun with light and radio waves and all kinds of radiation, and we'll have to distinguish one source from a thousand others, or from a million others. It can be done, but it's not easy, and it may take time. However, to get the precise electromagnetic radiation associated with neurons in complex relationship is quite unique. We are not likely to have more than one source exactly like that — or if we do, it's because Rotor has built another Settlement.

     'They've been gone almost fourteen years. With hyper-assistance, they can travel only at the speed of light. If they have reached any star and settled in its neighborhood, it would have to be at a star within fourteen light-years distance of us. There are not very many of those. At superluminal velocity, we can visit each of those. With neuronic detectors, we can quickly decide whether Rotor is in the neighborhood of any of them.'
     'They might be wandering through space between stars at this very moment. How would we detect them then?'
     'We wouldn't, but at least we increase our own chances just a bit if we investigate a dozen stars in six months with our neuronic detector, instead of spending that much time investigating one star in a futile search. And if we fail — and we have to face the fact that we might fail — then at least we will return with considerable data on a dozen different stars, a white dwarf, a blue-white hot star, a Solar look-alike, a close binary, and so on. We're not likely to make more than one trip in our lifetime, so why not make it a good one, and go down in history with a huge bang, eh, Crile?'
     Crile said thoughtfully, 'I suppose you're right, Tessa. To comb a dozen stars and find nothing will be bad enough, but to search a single star vicinity and return thinking that Rotor might have been somewhere else that was reachable but that we lacked the time to explore would be much worse.'

     'Another thing,' said Wendel. 'The neuronic detector might detect intelligence not of Earthly origin. We wouldn't want to miss it.'
     Fisher looked startled. 'But that's not likely, is it?'
     'Not at all likely, but if it happens, all the more reason not to miss it. Especially if it is within fourteen light-years of Earth. Nothing in the Universe can be as interesting as another intelligent life-form — or as dangerous. We'd want to know about it.'
     Fisher said, 'What are the chances of detecting it at all if it is not of Earthly origin? The neuronic detectors are geared for human intelligence only. It seems to me that we wouldn't even recognize a really odd life-form as being alive, let alone as being intelligent.'
     Wendel said, 'We may not be able to recognize life, but we can't possibly fail to recognize intelligence, in my view, and it's not life but intelligence that we're after. Whatever intelligence might be, however strange, however unrecognizable, it has to involve a complex structure, a very complex structure — at least as complex as the human brain. What's more, it's bound to involve the electromagnetic interaction. Gravitational attraction is too weak; the strong and weak nuclear interactions are too short-range. And as for this new hyperfield we're working with in superluminal flight, it doesn't exist in nature as far as any of us knows, but exists only when it is devised by intelligence.
     'The neuronic detector can detect an elaborately complex electromagnetic field that will signify intelligence no matter the form or chemistry into which that intelligence may be molded. And we will be ready to either learn or run. As for unintelligent life, that is not at all likely to be dangerous to a technological civilization such as ourselves — though any form of alien life, even at the virus stage, would be interesting.'

From Nemesis by Isaac Asimov (1990)
Butterfly Planet

On roof and windows there were portable recognition computers against which disguise, including ruthless surgery, was useless. The computer didn't check features, it checked mannerisms of movement, weight, the exact size of the head and many other features. The device was not content with that, it inspected the teeth, and certain internal functions which, it had been discovered, were as characteristic as finger prints.

From Butterfly Planet by Philip E. High (1971)
The Trouble with Tribbles

Dr. McCoy:

[scanning Darvin] Heartbeat is all wrong. His body temperature is... [realizing] Jim, this man is a Klingon.

Spy Rays

Spy rays or spy beams are a jolly science fiction idea, apparently invented by the legendary E. E. "Doc" Smith in his novel Triplanetary (1934). Adjust the setting and you too can see and hear everything that happens inside an enclosed room at a remote location. Currently they do not exist, but certain remote sensing technologies are getting real close.

Spy rays are popular with spies (of course), combat spacecraft trying to get intel on their opponents, military intelligence, criminal gangsters trying to get the inside dope on their targets and/or rival gangs, police especially in the same situations where they'd have an agent "wearing a wire", astromilitary trying to obtain the details of the enemy's new secret weapon breakthrough, industrial espionage, and so on. In E. E. "Doc" Smith's Lensman series, warships would used spy rays to locate the enemy ship's crew at their enemy control panels then use needlebeams to vaporize the control panels (and probably the hapless crewmember at the panel).

Naturally this leads to the an arms race, with the creation of "spy-ray blocks" to foil spy rays. And improved spy rays to defeat the spy-ray blocks.

Spy-ray blocks are naturally popular with the targets of the activites listed in the previous paragraph. Note that the target of spies includes diplomats, top-secret development labs, military planning offices, and enemy spies.

Technologies that are almost spy rays include:

  • LifeReader would use doppler radar and sophisticated computer algorithms to detect and monitor multiple subjects by their individual heart rates, even through walls. Project was apparently discontinued.
  • Range-R uses a similar principle to LifeReader. It is a motion detector using radio waves that can detect the presence of people and movements as small as human breathing at a range of 15 meters or so, even through solid walls. It will penetrate most common building wall, ceiling or floor types including poured concrete, concrete block, brick, wood, stucco glass, adobe, dirt, etc. However, It will not penetrate metal or walls saturated with water. These are actually currently being used by US police, which has raised the alarm of possible Fourth Amendment abuses by law enforcement personnel.
  • Laser Microphones uses a remote laser beam to monitor the vibrations of an object inside a room, to create an impromptu microphone suitable for obtaining intel on drug deals and other conspiracies. Rippled glass windows can defeat laser microphones. But in theory the principle can be adapted to use microwaves, which means it can only be defeated by acoustically isolating the room and surrounding it with a Faraday Cage.
  • Time-of-Flight Microwave Cameras use a parabolic antenna as a lens to actually capture crude images through walls.
Skylark Three

Our heroes are on the planet Norlamin near the center of our galaxy. They are using a "fifth-order projector" as a spy ray to observe events in the Fenachome throne room, located on their homeworld several thousand light-years away.

"If we have a minute more, there's something I would like to ask," Dunark broke the ensuing silence. "Here we are, seeing everything that is happening there. Walls, planets, even suns, do not bar our vision, because of the fifth-order carrier wave. I understand that, partially. But how can we see anything there? I always thought that I knew something about communications and television hook-ups and techniques, but I see that I don't. There must be a collector or receiver, close to the object viewed, with nothing opaque to light intervening. Light from that object must be heterodyned upon the fifth-order carrier and transmitted back to us. How can you do all that from here, with neither a receiver nor a transmitter at the other end?"

"We don't," Seaton assured him. "At the other end there are both, and a lot of other stuff besides. Our secondary projector out there is composed of forces, visible or invisible, as we please. Part of those forces comprise the receiving, viewing, and sending instruments. They are not material, it is true, but they are nevertheless fully as actual, and far more efficient, than any other system of radio, television, or telephone in existence anywhere else. It is force, you know, that makes radio or television work—the actual copper, insulation, and other matter serve only to guide and to control the various forces employed. The Norlaminians have found out how to direct and control pure forces without using the cumbersome and hindering material substance..."

From Skylark Three by E. E. "Doc" Smith (1948)
Spacehounds of IPC

Steve and Nadia are on board the space ship Arcturus. Steve is conducting a tour of the ship. They are currently in the nose section.

"We are standing upon the upper lookout lenses, aren't we?" asked the girl. "Is that perfectly all right?"

"Sure. They're so hard that nothing can scratch them, and of course Roeser's Rays go right through our bodies, or any ordinary substance, like a bullet through a hole in a Swiss cheese. Even those lenses wouldn't deflect them if they weren't solid fields of force."

As he spoke, one of the ultra-lights flashed around in a short, quick arc, and the girl saw that instead of the fierce glare she had expected, it emitted only a soft violet light. Nevertheless she dodged involuntarily and Stevens touched her arm reassuringly.

"All x, Miss Newton—they're as harmless as mice. They hardly ever have to swing past the vertical, and even if one shines right through you you can look it right in the eye as long as you want to—it can't hurt you a bit."

"No ultra-violet at all?"

"None whatever. Just a color—one of the many remaining crudities of our ultra-light vision. A lot of good men are studying this thing of direct vision, though, and it won't be long before we have a system that will really work."

"I think it's all perfectly wonderful!" she breathed. "Just think of traveling in comfort through empty space, and of actually seeing through seamless steel walls, without even a sign of a window! How can such things be possible?"

"Well, anyway, by the use of suitable fields of force it can be used as a carrier wave. Most of this stuff of the fields of force—how to carry the modulation up and down through all the frequency changes necessary—was figured out by the Martians ages ago. Used as a pure carrier wave, with a sender and a receiver at each end, it isn't so bad—that's why our communicator and radio systems work as well as they do. They are pretty good, really, but the ultra-light vision system is something else again. Sending the heterodyned wave through steel is easy, but breaking it up, so as to view an object and return the impulses, was an awful job and one that isn't half done yet. We see things, after a fashion and at a distance of a few kilometers, by sending an almost parallel wave from a twin-projector to disintegrate and double back the viewing wave. That's the way the lookout plates and lenses work, all over the ship—from the master-screens in the control room to the plates of the staterooms and lifeboats and the viewing-areas of the promenades. But the whole system is a rotten makeshift, and...."

From Spacehounds of IPC by E. E. "Doc" Smith ()
The Space Beyond

From a million miles he explored the fleet. It did not take him long to learn that two thousand ships were weakly powered, for Flame ships, and that one was a giant of power. It had not the power of the Prometheus, for Atkill had not had time to experiment with the Flame, and had not learned the trick of control that permitted Warren to get nearly fifty times as much power from a given Flame. But Atkill's ship was larger. Warren began to explore that larger ship. He had a television screen set up before him, and now there appeared on it pictures as of a glass ship, wherein the walls were bare, dim shadows, save where it was focused, and there perfect vision was obtained. Only about the neighborhood of the Flame was the device inoperative. Where, nothing showed beyond strange, distorted shadows. Atkill was in the control room at the bow. "He escaped!" gasped Warren.

Atkill had been manipulating instruments with sudden interest. Putney watched his a moment. "He's spotted us. Look—he's shocked. He realizes an Anlonian ship has him spotted—and has the Flame." Atkill was making more tests. Putney watched his instruments. "Examining the size and power of our Flame. Doesn't like it does he?" Atkill was pursing his lips thoughtfully. Suddenly dawning understanding spread over his face, and a wide grin split his features. His lips moved silently. "By God— Warren," quoted Putney, reading his moving lips.

Atkill was suddenly laughing, and turned to a radio set beside him. Warren snapped a tumbler that put his receiver in operation. "Warren—Warren—Warren—James Atkill calling Warren—" Atkill's voice came through.

"Looking at you now, Atty," said Warren quietly. Atkill jumped, and looked around him annoyed. "Don't jump, Atty, we won't bite you yet."

"Hmmm—you are watching me, aren't you. Warren, you are a good man. I don't see how you do it. I haven't spotted anything that will look through metal yet. Well— let's try this." Atkill's hand reached for a tumbler and through it. His image blurred, dimmed, and was scarcely visible. Warren increased his power a trifle, and the image was clear again. Atkill solemnly winked his left eye.

"The left," said Warren, slightly bored. "We call that field X-394-21. It won't stop this, though it will stop most radiation."

From The Space Beyond by John W. Campbell, jr. ()
The Witches of Karres

In spite of the Daal's rigid limitations on what was allowable nowadays, they weren't really far away from the previous bad pirate period. In the big store where he and Goth had picked up supplies for the house, the floor manager earnestly advised them to invest in adequate spy-proofing equipment. The captain hadn't seen much point to it until Goth gave him the sign. The device they settled on then was small though expensive, looked like a pocket watch. Activated, it was guaranteed to make a twenty-foot sphere of space impervious to ordinary eavesdroppers, instrument snooping, hidden observers, and lip-readers. They checked it out with the store's most sophisticated espionage instruments and bought it. There'd be occasions enough at that when they'd want to be talking about things nobody here should know about; and apparently no one on the planet was really safe from prying eyes and ears unless they had such protection.

She went up the winding stairway to the living room while the captain took the groceries they'd picked up in the port shopping area to the kitchen. When he followed her upstairs he saw an opaque cloudy shimmering just beyond the living room door, showing she'd switched on their spy-proofing gadget. The captain stepped into the shimmering and it cleared away before him. The watch-shaped device lay on the table in the center of the room, and Goth was warming her hands at the fireplace.

From The Witches of Karres by James Schmitz ()
The Light of Other Days

The Light of Other Days is a 2000 science fiction novel written by Stephen Baxter based on a synopsis by Arthur C. Clarke, which explores the development of wormhole technology to the point where information can be passed instantaneously between points in the space-time continuum.

Plot summary

The wormhole technology is first used to send digital information via gamma rays, then developed further to transmit light waves. The media corporation that develops this advance can spy on anyone anywhere it chooses. A logical development from the laws of space-time allows light waves to be detected from the past. This enhances the wormhole technology into a "time viewer" where anyone opening a wormhole can view people and events from any point throughout time and space.

When the technology is released to the general public, it effectively destroys all secrecy and privacy. The novel examines the philosophical issues that arise from the world's population (increasingly suffering from ecological and political disturbances) being aware that they could be under constant observation by anyone, or that they could observe anyone without their knowledge. Anyone is able to observe the true past events of their families and their heroes. An underground forms which attempts to escape this observation; corruption and crime are drastically reduced; nations discover the true causes and outcomes of international conflicts.

From The Light of Other Days entry in Wikipedia


Sure, a sensor can give you a reading on whatever it measures, but what does it mean?

Patterns of specific readings from one or more sets of sensors can indicated the presence of an object or event, giving meaning to the raw readings. These are called Signatures.

For instance, if your seismometer indicates a small earthquake and the atmospheric radiation meter records an abrupt rise in atmospheric radiation, you can be pretty sure that a nuclear explosion has happened. The two readings correlated in time is the signature of a nuclear detonation.

Spectral Signatures

Spectral signatures are a spectrum of intensities of various frequencies of electromagnetic radiation which identifies elements and compounds. The signature is the spectral "fingerprint" of an element. The instrument used is called an optical spectrometer or spectroscope.

In other words Meteor Mike the rock-rat can point his ship's spectroscope at an asteroid and say "Hot Rockets! Thar's gold in them thar rocks!"

In 1835 positivist French philosopher Auguste Comte foolishly defied Clarke's first law and stated: "We will never know how to study by any means the chemical composition (of stars), or their mineralogical structure."

Comte should have known better. Joseph von Fraunhofer had discovered the beginning of how to do just that in 1814, twenty one years earlier. The discovery is now called Fraunhofer lines, Joseph had basically invented the spectroscope.

The point is if you are a scientist in those primitive days before rocket propelled space probes, the only thing you can get from planets, the Sun, and the stars is electromagnetic radiation. Since that is all you get, you would do well to analyze that radiation with a fine-tooth comb. Which is what a spectroscope does.

Back in the 1660s people knew that you could make a rainbow by passing a ray of white light through a prism, and you should know that too if you have a copy of Pink Floyd's Dark Side Of The Moon album. But back then they figured that white light was white and the glass prism was somehow staining it with various colors. In 1666 super-genius Isaac Newton proved that the prism wasn't staining anything, the white light ray was actually composed of a mix of colored light. For one thing you could use one prism to turn a white ray into a rainbow, then use a second upside-down prism to turn the rainbow back into a white ray. This doesn't make sense if the prism is staining the light. Newton wrote up his results in a book called Opticks which is considered to be one of the greatest works of science in history. Arguably the greatest is Newton's other work Philosophiæ Naturalis Principia Mathematica, which among other things contains his law of universal gravitation and his laws of motion so near and dear to spacecraft astrogators. But I digress.

The point is the prism is taking all the various frequencies of light in the ray and separating them. Which allows you to analyze the ray with a fine-tooth comb. You can check which light frequencies are present and which are absent, and the relative strengths of each. This is the basis of the spectroscope.

Glass prisms have limitations when used in a spectroscope, they were eventually replaced with diffraction gratings once the latter had been invented.

Elements heated inside a blazing star emit a unique pattern of frequencies which is the signature (fingerprint) of that element. These are the Fraunhofer lines. Thus the spectroscope can stick its tongue out at Comte and routinely determine the chemical composition of stars and planets.

As a matter of fact, the element Helium was discovered by spectroscope on the Sun before it was discovered on Terra. Several astronomers spotted a previously unknown Fraunhofer line at a wavelength of 587.49 nanometers in the solar corona spectrum. It was named "helium" after the Greek word the Sun: ἥλιος (helios). It wasn't discovered on Terra until 27 years later. Chemist Sir William Ramsay discovered it in a chunk of cleveite when his spectroscope spotted the tell-tale Fraunhofer line.

Sometimes this backfires, though. In 1869 astronomers spotted another previously unknown line at 530.3 nanometers in the solar corona spectrum. Aha! Obviously another unknown element, discovered by the awesome power of spectroscopy! They named it Coronium, though Dmitri Mendeleev renamed it Newtonium.

However in the 1930s researchers discovered that the unknown line was actually due to highly ionized iron, not from a mystery element. Up until then scientists could not ionize elements quite as extremely as obtains in the solar corona. Bye-bye Coronium. This also explained quite a few other mystery lines that had been observed.

I first encountered Coronium in Fletcher Pratt's novel Alien Planet, which was written before Coronium was discovered to be a myth. In the novel the alien visitor needs Coronium to refuel his spaceship. Since it does not exist on Terra, he makes do with helium for a short hop to the planet Mercury. There he harvests the mythical Coronium from the solar wind. Which goes to show that reading science fiction can be educational, but you need a crib sheet to separate the good science from the obsolete science.

If a line is bright it is an emission lines, if it is black it is an absorption line. But either way they are the fingerprint signatures of the elements. When looking at the spectrographs, it doesn't matter if a line is bright or black, the position is the important thing.

Since objects like the Sun are composed of lots of elements, all their signatures will be overlaying each other. The spectrum will look like a herd of rainbow zebras. But astronomers have become quite skilled at untangling the signatures.

But spectroscopes can do so much more than just identify the elemental composition of the object.

In theory astronomers can tell if a star is approaching, receding, rotating, or orbiting another star by observing the doppler effect with a spectroscope. This is impossible to detect if you have a featureless spectrum of light from the star in question. Kind of like trying to measure something using a featureless ruler with no indicator marks.

Fraunhofer lines to the rescue! They put identifiable marks on the star's spectrum. Suddenly your ruler has lines on it. Now you can see a red shift or blue shift by looking at the position of an element's Fraunhofer lines.

So you can take a photograph through a spectrograph of a star's spectrum while simultaneously photographing the spectrum of (say) some burning sodium in the lab (a "comparison spectra"), side by side on the same piece of photographic film (yes, kiddies, back in the days when dinosaurs roamed the planet people used photographic film instead of digital cameras to take their selfies). On the photo you can then measure the displacement between the lab's sodium signature and the sodium signature in the star's spectrum. A quick calculation and you know how fast the star is moving relative to Terra.

Obviously nowadays they use electronic photosensors instead of photographic film but the principle is the same. Instead of a photo the spectra is displayed as a jagged line in a graph, more accurate but nowhere near as pretty as a rainbow.

Fraunhofer used sodium lines for his lab comparison spectra because they are easily produced by sprinkling common table salt into a Bunsen burner flame. Electronic photosensors do not need comparision spectra because they can directly measure the exact frequency of a given Fraunhofer line.

If the signature is shifted toward the red end of the spectrum (with respect to the comparison spectra), the object has a "red-shift" and is moving away from you (technically its vector has a radial component if you are nit-picky). Shifting the other way is a "blue-shift", meaning the object is approaching.

Due to Hubble's Law, for objects like galaxies which are further away than 10 megaparsecs or so, you can use the red-shift to measure the distance to the galaxy. Which is real convenient, other measurement techniques are a pain in the posterior to utilize, and give fuzzy results.

Not only can you use red/blue shift on objects, but also on parts of objects. Say Planet X is spinning according to the right hand rule. When you look at it through a telescope, if "north" is upward, then the right edge of the planet will be receding from you, and the left edge of the planet will be approaching you. So if you measure the red/blue shift of each limb of the planet, you can calculate how rapidly it is spinning.

There are some binary stars where the two stars are so close that the telescope cannot resolve them (translation: it looks like a single star to the scope). But a spectroscope can reveal the truth. Using the spectroscope, astronomers will see not one but two sets of Fraunhofer lines. By observing how the two sets move back and forth relative to each other the two star's orbital period can be determined. Such stars are called Spectroscopic binaries.

You can even tell if the star has a strong magnetic field. The Zeeman effect is when the signature Fraunhofer lines are split in the presence of a magnetic field. The stronger the field, the wider the split.

This is why books about amateur astronomy tell you a plain old telescope is only useful for seeing stars as bright dots (or watching the co-eds undress through the dormitory windows). But add a spectroscope to your telescope and suddenly you've got a real live scientific instrument that you can do real science with.

Nuclear Signatures

Patterns of sensor readings that will detect the detonation of a nuclear weapon


Patterns of sensor readings that will detect the presence of life on a planet


Patterns of sensor readings that will detect the presence of a technological civilization on a planet. This is very important, because contacting an alien civilization means you are gambling with the extinction of the human species.


Technosignature or technomarker is any evidence of the operation of advanced technology by an extraterrestrial civilization, with the exception of the radio messages which are traditionally searched for in the search for extraterrestrial intelligence (SETI). First covered in Paul Davies's 2010 book The Eerie Silence and later developed by Iván Almár, technosignatures are analogous to the biosignatures that signal the presence of life, whether or not intelligent. Various types of technosignatures, such as radiation leakage from megascale astroengineering installations such as Dyson spheres, the light from an extraterrestrial ecumenopolis, or Shkadov thrusters with the power to alter the orbits of stars around the Galactic Center, may be detectable with hypertelescopes.

Astroengineering projects

A Dyson sphere, constructed by life forms not dissimilar to humans dwelling in proximity to a Sun-like star, would cause an increase in the amount of infrared radiation in the star system's emitted spectrum. Hence, Freeman Dyson selected the title "Search for Artificial Stellar Sources of Infrared Radiation" for his 1960 paper on the subject. SETI has adopted these assumptions in its search, looking for such "infrared heavy" spectra from solar analogs. From 2005, Fermilab has conducted an ongoing survey for such spectra, analyzing data from the Infrared Astronomical Satellite.

Identifying one of the many infra-red sources as a Dyson sphere would require improved techniques for discriminating between a Dyson sphere and natural sources. Fermilab discovered 17 "ambiguous" candidates, of which four have been named "amusing but still questionable". Other searches also resulted in several candidates, which remain unconfirmed. In October 2012, astronomer Geoff Marcy, one of the pioneers of the search for extrasolar planets, was given a research grant to search data from the Kepler telescope, with the aim of detecting possible signs of Dyson spheres.

(ed note: there is one candidate here)

Shkadov thrusters, with the ability to change the orbital paths of stars in order to avoid various dangers to life such as cold molecular clouds or cometary impacts, would also be detectable in a similar fashion to the transiting extrasolar planets searched by Kepler. Unlike planets, though, the thrusters would appear to abruptly stop over the surface of a star rather than crossing it completely, revealing their technological origin. In addition, evidence of targeted extrasolar asteroid mining may also reveal extraterrestrial intelligence.

Planetary analysis

Artificial heat and light

Various astronomers, including Avi Loeb of the Harvard-Smithsonian Center for Astrophysics and Edwin Turner of Princeton University have proposed that artificial light from extraterrestrial planets, such as that originating from cities, industries, and transport networks, could be detected and signal the presence of an advanced civilization. Such approaches, though, make the assumption that the radiant energy generated by civilization would be relatively clustered and can therefore be detected easily.

Light and heat detected from planets must be distinguished from natural sources to conclusively prove the existence of intelligent life on a planet. For example, NASA's 2012 Black Marble experiment showed that significant stable light and heat sources on Earth, such as chronic wildfires in arid Western Australia, originate from uninhabited areas and are naturally occurring.

Atmospheric analysis

Atmospheric analysis of planetary atmospheres, as is already done on various Solar System bodies and in a rudimentary fashion on several hot Jupiter extrasolar planets, may reveal the presence of chemicals produced by technological civilizations. For example, atmospheric emissions from industry on Earth, including nitrogen dioxide and chlorofluorocarbons, are detectable from space. Artificial pollution may therefore be detectable on extrasolar planets. However, there remains a possibility of mis-detection; for example, the atmosphere of Titan has detectable signatures of complex chemicals that are similar to what on Earth are industrial pollutants, though obviously not the byproduct of civilisation. Some SETI scientists have proposed searching for artificial atmospheres created by planetary engineering to produce habitable environments for colonisation by an ETI.

Extraterrestrial artifacts and spacecraft

See also: Potential cultural impact of extraterrestrial contact § Extraterrestrial artifacts

Interstellar spacecraft may be detectable from hundreds to thousands of light-years away through various forms of radiation, such as the photons emitted by an antimatter rocket or cyclotron radiation from the interaction of a magnetic sail with the interstellar medium. Such a signal would be easily distinguishable from a natural signal and could hence firmly establish the existence of extraterrestrial life were it to be detected. In addition, smaller Bracewell probes within the Solar System itself may also be detectable by means of optical or radio searches.

From the Wikipedia entry for TECHNOSIGNATURE


Patterns of sensor readings that will detect the remains of annihilated civilizations. These could be the gravesite of Forerunners, with the promise/threat of valuable/civilization-killing paleotechnology.

The paper is focused on refining the "L" factor in the famous Drake equation, the average lifetime of a technological civilization. So the paper estimates some scenarios where a planetary civilization can destroy itself, and tries to figure out what their sensor signatures are. Then astronomers can see if they can spot any. If they see lots, it might mean L is quite short.

But for our purposes, keep in mind that many of these sensor signatures will also work if a civilization has been exterminated by external alien invaders.


…The aim of this paper is to use the Earth as a test case in order to categorise the potential scenarios for complete civilisational destruction, quantify the observable signatures that these scenarios might leave behind, and determine whether these would be observable with current or near-future technology.

The variety of potential apocalyptic scenarios are essentially only limited in scope by imagination and in plausibility according to our current understanding of science. However, the scenarios considered here are limited to those that: are self inflicted (and therefore imply the development of intelligence and sufficient technology); technologically plausible (even if the technology does not currently exist); and that totally eliminate the (in the test case) human civilisation.

Only a few plausible scenarios fulfil these criteria:

  1. complete nuclear, mutually-assured destruction
  2. a biological or chemical agent designed to kill either the human species, all animals, all eukaryotes, or all living things
  3. a technological disaster such as the “grey goo” scenario, or
  4. excessive pollution of the star, planet or interplanetary environment

Other scenarios, such as an extinction level impact event, dangerous stellar activity or ecological collapse could occur without the intervention of an intelligent species, and any signatures produced in these events would not imply intelligent life…

2.1 Nuclear Annihilation

Current estimates of nuclear weapons held around the world are of the order 6 million kilotonnes (kt) (2.5 x 1016 J)…

…Nuclear weapons produce a short, intense burst of gamma radiation with a characteristic double peak over several milliseconds. These gamma flashes could be detected using the same techniques as for the detection of gamma ray bursts (GRB)…

…Given that the world’s nuclear arsenal is equivalent to around 1019 J of energy, the resulting radiation from its combined detonation would be much fainter than a typical GRB. If we assume that the energy is released on a similar timescale and with a similar spectrum to a GRB, a nuclear apocalypse is equivalent in bolometric flux to a GRB detonating around a trillion times closer than its typical distance. If we take a nearby GRB such as GRB 980425 which is thought to have detonated around 40 Mpc away, then we would expect a global nuclear detonation event to produce a similar amount of bolometric flux only 8 AU away!

Therefore, for us to be able to detect nuclear detonation outside the Solar system, the total energy of detonation must be at least nine orders of magnitude larger, i.e. the ETIs responsible for the event must engage in massive weapon proliferation and concurrent usage.

However, the production of fallout from terrestrial size payloads, which persists for much longer timescales, may make itself visible in studies of extrasolar planet atmospheres.

For the purposes of estimating fallout, the weapon impacts are assumed to be evenly distributed across the entire land area of the planet (1.5 x 108 km2 ). This gives an equivalent of approximately one 25 kt (1011 J) weapon per square kilometre of land area. This is of the same order of magnitude as the weapon used in the Semipalatinsk Nuclear Test, for which the effects of radioactive fallout were measured over time. However, given the local climatic conditions at this site (which were very windy) and the fact that our estimates include nuclear events every square kilometre, the effects are likely to be much worse than the results of this test. From measurements of soil at a town near the test site and modelling of radionuclide decay chains, the dose rate due to fallout from the weapon test (not the dose from the blast itself) was shown to begin around 10 3 microgray/hour, decaying to background levels after around 100 days.

Fallout products of fusion weapons are typically nonradioactive, though they do produce a low yield of energetic protons and electrons. Most fallout products from fission weapons are beta emitters and decay to other beta emitting isotopes. Some radioisotopes produced by fission weapons are gamma emitters, but these have short halflives. Ignoring the effects on the health of humans or other lifeforms (which would be severe), the deposition of a large amount of betaradioactive material into the atmosphere would have a significant effect on atmospheric chemistry and would quickly ionise many atmospheric species, with high altitude nuclear tests increasing local electron density several times. This would give ionised air the distinct blue or green of nitrogen and oxygen emission. Given that spacecraft and Earth based telescopes have detected (faint) nighttime airglow on Venus and Mars it may be possible to measure what would be considerably brighter airglow features in exoplanets, given that the order of magnitude increase in electron density caused by a nuclear war would generate an order of magnitude increase in airglow brightness. The brightest airglow feature in the visible spectrum on an Earthlike exoplanet would be the green oxygen line at 558 nm, which would be enhanced by global nuclear war to a photon flux of up to 1400 rayleighs.

IR emission from exoplanets in their secondary eclipse phase has been measured by spacebased telescopes so in theory these measurements could be extended into the visible part of the spectrum in future, though this would require exquisite precision in our knowledge of the host star’s properties, and would most likely be dominated by reflected light from the planet itself, especially in the bluegreen spectral region. A tenfold increase in brightness at 558 nm would potentially be observable with only a modest increase in sensitivity over instruments observing exoplanets today, especially since the airglow maximum occurs well above the tropopause and would therefore be observable above even a very cloudy planet. Airglow caused by fallout products would last for several years before decaying to unobservable levels.

The thermal effects of nuclear explosions also affect atmospheric chemistry. For every kilotonne in yield, approximately 5000 tonnes of nitrogen oxides are produced by the blast itself. Blasts from higher yield weapons will carry these nitrogen oxides high into the stratosphere, where they are able to react with and significantly deplete the ozone layer. Ozone can be detected in the ultraviolet transmission spectrum of an exoplanet, as can other oxygen molecules, and so the disruption of an exoplanetary ozone layer presents another potential observational signature.

Global nuclear war therefore potentially offers several spectral signatures that could be observed: a gamma flash, followed by UV/visible airglow and the depletion of ozone signatures. However, the aftermath of a global nuclear war will also act to obscure these spectral signatures. Groundburst nuclear explosions generate a significant amount of dust that will be lofted into the atmosphere. Airburst explosions do not generate dust, but still introduce particulates into the atmosphere. Atmospheric effects of nuclear warfare have been extensively modelled in climate simulations, the global consequences being known as “nuclear winter”. Recent simulations have shown that even with reduced modern nuclear arsenals severe climate effects are felt for at least ten years after a global conflict, especially due to the long lifetime of aerosols lofted into the stratosphere. They show that the atmospheric optical depth is increased several times for several years. The worst effects are confined to the northern hemisphere given that the model includes conflict over the US and Russia, though the entire planet is affected to a lesser extent.

A nuclear winter would dramatically increase the opacity of the atmosphere. This process itself would be observable if a planet observed with a previously transparent atmosphere (perhaps with an Earthlike spectroscopic signature) was observed again and the atmosphere was opaque, this would be a sign of a large dust event. However, such an event could also be caused by a large impact and therefore would not imply a civilisation had caused the disaster (though would be interesting in itself). If the atmosphere had not been observed before the event, it would simply seem like the planet had an extremely dusty atmosphere. What would be crucial is measuring the relative change in atmosphere as a result of nuclear detonation, hopefully with an added bonus of identifying a weak gamma ray or other high energy emission in the vicinity of the planet.

Hence, to confirm that a planet had been subject to a global nuclear catastrophe would require the observation of several independent signatures in short succession. One on its own is unlikely to be sufficient, and could easily be caused by any number of other processes on planets with potentially no biological activity whatsoever. There are cases beyond a global nuclear catastrophe that a spacefaring civilisation might be able to inflict on itself, given that the destructive energy at their disposal would be far greater than nuclear weapons, including redirecting asteroids. These would be far more destructive than nuclear warfare but would generate observable signatures different than those of a naturally occurring impact event.

2.2 Biological Warfare

Biological warfare involves the use of naturally occurring, or artificially modified, bacteria, viruses or other biological agents to intentionally cause illness or death. The use of a naturally occurring pathogen in a global conflict would probably have a limited net effect on a global population. The destruction would be selflimiting; once a population is reduced in size, transmission from host to host would become more difficult and the epidemic eventually ends. Artificially modified or created biological agents however, could potentially push a civilisation to extinction…

…Assuming a global conflict took place that made use of this method of warfare on a planet that hosts an intelligent civilisation, we pose the question of whether the self-destruction of that species, via this method, could be remotely observable.

If we assume that the time between the release of the engineered virus and its global spread is very short and that the virus is potent enough that a civilisation becomes fully extinct, the environmental impacts of this scenario can be assessed. The actions of anaerobic organisms cause biomass to decay, releasing methanethiol, CH3SH (via production of methionine) as one of the products. This can be spectrally inferred and has no abiotic source. For a population with a similar biomass to the present human population (currently, in terms of carbon biomass, ~2.8x1011 kg), the decay products can be estimated. Since the dry mass of a cell is approximately 50% carbon, the total human biomass would be 5.6x1011 kg. With an estimated cell sulphur content of 0.3-1%, the maximum amount of S available to form CH3SH would be 5.6x109 kg. If 10% of this S is incorporated into methionine, all of which is then converted into methanethiol, this would result in a total CH3SH flux of ~108 kg.

At the current biological production rate on Earth, this would be released to the atmosphere over a period of a year and would rapidly photodissociate, making this a very shortlived biosignature. One of the products of the decay of methanethiol is ethane (C2H6), which can be spectrally detected, but has an atmospheric lifetime under Earth-like conditions of < 1 year, leading to a short window of time for detection. Additionally, if carrion-eating species were unaffected, this would reduce the amount of organic matter available for microbial decay, further reducing the final biosignature.

However, if the engineered virus could cross species barriers, then the total amount of dead biomass could be as high as 6x1013 kg (the total animal biomass on Earth), potentially producing 1011 kg of CH3SH, which would enter the atmosphere over a period of ~30 years. It is likely that, due to its short atmospheric lifetime, this atmospheric CH3SH would still not produce a detectable signature. However, the associated C2H6 absorption signature between 11-13 μm may lend itself to remote detection. This signature would be deeper and therefore more readily detectable if the CH3SH production rate was higher.

Other decay products include CH4, H4S, NH3 and CO2. The most promising biosignature gas for global bioterrorism is CH4. The CH4 flux to the atmosphere is related to ethane production, potentially increasing the C2H6 absorption signature…For the case where only humans can be infected, both signatures are shortlived, requiring observations to be taking place at exactly the right time for a detection to be made. In the case where the virus can cross species barriers leading the the total annihilation of animal life, persistently high levels of these gases could make a detection more likely.

2.3 Destruction via ‘Grey Goo’

The terrestrial biosphere offers many examples of naturally occurring nanoscale machines. Feynman extolled the advantages of engineering at atomic scales. In Engines of Creation, Drexler described “nanotechnology” as a means of fabricating structures at nanoscales using chemical machinery. While the word now has a broader meaning, we can still consider the possibility that such a machine can be sufficiently generalpurpose to be able to make a copy of itself.

Following Phoenix and Drexler we define an engineered system that can duplicate itself exactly in a resource-limited environment as a self-replicator. (NB: This strict definition excludes biological replicators, as they are not engineered). The engineers of such machines have two broad choices as to what resources the self-replicator might use: resources that are naturally occurring in the biosphere, and resources that are not. Engineers that make the former choice run the risk of a “grey goo” scenario, where uncontrolled self-replication converts a large fraction of available biomass into self-replicators, collapsing the biosphere and destroying life on a world. This may be an accident or failure of oversight, or it may be due to a deliberate attack, where the replicators are specifically designed to destroy biomass (what Freitas refers to as “goodbots” and “badbots” respectively). In Engines of Creation, Drexler notes:

“Replicators can be more potent than nuclear weapons: to devastate Earth with bombs would require masses of exotic hardware and rare isotopes, but to destroy all life with replicators would require only a single speck made of ordinary elements. Replicators give nuclear war some company as a potential cause of extinction, giving a broader context to extinction as a moral concern.”

Freitas places some important limitations on the ability of replicators to convert the biosphere into “grey goo” (land based replicators), “grey lichen” (chemolithotrophic replicators), “grey plankton” (ocean-borne replicators) and “grey dust” (airborne replicators). With conservative estimates based on contemporary technology, it is suggested that if the replicators are carbon-rich, around a quarter of the Earth’s biomass could be converted as quickly as a few weeks. Equally, Freitas estimates the energy dissipated by carbon conversion, implying that subsequent thermal signatures (local heating and local changes to atmospheric opacity) would be sufficient to trigger local defence systems to combat gooification. For example, In the case of malevolent airborne replicators, a possible defensive strategy is the deployment of non-self replicating “goodbots” which unfurl a dragnet to remove them from the atmosphere.

Phoenix and Drexler emphasise that all these variants of the grey goo scenario are easily avoidable, provided that engineers design wisely (and that military powers exercise restraint). Indeed, they indicate that fully autonomous self-replicating units are not likely to be the most efficient design choice for manufacturing, and that having a central control computer guiding production is likely to be safer and more cost-effective. Provided that the control computer is not separated by distances large enough to introduce time-lag, as would be the case on interplanetary scales, this seems to be reasonable.

However, this still leaves the risk of replicator technology being weaponised. We will assume, as we do throughout this paper, that prudence is not a universal trait in galactic civilisations, and that grey goo is a potential death channel that might be detected.

So what signatures might a grey goo scenario produce? If a quarter of the Earth’s biomass is converted into micron sized objects, how would this affect spectra of Earthlike planets? This situation shares several parallels with the nuclear winter scenario described previously. In the case of grey goo, we may expect there to be a substantially larger amount of “dust”, as well as a fixed grain size. This will be deposited as sand dunes or suspended in the atmosphere, with similar spectral signatures as previously discussed.

Depending on the grain size of the dunes, it may be possible to observe a brightness increase as the angle measured by the observer between the illumination source (the host star) and the planet decreases towards zero on the approach to secondary eclipse.

Surfaces that are composed of a large number of relatively small elements packed together will produce significant shadowing. This shadowing increases as the angle between the surface and an illumination source increases. As the angle decreases towards zero, these shadows disappear, resulting in a net increase in brightness. This is sometimes described as the opposition surge effect, or the Seeliger effect in deference to Hugo von Seeliger, who first described it. Seeliger saw this shadowhiding mechanism in Saturn’s rings, which grow brighter at opposition relative to the planetary disc. Coherent backscattering of light also plays a role in this brightening effect.

This phenomenon is observed in the lunar regolith, so it seems reasonable to expect that this phenomenon would also act in artificially generated regoliths such as those we might expect from a grey goo incident. During exoplanet transits, it may be possible to detect an increase in the brightness of the system as the planet enters secondary eclipse. The Moon’s brightness increases by around 40% as it moves towards the peak of opposition surge, so it may well be the case that grey-goo planets produce opposition surges of similar magnitude. Buratti notes that the wavelength dependence of the surge is relatively weak, which would suggest that nearIR observations may be sufficient to observe this phenomenon.

On what timescale might we expect this artificial nano-sand to persist on a planetary surface? If the planet has an active hydrological cycle, airborne replicators will be incorporated into precipitation and delivered to the planet’s surface. The Earth’s Sahara desert transports away of order a billion tonnes of sand per year. Deposition into rivers and streams may deliver the material to oceans, and eventually the seabed, effectively removing it from view at interstellar distances. This material will be subducted into the mantle and reprocessed on geological timescales, removing all trace of engineering. Using Freitas’s estimate of available biomass, and assuming the nano-sand can be processed out of view at a few billion tonnes per year (which we propose as an upper limit) then this suggests that a goo-ified planet may require several thousand years to refresh its surface. It is likely that processing rates may be accelerated or impeded by other physical processes, but it seems to be the case that goo-ified planets remain characterisable as such over timescales comparable to that of recorded human history.

2.5 Total Planetary Destruction

Finally, it is not inconceivable that a civilisation capable of harnessing large amounts of energy could unbind a large fraction (or all) of a planet’s mass. Kardashev Type II civilisations wishing to build a Dyson sphere require this capability to generate raw materials for the sphere — it is estimated that to create a Dyson sphere in the Solar System with radius 1 AU would require the destruction of Mercury and Venus to supply sufficient raw materials.

Equally, civilisations with access to this level of energy control and manipulation may decide to use it maliciously, destroying large parts of a planetary habitat while it is still occupied, and in the extreme case destroying the planet completely. This would release a significant fraction of the planet’s gravitational binding energy.

The Earth’s binding energy is of order 1039 ergs. This is again several orders of magnitude fainter than a typical supernova or GRB of 10 51 ergs, but is strong compared to the solar luminosity — the Sun would require several days to radiate the same quantity of energy. This would likely produce a gamma ray signature even stronger than expected from the nuclear winter scenario described previously, and we may expect afterglows similar to those observed in other astrophysical explosions.

The destruction of an orbiting body will produce a ring of debris around the central star, in a manner analogous to the production of rings when solid bodies cross the Roche limit of a larger body.

The subsequent evolution of this material will be similar to that of the debris discs. The remnants of the planet formation process, debris discs have been detected around a variety of stars, and the behaviour of grains of differing sizes under gravity and radiation pressure has been modelled in detail. It is likely that, if a terrestrial planet has been destroyed, the debris will be principally composed of silicates, and as such any detection of refined or engineered materials is unlikely, even if such matter survives the planet’s demise untouched.

The fate of the material depends largely on the local gravitational potential and the local radiation field, as well as the grain size distribution of the debris. Grains below the “blowout” size — typically a few microns — will be removed from the system via radiation pressure. Neighbouring planets may collect some of the remaining debris in resonances while the debris grinds into material of sufficient grain size that it either loses angular momentum through Poynting-Robertson drag and is consumed by the central star or a neighbouring planet, or gains momentum through radiative forces and is removed from the system.

In any case, this death channel does not appear to be amenable to detection by Earth astronomers. If we are fortunate to witness the instant of destruction, then we may be able to speculate on the energies released in the event, and search for a natural progenitor of such energy, i.e. another celestial body. Giant impacts between planet-sized bodies will produce the required energies to unbind or destroy one of the objects, as was the case for the impact which formed the EarthMoon system. If such efforts fail, and no other explanation fits the observations, then we may tentatively consider extraterrestrial foul play.

The timescale for observing destruction as it happens will be short — perhaps a few days. The debris can be expected to persist for several centuries, but observing this is unlikely to elucidate its origins as a destroyed planet.

Prospects for Observing Civilisation Destruction
Death ChannelDetection MethodSignature
of Active
Signature of
Timescale (yr)
Gamma ray
detection, Transit
YY0-5 years
BioterrorismTransit spectroscopyYY1-30 years
Grey GooTransit spectroscopy
and photometry
NY>1,000 years
Stellar PollutionAsteroseismology,
stellar abundance
YY>100,000 years
(depending on
Transit spectroscopy
Orbital Pollution
Transit spectroscopy
and photometry
YY<100,000 years
Total Planetary
Debris Disk Imaging
YY<100,000 years


For three hundred years, while its fame spread across the world, the little town had stood here at the river’s bend. Time and change had touched it lightly; it had heard from afar both the coming of the Armada and the fall of the Third Reich, and all Man’s wars had passed it by.

Now it was gone, as though it had never been. In a moment of time the toil and treasure of centuries had been swept away. The vanished streets could still be traced as faint marks in the vitrified ground, but of the houses, nothing remained. Steel and concrete, plaster and ancient oak—it had mattered little at the end. In the moment of death they had stood together, transfixed by the glare of the detonating bomb. Then, even before they could flash into fire, the blast waves had reached them and they had ceased to be. Mile upon mile the ravening hemisphere of flame had expanded over the level farmlands, and from its heart had risen the twisting totem-pole that had haunted the minds of men for so long, and to such little purpose.

The rocket had been a stray, one of the last ever to be fired. It was hard to say for what target it had been intended. Certainly not London, for London was no longer a military objective. London, indeed, was no longer anything at all. Long ago the men whose duty it was had calculated that three of the hydrogen bombs would be sufficient for that rather small target. In sending twenty, they had been perhaps a little overzealous.

This was not one of the twenty that had done their work so well. Both its destination and its origin were unknown: whether it had come cross the lonely Arctic wastes or far above the waters of the Atlantic, no one could tell and there were few now who cared. Once there had been men who had known such things, who had watched from afar the flight of the great projectiles and had sent their own missiles to meet them. Often that appointment had been kept, high above the Earth where the sky was black and sun and stars shared the heavens together. Then there had bloomed for a moment that indescribable flame, sending out into space a message that in centuries to come other eyes than Man’s would see and understand.

From THE CURSE by Arthur C. Clarke (1946)

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