SETI: Starship Radiation Signatures

by Paul Gilster on August 21, 2012

Yesterday we pondered the possibility of detecting an interstellar craft as a new kind of SETI. If the energies needed to drive such a vessel are as titanic as we think, there could be a detectable signature, as Robert Zubrin pointed out in a 1995 paper. Zubrin’s best case in visible light involved an antimatter engine whose exhaust could be detected from as far as 300 light years from Earth. That would cover a huge number of stars, as 100,000 exist within 200 light years of our planet.

I suppose the classic starship detection occurs in Larry Niven and Jerry Pournelle’s 1975 novel The Mote in God’s Eye, where human starfarers using the ‘Alderson Drive’ — which allows instantaneous jumps between stars — detect an alien, laser-pushed lightsail. The starship is a throwback, an older technology that human interstellar methods have long superseded, one that contains a strange, asymmetric alien being, the first extraterrestrial humans have encountered. It’s no surprise to learn that Niven and Pournelle fine-tuned the laser lightsail idea through conversations with Robert Forward, who studied such concepts intensively in the scientific literature.

And then there’s Gregory Benford’s 2006 novelette about an astronomer who begins to suspect the anomalous object he’s looking at isn’t natural. Whereas Zubrin looked at antimatter, fusion, fission and magsails, Benford’s astronomer thinks he’s found something like a Bussard ramjet:

“What you wrote,” she said wonderingly. “It’s a…star ship?”

“Was. It got into trouble of some kind these last few days. That’s why the wake behind it – “ he tapped the Fantis’ image – “got longer. Then, hours later, it got turbulent, and—it exploded.”

She sipped her coffee. “This is…was…light years away?”

“Yes, and headed somewhere else. It was sending out a regular beamed transmission, one that swept around as the ship rotated, every 47 seconds.”

Her eyes widened. “You’re sure?”

“Let’s say it’s a working hypothesis.”

The story is “Bow Shock,” which ran in Jim Baen’s Universe and is reprinted in Benford’s new collection Anomalies. Benford speculates that synchrotron radiation in the bow shock of a magnetically screened starship would be detectable at microwave and radio frequencies, and perhaps an easier catch than the torch of Zubrin’s antimatter spacecraft from a great distance. Ralph, Benford’s astronomer, had been examining what he thought was a runaway neutron star, ‘a faint finger in maps centered on the plane of the galaxy, just a dim scratch,’ and if we ever do make a starship detection, this could be more or less what we find, a changeable, ambiguous object.

Image: Note the bow shock in the binary star system BZ Cam, created as the system moves through surrounding interstellar gas. In most cataclysmic variables, matter from a normal star accumulates on the surface of the companion white dwarf star, eventually causing a nova-like flare. In BZ Cam, however, light appears to flicker unpredictably, and an unusually large wind of particles is being expelled. BZ Cam lies about 2500 light-years away toward the constellation of Camelopardalis. Credit: R. Casalegno, C. Conselice et al., WIYN, NOAO, MURST, NSF.

Magsail Braking and Detection

A magnetic sail (‘magsail’) that deflects charged particles is also an interesting possibility. In his 1995 paper, Zubrin points out that a sail like this would be of value because it could decelerate a starship without the use of propellant, allowing it to brake against the stellar wind of the destination star. Here again we get a bow shock which will heat the interstellar medium to a degree dependent on the ship’s velocity, creating a plasma that will encounter the magnetic field of the magsail and produce radiation. This is an ingenious analysis — Zubrin, having gone through the equations governing the rate of deceleration of a magsail in the interstellar medium, notes the changes that occur as the velocity of the ship decreases and the bow shock heating begins to fall, causing the cyclotron radiation emitted by the bow shock to fall at the same time.

A large enough starship decelerating as it neared its destination should, according to Zubrin, put out emissions in the X-ray and radio ranges. Thus we get our starship signature: “The decline in bremsstrahlung energies and cyclotron frequencies in time in accord with the form of equation (2) [governing the rate of deceleration of the magsail] would be a dead giveaway that the emitting object was a decelerating magsail.” Governing the radiation emitted is the ship’s velocity and the density of ions in the interstellar medium. Zubrin calculates that at a density of 1 ion per cubic centimeter, a ship decelerating from a cruise velocity of 0.1 c will produce radiation at about 12 kHz.

Would we have a real chance at such a detection? From the paper:

It can be seen that the magsail radiation of a characteristic fusion starship being decelerated from a cruise velocity of 0.1c could be detected by a 6 km orbiting antenna from a distance of 400 light years, while that emitted by a characteristic antimatter photon rocket in its deceleration phase could be seen as far away as 2,000 light-years. There are about 100,000,000 stellar systems to be found within the latter distance. This extended range detection capability combined with magsail radiation’s unique time-dependent frequency spectrum appears to make a search for magsail radiation the most promising option for extraterrestrial starship detection.

You’ll recall that Zubrin estimated the characteristic mass of his ‘standard’ starship as 1,000,000 tons, something he describes as ‘a speculative guess.’ But even if the guess is off the mark, the idea of starship detection remains sound, in his view, because a decrease of ship mass by two orders of magnitude only decreases the detectability distance by one order of magnitude. We’re still looking at detecting a 10,000 ton starship (using a magsail) at 40 light years via a 6 km antenna, while a 30 km antenna could make the detection at a range of 200 light years.

Further thoughts on starship detection tomorrow. But let me add one note before closing: Al Jackson pointed out in the comments to yesterday’s post that Zubrin does not reference an important paper. It’s D. R. J. Viewing, C. Horswell, E. W. Palmer, “Detection of Starships,” JBIS, 30, 99-104 (1977), and it’s not one I’ve seen yet either. Because of that, let me quote Al:

The Viewing , et.al., paper looks at two kinds of starships, Innocuous and Energetic. Almost the same propulsion systems are considered as in Zubrin’s paper. That paper got me interested in what truly relativistic ships would look like in the simple case when a propulsion system does is not 100% efficient, and loses energy by waste heat. Suppose that in its rest frame it radiates isotropically. A simple relativistic kinematic effect will be that the radiation will be beamed relative to another rest frame. Called the ‘head light’ effect in special relativity. See: Correspondence: A. A. Jackson, IV, “Ultra-Relativistic Starships,” JBIS, 32, 240 (1979).

The Zubrin reference is “Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft,” Progress in the Search for Extraterrestrial Life, ASP Conference Series Vol. 74 (1995). Available online.

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Christopher L. Bennett August 21, 2012 at 11:19

Would the detection of a magsail ship depend on whether it was decelerating? If it were coasting at a fairly high percentage of c, using its magnetic fields only for, say, shielding the crew from radiation rather than for braking, how detectable would its bow shock be?

Christopher Phoenix August 21, 2012 at 18:49

I think searching for the radiation signatures of starships is a much more logical way to go about searching for intelligent extraterrestrials than searching for the kind of radio signals most SETI astronomers hope for. We still can’t be sure that aliens haven’t developed propulsion systems based on physics breakthroughs we haven’t made yet- like zero-point energy engines or FTL drives, but searching for various kinds of fission, fusion, and antimatter rockets or magsails is a very good start.

It would be interesting to see how the general public would react to the discovery of a starship in deep space. With so many popular depictions of star travel, and such excitement over the possibility of habitable worlds, would people start to leap to conclusions about the inhabitants of the starship? Would there be panic over the possibility of an alien invasion, or would there be hope for a peaceful extraterrestrial contact? Would there be a call to attempt to communicate with the aliens or, technology allowing, send a starship of our own on a long journey to reach the alien civilization that launched this ship?

Wojciech August 21, 2012 at 21:35

With all this talk about detecting spaceships, I wonder if anybody ever made a theory that this:
http://en.wikipedia.org/wiki/Radio_source_SHGb02%2B14a

Was a signal from a probe or a spaceship, although completely unintentional nor aiming at humanity.

David August 22, 2012 at 2:57

Zubrin calculates a radio frequency of around 14 kHz from the Cyclotron process. The problem
is that this is below the plasma frequency of the solar wind in the vicinity of the Earth so that such a signal could not pass through to the inner solar system in the same way that there is a cutoff frequency for radio waves to pass through the Ionesphere. Using the often quoted
electron density of 7 cm^-3, the plasma frequency is around 24 kHz so a 14 Khz signal simply won’t reach the Earth’s vicinity. In other words, any such low frequency radio telescope would have to be in the outer solar system.

There would still be other potential problems such as confusion meaning that how do you separate your signal from something else which has a small angular separation. You could have a situation that the more sensitive you make your telescope, the more likely you will also detect faint unrelated signals close so that one can’t easily disentangle signal from background stuff.

James Jason Wentworth August 22, 2012 at 3:16

In his science fiction short story “The Fourth Profession” (it is included in the anthology “A Hole in Space” and in “Project Solar Sail”), Larry Niven described how the approach of a light sail starship to our solar system was detected:

“All right. The astronomers were studying a nearby nova, so they caught the intruder a little sooner. It showed a strange spectrum, radically different from a nova and much more constant. It got even stranger. The light was growing brighter at the same time the spectral lines were shifting toward the red.

“It was months before anyone identified the spectrum.

“Then one Jerome Finney finally caught wise. He showed that the spectrum was the light of our own sun, drastically blue-shifted. Some kind of mirror was coming at us, moving at a hell of a clip, but slowing as it came.”

nickCD August 22, 2012 at 4:30

Searching for the emissions, reflections, and absorptions of live artifacts generally is a far better way to go than searching specifically for messsages in a communications medium that works best communicating around the surface of a planet (radio) but quite likely works far less well than many other parts of the spectrum for space-to-space communications.

10 magsail radiation’s unique time-dependent frequency spectrum…1,000,000 tons

These are good examples of the two most important things to think about, generally:

(1) The ability to distinguish the spectral signature from natural spectral signatures (This is where the so-called “Wow signal” breaks down — it’s at a common wavelength of the most common element in the universe — yawn). It’s not just a matter of whether we can detect the photons at this range — it’s whether we can distinguish them from the many photons from the many natural sources.

(2) The size of the radiating surface, or a localized group of radiating surfaces (e.g. a fleet of starships, or a civilization full of starships), here expressed indirectly as the mass of one starship which implies the energy needed for a given level of deceleration. For pioneering interstellar missions, this relates strongly to the minimal size of a reasonable (i.e. non-poverty-stricken) self-sufficient economy. Which while indeed speculative, we have good economics reasons to believe (cf. Adam Smith on the benefits of specialization and the division of labor) is awfully big. 1 million tons may be a severe underestimate — the minimal size of a pioneering interstellar mission may be in the trillions of tons or more, resulting in a far more energetic signature.

Furthermore, we should think about what a galaxy full of such emissions would look like. In other words, if a significant fraction of the energy of a distant galaxy were used for interstellar travel, using fusion or antimatter and magsails, would a distinguishable portion of that galaxy’s spectra be distinctly artificial? We have billions of galaxies to look at, a far vaster search space than mere space within a few hundred or thousand light-years of ourselves.

And there are other spectral signatures from the emissions, reflections, and absorptions of an advanced and Malthusian-impelled civilization that would probably be even more striking:

* quantum devices for illumination, or radiative communication or energy transport, of which we know of a few examples (LEDs, lasers, etc.), which generally have blatantly artificial spectra.

* optically optimized surfaces (for radiation, reflection, certain thermal properties, etc.), which often have blatantly artificial spectra due to their surface engineering properties, especially use of uncommon elements (e.g. gold) or molecules (e.g. chlorophyll) at their optical surfaces.

* band gaps in solar cells and similar surfaces.

Such search strategies, looking for live artifacts (not archaeology — such surfaces if not maintained would soon degrade to natural-looking surfaces), would likely be many orders of magnitude more productive than radio SETI. And rather than be satisfied with a hodge-podge list of familiar things to look for (magsails, chlorophyll, lasers, solar cells, etc.), we should develop more general ways of distinguishing such artificial or highly-evolved spectral signatures from the natural background — using computers to search through petabytes of data for us.

Greg Parris August 22, 2012 at 14:07

Focusing our search for ETI on things like interstellar travel is a really good idea.

We don’t make energy, we harvest it. This harvesting is usually done in a low energy density fashion. Such as, sunlight on the leaves of a tree (or our Dyson Swarm), digging Uranium out of the ground, filtering seawater for Deuterium, punching holes in the ground and sucking out carbon compounds. Preferably the energy is harvested in a form that is easily stored but if not we must convert it into a storable form. Now, we harvest and store energy so we can expend it in a controlled fashion.

We can be rather ostentatious with our displays stored energy; the Saturn 5 Launch vehicle, nuclear detonations, interstellar travel. The latter being particularly energetic. There are few applications of energy (that we can think of) that are more energetic than interstellar travel. So, this is going to be a great place to look.

Also, given all the posts on energy requirements for interstellar travel, there are going to be only a handful of places to harvest this much energy and just a handful of ways to store this much energy. Surely, there is a Dyson Swarm or two out there to find.

Rob Henry August 22, 2012 at 18:56

Christopher Phoenix writes,
“It would be interesting to see how the general public would react to the discovery of a starship in deep space. … Would there be a call to attempt to communicate with the aliens or, technology allowing, send a starship of our own on a long journey to reach the alien civilization that launched this ship?”

But I think that if they’re there, they’re almost certainly here. After such a detection we should look to the Oort Cloud. Even more surprisingly, we only know of a huge 300m Earth Trojan because its orbit took it further from the sun than was usual for such objects.
http://www.centauri-dreams.org/?p=18953
If there are cavernous ETI observation posts at our L4 and L5, it seems then, that we would not have detected them yet. We would surely find them within months of that hypothetical starship detection though. In this unlikely series of events, I further suggest that such observation posts might explain Long Delayed Radio Echoes.

James Jason Wentworth August 23, 2012 at 3:02

Rob Henry wrote:

“Christopher Phoenix writes,
‘It would be interesting to see how the general public would react to the discovery of a starship in deep space. … Would there be a call to attempt to communicate with the aliens or, technology allowing, send a starship of our own on a long journey to reach the alien civilization that launched this ship?’”

“But I think that if they’re there, they’re almost certainly here. After such a detection we should look to the Oort Cloud. Even more surprisingly, we only know of a huge 300m Earth Trojan because its orbit took it further from the sun than was usual for such objects.
http://www.centauri-dreams.org/?p=18953
If there are cavernous ETI observation posts at our L4 and L5, it seems then, that we would not have detected them yet. We would surely find them within months of that hypothetical starship detection though. In this unlikely series of events, I further suggest that such observation posts might explain Long Delayed Radio Echoes.”

LDEs (Long Delay Echoes) have long been associated with the UFO phenomenon. However, even though this is only a slightly modified version of the ETH (Extra-Terrestrial Hypothesis) that was/is proposed as an explanation for some UFO sightings (“Aliens [or their automated vehicles] are present in our solar system, but they either have *not* visited Earth or did/do so very rarely”), I think the LDE phenomenon is well worth investigating, as even any as-yet-unknown *natural* cause that may be responsible for them would be extremely interesting–and perhaps even useful, once understood. Also:

While I am not a “UFO believer” (in the sense that that term is generally understood to mean), I find the subject interesting because the sighting reports *are* data on something (or more likely, many things) that may range from mental illness to heretofore-unknown plasma phenomena (such as the “red sprites” and “blue jets”). While I do not think it is terribly likely that alien spacecraft or automated probes (or inter-dimensional vehicles?) are an explanation for many–if any–UFO sightings, it is unscientific (not to mention intellectually cowardly) to dismiss the entire subject as unworthy of scientific study simply because of the “giggle factor.” There is no telling what we might learn, perhaps even–as fantastic as it is to contemplate–that the LDEs could indeed have come from a vehicle or vehicles not of human manufacture that received and re-broadcast terrestrial radio and television signals.

Rob Henry August 23, 2012 at 22:08

James Wentworth, anyone who is observant should find unidentified patterns in the skies occasionally – and such a study would more likely give data on how observant people were, and comparatively little information on the causes of rare optical phenomena. By comparison those LDE’s give solid evidence of something that is not well understood by our physical scientists.

I was wrong to say that LDE’s might be explained by ETI probes at L4 and L5. To match observations of the time delays it seems I must also posit their existence at L1 and (buried) in the Lunar surface. I wonder if we should look at high resolution photos of the exact centre of Earthward side of the Lunar surface. Though I admit that this is the longest of long shots.

Dennis August 26, 2012 at 17:08

Any starship crew on a long voyage will want to watch the latest news, educational, and sports programing from the home world. Signals aimed at a ship headed our way will get here before they do. We will unscramble the feed and have assimilated their culture and technologies before they know we exist! Evens the odds of us getting along with them!

ljk August 31, 2012 at 12:39

Cherenkov radiation seems to be a bother to Optical SETI, at least the one at Harvard:

http://www.planetary.org/blogs/bruce-betts/20120830-seti-update.html

Dan November 25, 2012 at 8:11

Zubrin’s paper is interesting and perfectly valid, but in describing a typical starship of mass 1 million tonnes, there is a fundamental problem. Starships like that are very expensive to build, and thus I think voyages by them will be very rare. If they were common, we would have detected them already.

But any civilisation that can build a starship of that kind will have a Kardeshev type II level of energy consumption in their own solar system. The waste heat from their starship fuelling facility alone would be enormous, and easily detectable, because it would be a continuous output 24 hours a day.

Detecting waste heat will work whatever the technology type of the civilisation concerned.

HughP November 26, 2012 at 9:50

“That would cover a huge number of stars, as 100,000 exist within 200 light years of our planet.”

This is another assertion of the number of stars within some specified distance. No authority asserted for the number.

Is there a compilation of the numberss of stars or star systems within specified distances? I hope the list includes some references to the authorities for various numbers or else an explanation of the computations.

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