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Starship Observational Signatures

Now and again in relatively rarefied SETI discussions the topic of starship detection comes up. Specifically, if there were a starship moving through the interstellar medium in the general vicinity of our perch in the Orion Arm, would we be able to detect any sort of signature in our astronomical data? Centauri Dreams regular Al Jackson has looked into this for a variety of starship types (and discussed the matter at Starship Congress in 2013), and so has Gregory Benford, whose 2006 novelette “Bow Shock” describes the detection of an object whose synchrotron radiation fits the signature of the bow shock of a craft something like a Bussard ramjet.

We also have a 1995 paper from Robert Zubrin on the spectral signatures of starships and, back in 1977, a JBIS paper by D. R. J. Viewing and colleagues on relativistic spacecraft detection. Various detection methods come to mind, but Al Jackson has pointed out that the simplest would be finding the signature of waste heat (see SETI: Starship Radiation Signatures). A common theme in these discussions is to look for starships moving in the interstellar medium and try to identify interactions between that medium and the ship.

The choice makes sense, for we’ve looked on many occasions at what happens as we move swiftly between the stars. The designers of Project Daedalus knew that at 12 percent of the speed of light, some kind of shielding would be a good idea to protect the craft from interstellar dust particles and larger objects. And a new paper from Ulvi Yurtsever and Steven Wilkinson (Raytheon Company) gives us further information. Get your spacecraft moving fast enough and you’re dealing with collisions like those in particle accelerators but at much higher energies.

Into the Intergalactic Deep


Relativistic effects can be described through the Lorentz factor, cited as γ (gamma) in the relevant equations. Gamma depends upon the speed at which an object is moving. We don’t see length contraction and time dilation in our everyday lives because we don’t move fast enough to notice them, but gamma increases exponentially as we approach the speed of light even though it is all but negligible at lower speeds. A spacecraft moving at 25 percent of lightspeed, for example, has a gamma of just 1.03, meaning time or length contraction of no more than 3 percent.

That gamma factor may be low, but even this speed is high enough to cause serious problems for a starship moving through interstellar dust, and as we go higher, the situation rapidly worsens. At a gamma factor of 2, our craft is moving at 86 percent of the speed of light. Now an object the size of a baseball with mass of 150 grams, note Yurtsever and Wilkinson, has an impact energy of 36 megatons. The authors define ‘extreme velocities’ as those with gamma greater than 9.1, which is well over 99 percent of the speed of light. Here we are in the range of the kind of proton/proton collisions used in particle accelerators to produce antiprotons.

The authors see no safe speeds in the interstellar medium greater than a gamma of 1.3, which is still more than 60 percent of the speed of light. Even the latter seems much too high: Shielding systems to protect a craft moving through the interstellar medium at 60 percent of c are well beyond anything I’ve ever seen discussed. But where the new paper really moves beyond earlier work is in its choice of destinations. What if an advanced civilization were to travel through not interstellar but intergalactic space? Here there is much less debris per unit volume and the chances of collision are sharply reduced. The interactions in question are with photons from the cosmic microwave background (CMB) as the craft moves close to c. From the paper:

The possibility of detecting radiation associated with distant relativistic spacecraft has been discussed in the literature before. These discussions mostly focus on detecting radiation from spacecraft engines or light from nearby stars reflecting off the spacecraft. Our approach is different in that we do not speculate on possible propulsion technologies but are interested in how a large relativistic object would interact with the interstellar/intergalactic medium and mainly with the CMB radiation. As a baryonic spacecraft travels at relativistic speeds it will interact with the CMB through scattering to cause a frequency shift that could be detectable on Earth with current technology.

Collisions with photons from the cosmic microwave background will appear in the spacecraft frame as highly energetic gamma rays. The paper assumes that a technology capable of reaching these speeds will be able to handle the effects of ionization and other interactions, but pair production is likely still an issue. Here we are looking at interactions between photons and matter in the hull of the spacecraft, with a single CMB photon creating an electron-positron pair when it collides with a nucleus in the hull, producing a characteristic signature. The paper looks at two special reference frames, the spacecraft frame and the rest frame of the cosmic microwave background, to understand how the CMB is distorted from these viewpoints.

The starship’s interactions with the CMB photons should produce a unique signature. The paper notes that the effect of CMB scattering from macroscopic relativistic objects like starships is similar to the Sunyaev-Zel’dovich effect in cosmology, in which low-energy CMB photons receive an energy boost when they collide with high energy electrons. The effects of this distortion of the cosmic microwave background have been used by cosmologists to study density perturbations in the universe and have been applied to the study of galactic clusters.

We don’t know if any civilization will learn how to travel at the kind of gamma factors discussed here, which take us well above 99 percent of the speed of light, but if a civilization is able to build spacecraft that can move this fast, they will have solved the problems of matter interactions that are much like those that occur in particle accelerators. The kind of technologies used would be well beyond our understanding but perhaps not beyond our detection. The paper concludes:

Our calculation for what an observer on earth could detect predicts a very unusual signature that is unlikely to be caused by any naturally occurring object in the known universe. This result is independent of propulsion technology, but the ability to detect the signal from Earth depends on available detector technologies. We are currently working to predict how far can we see this signature given our current capability.

The paper is Yurtsever and Wilkinson, “Limits and Signatures of Relativistic Spaceflight,” now available on the arXiv site.


Comments on this entry are closed.

  • william March 24, 2015, 14:45

    This begs the extremely interesting question as to how you could NOT failed to detect a starship moving at such near relativistic speeds (12% the speed of light or greater). After all, when you think about it in any great detail. You have to realize that there does not exist as far as we know any natural phenomenon in which radiation is emitted by a body moving at that speed under normal circumstances.

    Certainly there are phenomenon in which dust and particles enter into black holes and create such tremendous radiation signatures, but they are usually of fairly large angular extent and they do not move across the sky with any great proper motion. A true spaceship going somewhere would show the radiation source moving with a large degree of proper motion, which would suggest that it was a craft rather than a natural body.

  • RobFlores March 24, 2015, 15:26

    For a brief moment there I thought detection by Hubble of
    SCP O6F6 http://en.wikipedia.org/wiki/SCP_06F6 was a potential
    candidate interstellar ship decelerating w/ engine turned toward our solar system. Even though they have now think there’s a plausible explanation.
    it sure had elements that one could mistake for an artificial origin

    1) Light signature was SYMETRIC. 100 days to ON about 100 days to OFF
    2) It was not in the direction on any detectable stellar object.
    3) It’s spectral signature was peculiar.
    4) very powerful X-ray emissions
    5) did not match any known phenomena from Sloan Digital Sky survey.

    I think the most compelling feature is the symmetric light signature,
    which you have to dig deep to find in the internet.

  • Andrew Palfreyman March 24, 2015, 15:37

    Since reading Crane & Westmoreland’s paper on the black hole space drive, and having run many optimisations on staggered-lifetime drives, I’m sold on the idea that a micro black hole (uBH) drive is just about the best one can do with a sublight propulsion system, had one the technology to build it. No physics seems to prevent it, save putative quantum gravity having a say on the mode of its creation.

    The drive relies on directed Hawking radiation, and so the best detection case is to be staring up the tailpipe and detecting the gamma spectrum, which ought to be recognisable and identifiable as that from a uBH.

  • Wojciech J March 24, 2015, 16:10

    I remember a very long discussion on bautforum about speeds that a spacecraft could achieve before collisions will become too powerful for known materials to withstand them, IIRC the value was somewhere around 1/3 of the speed of light.

  • william March 24, 2015, 16:44

    @Andrew Palfreyman; do you know where you one can find this Crane & Westmoreland paper on the black hole space drive ?

  • Paul Gilster March 24, 2015, 16:46

    william, I’ve discussed this paper in a Centauri Dreams entry here:


    There’s a link within to the paper online.

  • Adam March 24, 2015, 17:16

    The speed required to butt heads with the CMB and make e+/e- pairs is quite staggering – the gamma factor is ~124 million. A starship accelerating at 1 gee would need to travel 120 million light-years to hit that speed. So other issues will be more relevant while puttering about the Local Group, but trips out of our Local Super-Cluster might need a careful watch on the speedometer.

    Of course, at 1 gee, the subjective time to reach that speed is ~18.75 years. An integrating accelerometer, which measures “speed” by adding up acceleration per unit time, would show a “speed” of ~19.3 c. That’s much too high for any rocket to achieve, since a perfect photon-rocket would need a mass-ratio of ~248 million, minimum. Even more if the thrust is increased to compensate for the CMB drag.

    The only way to push to such extreme speeds is for the rocket to pipe in energy at zero momentum cost and use it to push the specific impulse (Isp) of its propellant to values greater than c. The Isp is c when the energy isn’t carried all the way. Wikipedia has a nice discussion of this under “Relativistic Rockets”.

    Thus such speeds are likely to remain theoretical, until we can make energy ex nihilo. I discussed this recently in my blog under the title “Extreme Relativistic Rocketry”:


    but then saw the paper by Yurtsever and Wilkinson. Might make for a follow up post…

    Though highly speculative it makes for an interesting question and the CMB “rainbow” produced at increasing gamma-factors is a nice visual.

  • Matthew Barker March 24, 2015, 17:33

    Another site gives the amount of energy released as Gamma Radiation as being around 2 megajoules per second for a spacecraft with a cross-section of 100 meters squared. The radiation shielding would have to be phenomenal, unless a method is developed to refract the CMB photons around the star-ship possibly using a form of meta-material with a negative index.

  • James Stilwell March 24, 2015, 18:05

    A great post…the next century may dive wholeheartedly into this very subject…even with one confirmed detection…the realization that we are not alone in the galaxy might change the course of human history…
    William Blake wrote so long ago…the altered eye alters all…
    Clarke approached the subject in Rendezvous with Rama..

  • Erik Landahl March 24, 2015, 18:46

    Paul and/or Rob, will you please do a follow-up on Hubble SCP O6F6? I found the original 2006 paper authored by Barbary, Dawson, Tokita and others; the 2008 submission by Soker, Frankowski and Kashi; and the 2009 submission with a 2011 update, submitted by Quimby, Kulkarni, Kasliwal and others. The proposed solutions seem unsatisfactory. The 2009 Quimby paper trumpeted an “unmasking” of SCP O6F6 as a pulsational pair-instability outburst, but upon reading the paper that seems like a stretch, more speculative than the authors admit it is. None of the papers mention the possibility of a starship or other intelligently-designed artificial object. Is SCP O6F6 worthy of a Centauri Dreams article? Or have I missed an SCP O6F6 Centauri Dreams article in the archives? Thanks much!

  • william March 24, 2015, 20:07

    I’m really scratching my head here, and the less I made a mistake with my arithmetic something here doesn’t really add up number wise. The article states that a 150 gram baseball moving at 86% the speed of light will have a impact energy equal to 36 megatons of explosive power. However, according to the online conversion program. It shows the following:

    1 megaton [explosive] = 4 184 000 000 000 000 joule

    the energy by E equals MC squared would be numerically:

    = (. 15 kilograms) (3 X 10 + 8) ^ 2 = 1.35 x 10 + 15 joules
    , which yields about 3.22 megatons; I thought the figures quoted above sounded a little bit high!

  • william March 24, 2015, 20:10

    A little bit high but still at 3. 22 megatons still packs a wallop!

  • V. L. Teofilo March 24, 2015, 21:34

    The premise of this post is that “Starships” will travel with a propulsion system that permits relativistic velocities. However the probability that such a propulsion system would be feasible compared to one that uses a Alcubierre Warp drive with the minimal amount of negative energy as theorized by Harold White is indisputably small .

  • David W March 24, 2015, 22:36

    More mundane than the CMB ..but I remember many years ago reading in Science that we had a satellite end its run and the story ended by sayings it observations ruled out any spaceship using anti-matter in our solar system. Its something I wish I copied at times like these does anyone else recall this? it must have been 20 years ago

  • Paul Gilster March 24, 2015, 22:43

    Erik Landahl writes:

    Is SCP O6F6 worthy of a Centauri Dreams article? Or have I missed an SCP O6F6 Centauri Dreams article in the archives?

    Erik, I have but one article on SCP O6F6, written in 2009:


  • Rob Flores March 25, 2015, 1:39

    Well, here is the light curve for SCP 06f6
    I can’t tell if the post peak data points are not available because of
    instrument re-alingment due to Earth-Sun configuration OR
    it just was not dectected.
    A paper mentioned they were going to do follow up observations with the hubble widefield camera 3, but I don’t think anything was found and the
    probability that [it just happens to be a free floating black hole that co-incidentally ran into/captured a sun and shreded it] seems miniscule to me. but apparently ETI are not a plausible possibility.
    The object is in the middle of Nowhere astronomically speaking.
    Someone mentioned that the intergalactic medium has far less
    debris to create a bowshow for any fast moving spaceships of
    extra galatic object falling into the Milky Way. If this is a spacecraft it
    chose a very stealthy approach trajectory: only they must have assumed the neighborhood of our Sun had no Civilizations capable of detecting a deceleration plume. If They detect that plume again, and it had shifted a couple dozen arc seconds from the last position would that info be released to the public?

  • NS March 25, 2015, 1:41

    To David W, see this:


    This was linked in the Centauri Dreams 2012 discussion on starship detection:


  • Rob Henry March 25, 2015, 4:00

    William, first I note that the explosive energy density of TNT varies slightly according to how much stabilising agent you add to it. Secondly I note that the authors would have no interest in motion at a gamma of 2, and every interest in one at gamma 9.1.

    It thus seems an odd coincidence that the kinetic energy figure is such a good match for the velocity at which the authors had more interest.

  • william March 25, 2015, 10:17

    @Rob Henry – forgive me, but I am not following your line of reasoning here on what you’re saying concerning the kinetic energy figure. The kinetic energy of an object moving that 86 percent of the speed of light would still be negligible compared to the energy that would be released in the mass-energy conversion of E equals MC squared.

    I took it that the author of the paper was talking about the energy that would be released in the complete conversion of mass to energy and not kinetic energy in the pure sense, so I’m not following what you’re saying here. In addition, the author DID state that the value quoted was at a gamma of 2 and he wasn’t worried about the higher gamma factor. Why he chose gamma of two is not explained there in the paper, but he does take that value. As for the energy of TNT. It is merely just a conversion factor to get to the megaton equivalent range. What are your thoughts on what I have stated here?

  • Ron S March 25, 2015, 11:09

    V. L. Teofilo: “The premise of this post is that “Starships” will travel with a propulsion system that permits relativistic velocities. However the probability that such a propulsion system would be feasible compared to one that uses a Alcubierre Warp drive with the minimal amount of negative energy as theorized by Harold White is indisputably small .”

    You are comparing a technologically plausible mechanism that requires an enormous quantity of energy with a technologically and theoretically implausible mechanism that requires an enormous quantity of energy and unobtainium. Your use of ‘feasible’ here is insupportable.

  • Anthony Mugan March 25, 2015, 11:18

    V A Teofolo mentions the alternative possibility of technology based on some form of modification of the space-time metric such as the Alcubierre metric etc. There is an interesting paper by Puthoff in JBIS vol 63 ( 2010) which discusses the possible observed effects of this general class of possible technologies as they would appear to a nearby observer. One of the more striking effects could be a perception of speeded up or slowed down time compared to more distant observers. A local observer near such a device might think ( say) one hour had passed whilst a more distant observer woukd think a couple of hours had passed once the two observers met up away from the device to compare watches.

    Keeping in mind the need to leave all options open at the moment in terms of which technological model to look for signs of I do think this is a very interesting approach.

  • Eniac March 25, 2015, 13:03

    William: Above a certain gamma factor, the kinetic energy of an object is higher than it’s rest mass energy, and as gamma factor increases the kinetic energy keeps increasing ad infinitum.

    I think what Rob meant to say is that the figure cited is correct if you are making the calculation at gamma=9.1, and that most likely the context got mangled somehow, rather than the calculation.

  • Mark Zambelli March 25, 2015, 14:38

    William… I just did a quick relativistic kinetic energy calculation and I get around 3.1 megatons which, with my rounding, agrees very well [Ek= (mc^2 ÷ SQRT 1 – (m÷c)^2) – mc^2] … hope that’s right (?).

    Way back in the late 80’s IIRC, A. C. Clarke wrote in to New Scientist with a letter about staring down the tailpipe of a fusion engine may be similar to seeing a quasar… redshifted hydrogen/helium spectra. I’m thinking he wasn’t being 100% serious but I wonder how similar, if at all, any features would be. If there were a surplus of quasars laying in the plane of the milky way then they might be due to spacefaring civs traversing the galaxy but quasars aren’t distributed like that so that seems a tight constraint. Maybe one or two extra would slip under the radar and that might imply a few tens of ships out there going about their merry-way with only one or two aligned to us… but I highly doubt it. Fun thinking about though.

    This article above opens the way to more serious musings though.

  • Andrew Palfreyman March 25, 2015, 15:19

    A gamma source moving speedily across the heavens would be a place to start analysis of putative spacedrives. Such an observation isn’t known to me. Do we have data like that already?

  • Rob Henry March 25, 2015, 15:56

    @William, my first thought is that is that at 86% c we have a gamma of 2, and that makes rest mass exactly equal to mass due to kinetic energy.

  • Adam March 25, 2015, 16:36

    On the question of the warp-drive, a mathematical physicist friend opined that it makes for an amazing sub-light drive even if it runs into physics ambiguities if it goes above c. One chief advantage is its ability to accelerate without gee-forces being experienced by the vehicle. Thus extreme gamma-factors might be achieved without travelling for megaparsecs, but the warp could also provide protection against the CMB and ISM fluxes. Starships that take mere hours to fly between the stars, in ship-time, would use something like the warp-drive.

  • Wojciech J March 25, 2015, 16:54

    My question-if realistic light speed velocities are around 0.3c would such ships be significantly detectable ?

  • Ross Turner March 25, 2015, 19:56

    Would a warped drive spaceship deflect objects around the ship rather than running into them? Would the warp drive ship avoid the megaton hits from space debree altogether?

  • Rob Henry March 25, 2015, 20:56

    I want to hear more about SCP O6F6. Wikipedea writes ‘Because of its uncommon spectrum, the team was not able to determine the distance to the object using standard redshift techniques’.

    Now that is hardly as small detail. Science breakthroughs are made on such things, not from yawninging as you place it in the ‘too hard’ basket.

  • David W March 25, 2015, 22:03

    Thanks NS That was it! this is such a great site. Wish this list was running things!
    As to other comments I think some people have been looking for natural or artificial wormholes.
    I guess we need to push for the best and fastest near term precursor which should have been Sundriver

  • Rob Flores March 26, 2015, 1:17

    One more piece of information on SCP O6F6 to shake the cart.
    One part of the puzzling aspect of the object is very strong x-ray emissions.
    X-rays are one of the primary energy signatures of Atomic weapons
    detonated in space.

  • Michael March 26, 2015, 14:53

    @Wojciech J March 25, 2015 at 16:54

    ‘My question-if realistic light speed velocities are around 0.3c would such ships be significantly detectable ?’

    If bow shock developed over many years which was then illuminated by the radiation from the exhaust of the craft as it moved through it maybe, but it would be very faint indeed.

    P.S I managed to survive the ‘mosh pit’ and proved that at 42 I can still run with the lions, but it does feel like the entire pride ran over me and left me for dead!

  • Michael March 26, 2015, 14:56

    @Rob Flores March 26, 2015 at 1:17

    ‘One more piece of information on SCP O6F6 to shake the cart.
    One part of the puzzling aspect of the object is very strong x-ray emissions.
    X-rays are one of the primary energy signatures of Atomic weapons
    detonated in space.’

    I believe the absorption/emission spectrum involved a lot of carbon which is not present in great quantities in nuclear devices save for the explosives.

  • RobFlores March 26, 2015, 18:38

    Michael, Graphite composites are a promising radiation shield material
    Especially against X-rays. Carbon signature indeed.


  • Anthony Mugan March 27, 2015, 12:16

    @ Ross Turner
    My understanding of it is that a warped space time could, if achievable allow a vessel to travel relatively slowly by to all intents and purposes shrinking the distance it had to travel. Equally yes the shape of the warped space could potentially deflect oncoming material EBD this could even be useful in for example planetary atmospheres, allowing hypersonic velocities without the extreme heating and g forces associated with current technologies.
    The key bit is ‘if feasible’…. Some interesting work underway but we shall just have to wait and see.