While once living in Texas, my family and I had the privilege of watching a shuttle re-entry. It was breath-taking. I kid you not, it was like watching the finger of god drawing a line of fire through the sky. That memory and this question got me thinking.

The impact of even small rocks on the forward shield/armor/defense of a space ship traveling at 0.05c results in the release of an enormous amount of energy — at least it appears enormous in discussion and is certainly of great concern to the ship's occupants.

But is it enough to expose the ship to other dangers, such as space pirates (argh!), perturbed space monsters (hiss!), or even us (eeek!)?

Question: Is the energy created by the destruction of debris for a ship traveling at 0.05c sufficient to be detected by Earth's technology today or in the near future (10-20 years)?

  • Let's assume the ship is passing between us and Alpha Centauri, so we're detecting the destruction of debris from a distance of approximately 2 light years. I understand that what we would be detecting is an historical record — the record of a ship that passed years in the past, but that's till awesome, isn't it?

  • Ignore the idea that what you're not looking for is often hard to see.

  • Let's stick with a passing ship (detect the line drawn across our field of vision). Detecting an approaching ship (detect the growing dot) is too much to ask for.

  • I'm not sure what to do with the fact that today we really can't rotate things very fast (telescopes, especially radio telescopes, etc.) and therefore depend on the optics over the detection scheme to give us a wide-angle view. So, for the sake of the argument, let's ignore that and focus simply on the detection itself.

  • I am assuming that at 0.05c the ship would strike enough debris to create a reasonably constant stream of energy.

Irrelevant bonus question that need not be answered to be considered for answer acceptance... If the answer is "yes, we could detect that," does this suggest we are (a) alone in the universe, or at least in our corner of it, or (b) that transport tech will circumvent the issue of impactor energy signatures and we're actually surrounded and the only reason we get on with our happy little lives is we do... not... know about it? You don't need to answer this at all, but if you do want to have fun with it, I insist that you answer the actual question first. Answers that only address the bonus question will receive invitations for deletion. Of course, the real question may only be answered "nope, we can't see anything like that," in which case this bonus question is irrelevant and I'll be required to sit in a corner for an hour — but don't let that give you any ideas.


I think you're asking the wrong question. It isn't "Can we track it?" but "From what distance can we track it?" To answer that, we need to look at what kind of radiation would be emitted and at what intensity. I'll assume that we're talking about a ship of plausible size, and not something like a 10AU ram scoop field or something like that. Let's assume the ship is 100' across yielding a cross section of about 1000 square meters.

0.05c isn't really relativistic -- it's really fast by our standards, but not fast enough to pair-produce particles. Basically, you can think of the ship as stationary and the interstellar medium hitting it at 0.05c. The particles that hit will give up energy through Bremsstrahlung radiation which is well understood.

The density of the interstellar medium averages around a million atoms per cubic meter. So you have something like a thousand square meter cross-section ship encountering a 0.05c wind of a million atoms/cubic meter. That yields about 1016 collisions/second. A hydrogen atom travelling at 0.05c has a kinetic energy of about 10-13 joules, so the total flux would yield about 103 joules/sec. About one kilowatt of heating.

The detection distance would be measured in miles, not light years.

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    $\begingroup$ Just curious, but what order of magnitude of miles would you expect this to be visible from? Say, with modern technology? $\endgroup$ – Sean Boddy Apr 22 '18 at 20:09
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    $\begingroup$ That's a good question and it strains my ability to do a back-of-the-envelope estimate. Since the Bremsstrahlung is going to be directed into the structure of the leading edge of the ship, I'd expect the energy mostly to go towards heating the leading edge. A kW isn't going to heat it very much, so it's likely to be mostly infrared blackbody. Honestly, I'd expect it to be completely lost in the huge amount of heat that must be radiating from the ship. This waste ship heat has a far greater detection range than the Bremsstrahlung heat. $\endgroup$ – Mark Olson Apr 22 '18 at 20:44

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