Could not find answer via web search. To avoid motion blur we use high speed cameras. For seeing with high speed more light is needed lighting-for-high-speed.

If alien spaceship would not emit billions times per square space more intense light then our sun Visualizing video at the speed of light — one trillion frames per second, but say will only reflect nearly 100% light it got, could we see (or even detect) spaceship passing through our solar system near (say 99%) speed of light?

What size should it be for us to detect it if say it would pass as close as the moon?

  • $\begingroup$ Due to Lorentz contraction How small would it be if it was moving that fast? $\endgroup$
    – joseph h
    Commented Nov 16, 2020 at 4:37
  • $\begingroup$ @Dr jh. Lorentz factor is 7 here. But contraction is along direction of motion only AFAIK. $\endgroup$ Commented Nov 16, 2020 at 4:52
  • $\begingroup$ @Dr jh, I doubt it is of much relevance here, due to Penrose effect we should not see it contracted. it would take it about say 30 minuted to approach Earth from edge of our system. Question is what size should it be to take enough angle of the sky for enough time (given Earth is moving too ....). $\endgroup$ Commented Nov 16, 2020 at 4:58
  • $\begingroup$ it would take it about say 30 minuted to approach Earth from edge of our system @Martian2020 What “edge of our system” is only 30 light-minutes away? $\endgroup$
    – G. Smith
    Commented Nov 16, 2020 at 5:24
  • $\begingroup$ VTCing as there seems no conceptual question here and (IMO) it falls under the homework-type question rule (which covers more than homework). $\endgroup$ Commented Nov 16, 2020 at 8:55

2 Answers 2


The density of the solar wind is (at a low estimate) about 3 protons per cubic centimeter. This means that a macroscopic object moving at $v = 0.99 c$ will be colliding with protons at an approximate rate of $$ \left( \frac{3 \text{ protons}}{\text{cm}^3} \right) \left( 2.97 \times 10^8 \text{ m/s} \right) \approx 9 \times 10^{14} \frac{ \text{protons}}{ \text{s}\cdot \text{m}^2} $$ or about $10^{15}$ protons per second for every square meter of frontal cross section of the object.

Moreover, you have conveniently picked a speed ($0.99c$) for which moving protons can pair-produce with stationary protons (the protons in the solar wind are basically stationary compared to those of the ship): $$ p + p \to p + p + p + \bar{p} $$ The threshold energy for this reaction is that the incoming proton must have energy of $E = 7 mc^2$, but for $v = 0.99c$ the protons in the ship will have $E \approx 7.09 m c^2$ in the rest frame of the Sun. If this process occurred, the subsequent annihilation of the antiprotons would generate large numbers of 938-MeV gamma rays, which I suspect would be easily detectable (given the proximity of the source) by satellites such as Fermi, INTEGRAL, or AGILE, and possibly by ground-based gamma-ray observatories as well. Even if the aliens tapped the brakes a bit, the reaction $$ p + p \to p + p + e^- + e^+ $$ would still be possible, and the electron-positron pairs would then annihilate to emit 511-keV gamma rays. So this is a fundamental problem that the aliens would have to deal with.

If the aliens don't want their ship to ablate via pair-production during their voyage, then they need some sort of way to move the protons out of their way. But whatever technology the aliens have to do this, given the speed of the craft the protons will still have to accelerate rather quickly. They would thereby emit bremsstrahlung, which probably wouldn't be quite as easy to detect as $p\bar{p}$ or $e^- e^+$ annihilation but might still be quite noticeable.

  • $\begingroup$ Notes: (1) this is all ballpark estimation, so I'm glad to receive comments helping me refine this answer and/or telling me I'm full of crap. (2) Someday I'll spell bremsstrahlung correctly on the first try, but today is not that day. $\endgroup$ Commented Nov 16, 2020 at 16:00
  • $\begingroup$ You certainly touched opportunities I have not thought of! If you would add chances of ordinary people noticing it: in visual range (taking into account Doppler shift), gravitation effects (given there is no upper limit on size and mass). Thank you! $\endgroup$ Commented Nov 17, 2020 at 3:27
  • $\begingroup$ "you have conveniently picked a speed (0.99c)". Could you please add at what speed that effect starts? Also, if possible I would appreciate some thoughts on "observability chances" as a function of distance of closest approach. $\endgroup$ Commented Nov 17, 2020 at 3:33

I can envision two possibilities: 1. It is emitting some kind of signal and we happen to be looking for radiation at the right Doppler shifted frequency. 2. It passes through two monitored parallel laser beams which are being sent from the moon to the earth. (or the light from a star being monitored by two coordinated and well separated telescopes on earth).

  • $\begingroup$ My initial thoughts were how we would be able to exchange messages with it, however this question is purely for object passing passively. There is no upper restrictions on the size, do you see? I'm interested in estimating minimum size. The ship may well be size of the Moon itself, or even the Sun (if center will pass at Moon's Perigee it would fit ;-). $\endgroup$ Commented Nov 17, 2020 at 3:22

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