If extraterrestrial civilizations were sending probes by Earth similar to our plans for Breakthrough Starshot (where the probes are only a few meters in diameter and will be moving at up to 0.2c when they fly by the planet), would we be able to detect them, technologically speaking? And if so, what are the odds we would catch a glimpse of one?

EDIT/ADDITION: An additional question I thought of is whether astronauts would potentially be able to perceive such objects with the naked eye, or would the probes be moving far too fast to register?

  • $\begingroup$ This is a *really great question! There are issues of visibility (thermal IR and visible wavelengths) and orbital mechanics (how fast it will be moving as seen from Earth). $\endgroup$
    – uhoh
    Jul 16, 2019 at 1:13

1 Answer 1


I'm not in a position to toss real numbers around at the moment, but here's a thought experiment:

Iridium satellites reflect sunlight toward Earth's surface, producing bright flares, up to magnitude -8. That's about 1 million times brighter than the faintest light source a fully dark-adapted eye can perceive -- a five-magnitude difference in brightness is a factor of 100, and under really dark skies humans with excellent vision can see down to magnitude 7.

I think these reflections are coming off a panel of about 3 x 1.5 meters, so it's comparable in size to your probe.

These satellites orbit about 800km above Earth's surface. How far away could one be, and still produce a visible reflection? Brightness falls off as the inverse square of distance, so the greatest distance at which you could barely see a reflection would be 1,000 times greater, or 800,000km.

Light travels at 300,000km/s. A probe traveling at 0.2c would cover 60,000km/s. Traveling 800,000km would take about 13 seconds.

That means that something the size of an Iridium satellite, reflecting sunlight directly toward an observer during its entire passage, could be visible for up to almost half a minute -- blue-shifted for the first part of that, and red-shifted for the last part. (Unless it's quite far away at closest approach, which would make it dimmer, the transition from "basically heading straight for you" to "basically heading straight away from you" would take place in the literal blink of an eye, a matter of milliseconds.)

Now, this is probably an upper bound -- probes won't have the same shape and reflectivity as an Iridium satellite, and won't be oriented to reflect directly toward an observer during their entire transit. But, somewhat to my surprise, it looks like you might be able to see one approaching and receding.

On the other hand, it's also possible that an object making 0.2c might have a visible interaction with the solar wind. (That requires more numbers than I'm able to run at the moment, but having thought on it a little bit, it seems unlikely.)

  • 2
    $\begingroup$ A factor of $10^4$ further away means $10^8$ times less flux, which is 20 magnitudes fainter. Not observable. $\endgroup$
    – ProfRob
    Jul 17, 2019 at 18:28
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    $\begingroup$ @jeffB. I think -8 magnitude is about 4E5 times brighter than the naked eye limit (assuming 6.0), not 1E8 times brighter. $\endgroup$
    – JohnHoltz
    Jul 17, 2019 at 20:05
  • $\begingroup$ Well that was a whole bunch of stuff I never knew! So assuming the probe was reflecting at you, you would see it (the spot of light from the reflection) flying towards you and flying away. It may not be the 4-5 minute window if some of the values need correction, but the concept is still sound (?). Is this assuming we're viewing the probe from space, or down on the ground? $\endgroup$
    – user22038
    Jul 17, 2019 at 20:48
  • $\begingroup$ @RobJeffries Yep, I somehow slipped in an extra 5-magnitude difference. One edit coming up. $\endgroup$
    – jeffB
    Jul 18, 2019 at 3:49
  • $\begingroup$ @user22038 I was assuming a ground-based observer. In space you could probably see somewhat dimmer objects (no atmosphere to attenuate their light, less background light to swamp them), but that would also depend on what you're inside while you're observing. (If you're outside, your eyes are probably icing up, which will reduce your visual acuity.) $\endgroup$
    – jeffB
    Jul 18, 2019 at 4:02

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