The thread about creating a probe got me thinking about the problem in reverse. What if an alien civilization created a probe and sent it towards our solar system millions of years ago. Assume it got up to .1c at some point, but that now all internal workings have failed. It’s just a hunk of something heading in a straight line that will pass through our solar system around 1.c. How small could the probe be and still be detectable?
I image if it’s too small it would zoom right by and we’d never know. If it’s huge (moon size), would we see it?
Are there any design considerations that would make the probe easier to detect? What if it was like a huge rotating disco ball? Would the flicker make it easier to spot? Or are there other ideas that would make an inactive probe more detectable?
We detect things like that based on the movement of a point of light relative to background stars over multiple exposures. That movement can result from both parallax (the Earth moving, causing nearer objects to shift in apparent position) and from the object’s own movement.
The catch is that we have to be looking at the right part of the sky and comparing images from multiple days (or weeks or months). A single image isn’t enough. Plus, the sky is too big to look everywhere. Most of the things we are looking for are around the ecliptic, but there’s no guarantee that an alien probe would be anywhere near that.
A large/reflective* probe (like moon- or planet-sized) could conceivably be seen by the naked eye or amateur telescopes, making it very likely to be noticed. The smaller/less reflective it is, the more you rely on powerful telescopes that are in short supply.
What we really care about is the total amount of reflected light.
Obviously, that equation also depends on how close it gets to the Earth. A moon-sized object that got within the moon’s orbit would be essentially as bright as the moon (assuming the same reflectivity). Nobody could miss that. Put that same object out around Pluto and seeing it becomes like winning the lottery.
If the aliens wanted it to be easily visible, they’d make a retroreflector shape. Basically, put a big piece of foil along the XY plane, centered on the origin. Then put another piece of foil in the YZ plane, and one in the XZ plane, so no matter what direction you’re looking from, you’re looking into an inside corner. Then, any light that hits it (say, from a star very near a habitable planet, or from some sort of active radar) will be reflected straight back towards the source. This doesn’t have much advantage beyond visibility, though, so if that wasn’t one of their design goals, it probably wouldn’t be shaped that way.
Oh, and on recognizing this thing in the first place: You could do it in a single image, if you’re doing spectroscopy. If this thing is approaching us at 10% of the speed of light (heck, even at a tenth of a percent), it’s going to have a huge Doppler shift, enough that even a cursory examination would make it stand out. Any astronomer who saw an object that blueshifted is going to immediately give it es full attention, and devote more observations, of as many kinds possible, as quickly as possible. It’d probably get transient-event observing time on every major telescope in the world within days.