[Razengan]

If the craft is packed with instructions on how to get back to us, maybe we could get some sort of exchange going over multiple centuries.

On the other hand, if someone were to sent similar craft to us, would we even have a reasonable chance of detecting something that small without knowing exactly where, when and what to look for?

ʻOumuamua may have been a probe, yet even with our suspicions about it being something out of the ordinary, we more-or-less collectively yawned and went about our daily strife without making any effort to collect it.

[0] https://en.wikipedia.org/wiki/ʻOumuamua

[bencompanion]

On your other point - would we have a chance to detect something that small? No, absolutely no way, even considering that there would be a continuous 'stream' of these things spread out like beads on a string.

Say each one weighs a gram, and is 1m across - that's just impossibly small. The Hubble ST's resolving power means that it can resolve a 1m object (ie, make one pixel = 1m²) at something like 4000km away. In space terms, that is practically touching. In interstellar terms, that might as well be inside the planet!

Any transmitting that it might be doing would necessarily be very low power, which means it would need to be highly directional and pointed back the way it had come, behind what must be a reflector more reflective by far than anything we know how to build. Unless you managed to get its transmission pointed directly at you by being behind it, there'd be no way to see it.

After decades or more in the interstellar void, it would be about as equal to the ambient temperature as it's possible to be, and in any case, it's not made of very much 'stuff', so it would have no heat signature to detect.

Lastly, at 20% light speed, they would cover the average distance from Pluto to Earth in something like 36 hours. That's not a lot of time to search!

The only possible method I can think of would be to detect the results of these things crashing into dust within the solar system. How much of a 'puff' a gram of diffuse something travelling at 0.2c hitting a speck of rock makes I don't know. Still way too small to spot, I'm sure.

[bencompanion]

ʻOumuamua is fascinating, I agree - but the most remarkable thing about it is that it's the first object like it we've seen, followed closely by the fact that it's taken us so long to see one. Most models predict these are pretty common - it's just our lack of spotting capacity that's stopping us seeing more.

As far as collecting, I remember reading a back-of-the-envelope analysis that suggested that we might be able to catch it if we made a really concerted effort. We might then be able to hit it with an impactor (or just smack the probe into it) and do some spectroscopy on the resulting dust cloud, but it's going way too fast in an inconvenient direction for us to be able to collect anything, slow down and return it.

[madeuptempacct]

I have been obsessing over Oumuamua as well, but there's just not that much information, and we won't be getting more. Then there was the fact that it was tumbling and wasn't emitting anything - one would expect radar scans or radio transmissions, or at least something.

[jobigoud]

> Then there was the fact that it was tumbling and wasn't emitting anything

There is a range of the radio spectrum we can't really practically measure: the very, very low frequencies. Consider a nanohertz signal. Period of each sine wave: 31 years. You might also need an antenna the size of the solar system to grab anything.

[xj9]

passive sensors are safer if you are sending probes into unknown territory populated by potentially violent aliens.

[java-man]

violent aliens are much closer than you think ;-)