Almost zero, unless it has a beacon attached. Indeed some people think our solar system may be full of dormant spacecraft from other species.
It's one idea - that maybe the way that most species explore the galaxy is that they send out a self replicating spaceship. Something we might be able to do not so far into the future with nanoscale 3d printers.
It might stop off at the nearest star, make say 10 copies of itself - they all go off to the ten nearest stars and keep going.
With of the order of 100 billion stars, i.e. 100,000,000,000 - then within 11 or 12 generations - then all the stars in the galaxy have one of these spacecraft around them.
Also seems it could be made reasonably reliable. Only 12 generations.
Is true that there are lots of possibilities for it to go wrong, with 100 billion spacecraft - eventually - but seems reasonably possible that robust error correction could solve that issue also. I.e. if the reproduction goes wrong - it does a series of self tests - finds the flaw - and either fixes it - or you just scrap it and start the print out again.
As for time it would take, less than a million years probably, depending how fast they can travel, and how easily they can replicate.
So now suppose that of those 100 billion stars, say, that 1 in a million has evolved technological life already to the point that they make these self replicating spaceships.
Then our solar system would have about 100,000 of them, each from a different species of ET. Many may be non functioning relics from long extinct ETs but they would still be there.
So - is that possible? If they were large hundreds of meters things orbiting close to Earth we'd spot them by now.
But maybe designed to be hard to detect. Or just for practical reasons.
Either way, may be ultra miniaturized, just meters across or smaller.
Or - after dispatching their relatives to other stars, the remaining spaceship turns into a tiny seed, just a miniaturized 3D printer + antenna or telescope + data banks or some such - which would be again probably ultra miniaturized - probably also duplicated for redundancy.
It doesn't need rockets any more - and - if it does need anything, it can reconstruct it easily with its 3d printer. If it spots something interesting it might create all sorts of complex and interesting machines to investigate. Maybe it did do that billions of years ago when it first arrived. Maybe it does that every few millennia or every few million years to do a new survey to send results back to its constructors.
So meantime all it has to do is to watch, and maybe send reports to its originating solar system every few millennia or whatever it is.
Something like that - size of Voyager or smaller - is no way we'd be able to spot it yet.
Even if it came close to Earth frequently - we probably wouldn't see it at all, and if we did, would probably mistake it for an ordinary asteroid.
So - is possible. How probable it is, I have no idea.
This is an idea explored by David Brin in his novel Existence.
http://www.davidbrin.com/existen...
It's one idea - that maybe the way that most species explore the galaxy is that they send out a self replicating spaceship. Something we might be able to do not so far into the future with nanoscale 3d printers.
It might stop off at the nearest star, make say 10 copies of itself - they all go off to the ten nearest stars and keep going.
With of the order of 100 billion stars, i.e. 100,000,000,000 - then within 11 or 12 generations - then all the stars in the galaxy have one of these spacecraft around them.
Also seems it could be made reasonably reliable. Only 12 generations.
Is true that there are lots of possibilities for it to go wrong, with 100 billion spacecraft - eventually - but seems reasonably possible that robust error correction could solve that issue also. I.e. if the reproduction goes wrong - it does a series of self tests - finds the flaw - and either fixes it - or you just scrap it and start the print out again.
As for time it would take, less than a million years probably, depending how fast they can travel, and how easily they can replicate.
So now suppose that of those 100 billion stars, say, that 1 in a million has evolved technological life already to the point that they make these self replicating spaceships.
Then our solar system would have about 100,000 of them, each from a different species of ET. Many may be non functioning relics from long extinct ETs but they would still be there.
So - is that possible? If they were large hundreds of meters things orbiting close to Earth we'd spot them by now.
But maybe designed to be hard to detect. Or just for practical reasons.
Either way, may be ultra miniaturized, just meters across or smaller.
Or - after dispatching their relatives to other stars, the remaining spaceship turns into a tiny seed, just a miniaturized 3D printer + antenna or telescope + data banks or some such - which would be again probably ultra miniaturized - probably also duplicated for redundancy.
It doesn't need rockets any more - and - if it does need anything, it can reconstruct it easily with its 3d printer. If it spots something interesting it might create all sorts of complex and interesting machines to investigate. Maybe it did do that billions of years ago when it first arrived. Maybe it does that every few millennia or every few million years to do a new survey to send results back to its constructors.
So meantime all it has to do is to watch, and maybe send reports to its originating solar system every few millennia or whatever it is.
Something like that - size of Voyager or smaller - is no way we'd be able to spot it yet.
Even if it came close to Earth frequently - we probably wouldn't see it at all, and if we did, would probably mistake it for an ordinary asteroid.
So - is possible. How probable it is, I have no idea.
This is an idea explored by David Brin in his novel Existence.
http://www.davidbrin.com/existen...
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
4.8m answer views110.4k this month
Top Writer2017, 2016, and 2015
Published WriterHuffPost, Slate, and 4 moreHuffPost, Slate, Forbes, Newsweek, Business Insider, and Science 2.0