Asked by Rab Manderson
This is a question that provoked some discussion in the office. On Earth it is quite easy to detect our own spacecraft in the Solar System such as the most distant object from Earth, NASA’s Voyager 1 probe, but we have an advantage. We know approximately where to look and more importantly, we know what we are looking for. Scour the region looking for a specific radio frequency and Voyager sticks out like a sore thumb. But if an alien version entered our Solar System would detection be as easy?
Probably not. The Voyager probe is specifically built to get a signal back to Earth. Not only are we looking in the right direction, but it is constantly making sure its radio antenna are pointing towards Earth. This is no mean feat; at the edge of our Solar System the area of sky occupied by Earth will be vanishingly small. If an alien probe had a focused radio signal like Voyager then the chance of it pointing straight at us is close to impossible.
Ultimately it would be extremely difficult to find an alien probe, assuming it had similar communications technology to Voyager.
Answered by Josh Barker, Education Team Presenter for the National Space Centre.