Imagine you’re an alien civilization and you want to study our planet or “regard it with envious eyes”, where are the best places to position a probe? Well, physicist James Benford has an answer for you. There are regions around Earth where it’s possible for celestial bodies or artificial ones to co-orbit our planet. They are in a 1:1 resonance, so basically they remain at roughly the same distance from Earth as we go around the Sun.
In a paper published in The Astronomical Journal, Benford discusses how these natural objects in resonance with our planet make them ideal places to hide a “lurker” as he referred to the possible probes. He lists the three main orbits these objects can take as well as the stability of some of the known co-orbitals.
There is, for example, Cruithne, often referred to as Earth’s second moon (it isn’t), which is expected to leave its current orbit in about 5,000 years. And there is Kamoʻoalewa (or 2016 HO3) – the smallest, closest, and most stable quasi-satellite of Earth, which only came into its current orbit 100 years ago. There are more of these objects and many more are likely to be discovered in the future.
Benford argues that these objects and their neighborhoods are ripe for SETI (Search for Extraterrestrial Intelligence) investigations. Using wavelengths from radio to optical to infrared, it could be possible to study these co-orbital bodies and find out if lurkers are on them. The physicist also suggests sending a robotic probe to rendezvous with these objects to study them up close.
“What have we to lose by checking out these objects? Certainly resources such as time on telescopes, radio and optical. But we would be studying newly found objects, which could well be interesting astronomy. Nobody has really looked at these co-orbitals, other than orbital calculations and faint images. We know almost nothing about them,” Benford writes in the paper.
The paper also detailed regions not ideal for lurkers, such as near the Earth, on the Moon, or further out around the asteroids. That said, Benford states a simple truth: We don't know anything about the intentions or logic of alien civilizations. We don’t even know if these alien civilizations are out there.