Intelligent aliens could be spying on Earth using robotic probes tactically positioned on nearby space rocks, according to a bizarre report from a prominent American scientist.
The idea that "superior galactic communities" could be using interstallar probes to to observe and monitor other life forms was first proposed by Stanford radiophysicist Ronald Bracewell back in 1960.
Now physicist James Benford has suggested that the obvious place for these probes - or "lurkers" - is on near-Earth asteroids known as "co-orbital objects".
As the name suggests, these space rocks circle the Earth as the same time as orbiting the Sun - although they are too distant to be considered satellites.
Only a small number (less than 20) of these objects has ever been found.
The closest, a small asteroid known as 2016 HO3 , has been circling Earth for almost a century, and is described by NASA as "Earth's constant companion".
"2016 HO3 loops around our planet, but never ventures very far away as we both go around the Sun," NASA researcher Paul Chodas explained in 2016 .
"In effect, this small asteroid is caught in a little dance with Earth."
According to Benford, the close proximity of these space rocks to Earth makes them the ideal place for aliens to set up probes to spy on us.
"These near-Earth objects provide an ideal way to watch our world from a secure natural object," his paper explains .
"That provides resources an [extraterrestrial intelligence] might need: materials, a firm anchor, and concealment."
He suggests that astronomers should be investigating these co-orbital object as a matter of urgency.
"We should move forthrightly toward observing them, both by observing them in the electromagnetic spectrum and planetary radar, as well as visiting them with probes," he writes.
Perhaps unsurprisingly, not all space scientists agree with Benford's assessment of the space rocks.
"How likely is it that alien probe would be on one of these co-orbitals, obviously extremely unlikely," physicist and astrobiologist Paul Davies from Arizona State University told Live Science .
"But if it costs very little to go take a look, why not? Even if we don't find E.T., we might find something of interest."
As it happens, China is planning to send a spacecraft to asteroid 2016 HO3 in 2022, to collect rock samples, so that should provide a good opportunity to check for "lurkers".
The findings are reported in The Astronomical Journal .
