Satellite, once asteroid, in fact old rocket to the Moon

This tiny moving dot is a fascinating object that passes by Earth today at a distance of just 50 000 km (less than a tenth of the distance to the Moon). First thought to be an asteroid, it is now considered to be an old rocket part from a failed 1966 robotic mission to the Moon.

The object, now named 2020 SO, was first detected in September 2020 by the Pan-STARRS telescope which surveys the skies for potentially hazardous ‘near-Earth objects’ – asteroids or comets that could impact Earth.

Astronomers initially thought it could be a new asteroid or a ‘temporary natural Moon’ of Earth – a natural object that temporarily gets trapped in Earth-orbit. So, they began to track its movements to get a better idea.

“We followed it quite a bit for the very first few days, once there was a possibility for it to be natural,” explains Marco Michelli, Astronomer at ESA’s Near-Earth Object Coordination Centre.

“I got astrometry (measurements of its position) for a couple of weeks which showed that there was quite a strong solar radiation pressure signature, proving it was too light to have formed naturally.”

This apparently new object was being pushed by radiation from the Sun. Such solar pressure is so weak that astronomers knew the object in question must be very light, and this suggested it was in fact a human-made, artificial object (which tend to be far lighter and less dense than natural bodies formed in space).

Astronomers traced the orbit of this mysterious object back in time and now believe it is in fact the upper stage of a Centaur rocket that left Earth on 20 September 1966 carrying the Surveyor 2 lander bound for the Moon. The mission failed, and the upper stage of the rocket, 12-metres long by 3-metres wide, shot past the Moon and drifted off into orbit around the Sun.

It’s thought that, possibly, the old rocket has only been temporarily captured by Earth’s gravity, and in a couple of turns around our planet could eventually get back into orbit around the Sun.

“In some ways it has been and is hiding in the boundary between near-Earth object and space debris searches, a search region where there are very few objects distributed over a large volume of space” concludes Tim Flohrer, Head of ESA’s Space Debris Office.

“The life of this rocket part so far has similarities to an object called WT1190F, a small temporary satellite of Earth thought to be debris from the 1998 Lunar Prospector mission, that impacted in 2015. It is still to be assessed if this newly rediscovered object could return and re-enter Earth’s atmosphere one day.”