'Asteroid' 2020 SO was first detected on 17 September 2020, by the University of Hawaii's PANSTARRS telescope. The designation 2020 SO implies that it was the 20th asteroid (asteroid O - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 24, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 25, A2 = 49, etc., which means that O = 20) discovered in the second half of September 2020 (period 2020 S). The body was found to have a 386 day (1.06 year) orbital period, with an elliptical orbit tilted at an angle of 0.14° to the plain of the Solar System which takes in to 1.00 AU from the Sun (the distance at which the Earth orbits the Sun) and out to 1.07 AU (7% further away from the Sun than the Earth). As such it was classified as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer).
2020 SO regularly passes close to the Earth, passing us at a distance of 50 400 m (less than a tenth of the distance to the Moon) on 1 December 2020, and being due to pass us agian at a distance of 225 300 m on 2 February 2021. As such 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 about its nature. Now a press release issued by the European Space Agency on 12 January 2021 suggests 2020 SO is an old rocket part from a failed 1966 robotic mission to the Moon.
Series of images showing the motion of 2020 SO. The fixed objects are stars. European Space Agency.
'We followed it quite a bit for the very first few days, once there was a possibility for it to be natural,' explained Marco Michelli, Astronomer at the European Space Agency’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 European Space Agency’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.'
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