Earth’s phoney ‘second moon’ is about to disappear forever

The ‘moon’ is in fact a Centaur rocket booster from the 1960s, lost from the Surveyor 2 mission

Adam Smith@adamndsmith
Monday 01 February 2021 17:06
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Earth’s second ‘moon’ is set to leave the space around our planet for the last and final time.

Known as 2020 SO, the object was temporarily captured by Earth’s gravitational pull on 8 November 2020 as scientists attempted to identify it.

"Due to extreme faintness of this object following CNEOS prediction it was a challenging object to characterize" said Professor Vishnu Reddy said at the time.

"We got color observations with the Large Binocular Telescope, or LBT, that suggested 2020 SO was not an asteroid."

Nasa eventually confirmed that the object was not in fact any asteroid, but was a Centaur rocket booster from the 1960s that likely made its way into space on the 1966’s Surveyor 2 mission but failed to land on the Moon. 

This 1964 photograph shows a Centaur upper-stage rocket before being mated to an Atlas booster. A similar Centaur was used during the launch of Surveyor 2 two years later

One of the giveaways to the true nature of the ‘moon’ was the proximity of its orbit – coming close enough to the Earth that scientists believed it could have originated from the Earth in the first place.

Another was its comparatively slow speed. The rocket booster passed the moon at a speed of 1,880 miles per hour, which is particularly sluggish had it been an asteroid.

Nevertheless, the booster had actually passed by Earth several times – unnoticed by scientists – including one in 1966 shortly after it had launched.

The reason the booster came back into orbit was, strangely, due to the light of the sun.

"Solar radiation pressure is a non-gravitational force that is caused by light photons emitted by the Sun hitting a natural or artificial object," said Davide Farnocchia, a navigation engineer at JPL, who analyzed 2020 SO's trajectory for CNEOS.  

"The resulting acceleration on the object depends on the so-called area-to-mass ratio, which is greater for small and light, low-density objects."

Since an empty rocket has a very low density, combined with the lack of resistance in the vacuum of space, it was able to be moved easily – comparable to an empty soda can pushed around by the wind.  

The mini ‘moon’ will make one final close journey to the Earth on 2 February 2021, notes, travelling 0.58 lunar-distances (140,000 miles, or 220,000 km) from our planet before the grip of spatial forces will let the celestial body go and it sets off to begin orbit around the Sun.

The Virtual Telescope Project in Rome will be showing the object’s departure online on the night of 1 February.

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