Nasa has confirmed that an object spotted by its asteroid-hunting facilities was actually something else entirely.
The object, known as 2020 SO, is in fact a Centaur rocket booster from the 1960s, the space agency said.
It probably made its way into space with the 1966’s Surveyor 2 mission – which was intended as the second lunar lander to arrive on the Moon, but got lost on the way – and has been floating around since then. Earlier this year, that long orbit brought it back towards Earth, where it was discovered in September by astronomers who were looking for asteroids.
As scientists examined it, they found that its orbit would have brought it close to Earth a few times over the decades. One of those close passes, in 1966, was so close that it appeared to suggest that it might actually have set off from Earth in the first place.
New research saw scientists use Nasa’s Infrared Telescope Facility, or IRTF, which is based on Maunakea in Hawai’i. Vishnu Reddy from the University of Arizona and his team conducted spectroscopy observations of the object in an attempt to understand what it might be made of and what it could be.
"Due to extreme faintness of this object following CNEOS prediction it was a challenging object to characterize" said Professor Reddy in a statement. "We got color observations with the Large Binocular Telescope, or LBT, that suggested 2020 SO was not an asteroid."
The team then compared the object with 301 stainless steel, which was used to build the Centaur rocket boosters in the 1960s. Unexpectedly, they found that it was a little different – but wondered if the difference might be a result of the fact they were comparing steel from a lab with a sample that has spent half a century floating in space.
"We knew that if we wanted to compare apples to apples, we'd need to try to get spectral data from another Centaur rocket booster that had been in Earth orbit for many years to then see if it better matched 2020 SO's spectrum," said Professor Reddy.
"Because of the extreme speed at which Earth-orbiting Centaur boosters travel across the sky, we knew it would be extremely difficult to lock on with the IRTF long enough to get a solid and reliable data set."
In December, however, they found themselves in luck: another rocket booster that left Earth in 1971 was observed in Earth’s orbit. They could then compare the readings from 2020 SO – and found that they were remarkable similar after all.
2020 SO made its closest approach to Earth on Tuesday. It will stay within the Earth’s gravitational pull until March, when it will escape and fall back into a new orbit around the Sun.
Nasa noted that being able to tell the difference between natural and artificial objects will be key as it continues its efforts to catalogue the near-Earth objects that could threaten our planet, and as a variety of countries fill up the space above our heads with yet more launches.
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