A tiny mystery object is zipping past the Earth today, providing astronomers with an excellent opportunity to finally confirm it as being the upper stage of a Centaur rocket that was launched by NASA in 1966.
Is it or isn’t it? This is the question that astronomers have been asking since September, when scientists with the Pan-STARRS1 survey in Maui, Hawai’i, first spotted the object, named 2020 SO. Astronomers have good reason to believe it’s returning space junk, specifically a Surveyor 2 Centaur rocket booster dating back to the 1960s. Trouble is, they haven’t actually been able to prove it.
2020 SO normally orbits the Sun, but Earth’s gravity has, albeit temporarily, turned this object into an artificial minimoon. The object will complete a pair of orbits around our planet before it adopts a new orbit around the Sun, but today (December 1, 2020) is a special day, as the object is making its closest approach to Earth.
Gianluca Masi, an astronomer with the Virtual Telescope Project 2.0 — a group that uses remotely controlled telescopes to observe space — took the opportunity to capture a photo of the object last night.
“I managed to get a tracked image of the object, but also a trail [upper left in the photo] and the latter shows a dotted pattern, basically a bright dot, followed by a fainter one and so on,” Masi explained in an email. “This suggests the object was rotating, with a period of about 10 seconds.”
Masi said he’ll have more to share soon, so we’re looking forward to that.
Looking at the image, we still can’t be sure that we’re gazing upon the lost rocket booster, but we so want to believe that it is. The purpose of NASA’s Surveyor 2 mission was to examine the lunar surface prior to the Apollo missions. Launched on September 20, 1966, the mission started well, but on the second day, a thruster on Surveyor 2 failed to ignite, throwing the spacecraft into a spin. Surveyor 2 crashed onto the lunar surface, while the Centaur upper stage drifted past the Moon and into an unknown orbit around the Sun.
After that, no one gave it much thought.
NASA’s Surveyor program was actually quite successful, despite two failures out of seven attempts to perform soft landings on the lunar surface between 1966 and 1968. You can learn more about these missions here.
Soon after 2020 SO was spotted by PanSTARRs, astronomers at the Centre for Near-Earth Object Studies (CNEOS) at NASA’s Jet Propulsion Laboratory flagged the object on account of its unexpected behaviour. The object’s orbit was more Earth-like than asteroid-like, featuring a nearly circular orbit within Earth’s orbital plane. That’s not typically seen in asteroids.
Additional observations showed that 2020 SO had a trajectory that was being modified by a phenomenon known as solar radiation pressure, as NASA explains:
The pressure exerted by sunlight is small but continuous, and it has a greater effect on a hollow object than a solid one. A spent rocket is essentially an empty tube and therefore is a low-density object with a large surface area. So it will be pushed around by solar radiation pressure more than a solid, high-density clump of rock — much like an empty soda can will be pushed by the wind more than a small stone.
Given these oddities, CNEOS director Paul Chodas sought to determine the origin of the object, which he did by working its orbit backward in time to see where it was in the past. His calculations showed that 2020 SO performed some close approaches with Earth over the past few decades, but the object’s flyby in 1966 was close enough such that it could have originated from Earth.
“One of the possible paths for 2020 SO brought the object very close to Earth and the Moon in late September 1966,” said Chodas in a NASA statement. “It was like a eureka moment when a quick check of launch dates for lunar missions showed a match with the Surveyor 2 mission.”
Today’s close flyby presents an excellent opportunity to study 2020 SO even further, and for scientists to make an ironclad confirmation of its identity. In addition to optical images, astronomers can study the object with spectrometers to determine its chemical composition. We should know very shortly if this mystery object is indeed the Centaur upper stage.
Regardless of whether it’s artificial or natural, this object is most certainly a temporary minimoon. 2020 SO entered into this special phase of its orbit on November 8 and will exit Earth’s gravitational clutches in March 2021. Earth was recently visited by a natural minimoon, asteroid 2020 CD3, which spent the past 2.7 years as a temporarily bound satellite.