A return visit to Earth

What scientists originally believed to be an asteroid captured by Earth’s gravity was actually a blast from the past.

Researchers from NASA’s Jet Propulsion Lab on Dec. 2 said the previously unidentified object was in fact a 1960s-era Centaur rocket booster endlessly slingshotting through the solar system. While astronomers were excited to identify the rocket, the discovery of 54-year-old debris highlights growing concerns with space junk near Earth.

When a NASA telescope in Hawaii first spotted the near-Earth object named 2020 SO in September, scientists thought it was an asteroid. NASA astronomers keep track of comets and asteroids passing close to Earth because of the trouble they might cause if they break through the atmosphere.

But its small size and unusual orbit led NASA’s top asteroid expert, Paul Chodas, to speculate it might not be an asteroid at all. Initial size estimates put the entity’s dimensions in line with a booster stage from a Centaur rocket. Based on the size and approach path, Chodas reasoned it might be from the failed 1966 Surveyor 2 lunar mission.

University of Arizona planetary scientist Vishnu Reddy pointed telescopes at the object as it made its closest approach to Earth to test Chodas’ theory. Using spectroscopy, Reddy’s team hoped to identify 2020 SO by comparing its material makeup to a piece of stainless steel that would have been used in 1960s rocket construction.

But the data didn’t quite match. Reddy wondered if 54 years of space travel altered the rocket booster enough to account for the difference. “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,” Reddy said.

Reddy knew just where to look. His team took spectral readings from a confirmed 1971 Centaur rocket booster already in Earth’s orbit and found a match.

NASA’s 1966 unmanned Surveyor 2 mission sought to place a lander on the moon. But a crucial thruster failed during a course correction, and the lander crashed onto the moon’s surface. According to NASA calculations, the rocket booster that carried Surveyor 2 to space entered a solar orbit before Earth’s gravity briefly and temporarily captured it this year.

According to NASA, more than 6,000 tons of material circle in low-Earth orbit alone, much of it traveling at speeds about 18,000 mph. Other debris like the Centaur rocket orbits the sun, complicating and jeopardizing space missions.

The Centaur rocket booster, captured by Earth’s gravity on Nov. 8, will make one full but unusual orbit around the Earth before shooting back toward the sun. The American space agency predicts the space debris will escape Earth’s gravity in March, resume its solar orbit, then return to Earth briefly in 2036.