Observations by a telescope in Hawaii clinched its identity, according to NASA's Jet Propulsion Laboratory in Pasadena, California on Wednesday.

The object was classified as an asteroid after its discovery in September.

However NASA's top asteroid expert, Paul Chodas, quickly suspected it was the Centaur upper rocket stage from Surveyor 2, a failed 1966 moon-landing mission.

Size estimates had put it in the range of the old Centaur, which was about 10 meters long and 3 meters in diameter.

Chodas was proven right after a team led by the University of Arizona's Vishnu Reddy used an infrared telescope in Hawaii to observe not only the mystery object but - just on Tuesday - a Centaur from 1971 still orbiting Earth.

The data from the images matched.

"Today's news was super gratifying!," Chodas said via email.

"It was teamwork that wrapped up this puzzle."

The object formally known as 2020 SO entered a wide, lopsided orbit around Earth last month and, on Tuesday, made its closest approach at 50,476 kilometres.

It will depart the neighbourhood in March, shooting back into its own orbit around the sun.

Its next estimated return is in 2036.

Australian Associated Press