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LOS ANGELES — An amateur astronomer hunting for asteroids may have discovered a piece of the rocket that launched the Apollo 12 astronauts to the moon in 1969, a NASA scientist said Thursday. 

The object was first spied on Sept. 3 by Arizona astronomer Bill Yeung. Follow-up observations and calculations of its path suggest it is orbiting the Earth once every 48 days at a distance twice that of the moon. 

Although initially believed to be an asteroid, astronomers now suspect it is a rocket fragment, possibly the third stage of the massive Saturn V launched Nov. 14, 1969, with astronauts Charles “Pete” Conrad Jr., Richard F. Gordon and Alan L. Bean aboard. 

“It’s a detective story and we’re looking at the evidence here,” said Paul Chodas, an astronomer at the National Aeronautics and Space Administration’s Jet Propulsion Laboratory. 

Complex orbit calculations suggest the fragment, which stands nearly 59 feet tall, was captured into Earth orbit in April, explaining why it had not been spotted before. Prior to this spring, the rocket stage had likely spent three decades orbiting the sun, Chodas said. 

On the Apollo 8 through 12 missions, NASA designed the third stage, which boosted the astronauts from Earth orbit toward the moon, to sail past Earth’s lone natural satellite. 

That close passage by the moon was designed to swing the stage into solar orbit and away from Earth. 

The first four times, it worked perfectly, but NASA engineers made an error on Apollo 12, leaving the third stage stranded in Earth orbit. Eventually, in the early 1970s, it drifted from the Earth’s bounds and began orbiting the sun. 

In April, the Earth apparently snagged it back, Chodas said. 

Looking forward, NASA astronomers said there is a 20 percent chance the rocket will end up hitting the moon — as did the third stages of the Apollo 13 through 17 missions. 

There is also a 3 percent chance it could strike Earth, as did some Apollo stages in the 1960s. Most of the rocket body would burn up in the atmosphere, although some pieces could survive the fiery re-entry, Chodas said. 

In the past, astronomers have suspected other near-Earth bodies are actually Apollo rocket fragments. None has been confirmed, Chodas said.