NASA scientists say a recently discovered satellite of Earth is probably a Saturn V third stage rocket booster that has been travelling around the Sun in Earth's vicinity for decades and was only recently captured by Earth's gravity in April.
The object has a 20 percent chance of hitting the Moon next year, according to new computer calculations based on the most recent observations of the object's position and path.
The odd satellite orbits Earth every 50 days. It was discovered on Sept. 3 and scientists had wondered if it might be a space rock captured by Earth as a new moon. That idea has not been ruled out, but the new calculations, by Paul Chodas at NASA's Jet Propulsion Laboratory (JPL), lend further support to the more likely expectation that the object is human-made space junk.
"It could be a leftover Saturn V third stage from one of the Apollo missions, most likely the Apollo 12 mission, launched on November 14, 1969," Chodas and his colleagues explained in a statement.
Such junk can drift in and out of Earth's orbit over the years, spending time orbiting the Sun along roughly the same path as Earth's yearly circuit. Earth's new satellite, temporarily named J002E3, is about twice as far away as the Moon.
"It's most likely a spacecraft," said Donald Yeomans, an asteroid expert at JPL. "It's not likely to be a natural object, not in that kind of orbit."
J002E3 might have come within Earth's vicinity in the late 1960s or early 1970s, the computer analysis showed.
The satellite was apparently captured by the Earth when the object passed near Earth's L1 Lagrange point, a location where the gravity of the Earth and Sun approximately cancel each other out. This point serves as "portal" between the regions of space controlled by the Earth and Sun.
The JPL statement said J002E3 is the first known case of an object being captured by the Earth, although Jupiter has been known to capture comets via the same mechanism. Comet Shoemaker-Levy 9, which collided with Jupiter in 1994, had been captured by Jupiter decades earlier.
J002E3 was discovered by Bill Yeung from Arizona. It was first listed as a minor planet (typically meaning an asteroid) by the Minor Planet Center in Cambridge, Massachusetts, which serves as a clearinghouse for such objects. It was later removed from that list when it was found to be in Earth orbit.
The presumed hunk of metal is visible from Earth to experienced amateur astronomers with telescope of 8-inches or larger. It shines at magnitude 16.5 -- too faint for the naked eye or small telescopes.
The brightness of J002E3 seems to match the expected brightness of a Saturn rocket booster, the scientists say. "Further circumstantial evidence suggests that this object is in fact the Apollo 12 stage, which was left in a very distant Earth orbit after it passed by the Moon on Nov. 18, 1969. This spent rocket body was last seen in an Earth orbit with a period of 43 days, not much different from J002E3's current orbit."
The presumed hunk of metal could join a handful of others that have hit the Moon. NASA intentionally slammed five rocket boosters on the Moon in the early 1970s in an effort to study the structure of the Moon.
JPL scientists say J002E3 "has a surprisingly large 20 percent chance of impacting the Moon in 2003."
It also has a 3 percent chance of entering Earth's atmosphere in the next 10 years. It would almost surely burn up and not impact the planet, experts say. Larger pieces of space junk have fallen through the atmosphere harmlessly.
The odds of any impact will likely change in coming days as more observations are made and the path of J002E3 is further refined, JPL scientists said.
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