While searching for space rocks on Feb. 19, 2001, Robert McMillan made a routine discovery of a streak of light in the sky that appeared to be a relatively nearby object. McMillan hunts for asteroids that might collide with Earth. So following normal procedure, he reported the object to the International Astronomical Union's Minor Planet Center.

"I didn't know what the object was at the time of discovery, only that the Minor Planet Center's Web site did not list any known minor planet at that location," said McMillan, who scans the skies as part of the Spacewatch program at the University of Arizona's Lunar and Planetary Laboratory.

It is important work. If an asteroid is heading our way, we want to know.

Scientists at the Minor Planet Center were immediately intrigued by the object. They calculated where it was headed. The object was about 1.5 lunar distances from Earth. Anything this close needed to be watched carefully. But the scientists were suspicious. Something wasn't right. The orbit was a very unlikely one. It even seemed to share characteristics with Earth's orbit around the Sun.

"When you have small objects in orbits that resemble the Earth, there is always a suspicion that an object is artificial," said Gareth Williams, associate director of the Minor Planet Center.

Still, Williams gave the object an official space rock stamp of approval: Asteroid 2001 DO47.

Williams said yesterday that in the past, scientists have complained about how long it sometimes took to get an object listed, a critical step to circulating information so others can turn their telescopes toward the potentially dangerous rock and make orbital calculations. Efforts have been made to speed up the process since a new staff member was added midway through last year.

E-mails flew from coast to coast as researchers discussed the object's strange orbit and scrambled to pin it down.

After 2001 DO47 was announced, Jon Giorgini of NASA's Jet Propulsion Laboratory looked at the object's orbit. He ran some orbital solutions on his computer and determined that it had passed within 6,064 miles (9,782 kilometers) of the Moon last August. Giorgini also noted that the orbit was "difficult to obtain by accident." (Williams called it "weird.")

It looked like the object might be captured in a temporary Earth orbit. Sort of a second moon.

More data arrived on Friday, Feb. 23, and researchers were anxious. The object was not behaving as predicted. Nobody needs a near-Earth asteroid behaving erratically. But Giorgini and others became more and more suspicious that the object might not be a real space rock. Calculations continued.

On Sunday at the Minor Planet Center, Williams finally got word on the true nature of the object from the local satellite expert Jonathan McDowell. He broke the news to his boss, Brian Marsden: "2001 DO47 has switched on its engine!"

Williams suggested that confirmation of the presumed engine firing be obtained from Giorgini before announcing anything. Giorgini agreed that an engine had been activated, and the announcement was made that 2001 D047 was a machine.

Asteroid 2001 DO47 is actually NASA's WIND spacecraft, launched in 1994. NASA says the craft was to go into a "sun-ward, multiple double-lunar swing-by orbit ... followed by a halo orbit at the Earth-Sun L-1 (Lagrangian) point."

No wonder it confused researchers.

"We would have liked to check out the artificial possibility before announcing the object, but our local ... satellite expert, Jonathan McDowell ... was away observing in Arizona," Williams said in an electronic posting after the object's true identity was revealed.

WIND has tricked scientists before, Williams said, but the last time around it didn't undergo any official name changes.

"Part of the problem," Williams told SPACE.com on Wednesday, is that "many artificial satellites with highly elliptical orbits [making it easy to confuse them with asteroids at certain points in their orbits] are not tracked and so orbital elements are not available for them."

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