A large near-Earth asteroid discovered in 1937 but not seen since was found again last week.
The rediscovery ends an investigation, ongoing for 66 years, while the boulder the size of a modest town repeatedly slipped by unnoticed.
The space rock, called Hermes, is well known to asteroid experts for its passage about twice as far from Earth as the Moon back in 1937. At the time, astronomers didn't know of any object that had ever come closer.
Hermes orbits the Sun on an elongated path that crosses the orbits of Earth and Venus, and then curves well out into the solar system. The new observations suggest it may be larger than originally thought, perhaps about a mile wide (1-2 kilometers).
Steven Chesley of NASA's Near Earth Object Program Office at the Jet Propulsion Laboratory (JPL), called Hermes' elusiveness "the last real big remaining thorn in the side for this business" of large-asteroid detection.
Closer still
For six decades, researchers assumed Hermes was making other close approaches to Earth, but they didn't know enough about its orbit to determine whether the planet was at risk of some future direct hit.
Now they know.
Earth is safe from Hermes, at least for 100 years, according to computations by Chesley and his colleague, Paul Chodas. Thereafter, no one can yet say exactly what path Hermes will travel.
Hermes, also named 1937 UB, is capable of bringing civilization to its knees were it to smack into Earth. Scientists have now calculated that it came even closer to Earth during its years in hiding. The closest, in 1942, was about 1.6 times the Earth-Moon distance.
Why was the big boulder not found until last week?
"People weren't looking in a real systematic way until the 1990s," Chesley said in a telephone interview. "There haven't been any real close passes to Earth since the '40s and '50s."
Nowadays, various search programs routinely spot asteroids, even some no larger than football fields that zoom by closer than the Moon. In fact, the closest known pass ever by a space rock that didn't hit the planet was recorded last month.
The recovery
Hermes was originally discovered by German astronomer Karl Reinmuth. It was found anew in a collaborative effort.
Brian Skiff of the Lowell Observatory Near-Earth Object Search program (LONEOS) in Arizona first spotted Hermes.
"I didn't recognize it to be Hermes when I found it," Skiff told SPACE.com, "It was just an unusually bright fast-moving asteroid." Astronomers find asteroids by their motion against comparatively still background stars. The uncertainty in the sky position of Hermes at the time, however, "was the whole width of the sky," he said.
Skiff sent images of the then-unknown object to the Minor Planet Center, an international clearinghouse for asteroid data. There, Timothy Spahr tied it in with other recent LONEOS observations as well as from the Lincoln Near Earth Asteroid Research (LINEAR) project in New Mexico.
The object's identity was confirmed with the help of follow-up observations by James Young at the Table Mountain Observatory in California.
Astronomers were sure this was Hermes. But they couldn't quite get its current path to match up with the 1937 observations. That job was left to Chesley and Chodas at JPL. It wasn't easy.
"This was the most demanding trajectory computation I've ever run across," Chesley said. He explained that with each pass through the inner solar system, Hermes' path has been altered by the gravity of Earth and Venus. But none of those interactions was known with precision.
The problem was akin to seeing the final inches of a billiard ball's motion after it's been bouncing off other balls for awhile, Chesley said. "By the time a ball finally comes to rest, it's hard to know what its complete path amongst all those balls was."
He added that "you almost have to know the answer before you can compute the answer."
There is no indication that Hermes will ever hit Earth, but astronomers will keep an eye on it to get a better handle on the rock's orbit around the Sun and future passes near Earth.
"It will remain a potentially hazardous asteroid probably for many centuries," Chesley said. More data, including radar observations expected during the next couple of weeks, should help researchers better assess the past and future movements of Hermes, he added. At present, no pictures of Hermes exist that show it as more than a point of reflected light.
Inevitable find
The first asteroid to be discovered was Ceres, in 1801. But Ceres was an easy target, roughly 590 miles (950 km) wide. It stays in the main asteroid belt between Mars and Jupiter.
Astronomers had predicted about two years ago that Hermes might make an approach late this year that would be near enough to allow rediscovery.
Hermes will pass within about nine Earth-Moon distances later this year, the rediscovery team now says. Chesley said a little luck was involved in finding Hermes so soon on its current close approach to Earth. But someone would have found it before the end of the year, he's sure.
"There's no way this one would have passed through our net this time," he said.
The asteroid will be visible in 8-inch and larger backyard telescopes in late October, according to Roger Sinnott, senior editor at Sky & Telescope magazine [Software for locating Hermes].
This article is part of SPACE.com's weekly Mystery Monday series.
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