[meteorite-list] Lost & Found: Near-Earth Asteroid Spotted after 66 Years

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:28:32 2004
Message-ID: <200310201613.JAA17974_at_zagami.jpl.nasa.gov>

http://space.com/scienceastronomy/mystery_monday_031020.html

Lost & Found: Near-Earth Asteroid Spotted after 66 Years
By Robert Roy Britt
space.com
20 October 2003

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].
Received on Mon 20 Oct 2003 12:13:30 PM PDT