[meteorite-list] The Curious Tale of Asteroid Hermes

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:27:43 2004
Message-ID: <200311010106.RAA26381_at_zagami.jpl.nasa.gov>

http://science.nasa.gov/headlines/y2003/31oct_hermes.htm

The Curious Tale of Asteroid Hermes
NASA Science News
October 31, 2003

For the next few days backyard astronomers can see for
themselves the long lost asteroid Hermes.

October 31, 2003: It's dogma now: an asteroid hit Earth 65
million years ago and wiped out the dinosaurs. But in 1980
when scientists Walter and Luis Alvarez first suggested the idea to a
gathering at the American Association for Advancement of Sciences, their
listeners were skeptical. Asteroids hitting Earth? Wiping out species? It
seemed incredible.

At that very moment, unknown to the audience, an asteroid named Hermes
halfway between Mars and Jupiter was beginning a long plunge toward our
planet. Six months later it would pass 300,000 miles from Earth's orbit,
only a little more than the distance to the Moon. Rhetorically speaking,
this would have made a great point in favor of the Alvarezes. Curiously,
though, no one noticed the flyby.

                       
1980 wasn't the first time Hermes had sailed by
unremarked. Hermes is a good-sized asteroid, easy
to see, and a frequent visitor to Earth's
neighborhood. Yet astronomers had gotten into the
habit of missing it. How this came to be is a
curious tale, which begins in Germany just before
World War II:

On Oct. 28, 1937, astronomer Karl Reinmuth of Heidelberg noticed an odd
streak of light in a picture he had just taken of the night sky. About as
bright as a 9th magnitude star, it was an asteroid, close to Earth and
moving fast--so fast that he named it Hermes, the herald of Olympian
gods. On Oct. 30, 1937, Hermes glided past Earth only twice as far away
as the Moon, racing across the sky at a rate of 5 degrees per hour.
Nowadays only meteors and Earth-orbiting satellites move faster.

Plenty of asteroids were known in 1937, but most were plodding members of
the asteroid belt far beyond Mars. Hermes was different. It visited the
inner solar system. It crossed Earth's orbit. It proved that asteroids
could come perilously close to our planet. And when they came, they came
fast.

Reinmuth observed Hermes for five days. Then, to make a long story short,
he lost it.

Hermes approaches Earth's orbit twice every 777 days.
Usually our planet is far away when the orbit crossing
happens, but in 1937, 1942, 1954, 1974 and 1986, Hermes came harrowingly
close to Earth itself. We know about most of these encounters only
because Lowell Observatory astronomer Brian Skiff re-discovered Hermes...
on Oct. 15, 2003. Astronomers around the world have been tracking it
carefully ever since. Orbit-specialists Steve Chesley and Paul Chodas of
NASA's Jet Propulsion Laboratory (JPL) have used the new observations to
trace Hermes' path backwards in time, and so they identified all the
unnoticed flybys.

"It's a little unnerving," says Chodas. "Hermes has sailed by Earth so
many times and we didn't even know it."

"Hermes' orbit is the most chaotic of all near-Earth asteroids," he adds.
This is because the asteroid is so often tugged by Earth's gravity.
Hermes has occasional close encounters with Venus, too. In 1954 the
asteroid flew by both planets. "That was a real orbit scrambler," Chodas
says. Frequent encounters with Earth and Venus make it hard to forecast
Hermes' path much more than a century in advance. The good news is that
"Hermes won't approach Earth any closer than about 0.02 AU within the
next hundred years." We're safe for now.

Using the JPL ephemeris, we can look back and figure out what happened in
1937 when the asteroid was lost. With hindsight, it's understandable:

Reinmuth first spotted Hermes approaching Earth from the direction of the
asteroid belt. At first it was easy to see because the asteroid's sunlit
side was facing Earth. Speedy Hermes soon crossed Earth's orbit, however,
and began turning its night side toward us. Asteroids are nearly as dark
as charcoal, and their night sides are very dim. By Nov. 3rd, six days
after its discovery, the asteroid had faded from 9th to 21st magnitude, a
factor of 60,000. "Hermes was also heading into the sun's glare, which
only made matters worse," notes Chodas. Hermes literally vanished.

No one seemed to care, not much. In 1937, World War II was about to begin
in Europe, so people had a lot on their minds. Hermes failed to impress.

Says Chodas: "Astronomers of the day were somewhat biased, perhaps. They
had convinced themselves that collisions were too rare to consider.
Hermes didn't change their opinion because catastrophism was not in
vogue."

It's in vogue now--largely because of comet Shoemaker-Levy
9 (SL9), an object discovered by people hunting for Hermes.
Found in 1993 by Gene and Carolyn Shoemaker and David Levy, SL9 hit
Jupiter on July 14, 1994, with much of the world watching on CNN. Long
before the collision, SL9 had been torn apart by Jupiter's powerful
tides. The largest fragments, coincidentally about the same size as
asteroid Hermes, exploded with such force when they struck that dark
clouds formed in Jupiter's atmosphere as large as Earth itself.

A message from Jupiter: Catastrophes happen.

"Gene always felt that Hermes should have done more to excite the world
than it did at the time" recalls David Levy. "Indeed, he and his wife
Carolyn were always hoping to find it." Shoemaker was a visionary who
realized long before most others did that asteroids and comets posed an
ongoing threat to Earth. In the late 1970's he and a few colleagues began
to hunt for near-Earth objects using an 18-inch telescope at the Palomar
Observatory. For a long while it was the only such survey on Earth. They
discovered dozens of asteroids and comets, including SL9--but not Hermes.
"When Hermes passed by Earth in 1986 (an encounter identified post-facto
by Chodas) it should have been an easy target for us," notes Levy. "But
the telescope was down for repairs." Shoemaker died in 1997 not knowing
how close he came.

Now backyard astronomers around the world can do something Gene Shoemaker
never did--see Hermes.

Hermes is fast approaching Earth, and on Nov. 4th it will pass by our
planet 18 times farther away than the moon. Already the asteroid is about
as bright as a 13th magnitude star--an easy target for 8-inch telescopes
equipped with CCD cameras. Where should you point your 'scope? Consult
the JPL Ephemeris for details.

In recent days a group of NASA-supported astronomers led by Jean-Luc
Margot of UCLA have pinged the asteroid with radar pulses from the giant
Arecibo antenna in Puerto Rico. Hermes, it turns out, is a double
asteroid--two space rocks orbiting one another, each about 400 meters
across. No one knows how Hermes came to be this way. Margot and
colleagues hope to learn more when the asteroid passes by on Nov. 4th as
they continue their observations using both Arecibo and NASA's Goldstone
radar.

Now that Hermes has our attention, it might teach us a few things after
all.
Received on Fri 31 Oct 2003 08:06:48 PM PST