[meteorite-list] The Baffling Geminid Meteors

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:37:33 2004
Message-ID: <200012090034.QAA14379_at_zagami.jpl.nasa.gov>


The Baffling Geminid Meteors

NASA Science News
The Baffling Geminid Meteors

Most meteor showers are caused by comets, but the Geminids, which peak on
December 13th, seem to come from a curious near-Earth asteroid.

December 8, 2000 -- Early risers who venture outdoors before dawn next week
are in for a treat. The Geminid meteor shower peaks on Wednesday morning
when sky watchers could spot as many as 3 to 5 shooting stars every minute.

Geminids look like most meteors -- they tend to be fast-moving and yellow in
color. But there's something special about them. Other meteor showers happen
when Earth passes through the debris trail of a comet. Tiny bits of dust no
bigger than a grain of sand disintegrate high in our atmosphere and leave
behind dazzling streaks of light. But the parent of the Geminids isn't a
comet at all. It appears to be a curious near-Earth asteroid known as 3200

"The Geminids are a mystery," says Brian Marsden of Harvard's Minor Planet
Center. "Most meteoroids that we know of come from comets. They are set free
by solar vaporization of [cometary] ice. Geminid meteoroids, on the other
hand, appear to come from 3200 Phaethon, an asteroid. We're not sure why an
asteroid should have a debris trail, but this one does."

Sky watchers first noticed the Geminids in the mid-1800's, but for more than
a century the shower's source was unknown. Then, in 1983, NASA's Infrared
Astronomy Satellite spotted a new asteroid: 3200 Phaethon. Astronomer Fred
Whipple quickly realized that Phaethon and the Geminid meteoroid stream
follow nearly identical orbits. They move around the Sun in a one and a
half-year elliptical path that stretches from inside the orbit of Mercury
outward to the asteroid belt. [View the orbit of 3200 Phaethon in 3D,
courtesy of JPL.]

Every year in mid-December when the Geminid meteor shower is active, Earth
is barely eight lunar distances (~0.021 AU) from Phaethon's orbit. That
makes Phaethon a "potentially hazardous" near-Earth asteroid (NEA).

In most respects Phaethon appears to be an ordinary NEA, says Marsden, but
it is remarkable because it comes so close to the Sun. Its distance from the
Sun ranges from 0.14 AU at perihelion to 2.4 AU at aphelion. "The small
aphelion distance would be unusual for a defunct comet," he explained.

Phaethon's sungrazing orbit might be responsible, in part, for the Geminids.
"You could argue that a lump of dusty ice on the surface of Phaethon was
uncovered at some point and then vaporized by solar heating," he speculated.
Such an event might produce meteoroids in the style of a comet.

Phaethon doesn't have a tail now and there's no evidence that jets of
vaporizing debris are pushing the asteroid around. Whatever liberated the
Geminid meteoroids probably happened long ago.

"The Geminid meteor shower is very stable from one year to the next," notes
Robert Lunsford, Secretary General of the International Meteor Organization,
"and there is no evidence for outbursts that follow close encounters between
Earth and Phaethon." The debris trail seems to be spread rather uniformly
around Phaethon's orbit -- another indicator that the meteoroids are old.

In July 1996 astronomers saw something in the asteroid belt that could be
relevant to the past experiences of 3200 Phaethon.

"Four years ago Eric Elst contacted us from the European Southern
Observatory and reported a strange object (now known as 'Elst-Pizarro' after
its discoverers)," recalled Marsden. "It had a tail, like a comet, but no
coma. We calculated an orbit and it seemed to be a perfectly ordinary minor
planet in the asteroid belt. Furthermore, we found some older images of it
from 1979 and '85. There was no tail in those photos and by 1997 the tail
Elst saw a year earlier was gone."

Despite its brief appearance as a comet look-alike, Elst-Pizarro is probably
an asteroid, says Marsden. "We may have been seeing a cloud of dust that was
ejected by an impact with another asteroid or, perhaps, a small ice deposit
became uncovered and vaporized."

Elst-Pizarro spends all of its time in the main asteroid belt where
asteroid-asteroid collisions are most likely to happen. Phaethon spends less
time there, but it does visit the asteroid belt every 17 months when it
reaches its farthest point from the Sun. A collision between Phaethon and
some smaller object in the asteroid belt might account for the Geminid
debris stream. Detailed studies of Geminid orbits, however, indicate that
the meteoroids more likely crumbled away while Phaethon was close to the
Sun. Once again, there's no clear solution to the Geminid riddle.

The mysterious Geminids will be on display Wednesday morning, Dec. 13th,
when the ongoing shower reaches its day-long peak. Most years stargazers in
rural areas can see as many as 140 Geminids per hour. That number will be
substantially reduced by the glare of a nearly-full Moon on Wednesday.

"The brutal moonlight will prevent us from enjoying the Geminids as much as
usual," laments Lunsford. "I would estimate that rates will average 20 to 30
per hour for most observers."

No matter where you live, the best time to watch will be during the hours
before dawn on Wednesday. Meteors will streak away from a point (called the
"radiant") in the constellation Gemini. Geminids can appear anywhere in the
sky, but their trails will point back toward the radiant, which will lie
some 60 degrees above the southern horizon at 4 am as seen from mid-northern
latitude observing sites.

Even if the 2000 Geminids produce fewer meteors than usual, they're still
worth watching. The morning sky this month features an array of bright stars
and planets including Jupiter, Saturn, Sirius, the constellation Orion, and
even the subtle Pleiades. A trip outdoors before breakfast on Wednesday is a
no-lose proposition (weather permitting!).

And if you see a smattering of Geminids, just remember, each and every one
is a bona-fide enigma -- baffling and dazzling in equal measure!
Received on Fri 08 Dec 2000 07:34:34 PM PST

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