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.

Listen to this story (requires RealPlayer)

December 11, 2000 -- Early risers who venture outdoors before dawn this week are in for a treat. The Geminid meteor shower peaks on Wednesday, Dec. 13th, when sky watchers could spot as many as 3 to 5 shooting stars every minute. [observing tips]

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 Phaethon.

Above: Only the brightest Geminids will be visible this year, a result of the nearly-full Moon's overpowering glare. Nevertheless, the Geminids are worth watching. Each and every one is a bona fide enigma. Image credit: Duane Hilton.

Sign up for EXPRESS SCIENCE NEWS delivery

"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).

Left: Meteors are caused by tiny dust particles called meteoroids that burn up in Earth's atmosphere. This picture shows a meteoroid just 10 microns across captured by scientists high in the stratosphere. Geminid meteoroids are dense, 2 to 3 gm/cc, comparable to the density of rock. Typical cometary dust flakes are ten times fluffier, only 0.3 gm/cc.

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."

Right: ESO photo of the comet-like asteroid Elst-Pizarro in 1996.

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."

Above: Sky watchers in the northern hemisphere will see the Geminids radiant (denoted by a red dot) about 60 degrees above the southern horizon before dawn on Dec. 13th. Observers in the southern hemisphere, where the radiant will be just 30 degrees above the northern horizon at dawn, should rotate this sky map 90 degrees clockwise.

No matter where you live, the best time to watch will be during the hours before dawn on Wednesday, Dec. 13th, and again on Thursday, Dec. 14th. The nominal peak of the shower will occur at 1704 UT on Dec. 13th, but the Geminid maximum is relatively long-lasting. Meteor enthusiasts are likely to spot a good number of shooting stars on both Wednesday and Thursday mornings.

Geminid meteors will streak away from a radiant point 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!

Tune in to SpaceWeather.com for more information about the ongoing Geminid meteor shower.

See the orbit of 3200 Phaethon in 3D -- from JPL

Learn more about the Geminids -- compiled by Gary Kronk

"Comet" Elst-Pizarro -- the original ESO press release

The Geminid meteor shower active from Dec. 7 until Dec. 17, but the pre-dawn hours of Dec. 13 should be the best time to watch.

The radiant is in the constellation Gemini at RA=7^h30^m, DEC=+33^o.

Geminid meteoroids hit the atmosphere travelling at about 35 km/s.