 Space Science News home |
|
|
Space Science News home |
|
December 7, 1999: The 1999 Leonid meteor storm was a
rare treat for many skywatchers in Europe and the Middle East,
but a bit disappointing in other parts of the world. If you missed
the Leonid display because of poor weather, or perhaps because
you live in the wrong place, there's still one more chance in
1999 to see a good meteor shower: the Geminids.
Right: This video clip of two meteors in Orion was obtained by Tony Phillips in Aspendell, CA at approximately 1110 UT on December 13, 1998, the night before the maximum of the 1998 Geminid meteor shower. The 30 frame sequence spans 2 seconds of actual time. The dimmer of the two shooting stars, which travels straight down from the top of the image, is a Geminid. The brighter, which zooms in from the left is probably a sporadic meteor. The shower officially begins on December 7th, but it doesn't peak until the morning of the 14th around 3 a.m. PST (1100 UT). Unlike the Leonids, the Geminid's broad maximum lasts nearly a full day, so observers around the globe have a good chance to see the show. At its peak the Geminids could produce as many as one shooting star every 30 seconds.
Most well known meteor showers, like the Perseids and Leonids, are old. They've been observed for hundreds or even thousands of years. The earliest record of a modern-day meteor shower is probably a notation in Chinese annals dated 36 AD, regarding the Perseids, where it is said that "more than 100 meteors flew thither in the morning." [ref.] The Geminids are a different story. The first Geminid meteors suddenly appeared in the mid-1800's. Those early showers were unimpressive, boasting a mere 10-20 shooting stars per hour. Since then, however, the Geminids have grown in intensity until today it is one of the most spectacular annual showers. In 1998 observers counted as many as 140 per hour (zenithal hourly rate). Sky-watchers with clear skies should see at least that many this year if the Geminids continue to intensify. After the discovery of the Geminids in 1862 astronomers began searching for the parent comet. Most meteor showers result from debris that that boils off a comet's nucleus when it passes close to the sun. This debris orbits the sun along with the comet, forming a thin, elongated stream of meteoroids that become shooting stars when they hit Earth's atmosphere. Years of searching proved to no avail until finally, in 1983, NASA's Infrared Astronomical Satellite discovered a curious object moving in the same orbit as the Geminid meteoroid stream. The orbital match was so good that it had to be the source of the debris, but to the surprise of many it wasn't a comet. The source of the Geminids was apparently a rocky asteroid. 
3200 Phaethon, as the asteroid is now known, is in a highly elliptical
1.4 year orbit that brings it within 0.15 AU (astronomical units)
of the Sun. It made its closest recent approach to Earth in December
1997 when it passed within 0.31 AU of our planet.
Left: Click on the image to take a virtual tour of the solar system, complete with the curious asteroid 3200 Phaethon, the source of the Geminid meteors. But how does an asteroid produce a meteoroid debris stream? Comets do it easily whenever they pass close enough to the sun to heat their frozen nucleus. Tiny bits of ice and dust naturally bubble away into interplanetary space. Rocky asteroids are made of tougher stuff, however, so it is unclear how bits of 3200 Phaethon would break or boil off to form a meteoroid stream. One of the earliest ideas was that Phaethon might occasionally collide with other asteroids. Collisions would create a stream of pulverized rocks that would account for the Geminids meteor shower. Phaethon's orbit passes through the asteroid belt just beyond Mars, so at first this hypothesis seemed likely, but more detailed studies disagree. The orbits of individual Geminid meteoroids are not consistent with the idea that they broke free while in the asteroid belt. Instead, they appear to have crumbled away when Phaethon was closer to the Sun. In this respect Phaethon is behaving like a comet.
So, which is it?Is Phaethon a comet or an asteroid?There are arguments in favor of both. Phaethon's spectra look like those of a rocky asteroid, but its orbit is similar to that of a comet. When Phaethon passes by the sun it doesn't develop a cometary tail, but bits and pieces do break off to form the Geminid meteoroids. By studying photographic records of fireballs, scientists have estimated the density of the Geminid meteoroids to be between 1 and 2 gm/cc. That's less dense than typical asteroid material (3 gm/cc), but several times denser than cometary dust flakes (0.3 gm/cc). Many astronomers now believe that Phaethon is an extinct or dormant comet that has accumulated a thick crust of interplanetary dust grains. Phaethon's thick mantle gives it the outward appearance of an asteroid, but underneath lies the nucleus of a comet. The origin of the Geminids may not be fully understood until future space travelers pay a visit to the asteroid-comet 3200 Phaethon. Until then we can still enjoy the sky show and savor the mystery of the enigmatic Geminids. |
| Web Links |
|
Leonids Live! -site of the live webcast of the 1999 Leonids North American Meteor Network - home page Related Stories: Perseids
Live! Balloon Flight Planned
-- Aug 6, 1999. A NASA weather balloon will ascend to the stratosphere
for a live webcast of the 1999 Perseids. |
 Join our growing list of subscribers
- sign up for our express
news delivery and you will receive a mail message every
time we post a new story!!! Headlines
return to Space Science News Home
|
