Dec 7, 1999

Great Geminids!

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On December 13 and 14, 1999, fragments of the mysterious asteroid 3200 Phaethon will strike Earth's atmosphere and produce a beautiful sky show.
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.
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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.
Above: This image of Comet Hale-Bopp shows its spectacular tail stretching for millions of miles. The tail is composed of gas, dust, and icy meteoroids that bubble off the comet's frozen nucleus as a result of solar heating. Although 3200 Phaethon has some of the characteristics of a comet it does not develop a cometary tail when it passes close to the Sun.
Above: The three potato-shaped objects above are large rocky asteroids orbiting between Mars and Jupiter. Shown to the same scale from left to right are Mathilde, Gaspra, and Ida. Mathilde has dimensions of about 37 by 29 miles. If 3200 Phaethon is indeed an asteroid, it probably looks like these objects.

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.
Follow the shower at
Web Links

Leonids Live! -site of the live webcast of the 1999 Leonids

North American Meteor Network - home page

Related Stories:
Leonids on the Moon -- Nov 3, 1999. Leonid meteorite impacts on the Moon might be visible from Earth and provide a means for long-distance lunar prospecting.

NASA Meteor Balloon Rises Again -- Nov 1, 1999. NASA scientists are planning to launch a weather balloon into the stratosphere on November 18 to capture a recording of the Leonids meteor shower from 100,000 ft.

Leonids in the Crystal Ball -- Oct 27, 1999. Is 1999 the year for a Leonids meteor storm? Experts make their predictions.

Pop! Ping! Perseids! -- Aug 13, 1999. The Science@NASA meteor balloon popped before reaching the stratosphere but many meteor enthusiasts still saw and heard the Perseid shower.

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.

The Leonid Meteor Outburst of 1997 -- July 16, 1999.Newly released video shows a flurry of Leonids in 1997 that briefly rivaled the great meteor storm of 1966.

Tuning in to April meteor showers -- Apr. 27, 1999. Amateur astronomers capture radio echoes from fiery meteors in April 99.

April's Lyrid meteor shower -- Apr. 21, 1999. The oldest known meteor shower peaks this year on April 22.

A Wild Ride to the Stratosphere in Search of Meteors -- Apr. 14, 1999. The payload from the NASA Meteor Balloon has been recovered.

Meteor Balloon set for Launch -- Apr. 9, 1999. NASA scientists prepare to launch a weather balloon designed to capture micrometeoroids in the stratosphere.

Leonid Sample Return Update -- Apr. 1, 1999. Scientists will describe initial results from a program to catch meteoroids in flight at the NASA/Ames Leonids Workshop April 12-15, 1999.

The Ghost of Fireballs Past -- Dec. 22, 1998. RADAR echoes from Leonid and Geminid meteors.

Bunches & Bunches of Geminids -- Dec. 15, 1998. The Geminids continued to intensify in 1998

The 1998 Leonids: A bust or a blast? -- Nov. 27, 1998. New images of Leonid fireballs and their smoky remnants.

Leonids Sample Return payload recovered! -- Nov. 23, 1998. Scientists are scanning the "comet catcher" for signs of Leonid meteoroids.

Early birds catch the Leonids -- Nov. 19, 1998. The peak of the Leonid meteor shower happened more than 14 hours earlier than experts had predicted.

A high-altitude look at the Leonids -- Nov. 18, 1998. NASA science balloon catches video of 8 fireballs.

The Leonid Sample Return Mission -- Nov. 16, 1998. NASA scientists hope to capture a Leonid meteoroid and return it to Earth.

Great Expectations: the 1998 Leonid meteor shower -- Nov. 10, 1998. The basics of what the Leonids are and what might happen on November 17.


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For more information, please contact:
Dr. John M. Horack , Director of Science Communications
Author: Dr. Tony Phillips
Curator: Linda Porter
NASA Official: M. Frank Rose