Meteor showers occur when Earth passes through a stream of debris given off by a comet. Particles that make up this debris stream typically range in size from pebbles to sand grains. As they enter Earth’s upper atmosphere at high speeds, they vaporize 50 miles above our heads. When they vaporize, they produce slivers of light streaking across the sky that we call a meteor or “shooting star”.
Earth passes through a particular particle stream every year in December producing what is known as the Geminid meteor shower. The mystery connected with this shower began in 1862. Prior to 1862, there was no Geminid meteor shower. The first Geminid shower suddenly appeared in that year, surprising onlookers who observed meteors streaking away from the constellation Gemini.
Astronomers immediately began searching for a comet as a source of the particles. Their search was unsuccessful for more than a hundred years. In 1983, NASA’s Infra-Red Astronomy Satellite (IRAS) spotted an object moving in about the same orbit as the Geminid debris stream. The object, which is several miles across, was named 3200 Phaethon.
Oddly, Phaethon seems more like an asteroid than a comet. It is rocky, not icy, and has no obvious tail. If Phaethon is indeed an asteroid, how did it produce a stream of particles? Perhaps a collision with another asteroid or large meteoroid created a cloud of dusty material that follows Phaethon in its orbit around the Sun. Another possibility is that Phaethon could be a dead comet. Because its orbit takes it closer to the Sun than Mercury, the Sun’s heat could have vaporized all of Phaethon’s ice long ago.
In 2009, NASA’s STEREO-A spacecraft imaged Phaethon when it was close to the Sun. The asteroid showed an unexpected brightening that was consistent with a release of dust particles, possibly due to break-up of surface rocks caused by the Sun’s intense heat. If this was a release of dust, the amount of dust was small and not enough to replenish the meteor stream. More observations will be needed to determine how often this happens and whether this occurs only when Phaethon is near the Sun. Asteroid or dead comet? The mystery continues.
My next blog entry will have specifics on this years’ shower. For more in-depth information, the Gemind meteor shower will be the topic of Clark Planetarium’s Windows to the Universe on Thursday December 6 and Saturday December 8.