The Geminid Meteor Shower

The Geminids

The Geminid meteor shower officially begins on December 7th, but it doesn’t peak until the morning of the 13/14th. 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.

 As with all meteor observing, the best place to observe is well away from city lights and air pollution that could limit your ZHR. Wrap up warmly and bring a hot flask and some sugary food to help keep you warm and alert. 

Geminid Radiant

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.”

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). Meteor observers 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.

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.

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.