One of the old reliables of the night sky — the Geminid Meteor Shower — will soon make its annual visit and should put on quite a show.
Active for around a dozen days in December, it will peak on the evening of Dec. 13th and the morning of the 14th. The moon will then be three days past its third quarter phase and not rise until a bit after 3 a.m., so the sky will be dark (away from city lights), revealing up to 120 meteors per hour during the peak time (just after midnight).
On any given night, random meteors occur when celestial debris sporadically enters Earth’s atmosphere. Meteor showers are more predictable, typically include numerous meteors rather than a random few, and are usually linked to comets.
When comets come close to the sun, ices on the surface vaporize and stream away from the comet. Dust and other small particles are carried along with the gases. Over time this material spreads out over the entire orbital path of the comet.
If Earth's orbit happens to intersect the orbit of the comet, Earth can sweep up these particles which then burn up in our atmosphere, resulting in the dramatic streaks of light interchangeably known as shooting stars, falling stars, and meteors. In fact, many meteor showers are linked to specific comets. For example, both the Orionid and Eta Aquariid meteor showers derive from remnants of Halley’s Comet.
The Geminids, however, appear to be an exception to the link with comets. In 1983, astronomers discovered a celestial object now known as 3200 Phaethon. Its orbit matched that of the Geminids, thus giving reason to believe that it is the source of the Geminid Meteor Shower. The curious thing is 3200 Phaethon is not a comet, at least not in the typical sense. It has a rocky surface and scientists designate it as an asteroid, a type of body that, due to its rocky or metallic composition, doesn’t see its surface vaporize (meaning no source for potential meteor showers).
Perhaps 3200 Phaethon accumulated a debris cloud after bumping into another asteroid. Another possibility is that 3200 Phaethon is actually a dead comet, with all of its ice vaporized by repeated close approaches to the sun. A third idea, one that has gathered support in recent years, holds that it might be a relatively rare body known as a rock comet, discharging rock particles, rather than gases, when heated by the Sun.
While the origin of its debris cloud remains a mystery, other characteristics of 3200 Phaethon are pretty well established. It’s a type of asteroid, termed “Apollo”, whose orbit crosses that of Earth but in fact spends most of its time outside of this zone. 3200 Phaethon orbits the sun every 524 days, with an elliptical path that brings it within anywhere from 13 million to 223 million miles to the sun. It has the distinction of passing closer to the sun than any other named asteroid (of course that can change at any time as astronomers discover and characterize more asteroids).
Coincidentally, on Dec. 16 — just three days after the peak of the Geminid Meteor Shower — this three-mile-wide body will also make its closest approach to Earth, coming within 6.4 million miles. While this is relatively close by astronomical standards, it is still some 27 times the distance from Earth to the moon; scientists see no danger of it actually impacting Earth.
Whatever the true nature of 3200 Phaethon, the meteors produced by its debris are worth learning about and seeing. Lowell Observatory will offer a perfect opportunity to do this on Wednesday, Dec. 13, with special evening programming. The night kicks off at 6 p.m. with family-friendly activities that teach guests the cause of meteor showers. Then, at 7 p.m., an educator will give a lecture about the Geminids that culminates with viewing tips, including how, when and where to best see them.