Dec 12, 2014

Embers from a Rock Comet

Dec. 12, 2014: December has arrived, and for backyard sky watchers that means one thing:  It is time for the annual Geminid meteor shower. Every year in early December, Earth passes through a stream of gravelly, dusty debris from "rock comet" 3200 Phaethon.  This causes a meteor shower that sometimes lasts more than two weeks.

"This year's Geminid meteor shower will peak on Dec. 13th and 14th with as many as 120 meteors per hour," predicts Bill Cooke, head of NASA's Meteoroid Environment Office.  "I'm looking forward to a good display."

A new ScienceCast video previews the Geminid meteor shower of 2014. Play it

Everyone has heard of "comets"--icy visitors from the outer solar system that sprout long tails of gas and dust when they come close to the sun. But what is a rock comet?

A "rock comet" is a new kind of object being discussed by astronomers. It is, essentially, an asteroid that comes very close to the sun--so close that solar heating scorches dusty debris right off its rocky surface. Rock comets could thus grow comet-like tails that produce meteor showers on Earth.

Auroras Underfoot (signup)

The source of the Geminid meteor shower, 3200 Phaethon, looks a lot like an asteroid.  Indeed, it comes from the asteroid belt and its colors resemble the colors of other asteroids in the rocky zone between Mars and Jupiter. Yet 3200 Phaethon has an unusual orbit that brings it deep inside the orbit of Mercury.  When this happens, it brightens and sprouts a little tail in mimicry of a comet.  A team of astronomers led by Dave Jewitt of UCLA have been monitoring 3200 Phaethon using NASA's twin STEREO probes. They think that intense solar heating blasts the asteroid's rocky surface, causing 3200 Phaethon to shed meteoroids like embers spitting off a log in a roaring campfire.

The debris stream of 3200 Phaethon is broad and massive. "Of all the debris streams Earth passes through every year, the Geminids' is by far the most massive," says Cooke. "When we add up the amount of dust in the Geminid stream, it outweighs other streams by factors of 5 to 500."

Geminid meteoroids fly out of--you guessed it--the constellation Gemini.  They hit the top of Earth's atmosphere traveling about 35 km/s or 78 thousand mph.  That may sound fast, but it is actually somewhat slow compared to other meteor showers.  Geminid meteors tend to be leisurely and bright, producing many fireballs on the nights around the shower's peak. 

The best time to look, advises Cooke, is probably between local midnight and sunrise on Saturday, Dec. 13th, and again at the same time on Sunday, Dec. 14th, when the constellation Gemini is high overhead, spitting bright embers of a rock comet across a sparkling starry sky.

On Dec. 13, Cooke and a team of astronomers from Marshall Space Flight Center will host an overnight NASA web chat from 10 p.m. to 2 a.m. CST, answering questions about the Geminid meteor shower.  To join the webchat on Dec. 13, log into the chat page at:


Author:  Dr. Tony Phillips Production editor: Dr. Tony Phillips | Credit: Science@NASA