Rock comet sprouts a tail

Nov 28, 2013 by Dr. Tony Phillips
This STEREO image of 3200 Phaethon reveals a stubby but distinct tail.

Astronomers have long been puzzled by a certain meteor shower.

Every year in mid-December the sky fills with flashes of light shooting out of the constellation Gemini. The Geminids are fast, bright, and reliable. They never fail to show up and many observers count them as the finest meteors of the year.

But where do they come from? That is the puzzle.

Meteor showers are supposed to come from comets, yet there is no comet that matches the orbit of the Geminid debris stream. Instead, the orbit of the Geminids is occupied by a thing called "3200 Phaethon." Discovered in 1983 by NASA's IRAS satellite, Phaethon looks remarkably like a rocky asteroid. It swoops by the sun every 1.4 years, much like a comet would, but it never sprouts a dusty tail to replenish the Geminids.

That is, until now.

A group of astronomers led by Dave Jewitt of UCLA have been using NASA's STEREO probes to take a closer look at 3200 Phaethon when it passes by the sun. The twin spacecraft were designed to monitor solar activity, so they get a good view of sungrazing comets and asteroids. In 2010 one of the STEREO probes recorded a doubling of Phaethon's brightness as it approached the sun, as if sunlight were shining through a cloud of dust around the asteroid. The observers began to suspect 3200 Phaethon was something new:

"A rock comet", says Jewitt. A rock comet is, essentially, an asteroid that comes very close to the sun—so close that solar heating scorches dusty debris right off its rocky surface. This could form a sort of gravelly tail.

Indeed, in further STEREO observations from 2009 and 2012, Jewitt along with colleagues Jing Li of UCLA and Jessica Agarwal of the Max Planck Institute have spotted a small tail sticking out behind the "rock."

"The tail gives incontrovertible evidence that Phaethon ejects dust," says Jewitt.

Jewitt's team believes that the dust is launched by thermal fracturing of the asteroid's crust. A related process called "desiccation fracturing"—like mud cracks in a dry lake bed—may play a role too.

Seeing 3200 Phaethon sprout a tail, even a small one, gives researchers confidence that Phaethon is indeed the source of the Geminids—but a mystery remains: How can such a stubby protuberance produce such a grand ?

Adding up all of the light STEREO saw in Phaethon's tail, Jewitt and colleagues estimate a combined mass of some 30 thousand kilograms. That might sound like a lot of meteoroids but, in fact, it is orders of magnitude too small to sustain the massive Geminid debris stream.

Perhaps Phaethon experienced a "big event" in the recent past. "The analogy I think of is a log in a campfire," says Jewitt. "The log burns, makes a few embers, but occasionally will spit out a shower of sparks."

Continued monitoring by NASA's STEREO probes might one day catch the rock comet spitting out a shower of dust and debris, solving the mystery once and for all.

Until then, it's a puzzle to savor under the stars. This year's Geminid meteor shower peaks on the nights of Dec. 13-14 with dozens of "rock comet meteors" every hour. Bundle up and enjoy the show.

Explore further: Sandblasting winds shift Mars' landscape

Related Stories

Phaethon confirmed as rock comet by STEREO vision

Sep 10, 2013

The Sun-grazing asteroid, Phaethon, has betrayed its true nature by showing a comet-like tail of dust particles blown backwards by radiation pressure from the Sun. Unlike a comet, however, Phaethon's tail ...

The 2011 Geminid meteor shower

Dec 13, 2011

The 2011 Geminid meteor shower peaks on the night of Dec. 13-14, and despite the glare of a nearly-full Moon, it might be a good show.

Geminid meteor shower defies explanation

Dec 07, 2010

The Geminid meteor shower, which peaks this year on Dec. 13th and 14th, is the most intense meteor shower of the year. It lasts for days, is rich in fireballs, and can be seen from almost any point on Earth.

Geminid meteor shower 2011

Dec 07, 2011

Its the finale of this year’s meteor showers: The Geminids will start appearing on Dec. 7 and should reach peak activity around the 13th and 14th. This shower could put on a display of up to 100+ meteors ...

Asteroid Shower

Dec 03, 2007

Mark your calendar: The best meteor shower of 2007 peaks on Friday, December 14th.

Geminid meteors set to light up winter sky

Dec 13, 2012

(Phys.org)—On the evening of 13 and the morning of 14 December, skywatchers across the world will be looking up as the Geminid meteor shower reaches its peak, in potentially one of the best night sky events ...

Recommended for you

Video: Alleged meteor caught on Russian dash cam (again)

49 minutes ago

Thanks to the ubiquity of dashboard-mounted video cameras in Russia yet another bright object has been spotted lighting up the sky over Siberia, this time a "meteor-like object" seen on the evening of Saturday, Sept. 27.

Student to live in simulated space habitat

4 hours ago

A Purdue University industrial engineering doctoral student is among six "crew members" spending the next eight months in a domed habitat on a volcanic landscape mimicking life on a Martian outpost.

The wake-up call that sent hearts racing

7 hours ago

"But as the minutes ticked by, the relaxed attitude of many of us began to dissolve into apprehension. Our levels of adrenaline and worry began to rise."

US-India to collaborate on Mars exploration

16 hours ago

The United States and India, fresh from sending their own respective spacecraft into Mars' orbit earlier this month, on Tuesday agreed to cooperate on future exploration of the Red Planet.

User comments : 0