Geminid meteor shower: why spectacular light show puzzles scientists

By Pete Spotts, Staff writer / December 13, 2011

Gemenid meteor shower watchers can’t figure out where all the material in the event comes from. Asteroid 3200 Phaethon, the apparent source of the Gemenid meteor shower, doesn’t seem to shed enough rock and dust to account for the shower’s intensity.

The Gemenid meteor shower lights up the sky over the Mexican volcano Popocatepetl. Daniel Aguilar/REUTERS/File

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By now you’ve probably heard that tonight’s the night for what is arguably one of the most spectacular meteor showers of the year, the Geminids.

The shower earns its name from its apparent point of origin, or radiant, in the constellation Gemini.

But where many meteor showers represent Earth’s encounter with dust from a comet, the Geminids appear to have an odd duck of a source: an asteroid that some now call a rock comet.

And it’s not clear from recent observations whether the object, known as 3200 Phaethon, is kicking off enough material to account for the intensity of the meteor shower Earth encounters.

A team of astronomers identified the debris gap in a paper published in the Astronomical Journal in November 2010. And researchers are still puzzling over it.

Although the first recorded observations of the Geminids don’t appear until the early 1860s, modeling studies of the debris’ orbit suggests that the stream is anywhere from 200 to 6,000 years old.

3200 Phaethon was discovered by NASA’s Infrared Astronomical Satellite in 1983. Once scientists determined its orbit, the orbit closely matched with the orbit of the debris stream.

One of the asteroid’s key features is its proximity to the sun at closest approach. It comes nearer the sun than any known asteroid, well inside the orbit of Mercury. This allows surface temperatures to reach 1,400 degrees Fahrenheit.

In their 2010 paper, David Jewitt and Jing Li at the University of California at Los Angeles looked at images from one of a pair of NASA sun-watching satellites taken of 3200 Phaethon during its close approach to the sun. Its brightness increased suddenly, indicating that it was shedding material, just as a comet might.

Given the asteroid’s rocky makeup, the team posited that the object’s relatively quick swing into and out of this hot zone leads to rock fracturing when heated. Comets, on the other hand, shed material when the ices they contain heat and shift directly from solid to gas. In the process, a comet also ejects dust and rock bound up with the ices.

To account for the intensity of the Geminid stream, the duo calculated, the 30-mile-wide 3200 Phaethon would have to repeat the shedding process 10 times per orbit, something no one has observed.

Dr. Jewitt allows in an e-mail exchange that their mass estimate could be off, and that additional outbursts during each orbit could emerge with more-systematic observations.

But for now, the grit gap remains.

So, bundle up, grab a chaise lounge, a Thermos of hot chocolate, and head out to see the show overnight tonight or even Wednesday night. The peak actually is expected to occur at about 2 p.m. Eastern Standard Time Wednesday.

But the shower, which typically produces more than 100 meteors an hour during its peak under dark skies, will be a bit less spectacular than it otherwise might be. Blame the bright, if waning, moon. Moonlight will wash out the dimmer meteor streaks.

Geminid meteor shower: why spectacular light show puzzles scientists –


Asteroid (or Comet?) 3200 Phaethon (Strange Solar System Object Discovered by IRAS in 1983)

Last modified November 20, 2006 by Randy Russell

3200 Phaethon has a strange orbit. Sometimes it is far from the Sun, out past Mars, in the main asteroid belt. Other times it gets very close to the Sun, inside of the orbit of Mercury. This picture shows how Earth crosses 3200 Phaethon’s orbit in December. That is when we can see the Geminid meteor shower. Original Windows to the Universe artwork by Randy Russell using orbital information from NASA/JPL.

In 1983, a satellite named IRAS (Infrared Astronomical Satellite) discovered a strange, dark object orbiting the Sun. The object, which was given the name 3200 Phaethon, turned out to be an asteroid. It was the first time in history that a satellite discovered an asteroid!

Or so it seemed. 3200 Phaethon is a mysterious object, and astronomers are not quite sure whether it is an asteroid at all! Instead, it may be the burned out, skeletal remains of an extinct comet. In either case, 3200 Phaethon appears to be the source of the meteoroids that flash through Earth’s sky each year in mid December as the Geminid meteor shower. Scientists are trying to determine which of two possible stories about this object is true.

3200 Phaethon may be an asteroid. So how could it create a meteor shower, a phenomenon usually associated with the remains of comets? 3200 Phaethon’s orbit does at times send it into the main asteroid belt, a relatively crowded region in our solar system. Perhaps it collided with another asteroid, knocking a cloud of debris loose from the asteroids. This debris cloud would spread out along 3200 Phaethon’s orbit over time. When Earth crosses 3200 Phaethon’s orbit each December we would see the debris as a series of meteors.

A second possibility is that 3200 Phaethon is the burned out remains of a comet. The objects unusual orbit repeatedly swings it close to the Sun… very close. At perihelion, 3200 Phaethon’s closest approach to the Sun, the object swings well inside the orbit of Mercury. It gets to within about 1/7th of the distance between the Earth and the Sun (0.14 AU), or about 58% of the average distance between Mercury and the Sun. As you might guess, passing this close to the Sun heats 3200 Phaethon to very high temperatures. Astronomers estimate that its surface temperature may reach 750° C (1,385° F). After numerous passes by the Sun, this heat would boil all of the ices off of the comet, leaving its charred remains looking much like the surface of an asteroid. As is the case with other comets that produce meteor showers, the dust flung off the comet that in the past would have formed 3200 Phaethon tails would have gradually spread around the object’s orbit, and would have thus be the source of the Geminid meteors.

Scientists are not yet sure which of these two scenarios is true. They do know that 3200 Phaethon is a very dark object, and that it has a diameter of about 5.1 km (3.2 miles).

3200 Phaethon’s name is derived from its perilous passages near the scorching Sun. In Greek mythology, Phaëton was the son of the Sun god Helios. Phaëton insisted on driving his father’s chariot of the Sun, and did a rather poor job of it when his father gave him a chance. First, he veered too high, chilling the Earth. Next, he accidentally flew too low, turning most of Africa into desert and (so the story goes) burning the skin of the Ethiopians black. Eventually Zeus was forced to intervene, hurling a lightning bolt at the chariot. Even in the days of the ancient Greeks, parents got gray hairs when they handed over the keys to their kids!

Asteroid (or Comet?) 3200 Phaethon (Strange Solar System Object Discovered by IRAS in 1983).