Discovery date: October 11, 1983

3200 Phaethon (sometimes incorrectly spelled Phaeton) is an Apollo asteroid and a dead comet.

Simon F. Green and John K. Davies, while searching Infrared Astronomical Satellite (IRAS) data for moving objects, found 3200 Phaethon (1983 TB) in pictures from October 11, 1983. It was announced on October 14 in IAUC 3878 along with optical confirmation by Charles T. Kowal, who reported that it looks like an asteroid. It was the first asteroid to be found by a spacecraft. It measures 5.10 km in diameter.

Phaethon approaches the Sun closer than any other numbered asteroid. Its perihelion (closest approach to the Sun) is only 0.140 AU — less than half Mercury's perihelion distance. It is a Mercury-, Venus-, Earth- and Mars-crosser. The surface temperature at perihelion could reach ~1025 K, or 1400 F. For this reason, it was named after the Greek myth of Phaëton, son of the sun god Helios.

Phaethon approached to 18.1 Gm on December 10,2007. It will draw nearer in 2017, 2050, 2060, and closer still on December 14, 2093, passing within 0.0198 AU (3.0  Gm).

SOURCE: Wikipedia 3200_Phaethon

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Dec. 6, 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.

"It was a monster fireball," says photographer Wally Pacholka. "I caught it exploding over the Hercules Finger rock formation near Victorville, California, using a Canon 35 mm camera. This was one of 1522 photographs I took "

The fireball occurred during the Geminid meteor shower, which peaked on Dec. 13th and 14th when Earth passed through a stream of debris from 3200 Phaethon. In some places, people saw 200+ Geminids per hour. In the Mojave desert, one was enough!

"The Geminids are my favorite," he explains, "because they defy explanation."

Most meteor showers come from comets, which spew ample meteoroids for a night of 'shooting stars.' The Geminids are different. The parent is not a comet but a weird rocky object named 3200 Phaethon that sheds very little dusty debris—not nearly enough to explain the Geminids.

"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."

This makes the Geminids the 900-lb gorilla of meteor showers. Yet 3200 Phaethon is more of a 98-lb weakling.

3200 Phaethon was discovered in 1983 by NASA's IRAS satellite and promptly classified as an asteroid. What else could it be? It did not have a tail; its orbit intersected the main asteroid belt; and its colors strongly resembled that of other asteroids. Indeed, 3200 Phaethon resembles main belt asteroid Pallas so much, it might be a 5-kilometer chip off that 544 km block.

.. Credit: An artist's concept of an impact event on Pallas. Credit: B. E. Schmidt and S. C. Radcliffe of UCLA.

"If 3200 Phaethon broke apart from asteroid Pallas, as some researchers believe, then Geminid meteoroids might be debris from the breakup," speculates Cooke. "But that doesn't agree with other things we know."

Researchers have looked carefully at the orbits of Geminid meteoroids and concluded that they were ejected from 3200 Phaethon when Phaethon was close to the sun—not when it was out in the asteroid belt breaking up with Pallas. The eccentric orbit of 3200 Phaethon brings it well inside the orbit of Mercury every 1.4 years. The rocky body thus receives a regular blast of solar heating that might boil jets of dust into the Geminid stream.

Could this be the answer?

.. The path of 3200 Phaethon through STEREO's HI-1A coronagraph camera. False-color green and blue streamers come from the sun.

To test the hypothesis, researchers turned to NASA's twin STEREO spacecraft, which are designed to study solar activity. Coronagraphs onboard STEREO can detect sungrazing asteroids and comets, and in June 2009 they detected 3200 Phaethon only 15 solar diameters from the sun's surface.

What happened next surprised UCLA planetary scientists David Jewitt and Jing Li, who analyzed the data. "3200 Phaethon unexpectedly brightened by a factor of two," they wrote. "The most likely explanation is that Phaethon ejected dust, perhaps in response to a break-down of surface rocks (through thermal fracture and decomposition cracking of hydrated minerals) in the intense heat of the Sun."

Jewett and Li's "rock comet" hypothesis is compelling, but they point out a problem: The amount of dust 3200 Phaethon ejected during its 2009 sun-encounter added a mere 0.01% to the mass of the Geminid debris stream—not nearly enough to keep the stream replenished over time. Perhaps the rock comet was more active in the past …?

"We just don't know," says Cooke. "Every new thing we learn about the Geminids seems to deepen the mystery."

This month Earth will pass through the Geminid debris stream, producing as many as 120 meteors per hour over dark-sky sites. The best time to look is probably between local midnight and sunrise on Tuesday, Dec. 14th, when the Moon is low and the constellation Gemini is high overhead, spitting bright Geminids across a sparkling starry sky.

Bundle up, go outside, and savor the mystery.

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

SOURCE: NASA SCIENCE