It Wasn’t a Rock, It Was a Rock COMET!

Zoomed, contoured STEREO image showing the south-eastward extension of the image of (3200) Phaethon. Credit: Jewitt, Li, Agarwal /NASA/STEREO
Zoomed, contoured STEREO image showing the south-eastward extension of the image of (3200) Phaethon. Credit: Jewitt, Li, Agarwal /NASA/STEREO

Sometimes asteroids aren’t always what they seem. Such is the case with the asteroid Phaethon, which also happens to be the source of a well-known meteor shower in December.

As it turns out, Phaethon isn’t an asteroid at all — it’s a comet. But don’t think it’s as cut-and-dried as that… Phaethon isn’t the “dirty snowball” type of comet you typically hear about but rather a rock comet, spewing hot dust and rock as it approaches the Sun in the same way that other comets eject ices.

Pass the tannin’ butter!

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 doesn’t arise through the vaporization of an icy nucleus. During its closest approach to the Sun, researchers believe that Phaethon becomes so hot that rocks on the surface crack and crumble to dust under the extreme heat. The findings were presented by David Jewitt on September 10, 2013 at the European Planetary Science Congress (EPSC) 2013 in London.

Most meteor showers arise when the Earth ploughs through streams of debris released from comets in the inner solar system. The Geminids, which grace the night sky annually in December, are one of the best known and most spectacular of the dozens of meteor showers. However, astronomers have known for 30 years that the Geminids are not caused by a comet but by a 5 km diameter asteroid called (3200) Phaethon.

Until recently, though, and much to their puzzlement, astronomer’s attempts to catch Phaethon in the act of throwing out particles all ended in failure. The tide began to turn in 2010 when Jewitt and colleague, Jing Li, found Phaethon to be anomalously bright when closest to the Sun. The key to success was their use of NASA’s STEREO Sun-observing spacecraft. Phaethon at perihelion appears only 8 degrees (16 solar diameters) from the sun, making observations with normal telescopes impossible. Now, in further STEREO observations from 2009 and 2012, Jewitt, Li and Jessica Agarwal have spotted a comet-like tail extending from Phaethon.

Watch a video of Phaethon below:

“The tail gives incontrovertible evidence that Phaethon ejects dust,” said Jewitt. ‘That still leaves the question: why? Comets do it because they contain ice that vaporizes in the heat of the Sun, creating a wind that blows embedded dust particles from the nucleus. Phaethon’s closest approach to the Sun is just 14 per cent of the average Earth-Sun distance (1AU). That means that Phaethon will reach temperatures over 700 degrees Celsius – far too hot for ice to survive.”

Source: EPSC news release