New Insights On The 'Rock Comet' Phaethon

Lee Rannals for redOrbit.com – Your Universe Online

Phaethon is a confusing sun-grazing asteroid, complete with a tail of dust particles similar to what is seen on a comet. But what makes it an asteroid rather than a comet? Scientists at the European Planetary Science Congress in London have helped explain this apparent conundrum.

According to research presented by David Jewitt, Phaethon’s tail becomes so hot when it approaches the sun that rocks on the surface crack and crumble to dust under the extreme heat.

Astronomers have known that the Geminid meteor shower, which takes place in December, is caused by the asteroid. However, they had not been able to directly observe the comet-like asteroid shedding its debris until recently. In 2010 Jewitt and his colleague Jing Li used NASA’s STEREO Sun-observing spacecraft to find out more about the sun-grazing asteroid.

The team was able to spot a comet-like tail extending from Phaethon during these observations, initially generating just as many new questions as answers.

“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.”

The team said that thermal and desiccation fractures may be launching small dust particles that are then picked up by sunlight and pushed into the tail. These fractures can be likened to the mud cracks seen in a dry lake bed.

Astronomers have already detected unexpected amounts of hot dust around some nearby stars that could also come from asteroids similar to Phaethon.

“By the shape of its orbit, Phaethon is definitely an asteroid. But by ejecting dust it behaves like a ‘rock comet’,” said Jewitt.

The discovery about Phaethon brightens up the world of science a little bit after disappointing news about Comet ISON came in August. This comet was expected to become a fall spectacle like no other this year, with some scientists predicting that it would even be visible during the day. However, an amateur astronomer out of Arizona was the first to detect Comet ISON after it emerged from behind the sun, showing some disappointing data.

Using his 11-inch telescope, Bruce Gary found that the comet was about two magnitudes fainter than it should have been according to previous predictions. This means Comet ISON may not be any more spectacular from the backyard perspective than the rock comet Phaethon.

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