Just about everyone knows that meteor showers are produced by the dust from comets. But one of the year’s biggest displays of “shooting stars” has long puzzled astronomers because its parent appeared to be an asteroid. 

A composite image of bright Geminid meteors over Pendleton, Oregon. Credit: Thomas W. Earle

For 30 years, it has been clear that the body behind December’s Geminid shower is a rocky one, about 5 km in diameter, called (3200) Phaethon. Now new research confirms that Phaethon is indeed an asteroid and explains how it has managed to produce meteor dust.

The comets that produce most other known showers come from the frozen depths of space beyond Neptune and do so when their icy heads – the nucleus – vaporise as they venture into the inner Solar System and are warmed by the Sun.

They give off twin tails of gas and dust which are blown away from the comet’s nucleus by the pressure of the solar wind. Gradually the dust particles, or meteoroids, are spread around the orbit of the comet and if that orbit crosses the orbit of the Earth, they will enter our atmosphere when we pass through them every year.

Phaethon is unlike the majority of asteroids, which mainly orbit between Mars and Jupiter, because it passes so close to the Sun that it is termed a Sun-grazer. Researchers now believe that the asteroid becomes so hot during this close encounter that rocks on its surface break up and crumble to dust.

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

Phaethon was caught in the act of producing dust close to the Sun in 2010 by David Jewitt and colleague Jing Li, of the Department of Earth and Space Sciences, at the University of California, Los Angeles. They are revealing their findings at the European Planetary Science Congress (EPSC) 2013 in London today.

The researchers used NASA’s STEREO Sun-observing space telescope to monitor Phaethon as it rounded the Sun at a distance of just 16 solar diameters and found it to be brighter than they expected.

Now they have added observations made with STEREO in 2009 and 2012 by themselves and colleague Jessica Agarwal, of the Max Planck Institute for Solar System Research, in Germany. Combining their work revealed signs of a comet-like tail extending from Phaethon.

Jewitt said: “The tail gives incontrovertible evidence that Phaethon ejects dust. That still leaves the question, why? Comets do it because they contain ice that vaporises 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 (1 Astronomical Unit, or AU). That means that Phaethon will reach temperatures over 700° Celsius – far too hot for ice to survive.”

He added: “By the shape of its orbit, Phaethon is definitely an asteroid. But by

ejecting dust it behaves like a ‘rock comet’.”

The team believes that the Sun’s heat may be causing fractures in the rock like mud cracks in a dry lake bed. This then produces small dust particles that sunlight pushes away from the asteroid as a tail. It is the first time that such “thermal disintegration” has been found to play an important role in the Solar System, but astronomers have already detected unexpected amounts of hot dust around some nearby stars and say it might have been produced in a similar way.