It's big, it's blue, and it'll be raining down on you – it's 3200 Phaethon

Astroboffins baffled by mysterious repeat visitor


3200 Phaethon, a weird object that sends cosmic debris streaking through Earth's night skies during the Geminid meteor shower, is more puzzling than previously thought.

For starters, Phaethon appears to be blue. When the researchers viewed the object using telescopes in Hawaii and Arizona, USA, they found that it reflected more sunlight in the wavelengths that correspond to blue light rather than the typical red and dark grey colors for most asteroids.

Although Phaethon is considered an asteroid, it occasionally resembles and behaves like a comet. Its orbit brings it closer to the Sun than any other asteroid, heating its surface up to a whopping 800 degrees Celsius (1,500 degrees Fahrenheit). Then, as it whizzes through Earth’s orbit, bits and pieces of the object break off.

The space dust falls and burns up from the forces it encounters as it streaks through the sky. These bright flashes are seen as the Geminids meteor shower, which peaks every year in December.

Learning from the stars

Until 1983, scientists believed all meteor showers were caused by comets, but Phaethon changed their minds. It doesn’t have a clear glittering tail made from the vaporization of dust particles trapped in ice, a tell-tale sign of a comet.

"At the time, the assumption was that Phaethon probably was a dead, burnt-out comet," said Teddy Kareta, a graduate student at the University of Arizona. "But comets are typically red in color, and not blue. So, even though Phaeton's highly eccentric orbit should scream 'dead comet,' it's hard to say whether Phaethon is more like an asteroid or more like a dead comet."

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There are faint signs of a tail, however, when it gets closest to the Sun. There is only one other object dubbed 2005 UD found so far that is blue and also displays both comet and asteroid-like behaviours.

The team have been trying to work out the origin of Phaethon to get to the bottom of what the object really is. They believe it may have broken off from another blue asteroid dubbed Pallas, one of the largest asteroids in the Solar System.

"Interestingly, we found Phaethon to be even darker than had been previously observed, about half as reflective as Pallas," Kareta said. "This makes it more difficult to say how Phaethon and Pallas are related."

The next step is to see how Phaethon might be associated with 2005 UD to see if there are any other vital clues that might conclusively answer if Phaeton really is an asteroid or comet.

A team of international astronomers led by the University of Arizona are presenting their research at the 50th annual meeting of the American Astronomical Society's Division for Planetary Sciences in Tennessee, United States. ®

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