Meet Phaethon, a weird asteroid that thinks it's a comet
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4 min read
fairly easy
What's the difference between an asteroid and a comet? A comet is basically a dirty iceball composed of rock and ice. The classic image is of a bright "star" in the night sky with a long curved tail extending into space. This is what happens when they approach the Sun and start emitting gases and re...
leasing dust. It normally continues until there's nothing left but rock or until they fragment into dust. Asteroids, on the other hand, are primarily just rocks. They might conjure up notions of Hans Solo steering the Millennium Falcon through an implausibly dense "asteroid field" to escape a swarm of TIE Fighters, but mostly they just quietly orbit the Sun, minding their own business. Yet these two space objects are not always as mutually exclusive as this would suggest. Let me introduce Phaethon, a "rock comet" that blurs the definitions between asteroid and comet, and let me tell you why it will be worth paying attention to this fascinating object in the coming years. Phaethon was discovered by chance in 1983 by two astronomers at the University of Leicester, Simon Green and John Davies. They came across it orbiting the Sun while analysing images collected by a space telescope called the Infrared Astronomical Satellite (Iras). Soon after, other astronomers recognised that Phaethon is the source of the annual Geminid meteor shower – one of the brightest meteor displays in Earth's calendar. Every December, as our planet crosses the dusty trail left behind by Phaethon, we are treated to a brilliant spectacle as its dust grains burn up in our atmosphere. Yet Phaethon's behaviour is unlike that of any other objects responsible for a meteor shower. Unlike typical comets that shed substantial amounts of dust when they heat up near the Sun, Phaethon doesn't seem to be releasing enough dust today to account for the Geminids. This absence of significant dust emissions generates an interesting problem. Phaethon's orbit brings it extremely close to the Sun, much closer than Mercury, our innermost planet. At its closest approach (termed perihelion), its surface temperature…