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The Perilous Journey of Comet ISON

The vast dark past Pluto is a deep freeze filled with a trillion fragments of ice and rock, the left-over detritus from the formation of the Sun and planets. These tumbling iceballs are made of a loose rubble of rocks and dust, bound together by water ice and other exotic ices, such as carbon dioxide, methane, and ammonia. The orbits are fragile, and chance encounters with a neighbor or a close pass by a dwarf planet or distant star can send one of these iceballs on a long  journey toward the inner solar system.

When the solar system formed, the most distant reaches, far beyond Pluto, were populated with the left-overs. This cloud is the origin of comets.

When the solar system formed, the most distant reaches, far beyond Pluto, were populated with the left-overs. This cloud is the origin of comets.

On 21 September 2012, one particular iceball was cruising in the vast emptiness between the orbits of Jupiter and Saturn.  A robotic telescope in Russia was scanning the skies near the constellation Cancer and stumbled on a faint, growing glow — a new comet!  It is colloquially known as Comet ISON, after the International Scientific Optical Network, which discovered it. The iceball core, which astronomers call the nucleus, is about 5 kilometers across.  Like many comets, it has a lot of frozen carbon dioxide (“dry ice”) that is evaporating off the surface and forming the foggy cloud around the core (what astronomers call the coma).  These are ordinary properties that ISON shares with most comets.  But Comet ISON has a secret — it is really a member of an elite group of comets known as sungrazers.

On 28 November 2013, Comet ISON will skim low over the surface of the Sun. At its closest, it will be only 1.2 million km above the Sun (about 50 times closer than the planet Mercury). The temperature will soar to more than 2700 degrees Celsius, hot enough to melt iron, which could make the comet tremendously bright.  But the Sun’s gravity is also enormously strong, and could crush Comet ISON — ISON could disintegrate from the stress, much like a poorly packed snowball on a too-warm winter day.

Right now, ISON is spectacularly unpredictable. Will ISON shine brightly in the night, or simply suffuse the Earth with a subtle, diaphanous shower of comet light?  Will ISON survive its close slingshot ride around the Sun, or will the intense heat and gravity shatter it into a million tiny bits?  We don’t know the answers to these questions; all we can do is wait.  Either way, we have a chance to look at one of Nature’s great spectacles — a cosmic sky show featuring a little pile of rubble left over from the construction of the solar system. Left over rubble from the assembly of everything that we see around us — the Sun, Jupiter, Mars, the Moon, Earth, petunias, raccoons, and you.

Read the full post at WriteScience!  

Written by Shane Larson, Adler astronomer and research associate professor at Northwestern University.

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