Comet 67/P (Churyumov-Gerasimenko) reached its closest point to the Sun on August 14, 2015. It also became the first comet to enter the inner solar system and reach perihelion with a man-made companion, namely the magnificent Rosetta spacecraft operated by the European Space Agency. Rosetta has studied the comet for the past year, dropped the little Philae probe in late 2014 to land on the comet, and now enjoys a close-up view of the comet’s fulminating nucleus as it warms and ejects plumes of gas into space. Two weeks before perihelion, Rosetta captured a spectacular image of a brilliant jet erupting from “neck” of the bi-lobed comet (see above). The jet was the brightest yet seen by Rosetta, and the fireworks lasted nearly 20 minutes.
A comet’s nucleus becomes increasingly active as it approaches the Sun. Light and heat warm the dusty and icy surface of the comet. When a patch of subsurface ice gets warm, it turns to gas, builds pressure, then suddenly erupts into space like a geyser. Some geysers are so powerful, they can slightly modify the path of a comet in its orbit.
The top surface of the comet also sublimates (from ice directly into gas) near the Sun, and the gas is pushed into space far behind the dark nucleus by solar wind. From Earth, we see this stream of excited gas and associated bits of dust as the comet’s tail. While the nucleus of a comet is a few kilometers across, the tail can stretch for millions of kilometers in a direction away from the Sun.
Our understanding of comets has come a long way in the past few hundred years. These interlopers from the outer solar system were once feared unpredictable as omens of doom, and the sight of a bright comet terrified the superstitious masses. Their physical nature was uncertain, but that didn’t keep some from speculating, usually with the worst in mind. One 16th-century clergyman said of comets, that they are made from “the thick smoke of human sins, rising every day, every hour, every moment full of stench and horror, before the face of God, and becoming gradually so thick as to become a comet.” (*)
Rosetta will continue to follow Comet 67/P in the coming weeks. The comet is too faint to see in a small telescope, but more images are on the way from Rosetta as the comet continues its outbursts. Rosetta itself is getting buffeted by these gassy, dusty outbursts. On August 1, over a four hour period, the spacecraft endured 70 impacts by dust blasts, some of which hit at up to 67 mph. So far, the craft is uninjured and continues to make scientific history.
To stay tuned to what’s going on with Rosetta in this critical phase of its mission, check out the mission website.
(*) From Seeing in the Dark, Timothy Ferris, published by Simon and Schuster (2002).
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