The Comet Awakens

67P/Churyumov-Gerasimenko's coma extends out to 1300 kilometers. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

67P/Churyumov-Gerasimenko’s coma extends out to 1300 kilometers. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


Jun 4, 2014

67P/Churyumov-Gerasimenko recently formed a coma.

After almost ten years of travel time, the Rosetta Cometary Probe is preparing itself for orbital insertion around its target, 67P/Churyumov-Gerasimenko, (chur-ee-uh-moff, yer-uh-sim-yenko) or “Chury” for short. In the last few weeks, Chury began to exhibit signs of activity, even though it is still 640 million kilometers from the Sun.

The Rosetta Cometary Probe was launched on a rendezvous mission March 2, 2004 by the European Space Agency (ESA). It has been in hibernation for most of that journey, only powering up its instruments last January. The mission has been designed according to models of comet morphology that were first proposed in 1755 by Immanuel Kant. Kant suggested that comets were composed of some material that could easily vaporize, which caused them to shine brightly in the reflected light of the Sun.

The most commonly known comet model was devised in 1950, by Fred Whipple, who thought that comets were more like “dirty snowballs”—with more emphasis on the snowball than the dirt. Despite observations too numerous to mention, that description has remained in the astronomical lexicon. Various missions, such as Giotto and Deep Impact, revealed comets to be blackened, cratered, and fractured. No ice fields, reflective crust, or watery clouds were observed. The Giotto spacecraft’s close approach to Halley’s comet discovered the blackest object ever seen. The comet’s energetic plumes blasted out from a dark, dense nucleus.

Comet Tempel 1 resembled an asteroid more than a chunk of muddy slush. Craters, boulders, and cliffs were observed—nothing like a snowball venting. Water vapor was discovered near the comet, but there was too little ice on the surface to account for it.

Other comets defy convention: Shoemaker-Levy 9 exploded when it encountered Jupiter’s magnetosphere, and the pieces did not expel the volatile compounds astronomers expected to see. Deep Space 1 flew by comet Borrelly in 2001, finding it hot and dry instead of cold and wet. The Stardust mission to comet Wild 2 found a great deal of dust, but no trace of water could be found on its surface.

Comets form plasma sheaths that can become comas, often more than a million kilometers in diameter. Plasma filaments connect comets with the Sun’s electric field, generating “hot spots” on their surfaces; so hot that extreme ultraviolet light and X-rays were detected radiating from comet Hyakutake. In 1976, astronomer Robert Roosen photographed what he thought were plasmoids colliding in the tail of comet Kohoutek. A paper authored by J. C. Brandt and himself (Possible Detection of Colliding Plasmoids in the Tail of Comet Kohoutek (1973f)), took issue with Fred Whipple’s dirty snowball theory based on observational evidence.

Indeed, since comets travel through variable electric potentials as they move toward the Sun, those electric fields cause visible glow discharges. Rather than dirty snowballs, comets are electrically active, solid bodies, most like asteroids. With the discovery of “cometary astroids” the distinction between them is becoming harder to find.

Rosetta should help to resolve the discrepancies between Electric comets and those created by mainstream astronomers. Provided that the nucleus is not too energetic, the orbiter should survive until it and Chury enter deeply into the Sun’s fields of influence.

Stephen Smith

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