The comet that suddenly became about a million times brighter nearly two weeks ago continues to 鈥渟hine鈥� with abnormal luminosity, leaving observers puzzled over what caused the outburst and whether the comet will perform an encore in the coming months.
Comet 17P/Holmes is normally an invisible runt of a comet, about 3.3 kilometres across and about 25,000 times too faint to be seen with the naked eye.
But following its sudden brightening on 23 October, the comet鈥檚 coma, a surrounding shell of gas and dust, has been expanding at a rate of about 0.5 kilometres per second, making the comet appear as a fuzzy 鈥渟tar鈥� that can be seen with the naked eye in the constellation Perseus (see image at right and watch a ).
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The comet was actually discovered during a similar, but less spectacular, brightening event in November 1892. It faded after a few weeks, only to dramatically brighten again in January 1893.
The comet orbits the Sun every seven years on a path that takes it from the distance of Jupiter鈥檚 orbit to about twice that of Earth鈥檚. Interestingly, in both the 1892 event and the recent one, the comet initially brightened about five months after reaching perihelion 鈥� its closest approach to the Sun.
鈥淚t鈥檚 curious that the outburst came in the same period of orbit,鈥� says Brian Marsden, former director of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Massachusetts, US. 鈥淚t will be interesting to see if it behaves in the same general way [this time as before].鈥�
The common timing of the two mega-outbursts following perihelion suggests that the intensity of the Sun鈥檚 radiation is a key factor in the brightening. But that alone is not enough, as the comet reaches perihelion every seven years and hasn鈥檛 produced such an outburst in 115 years. There are also plenty of comets that make closer approaches to the Sun than Comet 17P/Holmes without brightening nearly as much.
Fresh ice
鈥淭he fact that it鈥檚 brightened by a factor of a million is just incredible,鈥� says David Jewitt at the University of Hawaii in Honolulu, US. 鈥淲hat鈥檚 special about this one? We don鈥檛 know.鈥�
Michael Mumma, director of the Goddard Center for Astrobiology in Greenbelt, Maryland, US, and colleagues are observing the comet with the 10-metre Keck II telescope鈥檚 infrared vision.
They find that the comet is still actively releasing gas. Most of it is composed of water vapour, but ethane, acetylene and hydrogen cyanide have also been seen in trace amounts. 鈥淲e hope to learn a great deal about the material that was once in the interior of the comet nucleus,鈥� Mumma told New 杏吧原创.
The standard explanation of comet outbursts says that a sudden event exposes fresh ices from within the nucleus to solar radiation. This causes them to vaporise, dragging dust along with them. Sunlight reflecting off that dust then magnifies the comet鈥檚 brightness.
Built-up pressure
In an electronic telegram distributed by the International Astronomical Union, Zdenek Sekanina of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, US, suggests that a flattened layer that was once part of the nucleus could have broken off and completely disintegrated to produce the outburst. The idea, which has not been peer-reviewed, is partially based on an estimate of the mass of particles in the coma.
Another hypothesis suggests that pockets of volatile gas somehow get trapped in the nucleus. Eventually, built-up pressure causes the surface to rupture, producing large outbursts.
Tom Van Flandern of Meta Research in Sequim, Washington, US, suggests a more outlandish possibility. He argues that comets are orbited by satellites and that occasionally a satellite crashes into its host comet, producing an outburst.
A similar idea was put forward more than 20 years ago by astronomer Fred Whipple, who suggested that the 1892 outburst of Comet Holmes was caused by a satellite grazing the surface of the nucleus and that the second brightening in 1893 was the result of the satellite crashing into the comet鈥檚 surface.
Race against the clock
Jewitt says the recent recurrence of the comet鈥檚 brightening strongly argues against the satellite model. 鈥淭he fact that this has happened after 115 years 鈥� with nothing in the intervening years 鈥� makes the model more ad hoc,鈥� he told New 杏吧原创.
Marsden agrees, and believes that whatever caused the comet to suddenly brighten has to be intrinsic to the comet. 鈥淲e need to continue observations and see how it goes,鈥� he adds.
Mumma says the comet could fade at any time. In the case of the event 115 years ago, the comet rapidly faded after the second outburst. 鈥淭hat鈥檚 part of the challenge 鈥� to try to learn as much as we can before the darn thing vanishes,鈥� he says.
Fortunately, the fading itself can also shed light on what caused the outburst. The comet is currently in the main asteroid belt, at a distance from the Sun of about two and a half times that of Earth (or 2.5 astronomical units). Water ice can no longer sublimate beyond about 3 astronomical units, so if the comet is still active past that point, something more volatile than water ice must be driving its activity.
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