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Comet Holmes

Comet 17P/Holmes

The short-period Comet Holmes follows an orbit that falls between those of Mars and Jupiter, on an inclined path that carries it out of the plane of the Solar System. Its orbit takes a little under seven years to complete, so that it regularly enters the inner parts of the Solar System, though it does not approach the Sun more closely than Mars' orbit. It is not, therefore, easily detectable from Earth under most circumstances, even on its closer approaches to the Sun.

At times, however, it can become much more easily visible, because this comet is given to intense temporary outbursts of material. During these outbursts the comet's coma, the thin layer of gases wrapping its solid nucleus, expand temporarily to form an immense bright 'bubble' around the tiny icy core of the comet. This cloud of outburst material is truly enormous, reaching an estimated diameter of a million kilometres (while the comet's solid nucleus is no more than four kilometres in diameter). The mechanism behind these outbursts is not known in detail, but it appears to be related to a build-up of pressure within the comet's nucleus, driven by accumulated energy from the Sun.

Comet Holmes displayed one of these outbursts in November 1892, when it brightened to a point when it was faintly visible to the naked eye, and was discovered by the amateur astronomer Edwin Holmes. The comet faded again, and after being tracked through two subsequent returns to the inner Solar System, it was lost for some sixty years before being rediscovered. In 2007, Comet Holmes once again underwent one of its periodic outbursts, reaching magnitude +2.8 as seen from Earth (comparable with, for example, the star Alcyone in the Pleiades). A further, though rather less dramatic, outburst occurred on its return in 2015.