A comet is a dirty snow ball of fused dust grains and frozen gases. On approaching the sun, it becomes hot and incandescent, the ice on the surface sublimates and the dust is released to form a glowing halo around the comet which is blown into a plume by solar radiation.
Comets come from those zones of our solar system beyond the planets where Pluto lurks with its band of dwarf planets, asteroids, protoplanetisimals, rocks, dust and gases, too thinly dispersed ever to accrete into real planets. Here, on the anarchic fringes, dirty snow balls form which can be catapulted towards us by the gravitational punch of our largest planets, Neptune, Uranus, Saturn and Jupiter. As they whip round the sun in oblique and elliptical orbits, they leave a thin trace of dust particles which flare up as falling stars when our planet ploughs through. The dust trail from Halley’s comet has left us with the Orionid meteor shower in October. While the Swift-Tuttle comet has given us the splendid Perseid shower in August.
Ignorance and credulity once saw bad omens or apocalyptic harbingers in the appearance of comets, a stupidity that can still manifest itself on the lunatic fringe of our society and which led to the suicide of 38 members of the Heaven’s Gate cult in 1997 in the belief that their souls would be transported to a space craft that was following the Hale-Bopp comet, a daft escape strategy from imagined terrestrial doom.
This month we have a little comet passing by, 103P/Hartley2, that can in theory be seen with the naked eye, though binoculars would help and only a telescope would give you a chance of seeing the halo and plume. This time, you can be sure that a space craft will be flying close by! It was launched by NASA in 2005 to hurl a missile at the 9P/Tempel comet to study the effects of the impact and gain insight into the core of the comet. It was dubbed the Deep Impact mission and it was a stunning success. Now the space craft has been recommissioned to fly past the Hartley comet at a mere 1000 km on 4th November. So if you don’t manage to see it in the night skies, you can view a NASA close-up picture instead.
This blog first appeared as a story for the Cairo BCA Magazine in November, 2011, to promote astronomy in Sinai using an 8″ Meade Telescope.