Giotto spacecraft

Flamsteed Astronomy Society

Comets — Messengers from deep space

October 10, 2005

Dr Robert Massey — Royal Observatory Greenwich

As with much of the solar system, our knowledge of comets has developed greatly in recent decades thanks to space missions:

In 1986 the Giotto mission achieved a rendezvous with Halley and took many photographs.  The nucleus of Halley was found to be much darker in colour than expected reflecting only 4% of the light.  Giotto was equipped with an armoured shield to stop the bombardment by dust and larger particles it received near the comet.  Even so Giotto’s camera was destroyed.  The Deep Space 1 mission encountered Comet Borrelly in 2001 and took pictures.  We can now see the shape of comet nuclei.  The technical terms are ‘peanut’ and ‘potato’.

Comets leave trails of dust and debris in their wake.   After several returns, the debris becomes evenly spread out around the orbit. When the Earth crosses a comet orbit a meteor shower can result from the debris hitting the atmosphere and burning up.  The Leonids are a  good example.  They appear in November each year and originate with comet Tempel-Tuttle which has a period of 33 years.   The Leonids in November 2006 should be a good display.  Occasionally larger chunks of rock arrive from space sometimes from comet debris.  In July 1994 astronomers watched enthralled when comet Shoemaker-Levy 9, which had fragmented into some 20 large pieces, impacted on Jupiter.  The impacts were expected to have little effect but huge scars were left in Jupiter’s atmosphere, a frightening demonstration of what might occur on Earth one day.  There are some 200 impact sites known on Earth including Meteor or ‘Barringer’ Crater.   In Siberia at Tunguska in 1908 an object from space caused an explosion that flattened the forest for 50 miles radius but left no detectable impact crater.  This was possibly a comet fragment which exploded in the air.

Our latest ideas about the composition of comets incline more to the dirty snowball ‘pile of rubble’ model.  Comets appear to have little crust and may be much more loosely assembled than we first thought.  The impactor fired by the Deep Impact spacecraft at comet Tempel 1 seems to have displaced much more powdery substance than the expected ices.

Great comets are only to be expected every 20 years or so.  In fact we’ve enjoyed two in quick succession — Hyakutake in 1996 and Hale-Bopp in 1997, so we may have to wait a while for the next!

                                                                                  cont/

Halley nucleus from Giotto

(NASA)

Barringer Crater

Comet Borrelly from Deep Space 1

Impact scars on Jupiter left by

Comet Shoemaker-Levy 9

(Hubble Space Telescope/NASA)

Comet Hale-Bopp 1997

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