Astronomy Picture of the Day

Explanation: What does a comet nucleus look like? Formed from the primordial stuff of the solar system, it is thought to resemble a very dirty iceberg. But for active comets, telescopic images only reveal the surrounding cloud of gas and dust, the comet's coma, and the characteristic cometary tails. In 1986, the European spacecraft Giotto encountered the nucleus of Halley's comet as it approached the sun. Data from Giotto's camera was used to generate this enhanced image of the potato shaped nucleus which measures roughly 15 kilometres across. It shows surface features on the dark nucleus against the bright background of the coma as the icy material is vapourized by the Sun's heat. Every 76 years Comet Halley returns to the inner solar system and each time the nucleus sheds about a 6 metre deep layer of its ice and rock into space. This debris composes Halley's tails and leaves an orbiting trail responsible for the Orionids meteor shower.