1. Comets, despite sometimes having a bright ‘tail’ in the sky when they come close to the sun, are actually as black as charcoal. They become visible near the sun because more dust and gas is liberated from their surface as they warm up, and this gas and dust scatters sunlight. We’re not really sure why comets are so black.
  2. Scientists often say that comets are  icy, but some of this ‘ice’ is not frozen water but rather frozen carbon dioxide – sometimes known as ‘dry ice’. Comet 67P is thought to be about 50% ‘dust’, 35% water ice and 15% dry ice.
  3. Some comets such as 67P are made of such light (low density) material that it would float on water. This is because the cometary material is highly porous (full of holes).
  4. The Rosetta team made ‘scratch and sniff’ postcards to give a feeling for how the comet would smell (in real life, the gases would be toxic and rather stinky with ammonia, sulfur compounds and cyanide).
  5. Most comets are a long way from the Earth and Sun, orbiting in the Oort cloud many thousands of times further from the Sun than the Earth is. Some of them only complete an orbit around the sun every few million years. All of the comets in the Oort cloud (maybe two trillion comets) together probably have a mass several times that of the Earth. It will take over 30,000 years for the old space probe Voyager one to pass through the Oort cloud
  6. As recently as 1910, residents of Chicago kept their windows and doors shut in order to protect themselves from a comet’s ‘poisonous tail’. Although there is no real danger from the tail on Earth, some meteor storms are associated with comets that have broken up (e.g. the Perseid meteor storm is linked to comet Swift-Tuttle)
  7. Some comets are destroyed when they pass too close to the sun during their orbit
  8. Many comets were first discovered by amateur astronomers, and this is still true for some types of comet, but many more have been found since 1998 by the LINEAR (Lincoln Near Earth Asteroid Research) project which uses telescopes in the USA. LINEAR has found over 280 comets and thousands of asteroids.


Churyumov-Gerasimenko trivia

  1. One ‘day’ on the comet is 12.4 hours (the time it takes to rotate once around its own axis). One ‘year’ on the comet (its orbital period) is 6.45 Earth-years.
  2. Summer in the northern hemisphere of comet Churyumov-Gerasimenko occus when the comet is far from the sun, and summer in the southern hemisphere occurs close to the sun (due to the elliptical orbit of the comet and the tilt of the spin axis relative to the orbital plane). This is (by coincidence) the same as for the Earth but the comet has a far more elliptical orbit and a greater obliquity (currently 23.4 degrees for Earth, but 43 degrees for the comet).

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