The dirty snowball: The comet 67P Churyumov-Gerasimenko which is now home to the Philae lander © ESA

The Rosetta mission has reached its goal, a comet currently half a billion kilometres from Earth. Soon we should know more than ever about these objects. But why are they so important?

  • Why are people talking about comets?

    Yesterday was one of the greatest days in the history of space exploration. After a ten year, 6 billion kilometre journey through the solar system the Rosetta spacecraft released its fridge-sized lander called Philae which then made the first landing on the surface of a comet.

  • Why is this mission important?

    Partly because of the astonishing engineering feat of simply getting there. The comet, which is called 67P Churyumov-Gerasimenko, is only a few kilometres across and travelling at 135,000 kilometres per hour. Launching a space probe and guiding it through the solar system, using both Mars and Earth (three times) to accelerate it on its way, to a rendezvous after ten years with such a small and fast-moving object is a technical triumph for the European Space Agency.

    But it is also important because it will give us so much more information about comets and what they’re made of.

  • What are comets?

    Comets are fossils, made of material left over as the planets came together from a giant cloud of dust whirling around the newborn sun 4.6 billion years ago. They are among the oldest objects in the solar system. There are billions of them.

    They are composed of rock and ice, like a very dirty snowball. When they approach the Sun its heat melts the surface and produces a great stream of gaseous debris, the comet’s ‘tail’, stretching for thousands of kilometres. Some of these can be very spectacular when seen from Earth.

    The dramatic appearance of a comet in the heavens used to be considered a sign of some forthcoming upheaval on earth, the death of a king, for example. One of the most famous, Halley’s comet, makes an appearance in the Bayeux Tapestry which records the Norman Invasion of England in 1066 and the Battle of Hastings in which King Harold was killed.

  • Why do we need to know more about them?

    Comets are of great interest to scientists because they are the oldest, most primitive bodies in the Solar System, preserving the earliest record of material from the nebula out of which it was formed. Planets have gone through chemical transformations, but comets have remained almost unchanged.

    Also it is likely they played an important role in forming Earth’s oceans and atmosphere. Where the sea came from is one of science’s great mysteries. Early on, the Earth is likely to have been hot, so water would have quickly disappeared into space. It may have oceans today only because it was bombarded by comets and carrying payloads of ice.

    Comets are also probably involved in the origin of life on Earth. Ground-based observations of about 150 comets have revealed that most have an abundance of organic compounds. There is little doubt that they helped to supply our planet with the molecular building blocks of life. Space may seem a great void, but out there are the remnants of the violent chemical reactions that took place in the birth and death of stars.

    Any organic molecules found by Rosetta will help show how much of a starter kit for life comets gave the Earth as a present from space.

  • How long will Rosetta’s mission last?

    It should end in December 2015. By then the comet will have reached its closest point to the Sun (in August 2015) and be heading back towards the outer Solar System. Rosetta will be the first spacecraft to witness, at close proximity, how a comet changes as it approaches the increasing intensity of the Sun’s radiation.

You Decide

  1. Is the Rosetta mission worth its cost of 1.4 billion euros?


  1. If it has taken Rosetta ten years to travel six billion kilometres to rendezvous with the comet, what has been its average speed on the journey?

Word Watch

European space agency
ESA is the European counterpart of NASA. It was created in its current form in 1975 from earlier joint European space ventures. Its members are Belgium, Denmark, France, Germany, Italy, the Netherlands, Spain, Sweden, Switzerland and the UK.
The name is taken from the Latin for ‘cloud’. Nebulas are vast interstellar clouds of dust, hydrogen and helium, sometimes stretching over hundreds of light-years.

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