“Excitement is rising and so is our nervousness,” said Hermann Boehnhardt, lead scientist on the Philae lander, which was released from its mothership Rosetta on Wednesday morning en route for its rendezvous with comet 67P/Churyumov-Gerasimenko.  Shortly after 1600 GMT, having travelled more than six billion kilometres, Philae successfully landed on comet 67P, which orbits the sun at speeds of up to 135,000km/h.  Andrea Accomazzo, Rosetta flight operations director, said: “We cannot be happier than we are now.”  But chances are you’re not one of the scientists who’ve been working on the mission for the last 20 years. Why should you be as excited as Boehnhardt and Accomazzo about the Rosetta space mission? Here’s what you need to know.  Very simply, this is the first spacecraft ever to land on the surface of a speeding comet - a huge landmark in the history of space exploration. Because comets are remnants from the formation of the solar system it is hoped that it will unlock further clues about the development of life on Earth.  Rosetta will tell us if a bombardment from comets billions of years ago created the Earth’s oceans and the building blocks of all living things.  Comets are the icy remnants of the phase of planet building in our solar system, some 4.5bn years ago. From these measurements we know that a large proportion of a comet is made of water-ice. This turns to vapour when heated by the sun, producing microscopic dust particles.  As the comet approaches the Sun it will heat up and some of this dust could be ablated to reveal more of the comet’s geology to scientists back here on Earth.  These measurements will help scientists answer the question of whether a bombardment from comets billions of years ago brought water and simple organic molecules to Earth, paving the way for life.  Rosetta has already made history by orbiting its target and gathering the most detailed information so far on the composition of the comet’s nucleus, the chemical composition of the gaseous coma surrounding the nucleus, and a detailed surface map of the nucleus.  The comet’s interior contains the initial composition of the solar system from when it was formed 4.5 billion years ago. These results will help improve models for the solar system and better understand how a planet becomes habitable.  The gravity on the comet’s surface is so weak that an astronaut could escape its gravitational pull just by jumping. On touchdown, Philae used two harpoons and icescrews on its three landing legs.  The harpoons were a one-shot deal, they could not be retracted and re-fired. Crucially, a thruster on the lander failed Tuesday night. It was designed to fire when Philae touched the comet preventing it bouncing off as the harpoons fired.  Dr Alan Duffy, a research fellow and astronomer at Swinburne University of Technology in Australia, said: “Without thrusters it [also] means that there’s no chance to change course if a lander-destroying shard of ice and rock is underneath.”  The Mont Blanc-sized comet is shaped like a rubber duck with precipices and slopes and a surface pocked with holes and strewn with boulders. When the shape was revealed in July, some thought the landing was going to prove impossible.  After ruling out several sites as too dangerous, the agency settled on a one-square-kilometre landing spot it has named Agilkia.  If Philae had strayed outside its landing zone, it had a greater chance of toppling over. To land in the right place, mission controllers must take into account the dust and water vapour billowing off the comet and its oddly shaped gravitational field.  Fred Jansen, the mission’s manager, gave the landing a 75% chance of success. Given the European Space Agency’s £1bn Rosetta probe has traveled six bn km and 10 years to reach comet 67 this isn’t a reassuring figure.  But overnight, that percentage went down when the important thruster on the lander failed to respond to commands sent from Earth.  “Twenty years ago we said we wanted to land on a comet we knew nothing about. There will be risks,” said Jansen.  “The mission is already a major achievement in space exploration history,” said Andrea Accomazzo, Rosetta flight director, and has worked on the mission for 18 years.  “Exploration implies risk. If you are not ready to take the risk, then you shouldn’t do exploration.”  Watch our interactive guide to the Rosetta mission.
#cometlanding is trending, and we have a live stream of this momentous occasion. But why is the Rosetta landing so exciting?