Rosetta mission: Joy as spacecraft lands on comet A SPACECRAFT developed by scientists at the European Space Agency has arrived at its destination - a four kilometre-wide comet. It is the climax of a decade-long mission to study a 2.5-mile wide lump of dust and ice known as 67P/Churyumov-Gerasimenko. It is also the end of a 6.4 billion-kilometre (four billion-mile) journey during which Rosetta carried its sidekick lander Philae piggyback to reach the comet. However, the journey was not without its problems, and fauly landing gear threatened to dereail the historic attempt. The ESA said that the lander's active descent system, which uses thrust to prevent the craft from bouncing off the comet's surface, could not be activated. Instead, the agency is relying on ice screws and a harpoon system to secure the lander. "The cold gas thruster on top of the lander does not appear to be working so we will have to rely fully on the harpoons at touchdown," said Stephan Ulamec, Philae lander manager at the DLR German Aerospace Centre. We'll need some luck not to land on a boulder or a steep slope. If all goes to plan, the washing machine-sized lander will have separated from Rosetta at 0835 GMT and drift down to the comet, where it will latch on using harpoons and screws. Comment: Does Rosetta hold key to mystery of life? During the descent scientists will be powerless to do anything but watch, because the vast distance to Earth - 500 million kilometres (311 million miles) - makes it impossible to send instructions in real time. Confirmation of a landing, if successful, should reach Earth by about 1603 GMT. The plan is that Rosetta and Philae will then accompany the comet as it hurtles towards the Sun and becomes increasingly active as it heats up. Using 21 different instruments they will collect data that scientists hope will help explain the origins of comets and other celestial bodies. The European Space Agency says that even if the landing does not succeed, the 1.3 billion euro (£1 billion) mission launched in 2004 will not be a failure. Rosetta will be able to perform 80% of the mission on its own.