THE last-minute postponement of the daring first attempt to place a probe on the surface of a comet may have unexpected benefits for the mission.
The European Space Agency is frantically studying a handful of comets as alternative quarries for its Rosetta spacecraft. The mission was postponed indefinitely last week, just eight days before launch, because of concerns over the reliability of the craft鈥檚 Ariane 5 launch vehicle.
The delay will prevent Rosetta from catching its intended target, Comet Wirtanen, as planned in 2012. The probe was designed to perform a series of slingshots around the Earth and Mars to gather enough momentum to rendezvous with Wirtanen near the orbit of Jupiter. But despite the last-minute postponement, some experts remain upbeat.
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鈥淩ather this than have the thing blow up,鈥� says Simon Green, at Britain鈥檚 Open University. 鈥淢ost people are taking things philosophically. It might be that you can find a target that you could get to before we would have got to Wirtanen.鈥�
Mark Bailey at the Armagh Observatory in Northern Ireland says researchers may even be able to find a more interesting comet. 鈥淲irtanen wasn鈥檛 perfect,鈥� he says. 鈥淪ome would say it was beginning to fade away. It would make it especially interesting if you could rendezvous with one that was just appearing.鈥�
In theory, Rosetta might still be able to catch Wirtanen by performing a more outlandish gravitational swing around Venus instead of Mars. But David Southwood, scientific director at ESA, says this would expose the craft to more extreme heat than it was designed to withstand. He told 快猫短视频: 鈥淢odifying the spacecraft is a terrific technical risk and I would need very strong persuasion to countenance it.鈥�
The replacement comet will have a lot to live up to. It must have been observed many times to give scientists enough data to model the complex rendezvous. The path it takes through the Solar System should begin far enough from the Sun to make landing feasible, preferably close to the orbit of Jupiter. But it should also come close enough for the Sun to make it active 鈥� around 1astronomical unit (AU), the same distance as Earth.
The target will also need to be at least 1 kilometre in diameter and stable enough for Rosetta to land safely. Yet it should not be so inactive that scientists won鈥檛 be able to study the processes that give comets their characteristic tails as they approach the Sun.
ESA has now come up with a list of contenders (see Table). A report outlining the alternatives will be given to the agency鈥檚 scientific committee in February. But even once the right comet has been found, complex calculations will still be needed to confirm that Rosetta can be sent to its quarry.
快猫短视频s hope the mission will eventually reveal the chemical and organic make-up of a comet 鈥渋n situ鈥� for the first time. That could show what conditions were like when the Solar System formed more than 4.6 billion years ago. The project could also provide compelling evidence that water or biochemical precursors of life arrived on Earth aboard comets.