Arrival of the comet Siding Spring at Mars Oct. 19, only four weeks after two orbiters designed to study the planet's atmosphere are set to arrive there, too, will be an unprecedented opportunity to study both the impact of its high-speed passage on the Martian system, and the chemistry of a primordial object from the outer reaches of the Solar system. Astronomers have been watching the comet since it was discovered in January 2013. Now as its closest approach to the Sun draws near, they have been joined by planetary scientists and spacecraft operators to take maximum advantage of what is essentially a free ride for once-in-a-lifetime close-up observations of an object from the mysterious Oort Cloud.
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