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On Wednesday Apophis will pass the Earth at a distance of 9 million miles, allowing astronomers to gather more data about this asteroid’s orbit and composition.
Having crossed outside Earth’s orbit, Apophis will appear briefly in the night-time sky. Wednesday 9 January will afford astronomers the rare opportunity to bring a battery of telescopes to bear: from optical telescopes to radio telescopes to the European Space Agency’s Infrared Space Observatory Herschel. Two of the biggest unknowns that remain to be established are the asteroid’s mass and the way it is spinning. Both of these affect the asteroid’s orbit and without them, precise calculations cannot be made.