However the practicalities of landing a spacecraft to monitor, explore or deflect an asteroid still face a number of challenges.

CNN: Space's threat to civilization

The large number and concentration of craters point to an old asteroid.

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They are then given a catalogue number (Bagehot's shows that it was the 2, 901st asteroid to be identified formally).

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One asteroid, the 300-m Apophis, will pass relatively close to Earth a number of times in the coming decades, including in 2029.

UNESCO: MEDIA SERVICES

Of course, this is just one of a number of methods and strategies being weighed by NASA and other agencies to prevent an asteroid from colliding with Earth.

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In 2005 Schweickart successfully campaigned for NASA to take the issue of NEOs seriously, the result has been a number of NASA centers developing prospective plans to land a spacecraft and even astronauts on an asteroid.

CNN: Space's threat to civilization

Since even a small mission to an asteroid costs hundreds of millions of dollars, this is not an easy choice, especially considering the growing number of potential collisions that a more detailed survey will probably uncover.

ECONOMIST: The American Association for the Advancement of Science