On Friday the 165-foot-diameter asteroid 2012 DA14 will pass by the planet just 18,000 miles above the atmosphere. The near miss of Earth is a dramatic reminder of the threat posed by its rogue relatives. Planetary scientists estimate there are close to a million near-earth asteroids the size of DA14 or larger. A cousin of DA14 probably strikes Earth every 1000 years or so, and does so with the explosive force of several megatons—enough kinetic energy to destroy a city.

We have the space technology for a mission to deflect a threatening asteroid: We could change its course with the gentle tug of gravity (gravity tractor), slam into it with a hypervelocity bullet (kinetic impact), or vaporize the “topsoil” of an asteroid to divert it from a collision course with us (nuclear explosive).

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