JUDY WOODRUFF: Now a different kind of space mission, finding asteroids and other large objects before they get close to Earth.

This Sunday, a comet will be making an unusually close fly-by near Mars. In fact, it will be coming closer to Mars than any other comet has come near Earth in recorded history. Its also a moment when scientists are assessing our own risk from such objects.

Science correspondent Miles OBrien has our report.

MILES OBRIEN: Traveling 40 times faster than a speeding bullet, it is a menacing messenger from the very distant past. The comet known as Siding Spring, a dirty snowball packed with four-billion-year-old leftovers from the dawn of our solar system, will streak ever so close to Mars and NASAs armada of spacecraft, for scientists, an unprecedented bonanza, for all of us, a stark reminder.

Jim Green is the space agencys director of planetary science.

JIM GREEN, NASA: Theres not only the scientific interest of where these objects fit in, in the origin and evolution of our solar system, but indeed ignorance is not bliss. We cant, in all consciousness, expect us to ignore the near-Earth population.

MILES OBRIEN: By that, he means the millions of comets and asteroids that come close enough to Earth that they could collide with the planet.

Don Yeomans runs the Near-Earth Object Program at NASAs Jet Propulsion Laboratory.

DON YEOMANS, NASA: Its just a matter of time before a large one is on an Earth-threatening trajectory. The only question then is, will we discover it well ahead of time and do something about it?

MILES OBRIEN: We humans got a stunning shot across the bow in February of 2013, when a 60-foot-wide asteroid exploded over Chelyabinsk, Russia. Equivalent to 30 atomic bombs, it shattered windows, injuring about 1,500.

Originally posted here:

Tracking killer comets before they strike