If we don't want to go the way of the dinosaurs someday, we need to protect ourselves against the threat of being hit by a big asteroid. According to NASA, typically about once every 10,000 years, a rocky or iron asteroid the size of a football field could smash into our planet's surface and possibly cause tidal waves big enough to inundate coastal areas.
But it's the real monsters -- asteroids about 100 meters (109 yards) across or bigger-- that we really have to fear. Such a collision would unleash a firestorm of heated debris and fill the atmosphere with sun-blocking dust, which would wipe out forests and farm fields and starve the human and animal life that it didn't immediately kill. A wisely funded space program enables us to spot a dangerous object long before it strikes Earth, and send a spacecraft to plant and detonate a nuclear explosion that could nudge it off its collision course [sources: NASA, NSS].