Two Asteroids to Make Fly-By of Earth Wednesday

One needs to be realistic about the asteroid impact hazard. These asteroids are too small (diameters of about 10 meters) to have been seen until they were within a few million kilometers of the Earth. Even if such a small asteroid hit the Earth, it would burn up or explode harmlessly in the upper atmosphere and cause little if any harm on the surface. NASA's Spaceguard Survey for Earth-approaching asteroids (http://neo.jpl.nasa.gov and http://impact.arc.nasa.gov) focuses on the larger objects that could do a great deal of damage if we collided with them. The objective of the survey is to provide decades of warning, so that we would have time to protect ourselves by deflecting the asteroid so that it misses the Earth. With less than a decade of warning there is not much we could do. For the asteroids with diameter more than 2 km, we have already learned that there are none on a collision course with our planet. No current survey is trying to find small asteroids just a few days or weeks before they come close to the Earth, but sometimes we do catch small asteroids such as these. The fact that we see these is an indication of how well the survey telescopes are working. A decade ago we would never have known that many small asteroids are sailing past the Earth every month.