It won't hit us, but the 1,300-foot-wide asteroid, 2005 YU55, that will slip between Earth and the moon's orbit on Tuesday night is a reminder that large chunks of rock and iron left over from the formation of the solar system are often whizzing by at tens of thousands of miles per hour.
The risk that one might actually hit--with catastrophic effects--keeps NASA's Near-Earth Object Observation program busy watching. The House and Senate approved $1.5 billion for NASA's planetary-science budget in fiscal 2012, which includes the observation program. A conference committee met on Thursday to begin resolving differences over funding NASA and a host of other federal agencies and programs.
Many of the objects that approach Earth are so small that if they did enter the atmosphere they would burn up before reaching the ground. Larger objects, however, can survive a trip through the atmosphere and strike with enough force to cause widespread damage. Such an event might have led to dinosaurs becoming extinct 65 million years ago.
Above is a NASA animation of the trajectory of Tuesday's asteroid. Below is a photo of the asteroid, along with the trajectories of some past asteroids.