A small asteroid will zip past the Earth later today. The asteroid, with the catchy name of 2009 TM8 (I think I'll call it Timmy), was discovered yesterday by the Catalina Sky Survey in Arizona. Timmy will pass within 216,000 miles of Earth. That's inside the orbit of the moon, or razor close by astronomical standards. Timmy is only about 30 feet across. It's pretty amazing that we are able to spot something that small moving towards us 18,163 mph. One day warning isn't enough time to send Bruce Willis up to stop it. However, a bigger asteroid would be spotted further out and a dinosaur killer would be spotted years, probably decades before it hit, giving us time to deploy dozens of Bruce Willises.
What would happen if Timmy hit? Not much. Timmy is small. Asteroids that small don't get very far into the atmosphere before they explode. Timmy would produce about a four kiloton (thousand tons of TNT) blast. The Earth gets hit by asteroids of this order of magnitude a couple dozen times each century. Last fall, 2008 TC3 exploded over Sudan with a force of a little over one kiloton. In 1993, an asteroid exploded over Northern Italy with a force of about ten kilotons producing no damage at all. If a Timmy sized asteroid hit the daylight side of the Earth, there is a good chance no one would notice--except for the Catalina Sky Survey and their crack team of Bruce Willises.