A bus-sized asteroid will “just” miss the Earth today around noon CDT. Asteroid 2011 MD, which is estimated to be 9-30 meters in size, will come within about 7,500 miles of the surface of the Earth, pretty close as far as asteroid encounters go, and well within the 22,000 mile radius of geosynchronous satellite orbits. But for comparison, it is well beyond the 220-mile orbit of the International Space Station. Even at its closest approach, the asteroid will not be visible to the naked eye, but may appear as a bright dot to sharp-eyed observers with medium-sized telescopes.

NASA scientists, who have been tracking asteroid 2011 MD since June 22, say there is “no chance” it will strike the Earth. Even if it did, its rocky composition means it would very likely break apart and burn up in the atmosphere (over Antarctica). The ones you have to watch out for are the rocks made up of iron, which are more likely to survive the trip through the atmosphere.

This isn’t the first close-encounter we’ve had this year. Asteroid 2011 CQ1 came within 3,400 miles of Earth in February, and set a new record as the closest recorded pass ever. NASA estimates that moderate-sized asteroids make close flybys about six times a year, and they have not been able to track all of them — asteroid 2011 CQ1 wasn’t discovered until hours before its closest approach. But the truth is, the Earth is struck by stuff from outer space all the time, and we haven’t experienced a catastrophic event for a very long time. Approximately 500 small meteors make it all the way to the surface of the Earth each year, and it is estimated that about one small asteroid (1-10 meters in size) reaches the Earth per year. The latter usually break up and vaporize in the upper atmosphere. Fortunately, there is an inverse relationship between the size of an object and the frequency with which it is likely to strike the Earth. In other words, the bigger they are, the less likely we are to be hit by one. Here is a comparison of asteroid radius and strike frequency:

• 10 m: every year
• 50 m: every 1,000 years
• 1 km: every 500,000 years
• 5 km: every 10 million years
• 10 km: every 100 million – 1 billion years

The last known very large impact event involving an asteroid about 10 km in size is the one that is believed to have wiped out the dinosaurs 65 million years ago. We’re not due for even a 1 km event until the year 2880 when asteroid (29075) 1950 DA has a possibility of hitting the Earth. So it’s probably okay to carry on with civilization for a while longer.

Related posts:

• Asteroid Apophis on its way to Earth
• Never tell me the odds!

Recommended reading:

• NASA: NEO Program
• Wikipedia: Impact event
• Curious About Astronomy? Comets, Meteors and Asteroids