Whew!

A couple of hours ago we got another near-miss here on Earth. Oh, astronomers knew it was coming for a few days, but still. 7,600 miles is pretty close!

"The object, called 2011 MD, zipped 7,600 miles above Earth’s surface at about 1:15 p.m. EDT on June 27. It was discovered on June 22 by the LINEAR project, which hunts for objects that pass close to Earth using robotic telescopes in New Mexico.

2011 MD is between 16 and 65 feet wide and is in a very Earthlike orbit around the sun, which made astronomers wonder if it was a stray rocket booster left over from a previous space mission. But running the object’s orbit backwards showed that the asteroid never came close enough to Earth during the space age to have started life as a rocket booster."

 

Actually these events are pretty common, but asteroids this small would probably burn up in our atmosphere if they hit us. And that happens frequently too.

 

They don't get really dangerous until they're 100 feet or so across, like the one that hit Tunguska in 1908. That one:

 

"leveled the forest for thousands of square kilometers and would have caused immense destruction if it had hit a populated area."

 

That kind is proportionally more rare, occurring about once every thousand years.

 

Fifty-thousand years ago an asteroid hit near what is now Flagstaff Arizona, creating a crater that is 4,000 feet across and 600 feet deep. The impacting asteroid was about 54 yards across.

 

No warning back then of course, but we're tracking these travelers very well now. And the bigger they are, the more warning we have. Once they get up to the 1-2 km size, we see them coming a long ways off:

 

How's this for advanced warning...We know that a 1km-diameter asteroid called (29075) 1950 DA will strike the Earth on March 16th, 2880. That is, unless our descendants destroy it first.

 

Rarer still are the big boys - asteroids than are capable of causing a life-changing, planet-wide catastrophe. These are the 5-10 mile-diameter monsters, like the one that wiped the dinosaurs out some 65 million years ago. The size of Halley's Comet. This type is even more rare, averaging 60 million or so years between events (please don't say "then we're due for one!", because that's not how statistics works :).

 

It is thought that direct hits by asteroids this size have caused several previous mass-extinctions. The most well known is the impact at the "K-T boundary" that hit the Earth with a million times more energy than every atomic bomb ever made, combined, and caused the extinction of the dinosaurs 65 million years ago. There's solid evidence for that one, including a 110-mile-wide crater in the ocean off the coast of the Yucatan peninsula that dates to exactly the right time period.

 

But there were others, long before that one, so long ago that all evidence of the craters would have been wiped clean by subduction or tectonics. The most spectacular was probably the event at the Permian-Triassic boundary 250 million years ago. That one took with it 90% of all life on Earth. So devastating was that event that the Earth's biology took some 30 million years to recover. This event is known as the Great Dying, the mother of all extinctions, and the only extinction to claim insects as well as plants and animals.

 

The photo above is today's tiny little nuisance, taken with long exposures and three color filters at different times so it appears as a series of colored streaks. It just quietly passed by and went on its merry way.