Planet earth got considerably more from outer space than it bargained for on Friday.

We were expecting the flyby of a small asteroid, about 150 feet across. And it sailed on by, right on schedule and on course, missing us by a little over 17,000 miles. That’s close. The moon is about 240,000 miles away and even geosynchronous satellites are about 22,000 miles out.  Had it hit the earth it would have devastated an area the size of a considerable city and caused seriously unpleasant effects over a far wider area. (Had it hit the ocean—more likely, as water covers a little more than 70 percent of the planet—it would have caused major tsunamis).

But while we were waiting for that one to pass, a totally unexpected asteroid suddenly slammed into the atmosphere above Siberia and exploded near the city of Chelyabinsk. While much smaller than the one we were expecting, it was no mere shooting star. Indeed, NASA now estimates that it was about 55 feet across and weighed about 10,000 tons. The explosion was equal to a nuke with a yield of 500 kilotons (the bomb that annihilated Hiroshima was less than 20 kilotons).

Fortunately, coming in at a very shallow angle (about 20°) it exploded about 25 miles up in the atmosphere. Had it come in at a steeper angle, it would have exploded nearer the surface with far more devastating effects. (Had it exploded five miles up, for instance, the shock wave would have been 25 times as strong). The shock wave was strong enough, however, smashing windows in thousands of buildings and setting off an infinity of car alarms. Over 1,000 people were injured, mostly by flying glass. (Lesson: If you see a sudden, dazzlingly bright flash of light, do not go to the window to check it out. Duck and cover and wait a minute.)

This is a quite unprecedented event.

While the earth sweeps up space dust and small meteoroids all the time (perhaps 80,000 tons worth a year, although estimates vary), asteroids this size hit earth, it is estimated, only about once a century. This one was the biggest that we know about since the so-called Tunguska Event of 1908 that flattened millions of trees in the Siberian wilderness but was witnessed by almost no one. This one was witnessed by a city of 1.1 million, many of them with video cameras at the ready.  And that means it will be a scientific bonanza. I imagine that planetary astronomers are descending on Chelyabinsk by the hundreds to gather information and as many meteorites produced by the asteroid as they can find. (Technically, while in space it’s an asteroid or, if small, a meteoroid. It becomes a meteor as it streaks through the atmosphere, and whatever survives to hit the ground is a meteorite.)  Locals who find fragments stand to make a bundle, as meteorites can sell in the hundred of thousands of dollars.

So, all in all, it was a great day. No one was seriously hurt; the news media and You Tube had a field day and planetary astronomers (and Chelyabinsk’s glaziers) are very happy. It also reminds us that, as the dinosaurs found out, you never know what tomorrow will bring.