Rocks in the Sky

Last week was busy in the world of close encounters with space rocks. Just a few hours before the closest pass with asteroid 2012 DA14, a meteoroid streaked through the early morning skies over Chelyabinsk, Russia, exploding at an altitude of about 12 to 15 miles. The latest estimates from NASA put the explosion at about 500 kilotons, which for comparison is about 30 times larger than the atomic blast at Hiroshima. NASA’s other numbers are equally impressive: the meteoroid struck the atmosphere going about 40,000 miles per hour, had a mass of some 10,000 tons, and was about 55 feet in diameter. Thankfully, though the blast caused around a thousand injuries from glass and debris, no lives were lost.

The Chelyabinsk meteoroid was unpredicted, and was virtually unpredictable with modern search techniques and technology. Smaller meteoroid impacts have been predicted in the past, but their discovery was serendipitous. Friday’s meteor could have been seen if someone had been looking in the right place at the right time, but no constant, all-sky monitoring program is currently in place to catch every such object headed for the atmosphere.

The much larger (150 feet) 2012 DA14 asteroid, on the other hand, was predicted in advance, and was known to be on a safe path that would carry it harmlessly past the planet.

Understandably, the proximity of the two events leads to questions about whether they were related. The answer is no, for a simple reason: the two objects were following entirely different paths through space when they encountered the Earth. This image from NASA illustrates:

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The green circle shows the Earth’s orbit. The orbit of 2012 DA 14 is the inner blue circle. This orbit is fairly well known—that’s how scientists were able to predict its close flyby. The orbit of the meteoroid that struck Russia is less precisely known, but can still be accurately reconstructed from the object’s flight path. Its path is wider and more elliptical, nowhere near the path of 2012 DA14. In fact, their orbits are even less similar than this image shows, given that it is only two-dimensional. The objects orbits are fairly highly inclined to one another: the asteroid moved from a south-to-north direction relative to the Earth’s surface, while the meteoroid moved east-to-west. So what we see is that while the Earth was still several hours from its closest encounter with 2012 DA14, it intersected the path of another, smaller object moving in a completely different direction.

The other question that deserves a little more attention, too, is the question of just how rare these events are. The Russian meteor strike is the sort of thing that happens once in several decades. The nearest comparison is the 1908 Tunguska impact, which was a much larger event at an estimated 10-15 megatons. So these things don’t happen every day, but they aren’t unprecedented. The flyby of asteroid 2012 DA14 is also not uncommon, but in the day and age when we can predict it in advance and provide intense real-time monitoring, it’s gotten a lot more attention (obviously) than previous encounters that took place in observed.

Finally, we have the news stories of fireball meteors seen in various places in the days since the meteor strike—in Florida, California, and elsewhere. These are bright meteors that are the size of large grains of sand up to pebbles and small rocks. They put on a bright show but are harmless. These are instances of an event that is in fact fairly common simply getting attention just now because of the media buzz. Bright fireball meteors occur nearly daily somewhere on the planet. Astrophotographers and videographers are often sharing new pictures and clips of these events. The American Meteor Society has over 2,000 fireball sightings documented for 2012, and 400 already for 2013. In other words, you won’t see one every night, but in a lifetime of focused observing, you should expect to see many—and you certainly shouldn’t be surprised if fireball meteors show up in the news frequently in the near future. It’s not that they are happening more often, it is just that they’re getting noticed.


Image: NASA

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