What Makes a Space Rock a Killer?

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Today's space rock scoreboard: one hit, one miss.

This morning's surprise meteor explosion over Russia and the near-Earth flyby of asteroid 2012 DA14 this afternoon constitute an amazing and confusing coincidence that underscores the need to keep monitoring the skies for dangerous objects and sort out the dangerous objects from the merely interesting.

Tune in to "Russian Meteor Explosion" Saturday, Feb. 16, at 8 E/P on Science Channel.

The first thing that needs sorting out, however, are the words: meteor, meteorite, meteoroid and asteroid. Meteoroids and asteroids are objects in space. Meteoroids can be bits of asteroids or bits of comets. When they are burning through Earth's atmosphere they are, for a few seconds, called meteors. If anything survives that fiery descent, the rocks found on the ground are called meteorites.

"The videos from Russia are all of the meteor," explained MIT planetary scientist expert Richard Binzel. "We'll have to wait and see if there are any meteorites" that fell to the ground in Russia.

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Next, there are the factors that separate the killers from the simply cool to watch: mass, composition, trajectory and speed.

The Russian meteor was probably a rock of several tons and about the size of an SUV, traveling at about 40,000 miles per hour (65,000 km per hour) from north to south across the sky. It's the sort of thing that probably hits Earth every 10 years or so. Its speed suggests it was a very small rocky asteroid, said Binzel, since pieces of comets move a lot faster (and account for most meteor showers).

An SUV-sized boulder sounds pretty dangerous, but at those speeds in Earth's atmosphere the meteor's surface gets terrifically hot and the rock starts to crack, which lets air inside and leads to the explosion, explained K.T. Ramesh, a planetary scientist at Johns Hopkins University. It's the explosion that can be heard in the videos and the shock wave from the blast which broke windows and probably caused the most injuries, said Ramesh.

In contrast, asteroid DA 14 is about 50 yards across, more massive, traveling south to north past Earth (opposite the Russian meteor's direction), and definitely not on a collision course. Other than that, the two objects probably have a lot in common: They are both rocky members of the inner solar system (unlike comets that swing in from the nether regions of the solar system) and both are probably made of similar kinds of rock.

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To rate the dangers of these different kinds of objects, scientists have developed a special zero-to-10 scale called the Torino Scale. Zero on this scale means there is no chance of collision, while 10 means a global catastrophe.

"Asteroid DA 14, if such a 50-meter object were to hit the Earth, would be in the mid-range of '8' on the Torino Scale," said Binzel, who helped to develop the scale. Category 8 on the Torino Scale means: "A collision is certain, capable of causing localized destruction for an impact over land or possibly a tsunami if close offshore. Such events occur on average between once per 50 years and once per several thousand years."

"The Russian event is right at the limit, but would be at the bottom limit of '8' on the Torino Scale," Binzel said.

As for why both events happened on the same day: "It's a coincidence, pure and simple," said Johns Hopkins University astronomer Richard Henry. "It's an extraordinary coincidence, but they happen."