In Cosmic Game of Billiards, What Are the Chances of an Asteroid Collision?
This image obtained by the framing camera on NASA’s Dawn spacecraft shows the south pole of the giant asteroid Vesta. Photo by NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
There’s been buzz in the news this week about two asteroids nearly sideswiping us. Boston Globe referred to one as a close flyby. The Washington Post said it just missed us. Wired called it a “cosmically close shave.” These headlines put a hazardous spin on the objects. And they kind of make you want to duck.
Just how close is 4.3 million miles, cosmically? That’s the distance from Earth of 4179 Toutatis, the Central Park-sized asteroid scheduled to zip by our planet sometime on Wednesday. The other — 2012 XE54 – was a closer, 140,000 miles away. So are we in danger?
In the foreseeable future, these asteroids pose very little risk, said Richard Binzel, a professor of planetary science at MIT.
On a geologic time scale, however, the risks increase. An asteroid like Toutatis, for example, might collide with the Earth every 10 million years or so. Plus, every now and then, when an asteroid gets close enough to a planet, the gravitational tug from that planet can create a so-called “slingshot effect,” flinging the asteroid out toward Jupiter or even out of the solar system entirely.
“Or they can hit the moon,” Binzel said. “It’s a cosmic game of billiards.”
Binzel directed my attention to an asteroid that really could be called a near-miss: On May 29, 2012 KT42 — “KT42” for short — streaked directly across the orbits of weather and television satellites, a mere 22,000 miles above Earth’s surface, making it the sixth-closest asteroid approach on record. Here’s what it looked like:
But let’s back up. Asteroids are leftover building blocks from the formation of the solar system. They’re the stuff that never turned into planets or stars, and they orbit in loose formation, between Mars and Jupiter, in a region known as “the asteroid belt.” Jupiter’s gravity stirs up these objects and tugs on their orbits, making them elongated and elliptical. They often look like peanuts or potatoes, and they hurl through space at speeds as fast as 24 miles a second.
“That’s really, stunningly fast,” said Timothy Spahr, who runs the Minor Planet Center at the Smithsonian Astrophysical Observatory.
As for how close the asteroids are, here’s some perspective. The asteroid belt contains about a million large asteroids. (Large means longer than a kilometer, or .6 miles.) About 1,000 of those are known as “near-Earth” asteroids. These are asteroids that cross the Earth’s orbit.
“Think about a four-way stop on a deserted country road,” Binzel said. “The Earth passes through the stop once a year. The asteroid passes through the stop every year or every few years.”
Remember the scene in “The Empire Strikes Back” in which Darth Vader, on the Star Destroyer, is pursuing Han Solo through the asteroid belt, while dodging and destroying asteroids in a death-defying feat?
The reality, he says, is those million large asteroids in the asteroid belt are each spaced a million miles apart on average. Which means chances are incredibly low that you’d see anything, not to mention be dodging several at once.
And as for risk, while the odds of a major collision in our lifetime is low, it’s not zero, Binzel said. The big question then, in his view, is how to deal with low-likelihood, high-consequence events. In other words, how do you address the possibility without sensationalizing it?
“How do you communicate that in a way that is factual, but doesn’t overemphasize the risk and doesn’t under emphasize the risk?” he asked. “It’s just an interesting problem.”
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Jeremy Blackman, Rebecca Jacobson, Patti Parson and David Pelcyger contributed to this report.