When a large asteroid comes to almost an Earth moon's distance to hitting Earth and another comes within 16,000 miles of smacking into all in the same week—you know asteroids present a real risk. But according to the latest research, we’re still in the early stages of understanding the risks that asteroids poise.
The asteroid TU24 was discovered by NASA's Catalina Sky Survey on Oct. 11, 2007, and is only one of an estimated 7,000 near-Earth objects identified to date. However, more than twice as many are estimated to exist, but have simply not yet been discovered. This particular space rock is lopsided and estimated to be about 800 feet across. Images of the asteroid were formed using several powerful telescopes.
"We have good images of a couple dozen objects like this, and for about one in 10, we see something we've never seen before," said Mike Nolan, head of radar astronomy at the Arecibo Observatory. "We really haven't sampled the population enough to know what's out there."
The one that passed by Earth recently is orbiting the sun. Most of the asteroids in our Solar System are found in the asteroid belt between and Jupiter. For the ones with orbits bringing them close to Earth, scientists are paying special attention. For this particular asteroid, scientists at NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, Calif., have determined that there is no real possibility of an impact with Earth in the foreseeable future, but there are many others out there. At this point, scientists have no idea how big the overall risk of asteroid impact is.
The latest study suggests that even the more common smaller asteroids poise a serious threat. According to supercomputer simulations by Sandia National Laboratories physicist Mark Boslough, the asteroid that destroyed the forest at Tunguska in Siberia in June 1908 was considerably smaller than TU24.
The asteroid that exploded over Siberia a century ago, left over 800 square miles of scorched or knocked down forest, wasn't nearly as large as previously thought. This finding implies a greater danger facing the inhabitants of planet Earth. Boslough has spent years trying to better understand what happened at Tunguska. He says a clearer understanding would help policymakers decide whether to try to deflect an asteroid, or evacuate people in its path.
"It's not clear whether a 10-megaton asteroid is more damaging than a Hurricane Katrina," Boslough said. "We can more accurately predict the location of an impact and its time better than we can a hurricane, so you really could get people out of there if it's below a certain threshold."
Even so, Boslough’s finding is bad news. Smaller asteroids approach our planet about three times more frequently than large ones. So if large asteroids approach about every 1,000 years, a smaller one would be about every 300 years. "Of course there's huge uncertainties," he noted.
But for now Boslough’s new model is the most reliable we have, and it indicates that even smaller asteroids can be more devastating that previously believed. The three-dimensional simulation better matches what's known of Tunguska than earlier models have. It shows that the center of the asteroid's mass exploded above the ground, taking the form of a fireball blasting downward faster than the speed of sound. But the fireball did not reach the ground, he says, which explains why miles of trees outside the epicenter were flattened, but those at the epicenter remained standing, albeit scorched with their branches blown off.
If the asteroid had been as large as previously thought, "it would have had really different effects on the ground," Boslough said.
Alan Harris, a planetary scientist at Space Science Institute in Boulder, Colo., agrees that Boslough's work is “very sound" and will be taken into account when revising estimates of risk and damage of smaller objects in the future.
Astrophycisist, Sir Martin Rees pf Cambridge University, has famously speculated that the asteroid risk is just one of many reasons why humankind has only a 50/50 chance of making it into the next century. Even so, he says comets are more frightening of a doomsday prospect. Pound for pound, comets are much more dangerous than asteroids, which have nonetheless gotten more media attention. Comets travel a lot faster through space than Asteroids, which travel at about 25-30 km per second. The speed of a comet approaches a much faster 70 km per second. A relatively small object of just one and a half km in diameter hitting the Earth would release more energy than all the atomic bombs ever detonated and then some. An object of 20 km or more would likely cause mass extinction. But hey, at least we’d go out with a bang.
Posted by Rebecca Sato.
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