Feb. 16, 2013 at 12:44 PM ET
Scientists have raised their estimates of the size and power of what turns out to be the most widely witnessed asteroid strike in modern history. The size estimate puts the object that caused Friday's meteor blast over Russia in a troublesome category of asteroids: big enough to cause damage, but small enough to evade detection.
The new estimates, based on additional readings from a sensor network built to detect nuclear blasts, suggest the meteor released the energy equivalent of nearly 500 kilotons of TNT. That's about 30 times the power of the Hiroshima atomic bomb.
Experts have been assessing the level of the meteor explosion using a network of infrasound sensors that were set up under the terms of the Comprehensive Test Ban Treaty to check for changes in atmospheric pressure caused by nuclear blasts.
"These new estimates were generated using new data that had been collected by five additional infrasound stations located around the world — the first recording of the event being in Alaska, over 6,500 kilometers away from Chelyabinsk," NASA's Jet Propulsion Laboratory said in a statement.
NASA now says the Chelyabinsk object must have been about 55 feet wide (17 meters wide) with a mass of 10,000 tons before it entered Earth’s atmosphere.
"We would expect an event of this magnitude to occur once every 100 years on average," Paul Chodas of NASA's Near-Earth Object Program Office said in the statement. "When you have a fireball of this size, we would expect a large number of meteorites to reach the surface, and in this case there were probably some large ones."
Searchers have been focusing on a frozen lake about 50 miles (80 kilometers) west of Chelyabinsk, where they suspect meteorite fragments made a 20-foot-wide (6-meter-wide) hole in the ice. Searchers have found fragments up to a half-inch wide (1 centimeter wide) that might have come from the meteor, but nothing bigger yet, according to reports from Russia.
Experts emphasized once again that the meteor's trajectory was significantly different from the path of asteroid 2012 DA14, a 150-foot-wide (45-meter-wide) space rock that passed harmlessly within 17,200 miles (27,600 kilometers) of Earth later Friday. Thus, 2012 DA14 was "a completely unrelated object," NASA said.
The space agency said Friday's Russian meteor was the largest reported since 1908, when an asteroid roughly the size of 2012 DA14 exploded over a remote wooded area in Siberia's Tunguska region. That blast flattened millions of trees over a 820-square-mile area, but was not widely seen. Friday's event, in contrast, took place over a city of 1.1 million inhabitants, and hundreds of millions more watched the videos that were distributed over the Internet.
As powerful as the meteor blast was, it's on the low end of the asteroid impact scale. Astronomers estimate that there are about a million potentially hazardous near-Earth objects smaller than 100 meters (330 feet) in width, and only about 1 percent of those have been cataloged. For the time being, NASA is focusing on detecting and tracking near-Earth asteroids wider than 100 meters.
But what about the smaller ones?
"Defending the Earth against tiny asteroids such as the one that passed over Siberia and impacted there is a challenging issue. That is something that is not currently our goal," Chodas told reporters on Friday.
The asteroid behind Friday's meteor blast would have been particularly hard to spot during its final approach, because it was coming from Earth's daylit side. The asteroid would have been lost in the sun's glare and undetectable by ground-based telescopes, said Bill Cooke, the head of the Meteoroid Environment Center at NASA's Marshall Space Flight Center.
Several programs on the horizon hold the promise of finding the smaller asteroids that could threaten Earth:
More about asteroids and meteors:
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.