Slideshow: Meteor streaks over Siberia

Yekaterina Pustynnikova / Chelyabinsk.ru via AP

Click through scenes from Russia's Chelyabinsk region, where a huge meteor fireball set off alarms, injured hundreds of people and caused a factory roof to collapse.

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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:

• NASA has just started funding the Asteroid Terrestrial-impact Last Alert System, or ATLAS, which aims to establish two telescopes in Hawaii dedicated to scanning the skies for potential threats.
• The non-profit B612 Foundation has been raising money to launch its Sentinel Space Telescope as early as 2018. Sentinel would scan Earth's surroundings from an outward-looking position in a Venus-like orbit, interior to Earth's orbit. Such a project could provide advance warning for asteroids like the one that blew up on Friday. Former Apollo astronaut Rusty Schweickart, one of the foundation's founders, said he was "overwhelmed" by requests for information after Friday's blast. "It's pretty bonkers at the moment," he told NBC News.
• Planetary Resources, a commercial venture, is developing a fleet of Arkyd-100 space telescopes to identify near-Earth asteroids, in hopes of sending mining operations to them in the decades to come. "As the company ultimately develops the capability and infrastructure for intercepting and mining asteroids, Planetary Resources expects to be able to help in the (slight) redirection of these rocks to keep the Earth safe," Peter Diamandis, the company's co-founder and co-chairman, said in a blog posting.
• Another commercial space-mining venture, Deep Space Industries, is proposing its own set of asteroid-hunting space telescopes. "Placing 10 of our small FireFly spacecraft into position to intercept close encounters would take four years and less than $100 million," David Gump, the company's CEO, said in a statement. "This will help the world develop the understanding needed to block later threats."

More about asteroids and meteors:

• Meteor? Asteroid? Terms get tangled
• Russian meteor blast outshone the sun
• System to vaporize asteroids is in the works
• NBCNews.com archive on asteroids

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.

What exactly fell on Russia's Chelyabinsk region on Friday? Was it an asteroid, meteor, meteoroid, meteorite or fireball? You could make a case for "any of the above," depending on your definitions and the precise part of the phenomenon you're trying to describe.

The Chelyabinsk incident is the biggest known cosmic impact since another Russian blast that occurred a century ago, the Tunguska incident of 1908. There's good reason for that notoreity: Hundreds of injuries were reported. NASA estimated that the energy released by the Chelyabinsk impact amounted to 300 kilotons of TNT, which suggests the blast was more than 10 times as powerful as the atom bombs that were dropped on Japan at the end of World War II.

NASA's assessment put the Chelyabinsk object's width at 15 meters (50 feet), and its mass at 7,000 tons. Much of that mass burned up during the object's atmospheric entry at a velocity of 40,000 mph (18 kilometers per second). "The fireball was brighter than the sun," the space agency said in a statement.

Astronomers use different terms to describe cosmic objects of different sizes: When the rock is no wider than a meter (3.3 feet), it's known as a meteoroid. But once you start getting into the 1- to 10-meter range, the term "asteroid" applies. Earlier estimates suggested the Chelyabinsk object was a meteoroid, but the latest assessment would put it in the class of a small asteroid.

Bill Cooke, who heads the Meteoroid Environment Office at NASA's Marshall Space Center, said the object was "a small asteroid or a large meteoroid, depending on how you want to define it."

When pieces of the meteoroid (or asteroid) survive their fiery fall through the atmosphere and hit the surface, those pieces are called meteorites. Russian authorities say a hole in the ice on Chebarkul Lake, near Chelyabinsk, marks a spot where at least one meteorite left its mark. There are already reports of Chelyabinsk meteorites turning up on online auction sites, but those are more likely to be "meteor-wrongs" — rocks wrongly assumed to be meteorites.

Slideshow: Meteorite showers in Siberia

Yekaterina Pustynnikova / Chelyabinsk.ru via AP

A huge meteor flares through the skies over Russia's Chelyabinsk region, triggering a powerful shock wave that injured nearly a thousand people, blew out windows and reportedly caused the roof of a factory to collapse.

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The term "meteor" refers to the fiery aerial display created by a falling meteoroid or asteroid. Meteors are called fireballs if they shine brighter than the planets in the night sky (magnitude -4), and bolides if the blast is even brighter (around magnitude -14). There's no question that the Chelyabinsk meteor qualifies as a bolide.

Some asteroids are made of iron and nickel, and survive their fall more easily. However, the fact that the Chelyabinsk object appeared to break up into pieces while it was still miles high indicates that it was made of less dense stuff. The stresses of atmospheric entry caused the rock to break apart explosively, creating the midair flash and generating a shock wave. The shock wave produced the loud "bang" that set off car alarms, blew out windows and apparently collapsed the roof of a zinc factory warehouse. Flying glass from all those broken windows caused many of the injuries that were reported.

What about the asteroid flyby?
The Chelyabinsk object streaked through Russian skies just hours before a 150-foot-wide (45-meter-wide) asteroid known as 2012 DA14 was due to make a remarkably close approach, coming within 17,200 miles of Earth's surface. However, the two objects were in dramatically different orbits, and that's one of the factors that led NASA to conclude that the two cosmic events were "not related."

"It's clearly coincidental, but it's a pretty amazing coincidence," said former Apollo astronaut Rusty Schweickart, a co-founder of the B612 Foundation. For years, Schweickart and his colleagues have been trying to raise awareness about the hazards posed by asteroids, and Friday's double dose of cosmic reality certainly serves as a consciousness-raiser.

"It's a torpedo across the bow," Schweickart told NBC News, "and it serves as an indication that these things really do happen."

Objects as small as the Chelyabinsk asteroid are difficult to detect — but the feat is not impossible, given the right circumstances. In 2008, a 2- to 5-meter-wide asteroid known as 2008 TC3 was spotted using the Catalina Sky Survey 1.5-meter telescope in Arizona, 20 hours before its impact in the Sudanese desert. The Chelyabinsk object would have been particularly hard to spot because it came in from the blind spot on Earth's sunlit side.

The Chelyabinsk object is no more, but there are still lots of other space rocks to be found. In 2011, NASA estimated that there are a million potentially hazardous near-Earth asteroids smaller than 100 meters (330 feet). Several organizations — including NASA, the B612 Foundation and Planetary Resources — are working on plans to detect and track more of the threats that are out there. To learn more about those efforts, click on the links below:

• Asteroid activists seek funds for space telescope
• Asteroid mining venture starts with space telescopes
• Asteroids vs. comets: NASA expert assesses threats

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.

This story was originally published on Fri Feb 15, 2013 1:34 PM EST

Asteroid 2012 DA14 has made its closest pass of Earth, just a scant 17,200 miles from our surface, and now astronomers are watching it recede harmlessly into the cosmos. You can watch it, too, thanks to a variety of webcasts.

The time of closest approach came at 2:25 p.m. ET, as scheduled, when the asteroid was zooming above the eastern Indian Ocean at a speed of almost 17,500 mph (7.8 kilometers per second). It was too dim to see with the naked eye, but observers in Australia, Asia and Europe could follow it with binoculars or small telescopes.

"It's on its way out now," said Paul Chodas, an astronomer with the Near-Earth Object Program Office at NASA's Jet Propulsion Laboratory in Pasadena, Calif.  

Read: Meteor vs. asteroid? Terms get tangled when describing space rocks 

Chodas and his colleagues monitored 2012 DA14 with high-powered optical telescopes and huge radar dishes to learn more about the asteroid's color, shape, spin and reflectivity. Such data could tell them what the object is made of, and perhaps provide insights into how similar objects could be diverted if they were on a threatening course.

Experts estimate that asteroids the size of 2012 DA14 hit our planet every 1,200 years or so, exploding with the energy of a 2.5-megaton atomic bomb: The last such impact struck a remote region of Siberia without warning in 1908, flattening 820 square miles of forest. If an object that big were to hit in just the wrong place, it could wipe out a city. Coincidentally, a much smaller space rock came down over Russia on Friday, sparking a fireball and a glass-shattering shock wave.

Even though the 150-foot-wide (45-meter-wide) asteroid 2012 DA14 is the biggest object of its kind to be seen coming this close to Earth, its orbit is so well-known that NASA's Near-Earth Object Program can rule out any chance of collision in the foreseeable future. And even though 2012 flew 5,000 miles closer than satellites in geosynchronous orbit, NASA said its mostly south-to-north orbital path went through a "sweet spot" that kept it far away from those satellites — as well as from other spacecraft that are in closer orbits, including the International Space Station.

Astronomer hope their observations of 2012 DA14 will provide insights into subtle phenomena such as seismic disturbances that are induced by Earth's gravitational kick, or characteristics of the asteroid's spin that are affected by radiation absorption and emission.

This animated set of three images shows 2012 DA14 as it was observed by the Faulkes Telescope South in Australia on Feb. 14 at a distance of 465,000 miles. The asteroid is the moving bright spot in the middle. NASA's website provides details. Credit: LCOGT / E. Gomez / Faulkes South / Remanzacco Observatory.

Radar readings provide the best way to get a fix on the asteroid's shape and spin, in part because observations from multiple radio telescopes can be combined to produce a clearer picture. During the 2012 DA14 encounter, scientists used radio telescopes in California and New Mexico to produce new sets of radar imagery.

The first pictures from NASA's 230-foot (70-meter) dish at Goldstone, Calif., are due to be released on Saturday, and eventually those radar images will be combined to produce a 3-D map of the space rock.

Other telescopes, spread out from Australia to Israel to the Canary Islands to the U.S., gathered optical data — and the images from some of those telescopes were shared over the Internet on Friday. NASA's Jet Propulsion Laboratory has been passing along pictures from a variety of telescopes via its Ustream video channel. Here's a rundown of other post-encounter webcasts:

5 p.m. ET: The Virtual Telescope Project 2.0 presents live video of the asteroid flyby from a telescope in Italy, weather permitting. Video site: Watch Virtual Telescope Project's webcast.

6 p.m. ET: Weather permitting, the Clay Center Observatory in Massachusetts will stream real-time, high-definition video from 6 p.m. ET until 4 a.m. ET Saturday. Watch Clay Center video on Ustream.

9 p.m. ET: Slooh Space Camera plans to present several live shows about the asteroid flyby, accompanied by expert commentary. Weather permitting, imagery will be beamed to Slooh HQ from telescopes on the Canary Islands and in Arizona. Watch the show on Slooh.com.

9 p.m. ET: A video feed of the flyby from a telescope at NASA's Marshall Space Flight Center will be streamed for three hours. During the live-streaming event, viewers can ask researchers questions about the flyby via Twitter or the Ustream chat window. Watch Marshall's Ustream channel.

Chodas said the initial observations confirmed scientists' estimates of 2012 DA14's size, but other revelations will have to wait until astronomers have had a chance to analyze the data collected on Friday. By that time, the asteroid will be long gone. Earth's gravitational influence has changed 2012 DA14's orbit to keep it farther away from our planet during future orbits.

"It won't return for many, many years," he said.

More about the asteroid encounter:

• How 2012 DA14 rates among asteroid hits and misses
• Would-be miners say asteroid is worth $195 billion
• Hooray! Earth will kick asteroid into a safer orbit
• Flash interactive: Close encounters of the asteroid kind
• NBCNews.com's archive on asteroids

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.

This story was originally published on Fri Feb 15, 2013 7:51 AM EST

When the asteroid known as 2012 DA14 zooms within 17,200 miles of our planet on Friday, it'll mark the closest approach by a massive space rock in more than a century (although the meteor that flared through the skies over Russia's Chelyabinsk region early Friday injured hundreds of people, it is only a fragment of the size of these monsters). Fortunately, the 150-foot-wide object will pose absolutely no risk to Earth — but over the course of millennia, other asteroids have literally rocked our world.

As safe as Friday's encounter will be, it's a reminder that Earth has been vulnerable to cosmic impacts in the past, and will continue to be in the future. That's why NASA and other agencies are spending millions of dollars to detect more of the estimated 1 million near-Earth objects that could be as threatening as 2012 DA14.

"We are looking at all kinds of partnership possibilities, across universities, space institutions and with the Air Force," said Lindley Johnson, program executive for the Near-Earth Object Observations Program at NASA Headquarters. This week, Johnson and other experts are gathering at a U.N.-sponsored conference in Vienna to discuss the creation of an international asteroid warning network.

Vienna is actually one of the places where 2012 DA14 can be seen in the night sky on Friday — not with the naked eye, but with binoculars or a small telescope. The best viewing opportunities will be available in Asia, Australia and Europe. (Follow the instructions at the bottom of this article to find out if it'll be visible from your location.)

The closest approach comes at 2:44 p.m. ET, when the asteroid will be zooming past at a speed of almost 17,500 mph, directly above the eastern Indian Ocean. It'll come 5,000 miles within the ring of communications satellites in geosynchronous Earth orbit, but those satellites are so widely distributed that experts say the chance of a collision is extremely remote.

NASA / JPL-Caltech

A NASA chart shows how asteroid 2012 DA14 will be deflected by Earth's gravitational field. Experts say the space rock will be put into a safer orbit after this week's encounter.

If 2012 DA14 were on a collision course, the shock of its rapid fall through Earth's atmosphere would cause it to explode, unleashing the power of a 2.4-megaton atomic bomb. In the worst-case scenario, that'd be enough energy to destroy an entire city. A similar cosmic blast in 1908 laid waste to 820 square miles of Siberian forest in the Tunguska region.

It's possible that other such blasts have occurred over the course of Earth's history without being recorded. Based on a statistical analysis, NASA estimates that asteroids the size of 2012 DA14 strike Earth every 1,200 years or so. The only reason we know about this encounter is because the capabilities for tracking near-Earth objects have improved so much in recent years.

A Spanish observation team discovered 2012 DA14 just last year during a more distant flyby. "We probably would not have found DA14 10 years ago," said Don Yeomans, the head of NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, Calif. Now that they know it's there, astronomers will be monitoring the asteroid with optical and radio telescopes, including the Arecibo Observatory's 1,000-foot-wide dish in Puerto Rico and NASA's Goldstone radio antenna in California.

Radar observations could provide insights into the space rock's shape and spin, while an analysis of the optical data could reveal what 2012 DA14 is made of. Think of Friday's encounter as a practice run for identifying and tracking the unknown asteroids that actually could threaten us in the years to come — and an incentive to figure out ways to deflect them in case we have to.

To get a better sense of how 2012 DA14 rates, here are a dozen more hits and misses involving near-Earth objects:

65 million B.C.: The most infamous asteroid is the 6-mile-wide rock that smashed into Mexico's Yucatan Peninsula 65 million years ago, sparking the global catastrophe that did in the dinosaurs. Experts say the explosion released as much energy as 100 trillion tons of TNT.

35 million B.C.: Geologists say a roughly kilometer-wide (0.6-mile-wide) asteroid or comet struck America's Eastern Seaboard millions of years ago, contributing to the formation of Chesapeake Bay and creating a biological crisis. Studies have shown that microbes deep underground in the blast zone are still adjusting to the ancient shock.

NASA

Barringer Impact Crater in Arizona — also known as Meteor Crater — is captured in this image taken in 1995 by space shuttle astronauts.

50,000 B.C.: A 150-foot-wide iron-nickel meteorite hits Arizona, creating the 0.75-mile-wide Meteor Crater. Asteroid 2012 DA14 is thought to be the same size as this meteorite, but made of less dense stuff that would break up before it hits the ground.

1490: Chinese accounts tell of a meteor shower during which "stones fell like rain" on the Qingyang (Ch'ing-Yang) district of Shaanxi Province (now Gansu Province), killing as many as 10,000 people. Experts are doubtful about the reported death toll, but they don't doubt that a dramatic event occurred, perhaps involving the breakup of an asteroid. 

1908: The Tunguska event in Siberia, which flattened millions of trees, is thought to have been caused by an asteroid similar to 2012 DA14 in size and composition. Tunguska has become a watchword for asteroid activists. "The greatest danger from an asteroid strike is from the ones we haven't yet found," former NASA astronaut Ed Lu, chairman and CEO of the B612 Foundation, told NBC News. "Of the asteroids larger than the one that struck Tunguska in 1908, we know less than 1 percent."

1937: Asteroid Hermes is observed to miss Earth by a distance of just 460,000 miles. Decades later, scientists found out that Hermes occasionally comes even closer to Earth, and in fact consists of two space rocks flying in tandem. Each of the objects is thought to be about 1,300 feet (400 meters) wide.

1972: The Great Daylight Fireball is witnessed blazing over the Rocky Mountains from the U.S. Southwest to Canada. Scientists say it was an Earth-grazing meteoroid that passed within 35 miles (57 kilometers) of Earth’s surface.

1997: Astronomers report that a mile-wide asteroid known as 1997 XF11 had a chance of hitting Earth in 2028. The report touched off a media tempest, but further observations reduced the chance of collision to zero. The news came amid a spate of asteroid disaster flicks, including "Deep Impact" and "Armageddon" as well as the TV miniseries "Asteroid."

2004: An 885-foot-wide asteroid known as 2004 MN4, later named Apophis, is initially given a 1-in-40 chance of hitting Earth in 2029. That collision risk was ruled out relatively quickly, but it took years longer to analyze the risk posed by a later encounter in 2036. Just last month, astronomers announced that Apophis will pose no threat to Earth in the foreseeable future.

2008: Asteroid 2008 TC3 explodes during atmospheric entry above Sudan’s Nubian Desert. The event marked the first time that a near-Earth object’s impact was successfully predicted, several hours in advance. 2008 TC3 was 2 to 5 meters wide, and broke up into fragments that were later recovered from the desert.

2011: Asteroid 2011 CQ1 makes the closest-ever flyby of Earth for a cataloged asteroid, passing within 3,400 miles of Earth’s surface. The asteroid was discovered just 16 hours before its super-close encounter, but because it's only a meter wide, it would have burned up in the atmosphere if it had been on a direct course.

2011: An asteroid the size of an aircraft carrier, 2005 YU55, sails past Earth at a distance of 198,000 miles, which is closer than the orbit of the moon. Earthlings marked the asteroid's passage with a barrage of picture-taking.

More about the asteroid encounter:

• Would-be miners say asteroid is worth $195 billion
• Hooray! Earth will kick asteroid into a safer orbit
• Flash interactive: Close encounters of the asteroid kind

Astronomers say asteroid 2012 DA14 won't be visible to the naked eye, but it is possible to watch it pass by through binoculars or a small telescope — if you know where and when to look. The Heavens-Above website can help you get a fix on the fast-moving rock. First, go to the website's location database and find the nearest city. Click on the link for that city. Then, click on over to the 2012 DA14 sky chart and look for the asteroid's track, with notations that indicate observation times. If you don't see the asteroid's track, you won't be able to see the asteroid. In some cases, the track is shown during daylight hours — which would generally rule out visual observations.

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.