5:02 p.m. ET: New information from Chelyabinsk, Russia, where the meteor that exploded in the sky today is estimated to have been about 50 feet wide and weighed 7,700 tons, entering the atmosphere at about 40,000 mph.
"People started to panic. Somebody screamed 'the end of the earth!" said one man in Russian, interviewed by a television crew.
Most injuries, at least according to reports today, were injured when the fragments of the meteor exploded in midair, sending shock waves through the air and shattering windows for miles around.
"There was panic. People had no idea what was happening," said Sergey Hametov, a resident of Chelyabinsk, reached by The Associated Press. The city has a population of 1 million.
"We saw a big burst of light, then went outside to see what it was and we heard a really loud, thundering sound," he said.
ABC News' Kirit Radia, based in Moscow, reports that 20,000 Russian soldiers have been sent to the area to help residents recover.
4:40 p.m. ET:NASA managers, at a teleconference, say the meteor that broke up over Russia had the force of a 300 kiloton bomb, and exploded 12 to 15 miles above the ground.
But they had no warning, they said, because they simply don't have the resources to look for objects that size.
"Defending the Earth against tiny asteroids is not currently our goal," said Paul Chodas, research scientist in the Near Earth Object Program Office at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Even DA14 is on the small side. Small objects are very difficult to detect, and therefore defend against."
Bill Cooke, who runs the Meteoroid Environments Office at NASA's Marshall Space Flight Center in Huntsville, Ala., said some calculations have now been done for the object that broke up over Russia, and it's likely an interloper from the asteroid belt between Mars and Jupiter, deflected our way, probably by a close pass with another object sometime in its past.
"It took 2.1 years to go around the Sun once," he said, so it was out in the asteroid belt about a year ago."
Cooke pointed out that meteorite impacts - smaller ones - are really quite common: "The Earth intercepts 80 tons of material per day." Car-sized objects from space fall to Earth every month or so.
An event like today's, doing damage that people notice, happens every 50 to 100 years, said Chodas. This was the biggest and most destructive since the Tunguska impact in Siberia in 1908.
4:10 p.m. ET: Peter Diamandis is one of the founders of Planetary Resources, Inc., a start-up company with grand plans to track asteroids - and mine them for platinum, other precious metals, and the ingredients of rocket fuel.
"There are platinum-rich asteroids, some of them hundreds of times richer than all the mines on Earth," he said to me in a telephone interview. He paints a picture of small, cheap robotic probes, waiting in orbit for an asteroid to come near enough. They would swarm over the asteroid, scraping the good stuff from its surface for return to Earth.
And by the way, he said, "It's not our mission to be in the planetary protection business, but we'll have the technology in orbit ready to go." They'd be able, he said, to give an asteroid a needed nudge if it's headed our way.
Why take these guys seriously? Well, they have backing from Eric Schmidt and Larry Page of Google, James Cameron of "Titanic" and "Avatar" fame, and Ross Perot Jr., son of the former electronics executive turned presidential candidate.
Last year when they announced their plans, ABC News asked physicist Michio Kaku what he thought of them.
"I think they are part crazy and part genius," he said. "But the bottom line is, they're rich."
3:25 p.m. ET: NASA has now posted a message saying, "The trajectory of the Russian meteor was significantly different than the trajectory of the asteroid 2012 DA14, making it a completely unrelated object." From the videos that came from Chelyabinsk, they say the Russian meteor "was traveling from north to south. Asteroid 2012 DA14's trajectory is in the opposite direction, from south to north."
3:18 p.m. ET: What if DA14 had actually hit us? Astronomers say it would have had the power of a hydrogen bomb. The Tunguska impact in Siberia in 1908 was probably caused by an object of about the same mass, they say, and it flattened the tundra around for 50 miles in every direction.
That's why today is of interest to scientists who say we ought to be watching more actively for the junk wandering around the inner solar system. With enough advance notice, they say we can launch a simple space probe to avert danger.
We asked former astronaut Ed Lu, now with the B612 foundation in California, which is trying to raise private funds for a space telescope to scan for objects decades before they hit.
If one is spotted, do we blow it up the way Bruce Willis and his crew did in "Armageddon"? Astronaut Lu says no - some sort of "space tug" to give it a gentle nudge would be less dramatic but would do the job. He said if we're smart, we'll have many years' warning. In that case, the slightest diversion would be enough.
"People have these crazy ideas because they get their physics from Hollywood movies," said Lu. "Never get your physics from Hollywood movies. In general, you only need to change the velocity of an asteroid by a millimeter per second. That's about the speed an ant crawls."
Lu proposes sending a spacecraft that would essentially act as a tractor - nudging the asteroid off its crash course with Earth.
"All you need to do to do is run into it with a small spacecraft, or tug at it with a space tractor," said Lu. "We are not powerless to prevent this, that part is reasonably well understood, what we need to do is find out where the asteroids are and that is what we have to solve first."
2:50 p.m. ET: The asteroid will now gradually slow, pulled backward and to the side by the Earth's gravity. it will continue to orbit the inner solar system, though NASA's Near Earth Objects program says this was the closest approach to Earth that they've projected for it.
2:27: p.m. ET: We're all safe here at ABC Headquarters in NYC. Safe from the asteroid, that is. There's still that nasty flu going around.
2:25: p.m. ET: This is the moment of closest approach. Asteroid 2012 DA14 has just now come with 17,220 miles of Earth. Looks like we're still alive.
NASA's Near Earth Objects Office at the Jet Propulsion Laboratory in Pasadena, Calif., actually was able to calculate the asteroid's path almost a year ago - and has been reassuring people since then that there have been no major changes.
Remember that Earth is a bit more than 7,900 miles in diameter, so the asteroid clearly missed by a fair amount.
But when you remember that the moon is 239,000 miles away, 17,220 miles don't feel like a lot to astronomers.
2:08 p.m. ET: Closest approach for the asteroid is 2:25 p.m. ET. Hold on to your…
1:27 p.m. ET: ABC News' Kirit Radia, based in Moscow, passes on notes from a conversation with Paul Abell of NASA's Orbital Debris Office at the Johnson Space Center in Houston:
"This was the BIGGEST meteor to hit since the 1908 impact that flattened a large part of Siberia. NASA's preliminary calculation is that this thing weighed in at 7,700 tons (not 10 tons as Russian scientists first said).
"NASA's preliminary estimate is that this item was about 15 meters in diameter (the 1908 one was 50 meters), larger than the one that streaked across the skies of California last year (though that one was probably heavier due to its composition).
"The blasts we hear in the videos are NOT sonic booms caused by the meteor breaking the sound barrier. Rather it is the meteor exploding in the atmosphere. Subsequent booms are the smaller fragments breaking up.
"It was the force of those explosions that blasted out the windows, again NOT sonic booms.
"This thing was moving VERY fast as it entered the atmosphere…about 40,000 miles per hour."
1:14 p.m. ET: Everyone was surprised by the Russian meteorite impact today. Google confirms to us that it put together a playful Google Doodle for its homepage - the second letter "g" ducking to get out of the way of a passing space rock - but after the reports from Russia, they decided it would no longer be funny.
"Out of respect for those injured in the extraordinary meteor shower in Russia earlier today, we have removed today's doodle from the Google homepage," a Google spokesperson told ABC News. "The doodle was created to mark Asteroid 2012 DA14 passing Earth."
Joanna Stern has written a post HERE.
1:10 p.m. ET:A thought offered by James D. Gleason, a research scientist at the University of Michigan who mostly studies meteorites believed to have come from Mars:
"While fireballs explode in Earth's atmosphere and rain debris on the land and ocean with regularity, it is extremely rare for people to be injured in such an event," he says in a statement from the university. It's been pointed out that while the Earth has 7 billion people, they are clustered together - in cities, on coastlines, along rivers. Most of the Earth's surface is open ocean, tundra, farmland, mountains, etc., which is why the injuries in Russia are so unusual.
12:57 p.m. ET: The U.S. Geological Survey reports it detected the meteor explosion near Chelyabinsk, Russia. It posted information on its earthquake hazards page. As an earthquake, it had a magnitude of…zero.
Still, a lot of people reported injured as the object entered the atmosphere at supersonic speed and broke up in midair.
12:40 p.m. ET: Here's some insight on why, after all the attention given to asteroid 2012 DA14, the Russian meteor impact was a surprise. The answer, Gina Sunseri of our staff reports, is that we earthlings simply do not have the resources to track everything wandering our part of the solar system.
She spoke with former Apollo astronaut Rusty Schweikart, who's out to remedy that. He's one of the heads of the B612 Foundation, a nonprofit group trying to raise private funds to launch an asteroid-watching telescope into space. They would call it Sentinel, and it would look for objects smaller than 2012 DA14. The bigger ones are already being found and tracked - but there are still smaller ones that could do a lot of damage, say Schweikart and his colleagues.
"The truth of the matter is of all the asteroids that are out there and come near the Earth and can do harm and hit the Earth we only know one percent of them now; 99 percent of them, we don't even know where they are," Schweikart said.
"We've got to find those 99 percent of asteroids that are missing, DA14 was one of those missing asteroids up until a year ago; then it was discovered by some of the guys with their telescopes in Spain, and that's the only reason we know this one is going by. A year ago this one would have gone by and no one would ever have seen it, let alone know we could protect ourselves against it."
12:18 p.m. ET: NASA's Jet Propulsion Laboratory is running video showing some images of the asteroid as they come in, mostly from Australia, where it is night. Take a look:
12:02 p.m. ET: Just spoke with Prof. Jay Melosh of Purdue University, who said he's doubtful the Russian impact and the asteroid are related - but he'd like to know more before saying for sure.
"Is there any chance they're the same? Probably not, but we're not sure," he said. Based on the amateur video of the flying debris in Russia, it appeared to be coming from east to west - the asteroid is moving from south to north - but he'd like to look at the video more carefully.
All that said, it's quite possible that today will be a bad day for asteroid 2012 DA14. The Earth's gravity could force it to split in two or shed debris, most of which would follow the same path, missing us by 17,000 miles. "It's outside the Roche limit where objects are pulled apart, but not much."
11:24 a.m ET: We're not absolutely sure what crashed in Russia - the Russians are the ones calling it a fragmenting meteor - but most scientists say the Russian impact and the passing asteroid are not related. They're separate pieces of space debris. Astronomers we've spoken with say they're pretty sure the asteroid - called 2012 DA14 - is solid, and not shedding pieces. We are looking for info on the direction from which the objects came in Russia.
Bob Berman, an astronomer who writes for Astronomy magazine and works with the space website Slooh.com, pointed out that there's stuff hitting us all the time:
"Six times per hour, there are smaller objects that enter the atmosphere and do no harm," he said. "They're very much like the asteroid we're talking about, except that they're too small for us to worry about."
10:52 a.m. ET:The European Space Agency has posted an image of the asteroid about seven hours before closest approach to Earth. The picture is a negative, so it shows the stars as black dots on a white background. The asteroid, ESA says, is the straight-line streak running across the top center of the frame.
This was a three-minute exposure, and Herald's telescope was mounted to stay locked on the stars in the background, so what we're seeing is the asteroid moving from south to north. If it were elongated, tumbling quickly, or less than solid, the streak would vary in width over the three minutes. It's not.
[/caption] Asteroid2012 DA14 will miss the Earth today, scientists assure us - but it won't miss by much. At 2:25 p.m. ET it is expected to pass 17,220 miles from us, flying from south to north in a path that brings it closer than the geosynchronous satellites we use for communications and many weather observations.
We'll be live blogging as the asteroid goes by, with updates, images and commentary from NASA, independent astronomers and others. Keep it locked here for the latest.
What does it matter if a hunk of rock, about 150 feet across, goes flying harmlessly by? The problem is that every now and then, an object from space actually hits the Earth, with devastating consequences. The most famous, 66 million years ago, was much larger than 2012 DA14 - about six miles in diameter - kicking up so much dust and ash that it is believed to have finished off the last of the great dinosaurs.
There was an impact as well in 1908, striking near the Tunguska river in Siberia with the force of a two-megaton nuclear weapon. It leveled the tundra for 50 miles in every direction.
Scientists say they would rather not think about what would have happened if it had hit a city or coastline. That is why many of them think it is worth the effort to track near-Earth asteroids so that if one is on a crash course, we can do something about it.
So join us. While scientists say they're confident the Russian impact was unrelated to 2012 DA14, they also say the asteroid is something of a shot across our bow. Here comes….