The following script is from "Cosmic Roulette" which originally aired on Oct. 6, 2013, and was rebroadcast on June 8, 2014. The correspondent is Anderson Cooper. Andy Court, producer.
For a long time, astronomers saw the asteroids and comets that come close to Earth as useless debris -- space rocks that blocked our view of distant galaxies. Not anymore. They're now viewed as scientifically important and potentially very dangerous if they were to collide with our planet. The odds of that happening on any given day are remote, but as we first reported last fall, over millions of years scientists believe there have been lots of impacts, and few doubt there are more to come. A former astronaut told us it's like a game of "cosmic roulette," and one mankind cannot afford to lose.
Concern over our ability to detect these objects grew after an incident in Russia last year, when an asteroid crashed into the atmosphere with many times the energy of the bomb dropped on Hiroshima, narrowly missing a city of one million.
This is video of that asteroid in Russia, barreling toward Earth at 40,000 miles an hour. It exploded into pieces 19 miles above and 25 miles south of the city of Chelyabinsk. People thought it had missed them entirely, until minutes later, when the shock wave arrived.
Shattering glass, crushing doors, and knocking some people right off their feet. More than a thousand were injured.
Anderson Cooper: How much warning did people in Chelyabinsk have?
Paul Chodas: None.
Paul Chodas is a scientist at NASA's jet propulsion laboratory in Pasadena, Calif. He and his boss Don Yeomans have been trying to track near-Earth objects for decades.
Paul Chodas: We didn't see it coming. It was coming from the general direction of the sun, so it was in the daytime sky as it approached.
Anderson Cooper: So how did you find out about it?
Paul Chodas: Twitter and YouTube-- when we first saw the images.
Anderson Cooper: So the first people at NASA that heard about it was Twitter?
Paul Chodas: Exactly.
Chodas says an object this size hits Earth once every hundred years, on average. Yet the same day, purely by chance, another asteroid twice as large came within 17,000 miles of Earth, passing between us and the satellites that are bringing you this broadcast. The only reason there was any advance warning was because an amateur astronomer in Spain, an oral surgeon by day, noticed it just before it moved out of view.
Amy Mainzer: We know about some of the most distant galaxies in the known universe, and yet, we don't really know everything that's right in our own backyard.
Amy Mainzer is a NASA scientist who focuses on detecting asteroids.
Amy Mainzer: So we gotta move the dome out of the way. And then we're gonna start to follow the asteroid as it tracks across the sky.
"We know about some of the most distant galaxies in the known universe, and yet, we don't really know everything that's right in our own backyard."
This telescope at the Table Mountain Observatory in California is one of dozens all over the world that are used to track and study near-Earth objects. Mainzer told us they're often very hard to find.
Amy Mainzer: Some of these asteroids are really, really dark. Darker even than coal in some cases. Kinda like the soot at the bottom of a barbecue grill.
Anderson Cooper: So you're looking for something that's darker than coal against a black sky.
Amy Mainzer: Exactly. And now you see the problem.
Another problem is that ground-based telescopes can't see objects coming from the direction of the sun because they're in the daytime sky, like the asteroid that hit Russia. Astronomers find asteroids by taking repeated pictures of the night sky and looking for things that change position. Professionals and amateurs all over the world work together, sharing information. Once Paul Chodas and his NASA colleagues have multiple sightings, they can predict an object's location as far as 100 years into the future.
The asteroid Amy Mainzer was observing the night we visited didn't look like much on her screen.
Anderson Cooper: That little thing?
Amy Mainzer: Yep, that's it.
...but it's nearly half a mile wide and capable of destroying an entire continent.
Anderson Cooper: So that's actually-- that's a huge asteroid.
Amy Mainzer: That's a huge asteroid. If something this size hit the Earth it would be devastating. It would be very bad.
Asteroids are composed mostly of rock; comets - ice and dust. They come in all shapes and sizes. Some look like small planets. Others -- giant dog bones. For a long time, nobody thought they were worth tracking at all.
Amy Mainzer: It wasn't thought that they really did hit the Earth. Astronomers debated for a long time about the nature of the craters on the moon. And--
Anderson Cooper: They thought that the craters on the moon were volcanic.
Amy Mainzer: Possibly, yeah. And it's only been fairly recently, within, you know, the last 50 years or so, that the field has really recognized that yeah, impacts actually do happen. And not only do they happen on geological timescales, you know, millions and billions of years, but on human timescales in some cases.
The last major asteroid to collide with Earth hit in 1908, in the Tunguska region of Siberia. It's believed to have been 40 yards wide and to have exploded in the air like a nuclear bomb, leveling 80 million trees in an area the size of metropolitan Washington. This crater in northern Arizona was created 50,000 years ago. It's one of more than 180 impact craters geologists have found so far. They think there are many more, hidden by water and vegetation-- more even than on the moon, because the Earth's gravity is greater.
The most famous impact of all is the one that may have wiped out the dinosaurs more than 65 million years ago. The theory is that an enormous asteroid or comet collided so violently with Earth it created a cloud of debris that blocked out the sun, killing off 75 percent of all species, and leaving behind a crater in Mexico more than 100 miles wide.
Don Yeomans: These are objects that were once in space...pieces of asteroids.
Yeomans and Chodas showed us some of the remarkable things that have fallen from the sky.
Anderson Cooper: This is a piece of Mars?
Don Yeomans: You have it in your hand. It wandered around the inner solar system for a few million years, and a 40-pound stone came down in Africa, about 10 feet from a farmer, in--
Anderson Cooper: Really?
Don Yeomans: --October of 1962.
Anderson Cooper: It's amazing, to think this is from Mars.
They played a trick on me as well.
Don Yeomans: Would you hand that one to me? That big one?
Anderson Cooper: That one?
Don Yeomans: Yeah. Come on.
Anderson Cooper: Oh my god.
This one was iron-nickel and heavy as an anvil. Not all asteroids are made of such dense stuff, but many contain high concentrations of valuable minerals, like platinum, that might some day be mined in space.
[President Obama: We'll start by sending astronauts to an asteroid.]
President Obama's budget for NASA includes a plan to capture a tiny asteroid so that astronauts could rendezvous with it by 2025. The idea is to perfect techniques needed to explore deep space and perhaps find a way to exploit the water resources that many comets and asteroids have.
Don Yeomans: You could extract water from them. You could break the water down into hydrogen and oxygen which is the most efficient form of rocket fuel. So asteroids may serve as the fueling stations and watering holes for future planetary exploration.
But as the scriptwriters of the Hollywood blockbuster "Armageddon" vividly imagined, asteroids have the potential to harm mankind as well.
For better or worse, this is what many of us know about near-Earth objects: that if Bruce Willis hadn't nuked one, it would have destroyed the world.
Anderson Cooper: I mean-- you see these movies with Bruce Willis where any asteroid is coming and is gonna destroy the world. Is that likely?
Don Yeomans: No. No. We've found 95 percent of the large ones and none of them represent a threat within the next 100 years or so.
Anderson Cooper: What about the other five percent?
Don Yeomans: We're still looking.
He's talking about objects over half a mile wide that are big enough to cause global destruction. The problem is there are lots of smaller objects -- over 40 yards in diameter -- that are unaccounted for and potentially very dangerous.
Ed Lu: If you look at the light green dot that's the orbit of the Earth.
Ed Lu is a former astronaut who spent six months on the International Space Station. He showed us a computer-generated representation of our solar system - that's the sun in the center and those green dots are 10,000 near-Earth objects astronomers have found so far.
Ed Lu: So these green dots are the asteroids that could hit the Earth.
Anderson Cooper: This is the-- about the 10,000 known asteroids?
Ed Lu: Yes. These are the 10,000 known asteroids. Here's the problem. There's about a factor of 100 more. The real solar system looks like this. And we know this because we've only been able to observe a small fraction of the sky, and we know that there's about 100 times more asteroids than we've found.
Anderson Cooper: Wait-- wait, this is all the asteroids that are--
Ed Lu: There are about a million asteroids large enough to destroy a city out there.
Anderson Cooper: And right now, we only know of what percent of those asteroids?
Ed Lu: About one half of one percent.
Anderson Cooper: Does it worry you that you only know one percent of these asteroids that are big enough to destroy a city?
Paul Chodas: Well, most of those are really small. And the odds are that many of these would hit in a remote area or they could hit in an ocean. So that is why the larger ones are those that we were paying attention to first. Now the next size range is the one to concentrate on, those that can cause, you know, continent-wide extinction or destruction.
Anderson Cooper: Yeah. That would be pretty good to prevent that-- continent-wide destruction.
Paul Chodas: Those are the next ones we'll continue to find those. And w-- and we work our way down to the small ones.
Anderson Cooper: But right now an object that could wipe out the eastern seaboard or New York City could be a day away and there's a very good chance we wouldn't know about it.
Don Yeomans: Well, we're working to make sure that we will know about it.
Anderson Cooper: But right now we wouldn't know about it.
Paul Chodas: It's possible, yes.
[Congressional hearing: The Committee on Science, Space and Technology will come to order. ]
NASA administrator Charles Bolden faced similar questions from Congress after the near-miss in Russia last year.
[Congressman: What would we do if you detected even a small one, like the one that detonated in Russia, headed for New York City in three weeks? What would we do?
Charles Bolden: If it's coming in three weeks -- pray. ]
Anderson Cooper: Is there anything you can do to deflect an asteroid that's-- that's going to hit besides evacuating a city?
Don Yeomans: If you find it 10 or 20 or 30 years in advance, then yes-- you could actually send a spacecraft up-- run into it, slow it down a millimeter or two per second so that in ten or 20 years when it was predicted to hit the Earth, it wouldn't.
Anderson Cooper: Just slam a spacecraft into it.
Don Yeomans: Just slam into it.
In 2005, NASA did just that, as an experiment, firing a small unmanned spacecraft into a comet called Tempel 1. But you can't deflect what you don't detect -- which is why former astronaut Ed Lu has taken on a new mission.
[Ed Lu: So here's the telescope that we're building...]
He's now chairman of the B612 Foundation, which has designed a space-based telescope to speed up the discovery of near-Earth objects. NASA's Amy Mainzer has been developing one too. Both telescopes would be able to find asteroids by using infrared sensors that detect heat rather than light. But a telescope like this would cost roughly half a billion dollars, and so far neither the United States nor any other government has committed significant funds. So the B612 Foundation is trying to raise the money privately by reaching out to individual donors.
Ed Lu: I don't think there's any other global catastrophe, global scale catastrophe that we can prevent. This is the only one that I know of. We can solve this particular issue for the cost of building a freeway overpass. I mean, and that's literally what it is.
Anderson Cooper: But nobody has been killed by an asteroid.
Ed Lu: Yeah. And what I'm saying is that you can't wait 'til that point afterwards, when you say, "We shoulda done it." You have to think of this as cosmic roulette, right?
Ed Lu: The phrase that they have in Vegas is that the house always wins, right? And you know, the sort of secret to all this is that we're not the house. Right? At some point, you know, the solar system's gonna get you.
Don Yeomans: They're very low probability events but very high consequence events.
Anderson Cooper: The problem it seems like is you're asking people to care about something which may not affect their lives, may not even affect their children's lives.
Don Yeomans: That's true. It's a tough concept to get across because, as you say, it's something that may not happen for another 100 years, 200 years. It may happen tomorrow morning.
After this story first aired, new research came out suggesting small asteroids crash into our atmosphere much more frequently than previously thought. This year, Congress increased NASA's budget for asteroid detection, but there's still not enough money, public or private, to launch one of those proposed infrared telescopes into space.