The newly discovered asteroid 2002 EM7 spends much of its time inside Earth's orbit during its 321-day circuit around the sun. On March 8, the asteroid came almost as close to Earth as the moon. Click here to see a 3-D version of the orbit from NASA's Jet Propulsion Laboratory.
Asteroid takes Earth by surprise
Close call went undetected until after it happened
By Alan Boyle
March 19 — An asteroid as wide as a Boeing 747 narrowly missed Earth this month — and we never knew it was coming. The case of asteroid 2002 EM7 has drawn attention to the gaps in the planet’s infant system for monitoring potential threats from space.
AT ITS CLOSEST, the space rock was about 288,000 miles (463,000 kilometers) from Earth on March 8, according to asteroid-watchers at NASA’s Jet Propulsion Laboratory and Italy’s University of Pisa. That’s just a bit farther away than the moon — spitting distance in astronomical terms.
But it wasn’t detected until four days later, by the Massachusetts Institute of Technology’s Lincoln Laboratory. There are two reasons why it was completely missed, scientists say.
For one thing, the rock came at us literally “out of the blue,” from the big blind spot on Earth’s sunward side. Objects that pass through Earth’s orbit almost always have to be spotted in the night sky first.
“You have to remember that the objects are only in that ‘blind spot’ for a non-infinite time,” said Gareth Williams of the Harvard-Smithsonian Center for Astrophysics. “The key is to detect them while they’re outside the blind spot.”
The second problem has to do with the asteroid’s size. Asteroid 2002 EM7 is thought to be 165 to 330 feet (50 to 100 meters) wide, or in the same ballpark as the 196-foot wingspan of a 747. That’s less than a tenth as wide as the asteroid that may have killed off the dinosaurs 65 million years ago, but still big enough to create a blast as powerful as a nuclear bomb if it were to hit Earth.
“It’s the most likely size of object that’s going to hit us in our lifetime,” said Benny Peiser, an anthropologist at Liverpool John Moores University who is an expert on the social impact of cosmic collisions.
It’s also the hardest size to spot. An asteroid that small is so faint that it can’t be seen unless it comes very close to Earth.
“It’s possible to spot such an object five, six days before it hits the atmosphere,” Peiser told MSNBC.com. “But it’s highly unlikely, because the search programs aren’t looking for them, and technically, they are so faint and small.”
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THERE’S MORE WHERE THAT CAME FROM
Today, enough observations have been made of 2002 EM7 to calculate its orbit in detail, and astronomers see no significant chance of the asteroid hitting Earth in the next century. But there could be hundreds of thousands of similar-sized objects crossing Earth’s orbit, Peiser said.
One asteroid, a little smaller than 2002 EM7, blew up in the atmosphere over a remote region of Siberia known as Tunguska in 1908 and flattened trees for hundreds of square miles around the blast point. A similar-size asteroid, made of iron, blasted out the 4,000-foot-wide (1,200-meter-wide) Meteor Crater in Arizona 50,000 years ago.
If something as big as 2002 EM7 were to come down over New York, it would have an effect far more devastating than the Sept. 11 terror attacks. The chances of that happening are, well, astronomical. But Peiser argues that even a Tunguska-level blow-up in the remote Pacific or the Arctic would pack a powerful psychological punch.
“If you think about 9/11, and the kind of knock-on effect on the country, and indeed the whole world ... the psychological, political and sociological effects can be much worse than the physical effects. People would be traumatized. They’d feel let down by the government, let down by NASA and the scientists,” he said. “There would be immediate blame laid on the scientific community for not doing enough.”
On that score, Peiser said there just might be a benefit to the recurring asteroid alerts that began four years ago — about the time that the Hollywood movies “Deep Impact” and “Armageddon” came out. Scientists as well as the general public are becoming more aware of the potential threat, and the limitations of today’s monitoring systems.
"All these stories about near misses, in a way, have a positive effect in that people get used to the idea that we might one day actually be hit by an object,” he said.
In the past four years, astronomers have put more resources into tracking the biggest asteroids that could cross Earth’s orbit, but it will take much more effort to extend the monitoring system down to the level of 2002 EM7.
“We need bigger telescopes with wide fields,” Williams said. And even if telescopes get bigger and more sensitive, “we may be limited fundamentally by the physics of the detectors.”
Peiser voiced confidence that “within the next 20-some years we will have satellite-based search programs that will be able to detect objects that come out of the blue.”
“That loophole, I am confident, will eventually be closed.”
At the same time, scientists are studying comets and asteroids with an eye toward developing the best plan for diverting any big ones that might come our way.
“For the time being, we just have to cross our fingers,” Peiser said.
Crossing fingers and watching the skies has become second nature for asteroid-watchers like Williams.
“I certainly wouldn’t worry about this. Eventually we are going to get hit by something Tunguska-sized, but I’m not losing any sleep over this,” Williams said.