To qualify, the bodies must be at least 500 feet (150 meters) wide and be capable of approaching Earth within 4.7 million miles (7.5 million kilometers).
By some accounts, the object that might have caused the Tunguska blast was not particularly large in the world of NEOs, highlighting the fact that larger objects need not hit populated areas to cause damage.
(Related: "Small Asteroid Caused Mysterious 1908 Blast, Study Says" [December 21, 2007].)
On average, mid-size meteor strikes occur once every 300 years, NASA estimates.
What's more, only a tiny fraction of Earth's surface is heavily populated, and about 70 percent is covered with water. So the odds of a direct strike on a populated area remain quite remote.
Still, meteorites that splash down in oceans can cause tsunamis. And land impacts even in remote locales can raise debris and dust that may alter climate by blocking sunlight and spawning acid rain.
"The chances are not large, but they are not zero either," said William Ailor, an NEO expert with the Aerospace Corporation, a federally funded research center in El Segundo, California.
Under the Radar
Alan W. Harris of the Space Science Institute in Boulder, Colorado, has participated in numerous studies supported by NASA to evaluate the NEO hazard.
He reported that impacts capable of causing global climate disasters that could claim a billion lives are likely to happen once every million years.
And once every billion years or so the possibility exists for a massive asteroid impact to wipe out much of the life on Earth.
Harris noted that survey work on large NEOs—those greater than 0.6 miles (a kilometer) in diameter—during the last decade has ruled out such apocalyptic events in the foreseeable future.
"The good news is we have conducted the 'Spaceguard' survey, which has detected most of these very large objects," he said.
"We think we've detected all the NEOs big enough to cause a mass extinction and pretty much eliminated that risk."
Meanwhile smaller objects that can cause significant destruction are currently flying under the radar.
That's why Rohrabacher and the Planetary Society have called for increased federal funding to find and track dangerous NEOs and develop strategies to deflect them.
"We now have the technology to prevent the next [strike]," said Ailor, the Aerospace Corporation's NEO expert. "But we have to be looking for it, and we're not looking at things that small."
Many NEO experts stress that the National Science Foundation's Arecibo Observatory in Puerto Rico is key to finding and tracking potential Earth-bound objects.
NEO hunters such as Don Yeomans, manager of NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, California, say that Arecibo is critical to their work.
It's not enough to know that an asteroid is headed toward Earth, Yeomans said.
"You have to know how big it is, its shape, its mass, and its rotation characteristics," he said. "One of the very few ways you can do that is with the radio telescope techniques."
But funding cuts mean that the massive radio telescope could soon be shut down.
As for deflecting NEOs that pose a threat, most experts agree that decades of lead time will be key to dealing with the problem.
"The main risk lies with those objects that we have not even found yet," the Space Science Institute's Harris said. "So the most important thing is to continue [surveillance]."
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