In the first year of their hunt for killer asteroids, NASA and the U.S. Air Force have detected seven objects larger than half a mile across that could smash into the Earth, causing worldwide devastation.
NASA officials hastened to assure the public that none of those asteroids is on a course that would hit the Earth within at least the next 200 years.
However, they cautioned that only 10 percent of the sky has been surveyed for asteroids, and most of the potentially hazardous objects remain unknown.
The most recent discovery was made on July 5, Eleanor Helin, a veteran NASA asteroid hunter, said in Cambridge on Wednesday. A milewide “Earth-crossing asteroid” was spotted between the Earth and the sun in an orbit that will bring it within 5 million miles of our planet - a stone’s throw in astronomical terms.
The asteroid is 20 times bigger than the small comet that crushed hundreds of square miles of Siberian forest 90 years ago. It is one-tenth the size of the huge comet that plowed into the Caribbean and may have wiped out the dinosaurs 65 million years ago.
Helin said the new object, named 1997 NC1, joins a growing list of ‘potentially dangerous minor planets, although it does not pose an immediate threat to the Earth.”
When the survey is complete, about 20 years from now, scientists expect to have found at least 2,000 objects larger than half a mile in diameter in Earth-crossing orbits. Trackers already have seen some as big as 3 miles in diameter.
Most asteroids are confined safely to a belt between Mars and Jupiter, 300 to 400 million miles away. Astronomers say they believe about a third of them are dead comets that have lost their glowing tails. The rest are fragments of the original planetary disk that failed to coalesce into one of the regular planets in the solar system.
But some asteroids were born - or have drifted - into orbits closer to Earth. Those inside Earth’s orbit, like 1997 NC1, are constantly being jostled by the gravity of Venus and Mercury, making it impossible to predict their trajectory accurately.
According to Helin, the odds are that an object the size of the one that hit Siberia in 1908 will strike the Earth every 100 years. A monster half a mile or more across comes along every 100,000 or so years.
The trouble is, “the clock is running, and we don’t know where we are in that cycle,” she said.
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