Near-Earth Objects (NEOs) are comets and asteroids that have been nudged by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth's neighborhood. Composed mostly of ice and dust, comets originally formed in the cold outer planetary system, while most of the rocky asteroids formed in the warmer inner solar system between the orbits of Mars and Jupiter.
The scientific interest in comets and asteroids is due largely to their status as the relatively unchanged remnant debris from the solar system formation process some 4.6 billion years ago. The four giant outer planets formed from an agglomeration of billions of comets; leftover bits and pieces from this formation process are the comets we see today. Likewise, today's asteroids are the bits and pieces left over from the initial agglomeration of the four inner planets.
The Earth has always been subject to impacts by comets and asteroids, although big hits are very rare; the last big impact was 65 million years ago, theoretically leading to the extinction of dinosaurs. Today, NASA regularly detects, tracks and characterizes asteroids and comets passing relatively close to Earth using both ground- and space-based telescopes.
The Near-Earth Object Observations Program, commonly called Spaceguard, discovers and characterizes these objects, and predicts their paths to determine if any could be potentially hazardous to our planet. They have already determined that there are no threatening asteroids as large as the one that killed the dinosaurs.
“The risk of a really large asteroid impacting the Earth before we could find and warn of it has been substantially reduced,” said Tim Spahr, the Director of the Minor Planet Center at the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts. The situation is different for the mid-size asteroids, which could destroy a metropolitan area if they were to impact in the wrong place.
New observations by NASA's Wide-field Infrared Survey Explorer, or WISE, show there are significantly fewer near-Earth asteroids in the mid-size range than previously thought. The results of the survey project called NEOWISE find a larger decline in the estimated population for mid-size asteroids than what was observed for the largest asteroids. Astronomers now estimate there are roughly 19,500, not 35,000, mid-size near-Earth asteroids; they say this improved understanding of the population may indicate the hazard to Earth could be less than previously thought.
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(Nasa's Wide-field Infrared Survey Explorer. Wikipedia)
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