One of the largest asteroids known to have made a close approach to Earth flew past about 300,000 miles away on March 8, but nobody noticed it until four days later. When the object, which has been named 2002 EM7, passed closest to the Earth, it was too close to the Sun to be visible. A telescope operated by the Lincoln Laboratory at M.I.T. first recorded the new asteroid on March 12, as it moved away from the Earth and more of its bright side came into view.

Asteroids approaching from a blind spot cannot be seen by astronomers. If an object passed through this zone on a collision course with Earth, it would not be identified until it was too late for any intervention.

Astronomers have made numerous pleas in recent years for more funds to catalogue near-Earth objects and their orbits. This would reduce the number of unknown objects that could possibly take us by surprise, and give us early warning about potential future collisions.

Further observations by Timothy Spahr of the Harvard-Smithsonian Center for Astrophysics revealed that 2002 EM7 has a 323-day orbit. It is invisible to the naked eye, and is too small to be classed as a ?potentially hazardous asteroid.? But it?s probably between 55 and 100 yards across, meaning it?s bigger than the object that exploded in 1908 over the Tunguska region of Siberia, flattening trees for 1,200 square miles.

Brian Marsden of Harvard-Smithsonian says preliminary calculations indicate that 2002 EM7 will have several more chances to hit the Earth during the next century, with odds of one in six million to one in a billion of an actual impact.

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NASA announced this week a new Web-based asteroid monitoring system called Sentry that can monitor and assess the threat of space rocks that could possibly strike the Earth.It will help scientists communicate with each other about their discoveries of new, potentially threatening asteroids.

While no large asteroid is currently known to be on a collision course with our planet, experts say an eventual impact is inevitable and the consequences could be serious, up to and including global devastation that might destroy civilization as we know it. The odds of such an impact in any given decade are extremely low, and most experts agree that there would likely be at least 10 years of warning if such an object were ever spotted.

However, smaller asteroids like 2002 EM7 are more likely to hit Earth in any given year and could cause significant local or regional damage. The odds of a locally or regionally destructive asteroid hitting an inhabited area in a given 50-year period are about 1-in-160, according to experts.

In recent years, asteroid experts worldwide have struggled to develop a system to catalogue and track newly spotted Near Earth Asteroids and communicate any possible threats to the public.

However, asteroids move so slowly against the background of stars that when one is first discovered, astronomers cannot pin down its exact path. Recent false alarms, when scientists said there was a threat that a particular asteroid would hit Earth in a certain year, have made headlines and frightened the public.

The new Sentry system, developed over the past two years, is operated out of NASA’s Jet Propulsion Laboratory. The system’s online ?Risks Page? listsed 37 asteroids last week.

?Objects normally appear on the Risks Page because their orbits can bring them close to the Earth?s orbit and the limited number of available observations do not yet allow their trajectories to be well-enough defined,? says JPL?s Donald Yeomans, manager of NASA?s Near-Earth Object Program Office, which oversees Sentry. ?By far the most likely outcome is that the object will eventually be removed as new observations become available, the object?s orbit is improved, and its future motion is more tightly constrained.? Several new asteroids will be added to the list each month, only to be removed to the ?no-risk? page soon afterwards.

A color-coded measurement called the Torino Scale, developed in 1999, gives each asteroid a number. Zero or one represents a remote risk, and a 10 means there will be an impact. All but one of the asteroids currently on the Sentry list are zeros on the Torino Scale. At the top of the list, however, is space rock 2002 CU11, discovered February 7. It presently has a 1-in-100,000 chance of hitting the Earth on August 31, 2049. But as its orbit is refined, it?s possible this asteroid, like many others, will be re-categorized as harmless.

Asteroid detections have increased in recent months because Congress has asked NASA to find 90 percent of all Near Earth Objects larger than 0.6 miles by 2008. About 500 of the these asteroids have been found, and an estimated 500 remain undiscovered.

To see the list of Sentry asteroids,click here.

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