NASA's goal is to find 90 percent of "Near Earth Asteroids" larger than 1000 yards (1 kilometer) by 2009. In recent years the discovery rate has improved dramatically. In figures just released by NASA, the LINEAR system in New Mexico still dominates the discovery statistics.

Discoveries of large Near Earth Asteroid by major search programs

Near Earth Asteroids (NEA) orbit the sun in the same region as Earth. But in most cases, the orbits are highly elongated and stretch out to the asteroid belt between Mars and Jupiter. Recent estimates suggest there are about 900 of these objects larger than 0.6 mile (1 kilometer) in diameter. A collision with a space rock of this size could destroy civilization. In an e-mail newsletter posted this week, David Morrison from NASA reports that 410 large NEAs have now been found. He points out that this is just past the halfway mark -- assuming that the 90-percent goal is achieved when 810 large NEAs have been found.

This may be a halfway point when measured by the number of objects discovered, but this is not the same as measuring the time it takes to reach the goal. Although the graph of discovery rate suggests this goal can be achieved by 2009, the task will become more difficult once the remaining easily-detected NEAs have been found. More telescopes will probably be needed to keep the graph on target.

The claim that a milestone has been reached is expected to draw a mixed reaction from scientists. Also, smaller NEAs and fast-moving comets are also still a threat. A collision by a NEA just 200 yards (180 meters) across could devastate a small country, disrupt global weather for months and, perhaps, bring down the global economy.

Earlier this year NASA's Don Yeomans told SPACE.com: "The current [NASA] plan is to find the big ones first, then as the detection technology improves, extend the search to smaller and smaller objects."

But it would likely take a major international effort to find a good proportion of the remaining threatening objects. Scientists are concerned about a lack of a major international program to follow-up on discoveries and to ensure they are not lost. In the words of astronomer Duncan Steel, "It is like throwing the needles back into the haystack."