Near Earth Objects (NEOs) are asteroidal or cometary bodies in the solar system whose orbits sometimes take them close to the earth's orbit. An NEO--and there are an estimated 4668 of them--could therefore someday collide with the earth. Since the impact of even a one-kilometer-sized NEO would probably destroy a state, NASA in 1998 set itself a ten-year goal of finding and measuring the parameters of 90% of the total estimated number of NEOs larger than one kilometer. The problem is knowing how many NEOs to expect, and when to feel safe that they have all been accounted for.
There are currently ten telescopes around the world dedicated to regular searches for NEOs, and their task is not a particularly easy one. NEOs are small and very faint until their orbits eventually bring them close to the sun, but then they are often difficult to see because they can lie in the direction of the bright sun.
In a new paper in this month's Icarus, SAO astronomer Timothy Spahr and three of his colleagues present a new simulator to estimate how many NEOs of different sizes there might be; they include in their computations the properties of the survey telescopes, allowing them to estimate how long it will take to detect these NEOs. They conclude that NASA's ten year goal was unrealistic, and that with the current instruments it will take until 2011 to account for 90% of the one-kilometer NEOs. They find that their simulator is remarkably accurate in estimating the number of NEOs that should have been detected so far, 763. Finally, they note that with the current facilities, about 8% of the kilometer-sized NEOs will not be detected even when extending the search time until 2016, and that space-based telescopes are needed to do the job more completely.