WASHINGTON, March 11  NASA can find and track most of the nearby asteroids that could hit and damage the Earth, but there is not enough money in its budget to finish the project within a 15-year deadline mandated by Congress, according to an agency report released Friday.

The report said there were about 20,000 asteroids and comets orbiting relatively close to our planet that could deliver blows ranging from destroying cities to ending all life.

These objects, 150 yards to more than a mile in diameter, represent about 20 percent of the asteroids and comets whose paths routinely pass between the Sun and the Earth’s orbit, it said. Rather than trying to detect, track, catalog and characterize all of the more than 100,000 “near Earth objects,” as Congress asked in a 2005 NASA authorization bill, the study said it would be more realistic to focus on those representing a real potential hazard.

But accomplishing this by 2020 would require using ground-based telescopes sponsored by other agencies for other purposes, possibly building a dedicated observatory for finding and tracking hazardous bodies and launching a spacecraft to observe the space around Earth from Venus. Such an undertaking, the report said, would cost more than $1 billion that the agency does not have.