That is because even though Congress assigned the space agency that mission four years ago, it never gave NASA the money to build the necessary telescopes, according to the report released Wednesday by the National Academy of Sciences.
Specifically, the mission calls for NASA, by the year 2020, to locate 90 percent of the potentially deadly rocks hurtling through space. The agency says it has been able to complete about one-third of its assignment with the current telescope system.
NASA estimates that there are about 20,000 asteroids and comets in our solar system that are potential threats. They are larger than 460 feet in diameter - slightly smaller than a sports stadium in New Orleans. So far, scientists know where about 6,000 of these objects are.
Rocks between 460 feet and 3,280 feet in diameter can devastate an entire region, said Lindley Johnson, NASA's manager of the near-Earth objects program. Objects bigger than that are even more threatening, of course.
Just last month astronomers were surprised when an object of unknown size and origin bashed into Jupiter and created an Earth-sized bruise that is still spreading. Jupiter does get slammed more often than Earth because of its immense gravity, enormous size and location.
Disaster movies like "Armageddon" and near misses in previous years may have scared people and alerted them to the threat. But when it comes to monitoring, the academy concluded "there has been relatively little effort by the U.S. government."
And the United States is practically the only government doing anything at all, the report found.
"It shows we have a problem we're not addressing," said Louis Friedman, executive director of the Planetary Society, an advocacy group.
NASA calculated that to spot the asteroids as required by law would mean spending about $800 million between now and 2020, either with a new ground-based telescope or a space observation system, Johnson said. If NASA got only $300 million it could find most asteroids bigger than 1,000 feet (300 meters) across, he said.
But so far NASA has gotten neither sum.
It may never get the money, said John Logsdon, a space policy professor at George Washington University.
"The program is a little bit of a lame duck," Logsdon said. There is not a big enough group pushing for the money, he said.
At the moment, NASA has identified about five near-Earth objects that pose better than a 1-in-a-million risk of hitting Earth and being big enough to cause serious damage, Johnson said. That number changes from time to time, as new asteroids are added and old ones are removed as information is gathered on their orbits.
The space rocks astronomers are keeping a closest eye on are a 430-foot (130-meter) diameter object that has a 1-in-3,000 chance of hitting Earth in 2048 and a much-talked about asteroid, Apophis, which is twice that size and has a one-in-43,000 chance of hitting in 2036, 2037 or 2069.
Last month, NASA started a new Web site for the public to learn about threatening near-Earth objects.