© REUTERS/NASA/JPL-Caltech/HandoutThe passage of asteroid 2012 DA14 through the Earth-moon system, is depicted in this handout image from NASA.
That's about all the United States - or anyone for that matter - could do at this point about unknown asteroids and meteors that may be on a collision course with Earth, Bolden told lawmakers at a U.S. House of Representatives Science Committee hearing on Tuesday.
An asteroid estimated to be have been about 55 feet in diameter exploded on February 15 over Chelyabinsk, Russia, generating shock waves that shattered windows and damaged buildings. More than 1,500 people were injured.
Later that day, a larger, unrelated asteroid discovered last year passed about 17,200 miles from Earth, closer than the network of television and weather satellites that ring the planet.
The events "serve as evidence that we live in an active solar system with potentially hazardous objects passing through our neighborhood with surprising frequency," said Representative Eddie Bernice Johnson, a Texas Democrat.
"We were fortunate that the events of last month were simply an interesting coincidence rather than a catastrophe," said Committee Chairman Lamar Smith, a Texas Republican, who called the hearing to learn what is being done and how much money is needed to better protect the planet.
Comment: At least one eyewitness has reported that the loud boom came from a meteor:
Bright blue fireball reported across several states, including loud boom above New York, 19 March 2013