NASA calculated that to spot the asteroids as required by law would cost 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 across, he said. But so far NASA has gotten neither sum
A comet or asteroid as small as 30 metres across is thought to have exploded in the atmosphere over Siberia in 1908, unleashing hundreds of times the energy of the nuclear bomb dropped on Hiroshima, Japan, in 1945, and flattening trees in a zone dozens of kilometres across.
Astronomers have now found 784 asteroids larger than 1 kilometer across (about 2+ billion tons), mostly using telescopes funded by NASA. That works out to 83 per cent of the 940 estimated to be out there by astronomer Alan Harris of the Space Science Institute in Boulder, Colorado.
Congress told NASA in 2005 to find 90 per cent of the near-Earth asteroids larger than 140 metres (460 feet) across by 2020.
From Discovery channel news, NASA estimates that there are about 20,000 asteroids and comets in our solar system that are potential threats to Earth. They are larger than 460 feet in diameter -- slightly smaller than the Superdome in New Orleans. So far, scientists know where about 6,000 of these objects are.
The report also points out that existing surveys are designed to gradually build up a catalogue of near-Earth objects over time, not to watch out for incoming asteroids that are just days or weeks from colliding with our planet.
Small asteroids could easily slip past existing surveys unnoticed until the moment of collision because telescopes currently devoted to the task are only capable of imaging a small part of the sky each night. And even then, clouds can prevent them from spotting asteroids, says Timothy Spahr of the Minor Planet Center in Cambridge, Massachusetts, a central clearinghouse for asteroid and comet data.
Asteroids approaching from the direction of the sun would also be missed, at least by ground-based telescopes.
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.