NASA astronaut and former University of Hawaii solar physicist Edward Lu is calling for a new spacecraft that would divert asteroids on a path to slam into Earth.

The small space tractor, estimated to cost between $200 million US and $300 million, would hover near an asteroidand exert enough gravitational pull that the space rock's orbit would change and a collision with our planet would be averted, Lu said in a speech at the University of Hawaii-Manoa Monday night.

"We're only trying to get a really tiny change in the velocity of the asteroid to prevent an impact," he said.

Lu was part of a panel including three Hawaii scientists who characterized the chances of an asteroid colliding with Earth as rare, but deserving of the same level of attention as major earthquakes, tsunamis and hurricanes.

Impending threats

Objects the size of a grain of sand frequently hit the Earth's atmosphere, appearing as shooting stars in the night sky. But a larger impact could be devastating. Asteroids are blamed for the death of the dinosaurs 65 million years ago and an explosion over Tunguska, Russia, in 1908 that wiped out 60 million trees over a 1,335-square-kilometre area.

Sky-watchers track the paths of known asteroids. The asteroid Apophis, for example,will pass within about32,000kmof Earth on Friday, April 13, 2029.

"It's going to come so close to the Earth in 2029 that its orbit will change and it might change enough so that it comes back and hits us in 2036," said Hawaii planetary astronomer David Tholen, who discovered Apophis. During the asteroid's next close pass to the sun in 2013, that risk will be assessed in radar surveys, he said.

Eyes on the sky

The University of Hawaii's Pan-STARRS program is working on a project that would train four powerful digital cameras toward the heavens to watch for asteroids. Officials from the project are hoping to garner public support for a plan to locate on Mauna Kea in Hawaii, butenvironmentalists and Hawaiian activists have argued against additional development there and some scientists have expressed concern about additional construction, as the volcano already hosts 13 telescopes.

The telescopes also could be built at two sites on Haleakala, where a prototype is alreadybeing constructed, but scientistssay the project would take twice as long to complete there.

The program would be able to provide decades of warning of an impending impact, the scientists said. That would be enough time to launch a tractor spacecraft to knock the asteroid into a safe orbit, said Lu, who spent six months aboard the International Space Station in 2003 and was a postdoctoral fellow at the University of Hawaii's astronomy institute from 1992 to 1995.

"If we are wiped out by an asteroid, that will be our own fault at this point," he said during his presentation.