The rationale was statistical. They knew about a number of close
calls and calculated that many other rocky threats whirling about
the solar system had gone undetected.
In 1996, with little fanfare, the U.S. Air Force also began scanning the skies for speeding rocks, giving credibility to an activity once seen as reserved for doomsday enthusiasts. It was the world's first known government search.
NASA took a lead role with what it called the Spaceguard Survey. In 2007, it issued a report estimating that 20,000 asteroids and comets orbited close enough to the planet to deliver blows that could destroy cities or even end all life. Today, with limited financing, NASA supports modest telescopes in the Southwestern United States and in Hawaii that make more than 95 percent of the discoveries of the objects coming near the Earth.
Scientists lobbied hard for a space telescope that would get high above the distorting effects of the Earth's atmosphere. It would orbit the Sun, peering across the solar system, and would have a much better chance of finding large space rocks.
But with the United States immersed in two wars and other earthly priorities, the government financing never materialized. Last year, Dr. Lu, who left the NASA astronaut corps in 2007 to work for Google, joined with veterans of the space program and Silicon Valley entrepreneurs to accelerate the asteroid hunt.
The B612 Foundation refers to its planned telescope as the world's first private mission to deep space. Private groups, it says on its Web site, can carry out "audacious projects that previously only governments could accomplish -- and at lower cost."
The plan is to launch a large telescope known as Sentinel that can find 90 percent of the asteroids larger than 140 meters, or 460 feet, in diameter that pass through the Earth's part of the solar system. They also want to discover smaller asteroids down to a diameter of 30 meters.
Such asteroids are much bigger than the meteor that hit the atmosphere over Russia, and the telescope would thus be blind to those kinds of smaller threats.
Last October, the B612 Foundation, based in Mountain View, California, signed a contract with Ball Aerospace to create prototype sensors for the Sentinel mission. The space telescope is to have a diameter of 50 centimeters, or 20 inches. In theory, the system could be ready for launching by 2017 or 2018.
In an interview, Dr. Lu said the overall cost of the mission was now estimated at $450 million, which covers its launching, insurance and operations. The group, far from that goal, has been soliciting money from citizens.
The close approach to Earth of an asteroid named 2012 DA14 on Friday, as well as the much smaller object whose shock wave broke windows and bones in Russia, prompted thousands of hits to the foundation's Web site and Twitter account, said Diane Murphy, a spokeswoman for the group.
"Everybody is calling," she said. "They see us as the solution. They're saying, 'When are you going to have the telescope up?"'
B612 is just one player. Last April, Planetary Resources unveiled plans to mine asteroids that zip close by Earth, both to provide supplies for future interplanetary travelers and to bring back metals like platinum.
The venture attracted some big-name investors, including Larry Page and Eric Schmidt of Google. The company also has plans to develop telescopes that would hunt for rocky intruders coming near the planet.
Dr. Green of NASA said the agency was preparing to launch a mission in 2016 that will fly to an asteroid and, in 2023, return a sample to Earth for detailed analysis. The insights are expected to help scientists learn more about the makeup of the threats whizzing through the cosmic shooting gallery.
"If you're going to protect the planet, you have to know your enemy," he said. "You have to get up close and personal."
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