A proposed mission to capture an asteroid and bring it into orbit in the Earth-moon system is a stepping stone to sending humans to Mars, NASA administrator Charles Bolden said Thursday at the Jet Propulsion Laboratory.

Bolden visited the La Cañada Flintridge facility to visit with asteroid experts and see an ion thruster — which could potentially nudge an asteroid — in action at the lab.

The public's interest in asteroids was heightened recently after JPL tracked a near-Earth object last year on the same day a fireball exploded over Russia, injuring hundreds and scattering meteorites across the region. But JPL has been working on identifying and tracking asteroids for years, as well as on the ion propulsion system that can move objects.

The Asteroid Retrieval Initiative would take about five to eight years from launch until the asteroid came into the Earth-moon orbit. A separate mission would then send astronauts to explore the captured asteroid and bring back samples.

The rock would be about 23 to 32 feet in diameter and slightly larger than the 340-ton "Levitated Mass" boulder on display at the Los Angeles County Museum of Art.

"This is a transportation problem like no other," said John Brophy, an electric propulsion engineer at JPL. "Space is big; asteroids are heavy."

The mission would use an unmanned spacecraft with four ion thrusters that would "lasso" the asteroid. The solar-powered propulsion system would ionize 12 tons of xenon gas and shoot the positive particles toward the asteroid at 80,000 mph.

JPL developed a prototype of the thruster with NASA's Glenn Research Center in Cleveland, which engineers showed off to Bolden at the lab.

"Moving an asteroid is hard, so you have to thrust on it for a long time with a very efficient device," Brophy said. "You can't have those devices wearing out, and they have to be able to process a lot of power. This is the only technology that can do that."

The mission will be an expensive one. NASA officials have so far set aside $105 million to study the proposed mission, which has an expected overall budget of $2.6 billion.

In 2010, President Obama expressed interest is sending astronauts to an asteroid, and eventually to the Red Planet. Obama set 2025 as a possible time frame in which to have humans explore the asteroid, but amid budget cuts at the agency, it didn't seem possible.

That's when engineers dreamed up a way to slash costs and bring an asteroid near the moon so humans could explore the rock by 2025.

"If you can't get to the asteroid, bring the asteroid to you," Bolden said. "So, sort of simplistically, that's what we're going to do."

After that, he added, the space agency would focus on sending humans to Mars, which Obama hopes will happen sometime in the 2030s.

"We're using this asteroid as a driver to filling the technological gaps that we need to put humans on Mars," Bolden said. "This is totally different than saying we're going to go back to the moon. We don't need to develop any of this technology to go back to the moon, but going back to the moon doesn't help us get to Mars."

--

Follow Tiffany Kelly on Google+ and on Twitter: @LATiffanyKelly.