Nasa chief Charles Bolden has inspected a prototype spacecraft engine that could power an audacious mission to lasso an asteroid and tow it closer to Earth for astronauts to explore.
Mr Bolden checked on the progress a month after the Obama administration unveiled its 2014 budget that proposes 105 million dollars (£69 million) to jump-start the mission, which may eventually cost more than 2.6 billion dollars (£1.7 billion).
Engineers at the Jet Propulsion Laboratory (JPL) in southern California and Glenn Research Centre in Ohio are developing a thruster that relies on ion propulsion instead of conventional chemical fuel.
Once relegated to science fiction, ion propulsion - which fires beams of electrically charged atoms to propel a spacecraft - is preferred for deep space cruising because it is more fuel-efficient. Engine testing is expected to ramp up next year.
During Thursday's visit to the JPL campus, nestled in the foothills of the San Gabriel Mountains north-east of Los Angeles, Mr Bolden viewed an engineering model of the engine and peered through a porthole of a vacuum chamber housing the prototype.
Nasa is under White House orders to fly humans to an asteroid as a stepping stone to Mars. Instead of sending astronauts all the way to an asteroid, as originally planned, the space agency came up with a quicker, cheaper idea: haul the asteroid close to the Moon and visit it there.
The space agency would launch an ion-powered unmanned spacecraft to snare a yet-to-be-selected small asteroid in 2019 and park it in the Moon's neighbourhood. Then a spacewalking team would hop on an Orion space capsule that is currently under development and explore the rock in 2021.
Besides preparing astronauts for an eventual trip to Mars, Nasa said the asteroid-capture mission is designed to test technologies to deflect threatening space boulders on a collision course with Earth.
Scientists have said the redirected asteroid would pose no threat to Earth. If it inadvertently plunged through the atmosphere, it would burn up, they said.
Mr Bolden's JPL stop is part of his annual spring tour of Nasa centres around the country. His California journey began on Wednesday at the Dryden Flight Research Centre in the Mojave Desert where Sierra Nevada Corp is preparing its Dream Chaser spaceship for test flights later this year before it can make supply runs to the International Space Station.