* Obama wants $105 million to kick off project
* Asteroid could be put in orbit near moon
* Visit by astronauts targeted for 2021
President Barack Obama wants NASA to start work on finding a small asteroid that could be shifted into an orbit near the moon and used by astronauts as a stepping-stone for an eventual mission to Mars, agency officials said on Wednesday.
The project, which envisions that astronauts could visit such an asteroid as early as 2021, is included in Obama's $17.7 billion spending plan for the U.S. space agency for the 2014 fiscal year.
It is intended as an expansion of existing initiatives to find asteroids that may be on a collision course with Earth, and preparations for a human expedition to Mars in the 2030s.
"This mission allows us to better develop our technology and systems to explore farther than we've ever been before - to an asteroid and to Mars - places that humanity has dreamed about but has had no hope of ever attaining," NASA administrator Charles Bolden told reporters during a conference call.
"We're on the threshold of being able to tell my kids and my grandkids that we're almost there."
In 2010, Obama proposed that NASA follow the International Space Station program with a human mission to an asteroid by 2025. The agency has been developing a heavy-lift rocket and deep-space capsule capable of carrying astronauts beyond the station's 250-mile (400-km) high orbit.
The system would be capable of traveling to the moon, asteroids and eventually to Mars, the long-term goal of the U.S. human space program.
"I think the asteroid-retrieval mission lays out a place for us to go," Kennedy Space Center director Bob Cabana told reporters in a separate conference call.
"It does everything that needs to be done as far as developing the technologies and the skills that we need for exploration beyond planet Earth."
Obama's 2014 spending plan proposes $105 million to start work on the new mission, which entails finding a 23- to 33-foot (7 to 10-meter) wide asteroid and robotically towing or pushing it toward Earth so it ends up in a stable orbit near the moon.
Astronauts aboard an Orion capsule would then blast off, land on the asteroid and bring back soil and rock samples for analysis.
"The plan combines the science of mining an asteroid, along with developing ways to deflect one, along with providing a place to develop ways we can go to Mars," U.S. Senator Bill Nelson, a Florida Democrat, told reporters last week.
Obama's budget proposal calls for a doubling of the $20 million NASA currently spends hunting and tracking asteroids; adding $38 million to speed development of a solar electric propulsion system that would be used to move an asteroid; $40 million for work on rendezvous and capture technologies; and $7 million for hazard-avoidance systems.
NASA has not yet estimated the total cost of the mission, but expects it to be less than the $2.65 billion estimated last year by the California Institute of Technology's Keck Institute for Space Studies.
"We do not think at this point that it will be that expensive," NASA Chief Financial Officer Elizabeth Robinson told Reuters.
The Keck-led "Asteroid Retrieval Feasibility Study" proposed relocating a 500-ton asteroid closer to Earth to give astronauts a "unique, meaningful and affordable" destination in the next decade, meeting Obama's deadline.
Robinson said Keck's cost estimate did not take into account projects already under way at NASA and proposed retrieving a type of asteroid that orbits farther away which would require a longer and more expensive mission.
NASA also would look to partner with fledging space mining companies, such as startups Planetary Resources and Deep Space Industries, as well as agencies interested in planetary defense.
"Obviously we're looking all sorts of interests in this asteroid mission in terms of the kinds of scientific and industrial uses that could be spawned from it," Robinson said.
Interest in potentially threatening asteroids sky-rocketed after a small asteroid exploded over Chelyabinsk, Russia on Feb. 15, shattering windows and damaging buildings. About 1,500 people were injured by flying glass and debris.
The same day another larger asteroid passed about 17,200 miles (27,680 km) from Earth - closer than the television and communication satellites that ring the planet.
The incidents had created an imperative "to develop techniques and technology that will help deter or to keep an asteroid or other type of body from impacting Earth," Bolden said.
"One of the serendipitous results from this (asteroid-retrieval) flight we hope will be the demonstration of a capability to move an asteroid, to deflect it ever so slightly."
Obama is also requesting $822 million to support efforts to develop commercial space taxis in hopes of breaking Russia's monopoly on crew transportation to the space station by 2017. The United States has been unable to fly astronauts since it retired its space shuttle fleet in 2011.