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 (April 10).

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.

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.

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.

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.

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.

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