WASHINGTON, March 25-- U.S. space agency NASA announced Wednesday it has chosen to capture a boulder from a larger asteroid rather than move a single smaller asteroid for its ambitious Asteroid Redirect Mission (ARM) that will test capabilities needed for future human expeditions to Mars.

NASA has identified three valid candidates for the 1.25-billion- U.S.-dollar mission so far: Itokawa, Bennu and 2008 EV5, but the space agency said a decision may not be made until 2019, about a year before launching the robotic spacecraft.

NASA seemed to have a special interest in 2008 EV5, which is a carbonaceous chondrite asteroid, but still expected to identify one or two additional candidates each year leading up to the mission.

Following its rendezvous with the target asteroid, the uncrewed ARM spacecraft will deploy robotic arms to capture a four-meter boulder from its surface and then redirect it into orbit around the moon in six years, NASA said.

In 2025, NASA's Orion spacecraft will launch on the agency's Space Launch System rocket, carrying astronauts on a mission to rendezvous with and explore the boulder.

The current concept for the crewed mission component of ARM is a two-astronaut, 24-25 day mission.

"The option to retrieve a boulder from an asteroid will have a direct impact on planning for future human missions to deep space and begin a new era of spaceflight," NASA Associate Administrator Robert Lightfoot said.

One of the key technologies that will be tested during the mission is Solar Electric Propulsion (SEP), which converts sunlight to electrical power through solar arrays and then uses the resulting power to propel charged atoms to move a spacecraft.

NASA said future SEP-powered spacecraft could pre-position cargo or vehicles for future human missions into deep space, either awaiting crews at Mars or staged around the moon as a waypoint for expeditions to the Red Planet.

Before the asteroid boulder is moved to lunar orbit, NASA will use the opportunity to test planetary defense techniques to help mitigate potential asteroid impact threats in the future.

In 2005, NASA's Deep Impact comet science mission tested technology that could assist in changing the course of a near- Earth object using a direct hit with a spacecraft.

The ARM robotic spacecraft will open a second option for planetary defense using a technique called a gravity tractor. According to NASA, the spacecraft will spend about a year in the so-called halo orbit to see if the asteroid's trajectory will be deflected through the gravitational attraction.

This crewed mission will further test new capabilities, including new sensor technologies and a docking system that will connect Orion to the robotic spacecraft carrying the asteroid boulder.

Astronauts will conduct spacewalks outside Orion to study and collect samples of the asteroid boulder wearing new spacesuits designed for deep space missions, NASA said.

In addition, collecting these samples will help astronauts and mission managers determine how best to secure and safely return samples from future Mars missions.