CAPE CANAVERAL, Florida (Reuters) - NASA plans to add a $370 million unmanned test flight of a new deep-space capsule designed to send astronauts to asteroids, the moon, Mars and other destinations in the inner solar system, officials said on Tuesday.

The Orion capsule, which is being developed by Lockheed Martin Corp, would fly aboard a Delta 4 or Atlas 5 rocket from Cape Canaveral Air Force Station in 2014, said NASA spokesman Josh Byerly.

The test capsule would be launched into an orbit that soars as high as 5,000 miles above the planet. After circling Earth twice, it would slam back into the atmosphere at more than 20,000 mph, eclipsing the 17,500 mph speed of a returning space shuttle.

The fastest a spaceship has returned to Earth with astronauts aboard was the 24,700 mph re-entry of Apollo 10 in 1969, Byerly added.

"We will learn through this test that Orion would survive re-entry, being as fast as it is," he said.

The unmanned test, which will cost NASA an extra $370 million including the launch vehicle, will bump back a test of Orion's launch abort system from 2015 to 2016.

Along with developing Orion, a legacy program from NASA's canceled Constellation moon exploration initiative, the United States is developing a heavy-lift rocket based in part on space shuttle hardware. The goal of the program is to send astronauts to explore an asteroid by 2025.

NASA already has spent about $5 billion on the Orion program.

(Editing by Tom Brown and Eric Beech)