Three aerospace firms have scored a total of $1.1 billion in NASA contracts to compete to build the next spacecraft to carry astronauts to the International Space Station after the shuttle program's end.

The awards, announced Friday, went to SpaceX, Boeing and Sierra Nevada Corporation, and aim to support efforts to return astronauts to space via US-made transport in the next five years.

The retirement of the US space shuttle fleet last year left Russia as the sole nation capable of transporting astronauts to the ISS, three at a time aboard its Soyuz capsules.

The latest three grants are part of NASA's Commercial Crew Integrated Capability (CCiCap) initiative, "intended to ultimately lead to the availability of commercial human spaceflight services for government and commercial customers," the US space agency said in a statement.

The awards amounted to $460 million for Boeing, $440 million for SpaceX and $212.5 million for Sierra Nevada, based in Colorado.

SpaceX is the leader of the pack so far, having successfully sent its Dragon capsule on the first private cargo mission to the ISS earlier this year.

The California-based company is working to refine the Dragon capsule so it can carry seven astronauts to the ISS by 2015.

Meanwhile, NASA is developing a multi-purpose crew vehicle and space launch system that may one day carry humans to deep space destinations like Mars or an asteroid.