Nasa will pay more than $1bn to three companies to develop commercial spaceships to fly to the International Space Station.
The lion's share of the $1.1bn allotted for the next phase of Nasa's so-called 'Commercial Crew' programme will be split between Boeing and Space Exploration Technologies, a privately held firm run by Internet entrepreneur Elon Musk.
Boeing will receive $460m to continue developing its CST-100 capsule, which is intended to fly aboard a United Launch Alliance Atlas 5 rocket.
ULA is a partnership of Boeing and Lockheed Martin.
Space Exploration Technologies, or SpaceX, was awarded $440m to upgrade its Dragon cargo capsule, which flies on the firm's Falcon 9 rocket, to carry people.
In May, a Dragon capsule became the first privately owned spacecraft to reach the station, a $100bn outpost that flies 240 miles above Earth.
The test flight was part of a related Nasa programme to hire commercial companies to fly cargo to the station.
Privately held Sierra Nevada Corp received a partial award of $212.5m for work on its Dream Chaser, a winged vehicle that resembles a miniature space shuttle, and which also launches on an Atlas 5 rocket.
All three firms are prior recipients of Nasa space-taxi development work.
Since the space shuttles were retired last year, Nasa is dependent on partners Russia, Europe and Japan to reach the station. Russia will remain the sole entity capable of flying crew until US companies develop systems, which Nasa hopes will be within five years.