The U.S. space agency awarded $10 million to privately owned Sierra Nevada Corp, $9.99 million to Boeing and $9.59 million to privately owned Space Exploration Technologies to help the firms get their vehicles certified to fly to the station.
Since the retirement of the space shuttles last year, NASA has been dependent on Russia to fly astronauts to the $100 billion orbital outpost, a research laboratory owned and operated by 15 countries that flies about 250 miles above Earth.
The companies have separate agreements with NASA to develop space transportation systems, with the aim of breaking Russia's monopoly by 2017. Boeing was awarded $460 million for its CST-100 capsule, Space Exploration Technologies received $440 million to upgrade its Dragon cargo capsule to carry people and Sierra Nevada was awarded $212.5 million for work on its winged Dream Chaser.
The earlier awards were to help fund the spaceship designs, while the new awards will help fund the process of certifying that they meet NASA safety requirements to carry humans.
"These contracts represent important progress in restoring human spaceflight capabilities to the United States," Phil McAlister, who oversees NASA's commercial spaceflight programs, said in a statement.
"NASA and its industry partners are committed to the goal of safely and cost-effectively launching astronauts from home within the next five years," he said.
The contracts run from January 22, 2013, through May 30, 2014.
Our top photos from the last 24 hours.