Private companies, aided by NASA's cash and expertise in human space flight, are rushing to be the first to build a space capsule to replace the retiring US shuttle in the next few years.

With Atlantis wrapping up its final mission and the end of the 30-year US program just days away, NASA is pinning its hopes on commercial industry to build the next low-cost vehicle to take astronauts to low Earth orbit.

"We are transferring 50 years of human space flight experience from NASA to the private sector," said Phil McAlister, acting director of commercial spaceflight development at NASA.

Faced with mounting criticism over its lack of a replacement for the shuttle, the US space agency insists it is focused on building a deep space vehicle while it "partners" with the private sector on a spacecraft to tote astronauts to familiar destinations like the International Space Station (ISS).

"We are bringing financial resources so we are going to invest in these systems, and we are also helping them technically," McAlister said.

Earlier this year, the US space agency distributed nearly $270 million in seed money to four companies -- Boeing, SpaceX, Sierra Nevada and Blue Origin -- to boost their bids to be first in the new space era.

President Barack Obama's budget request for fiscal year 2012 includes $850 million for such efforts and would mark the third round of funding so far.

A host of former NASA astronauts have already joined the private sector as highly paid consultants to companies in the space race.