Two US rocket companies are readying the first private space missions to the International Space Station (ISS).

SpaceX and Orbital both have multi-billion dollar Nasa contracts to supply cargo to the station, filling the void left by the retirement last year of the space shuttle.

California-based SpaceX has set the pace so far, having successfully launched its Falcon 9 rocket and Dragon capsule into orbit, and safely returning the capsule to Earth, in December 2010.

The company says it could launch for the ISS as soon as 30 April.

You wouldn't want to drink water in space if you knew what it cost to put it there Prof Howard McCurdy , American University

Orbital Sciences Corporation is still to test fire its Antares rocket, and its Cygnus capsule has yet to make it to space.

But the company is now looking to move quickly, with a static launch test and a first launch into orbit scheduled for the summer, and a possible rendezvous with the ISS in the autumn.

Frank Culbertson, a former astronaut and senior vice president at Orbital, admits it does feel a bit like a race.

"A little bit. They actually started the development of their system about a year-and-a-half before us, but we're almost neck-and-neck in terms of who's going to launch next, who's going to get to the station first.

"They're probably three or four months ahead of us on the schedule that we see, but who knows how things will work out?

"Nasa needs both companies to succeed on every single mission if at all possible."

The two companies have very different pasts, and have built very different spacecraft.

While Orbital has been in business since 1982, building satellites, small rockets and missile interceptors, SpaceX is just a decade old, even if its plans are big.

SpaceX designs and builds much of its Falcon 9 rocket system in-house, while its Dragon capsule can carry cargo or crew, say its makers.

The company's chief executive, PayPal co-founder Elon Musk, says Dragon could be used to ferry astronauts into space by 2015, and the company is one of four companies that Nasa has awarded seed funding to develop commercial crew vehicles.

Orbital has chosen a different path, buying in many of the components from across the globe.

Much of the the liquid-fuelled lower-stage rocket is designed and built in the Ukraine, while its engines are 40 years old and come from the Soviet Union's ill-fated N-1 lunar rocket.

Orbital's Cygnus capsule, meanwhile, is a purpose-built cargo vehicle that is designed to ferry supplies and not astronauts to the ISS. After being filled with waste from the station, it will burn up on its return to Earth.

While SpaceX will launch from Cape Canaveral in Florida, Orbital has chosen the lesser-known Wallops Island, a Nasa-operated facility just four hours drive from Washington DC.

The Mid-Atlantic Regional Spaceport, or MARS, to give the launch site its proper name, is still a building site as final work is completed to ready the site for a static test firing of the Antares rocket.

The rockets themselves lie on their sides in a massive hangar just over a mile away from where they will be transported to the pad aboard an equally massive "erector launch vehicle" which will lift them into position on the surface of MARS.

Antares' launch and flight into space will be controlled from a nearby Nasa control room. There, amid the screens and consoles, Jay Pittmann, Nasa's range commander at Wallops, says the agency is learning the art of letting go.

"It is a risk to hand that off to a commercial entity and to give up control. That is the biggest difficulty for us, not being in every part of that, assuring that every piece is going to work.

"In the end, we're taking our lifeline and we're handing it to these commercial companies, but we have confidence that they're going to get there."

There is also an eagerness to have access to American rockets, even if they're not Nasa's.

"We're relying on the Russians and other nations to get equipment and material to the International Space Station," he says. "Quite honestly, that's not as comfortable a position as we'd like to be in as a nation."

It's not just the agency which is conscious of the risks involved. Orbital will have the task of guiding the Cygnus capsule to the ISS from the company's control facilities at its Dulles HQ, near Washington, and the design of the entire rocket system is its responsibility.

Frank Culbertson says things could go wrong.

"Nasa has to bless us, and Nasa has full veto power when we get within the vicinity of the station, but the companies are responsible for their performance," he says.

"Since it's a fixed-price contract, we also have to control our costs, control our resources, and if we're ever going to make any money on it, we've really got to do it efficiently.

"That means in reality there may be some failures. If that's the case, we just learn from them and we go on and we get the next one going."

What all parties are hoping is that Orbital and SpaceX can not only get to the station safely, but that they can lead the way in cutting the cost of space travel.

"What I'd like to see is that the competition between the two firms spurs some technical breakthroughs that result in lower cost for getting to space," says Prof Howard McCurdy of the American University in Washington DC.

"It just costs too much right now. You wouldn't want to drink water in space if you knew what it cost to put it there."