NASA has tested a huge rocket booster, the biggest rocket ever built, in the Utah desert as part of the Space Launch System that might eventually take humans to Mars.

The SLS system is a vital part of the space agency's Orion program intended to take astronauts to the moon, to a rendezvous with an asteroid and possibly to Mars by 2040.

That's if NASA can find the funding; its current budget is severely constrained, while a number of private ventures are going forward with programs to send rockets into space at a small fraction of the money NASA says it must spend.

That has a number of people suggesting NASA should rethink its plans.

"If you look at the timeline for going to Mars, they keep talking about 25 years out - and it's 25 years out because of their budgets and because they keep spending it on their huge rocket at ridiculously higher prices than the private market," says Henry Vanderbilt, executive director of the Space Access Society, a research group campaigning for more cost-efficient approaches to going into space. "You can make the argument that with that rocket as a drag on NASA's budget, we're never going to be at Mars."

In a 2-minute static test in Utah - the same duration it would boost for during an actual launch -NASA fired the rocket booster, two of which would power the SLS.

The boosters are enlarged versions of those used to launch NASA's Space Shuttles, and their selection is another sign of the space agency having to work within reduced budgets.

One of the observers at the test firing, astronautics lecturer Adam Baker from Great Britain's Kingston University, said that although the test was impressive visually, it was troubling that it was using technology going back many decades.

Many people, including some former NASA officials, have echoed that criticism.

"Do we really think that technology from the 1970s is what we should be going to Mars with? It boggles my mind," says Lori Garver, former NASA deputy administrator and a vocal critic of the SLS program. "Let the transport part be handled by the private sector."

NASA, for its part, remains committed to the program and says it intends to conduct initial test flights of its SLS in 2018.

"Once qualification is complete, the hardware will be ready to help send the rocket, along with NASA's Orion spacecraft, on its first flight test," NASA says.

"When completed, two five-segment, solid-rocket boosters and four RS-25 main engines will power the SLS as it begins its deep space missions," the agency says.