Nasa Deep-Space Rocket, SLS, to Launch in 2018

The SLS has been in development for three years already, and when finished it should propel spacecraft beyond Earth's orbit and eventually launch crew vehicles to Mars by the 2030s.

Nasa has now completed a thorough review of the project, signifying formal space agency commitment to the 70 metric ton version of the SLS at a cost of $7.021 billion from 2014 to 2018.

"The program is making real, significant progress," said William Gerstenmaier, associate administrator for the Human Explorations and Operations Mission Directorate at Nasa.

"We will keep the teams working toward a more ambitious readiness date, but will be ready no later than November 2018."

The Government Accountability Office (GAO), however issued a report last month that called into question the space's agency's current funding plan for SLS, saying it "may be $400 million short of what the program needs."

The GAO also raised concerns about the development schedule and how engineers will integrate hardware that was designed to fly on a cancelled Nasa program known as Constellation that would have returned humans to the Moon.

Gerstenmaier said Nasa was taking those concerns into account and is seeking to address the GAO's recommendations.

The SLS is Nasa's first heavy-lift launch vehicle in over 40 years, and the space agency has estimated total costs in developing the first of three SLS variants at $12 billion.

The SLS will "provide an unprecedented lift capability of 130 metric tons (143 tons), which will enable missions even farther into our solar system, including such destinations as an asteroid and Mars," Nasa said.

The Orion multi-purpose crew vehicle is a separate project under development that aims to launch atop the SLS and carry people on a months-long journey to the Red Planet.

The first space flight test for Orion is set for December.

"We are moving. We are going now," said Nasa associate administrator Robert Lightfoot.

"After rigorous review, we're committing today to a funding level and readiness date that will keep us on track to sending humans to Mars in the 2030s and we're going to stand behind that commitment."