©Agence France-Presse

Despite funding uncertainty, NASA is on track to return humans to the moon by 2020 and set up a lunar outpost to serve as a springboard to explore Mars, officials said Monday.

"Our job is to build towns on the moon and eventually put tire prints on Mars," NASA's Rick Gilbrech told reporters here, one year after the US space agency unveiled an ambitious plan to site a solar-powered, manned outpost on the south pole of the moon.

"We have the International Space Station; we're going to have a lunar outpost, and someday, certainly, somebody will go to Mars," said Jeff Hanley, head of NASA's Constellation program, which is developing the tools to return humans to the moon.

"Thirty-five years ago this week, Gene Cernan, Ron Evans and Jack Schmitt were on the surface of the moon. We are working hard to return a future generation of astronauts to the moon," said space flight veteran Carl Walz, who now works for NASA's exploration systems mission directorate.

Despite budgetary constraints, NASA hoped to have Constellation fully operational by 2016, Gilbrech said.

"We're hoping we get a budget passed by Congress," he said, pointing out that only six-tenths of a penny of every tax dollar went to funding NASA's space programs.

"We're making plans to be ready for any and all scenarios. The (budget proposal) we put in keeps our program on track for the March 2015 initial operating capability... and full operating capability a year later," Gilbrech, who leads new spacecraft development at NASA, said.

"That will enable the human-moon return by the 2020 date that the president envisioned."

President George W. Bush in 2004 announced a plan to resume human flights to the moon after a decades-long gap.

"We're doing this effort to get back to the moon in phases," said Hanley.

He recalled that the first phase involved retiring NASA's space shuttle in 2010 after completing construction of the International Space Station.

Following that, attention will be focused on building the shuttle's successor, the Orion crew vehicle and Ares launcher of the new Constellation program, he said.

"Very soon, and we have already begun at a low level, we will ramp up rapidly to build the big systems which are the Ares V rocket and the lunar lander and the surface systems that will be put in place on the moon," Hanley said, again stressing budgetary constraints faced by NASA.

"We will begin that build-up after 2010 because that's how our budget is put together. We're going as we can pay, so to speak. This is a long-range strategy we're rolling out over the next two decades and more," he said.

"It will be a challenge, a challenge to do it on budgets the Apollo missions didn't have to deal with, and to get people interested and keep them interested," he said.

"In May 1961, then US president John F. Kennedy proposal to send astronauts to the moon before the end of the decade launched the Apollo era."

The pioneering program achieved its goal eight years later, on July 20, 1969, when Apollo 11 commander Neil Armstrong became the first man to set foot on the moon.

The December 1972 flight of Apollo 17, with Cernan, Evans and Schmitt on board, was the last manned mission to the moon for the United States.