Forty-five years after the first Apollo lunar landing, the United States remains divided about the moon's role in future human space exploration.
Ten more U.S. astronauts followed Neil Armstrong and Buzz Aldrin's July 20, 1969, visit to the moon before the Apollo program was canceled in 1972. No one has been back since.
The most recent effort to return astronauts to the moon ended in 2010 when the Obama White House axed an underfunded program of the previous administration called Constellation. Instead, NASA was directed to begin planning for a human expedition to an asteroid.
That initiative, slated for 2025, also includes a robotic precursor mission to redirect a small asteroid or piece of a larger asteroid into a high lunar orbit.
Astronauts would then rendezvous with the relocated asteroid and pick up samples for return to Earth. The missions are intended as steppingstones for eventual human expeditions to Mars.
This path, however, is fraught with technological cul-de-sacs that do not directly contribute to radiation protection, landing systems, habitats and other projects needed to build the road to Mars, a National Research Council panel concluded in June.
After a three-year study of different options for human space exploration, the panel said a more viable and sustainable path would be to return to the moon.
"The moon, and in particular its surface, (has) significant advantages over other targets as an intermediate step on the road to the horizon goal of Mars," the council’s Committee on Human Spaceflight wrote in a report.
"Although some have dismissed the moon as no longer interesting because humans have visited it before, this is similar to considering the New World to have been adequately explored after the first four voyages of Columbus."
NASA considers the moon "the purview of other nations' space programs," and "not of interest to the U.S. human space exploration program," the report said.
"This argument is made despite the barely touched scientific record of the earliest solar system that lies hidden in the lunar crust, despite its importance as a place to develop the capabilities required to go to Mars, and despite the fact that the technical capabilities and operational expertise of Apollo belong to our grandparent's generation," the report added.
Under current plans, it will be another 11 years before U.S. astronauts travel beyond the International Space Station, a permanently staffed research laboratory that flies about 260 miles (420 km) above Earth. A mission to Mars is at least a decade or more beyond that – if it happens at all.
"It is clear to me that we will not be able to build a long-term research base on Mars if we don't first do it on the moon," planetary scientist Chris McKay wrote in a paper entitled "The Case for a NASA Research Base on the Moon" that was published last year in the journal New Space.
"New technologies and approaches … and increased international interest in the moon make the time right to consider pushing for a base that is 10 times less expensive than previous base designs," McKay added in an email.
Development of the Orion space capsule, Space Launch System heavy-lift rocket and launch pad renovations at the Kennedy Space Center in Florida currently cost NASA more than $3 billion a year.
Ultimately, the hurdles on the path to Mars are political, not technical, in nature, the National Research Council report concludes.
"Probably the most significant single factor in allowing progress beyond low Earth orbit is the development of a strong national (and international) consensus about the pathway to be undertaken and sustained discipline in not tampering with that plan over many administrations and Congresses," the panel said.
(Reporting by Irene Klotz; Editing by Rosalind Russell)