The science

It only takes a glance at a planetary exploration roadmap to see Mars is currently one of humanity's leading targets of interest. There are eight spacecraft operating on or around Mars; five belong to NASA.

Following the loss of the Mars Observer spacecraft in 1993, NASA established the Mars Exploration Program, a systematic series of missions to determine whether Mars "was, is, or can be, a habitable world," and to pave the way for eventual human exploration. The program has been wildly successful; in 2013, drill samples analyzed by the Curiosity rover confirmed the answer to the "was" question is yes.

Whether or not life actually existed there—or still does—will be harder to answer. Back in late 2014, Ellen Stofan, who served as NASA's chief scientist for three years, told me the question would be best solved by astronauts.

"I have a bias as a field geologist that it's going to take astrobiologists, geologists, and chemists on the surface of Mars, being able to go out and read the landscape, pick up rocks, and take them into a lab, to really resolve the question of whether life arose on Mars," she said.

NASA's Opportunity rover has been exploring the surface for 13 years. Mike Seibert, an Opportunity driver and a lead spacecraft systems engineer at NASA's Jet Propulsion Laboratory, said a big advantage of humans over rovers is their ability to improvise quickly. An example of this, he told me, was a meteorite Opportunity inadvertently discovered while the rover was driving between two waypoints.

"We were taking pictures behind us just to record and document, and we downlinked them several sols (Mars days) later," he said. "A half-kilometer down the road, we saw an image of a meteorite. So we doubled back to go see it."

Astronauts haven't been to the Moon since 1972, and if they go back, they won't be searching for signs of life. But that doesn't mean our celestial neighbor doesn't have other secrets worth uncovering.

The National Research Council's decadal survey, which is published every 10 years to define priorities for planetary science, lists a lunar south pole sample return as one of five top targets for an upcoming mid-tier science mission. The survey says there are important questions to be answered about the Moon's internal structure, and the composition of its mantle.

"There's good science to be gained on both the Moon and Mars," Seibert said. "We haven't had that many surface assets on the moon—or Mars, for that matter. It's like saying you've visited Earth because you've been to Beijing and Lawrence, Kansas."

I put this question to a NASA scientist who works at a key NASA human spaceflight center. The scientist, who preferred to remain anonymous, agreed, telling me, "There's lots of science to do on the moon. But that's not likely going to be the motivating reason we go there."

If not for science, why?

There's a more practical reason some people prefer the Moon over Mars: it's easier to get there.

NASA's current "Journey to Mars" plan is pyramid-shaped. The base includes the International Space Station, Space Launch System and Orion. The middle layer involves learning to live and work around the Moon. And near the top are Mars transfer vehicles, landers, habitats and the all-important ability to come back to Earth.

It's the top of the pyramid that worries many people; right now, it's fuzzy, and without the funding to work substantially on more than one thing at a time, NASA's fully formed plans won't be ready anytime soon.

"There's a number of things that don't exist yet that need to happen," the NASA scientist told me. "And they may require several miracles."

Moon landings enjoy a moderate amount of international support; namely, from European Space Agency Director General Jan Woerner, who has vigorously promoted the idea of an international "Moon Village."

To what degree ESA could financially back this effort is unclear. The agency's 2017 human spaceflight and robotic exploration budget is just $675 million; by comparison, NASA spends almost $3 billion annually on SLS and Orion alone, and another $1.5 billion on the International Space Station.

China and Russia have also indicated an interest in moon landings, but the U.S. is prohibited from cooperating with the former. As for the latter, Russia's 10-year space budget was recently slashed by 64 percent, its long-term plans are uncertain at best, and it has more pressing problems at the moment: severe quality control issues plaguing its launch fleet.

I asked Dave Belcher, an analysis manager at the Washington, D.C.-based Avascent consulting group, whether he thought it was realistic for NASA to expect international partners to make meaningful contributions to lunar landings.

He framed the answer a different way: "I would say that if lunar surface operations would be a financial stretch for international partners, Mars would be even a larger financial stretch," he said.