THE WOODLANDS, Texas — To commemorate the 45th anniversary of Apollo 17 — the last time astronauts walked on the moon — three key figures from the historic mission held a panel discussion here at the 49th annual Lunar and Planetary Science Conference on Wednesday (March 21). Apollo 17 flew to the moon in December 1972 and marked the end of NASA's Apollo program. The lunar module pilot Harrison "Jack" Schmitt and Cmdr. Gene Cernan would become the last people to set foot on the moon. Now, 45 years later, NASA is finally making plans for the long-awaited return.

At the panel, Schmitt (a geologist who became the first trained scientist on the moon), Apollo 17 flight director Gerry Griffin and backroom scientist Jim Head reminisced about some highlights of the mission, like the "orange" soil Schmitt discovered on the moon and the time the crew repaired a lunar rover with duct tape. But they also looked to the future and explained why they believe that sending humans back to the moon is a crucial step for sending humans to Mars.

"Mars ain't gonna be easy," Schmitt said during the panel. "There are a whole bunch of operational issues related to not only landing on Mars, but also working on Mars, that we really need to work out closer to Earth, and the moon is a place to do that." [Apollo 17: NASA's Last Apollo Moon Landing Mission in Pictures]

All Apollo astronauts have long supported this idea of using the moon as a "stepping-stone" to Mars, Head added. Now that President Donald Trump has signed his Space Policy Directive 1, which directs NASA to prep for crewed Mars missions by first sending astronauts to the moon, it seems more likely that their wish will come true. However, some members of the space science community have expressed doubt in Trump's ability to make such an ambitious goal come to fruition.

"We've got to go back to the moon for a lot of reasons," Griffin said, "but we've got to get our mojo back." Sending humans out into deep space is a lot different from sending them into low Earth orbit, where NASA's human spaceflight program has the most experience from working on the International Space Station, he said. We'll need not only different kinds of spacecraft and life-support systems, but also better ground support systems, to deal with the long distance and communication delays.

"Even when we're working on Mars, I think that the science backroom is going to be even more important than it was for Apollo because of the planning activity that's going to have to go on there due to the communication delays," Schmitt said.

The longer distance also makes it far more difficult, if not impossible, for a crewed spacecraft to turn around and come back to Earth if the mission had to be aborted midflight. "You'd need to engineer your landing craft so that you can abort to land [on Mars] and work out the problems there," he said. "To me, it's just common sense that you would not do that. And that's a challenge."

Schmitt suggested working out those kinds of issues by doing lunar missions "partly in a simulation mode" to "figure out what are we missing operationally that we didn't think of with respect to preparing for Mars."

"The nice thing about the moon is, you can work most of these kinds of issues out … while you're doing good science," Schmitt said. "So, you get a double return from it when you're doing your preparation for Mars."

Even before Trump directed NASA to send astronauts back to the moon, the agency had been working on a plan to build a small space station in lunar orbit, which would serve as a stepping-stone for human missions to Mars. The Lunar Orbital Platform-Gateway concept (formerly known as the Deep Space Gateway) could serve as a staging ground for crewed landings on the lunar surface.

While some experts believe a lunar outpost will give NASA an opportunity to practice for crewed missions to Mars, others argue that it's an unnecessary detour on the road to the Red Planet that will only cost more time and money than a more direct approach.

Still, every person who has been to the moon agrees that we need to go back before pursuing more distant journeys, Head said. Apollo astronauts might be biased toward a return to the moon, but they are also the only people who can speak from experience about what it's like to travel in deep space.

Email Hanneke Weitering at hweitering@space.com or follow her @hannekescience. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.