One of the most promising discoveries on both Mars and the moon is water, but how that water is situated varies between the two. Spacecraft and samples from the Apollo missions have shown that the moon has potentially massive amounts of water frozen as hard as granite in deposits deep at its poles, which future astronauts could mine for their life-support systems. But the icy water on Mars may be more evenly distributed just beneath the surface. The technology that future astronauts would use to extract water from ice on the moon could be overkill on the red planet.

After half a dozen landings that supplied hundreds of pounds of lunar samples that have been carefully studied for decades, the moon may feel pretty familiar to us. But it still has its mysteries, and scientists are eager to probe them. Mars has its mysteries, too, including one that might answer one of humankind’s most existential questions. While the moon is generally understood to be lifeless, on Mars “you can actually go look for signs of existing life on the surface,” says Briony Horgan, a planetary-science professor at Purdue University who works on NASA’s Mars missions, including a rover that is scheduled to launch toward the planet in July. “We can do that with robots, but it’s hard.” (Take it from the little Mars spacecraft that landed last year, ready to drill into the soil to measure seismic activity, and became stuck almost immediately.)

Grunsfeld, the former astronaut, believes that NASA has already shown it doesn’t need the moon to go to Mars. The agency has proved it can keep astronauts healthy for long stretches in space; Christina Koch just came home after nearly a year on the International Space Station, a record for a woman. A journey to Mars would expose astronauts to more cosmic radiation than they experience on the ISS, though, which could increase their lifetime risk of getting cancer. When I asked Grunsfeld about that, he launched into a list of dangers so harrowing, it would be difficult to fault aspiring astronauts for quietly rescinding their applications: “How does that compare to the risk of blowing up on the launchpad or on ascent; getting hit by a meteor, asteroid, debris, some kind of space junk on the way there; burning up in the Mars atmosphere; burning up in the Earth’s atmosphere on the way back; or missing the Earth? You add up all those risks, and the [risk of radiation exposure] is kind of just another one.”

Even Apollo astronauts think it’s time to shoot for Mars. Michael Collins has said that he sees “more moon missions as delaying Mars, which is a much more interesting place to go.” Buzz Aldrin has been writing op-eds for a decade urging the nation to focus on Mars. Outside NASA, Elon Musk’s SpaceX is currently developing a spaceship and rocket powerful enough to head right to Mars. The pro-Mars camp has even included, at least for a time, Trump himself, when, in an uncomfortable rebuke of his own administration’s policy, he tweeted last summer, “For all of the money we are spending, NASA should NOT be talking about going to the Moon. We did that 50 years ago.”