For the last few years, we’ve been able to frame NASA’s research and exploration with the goal of reaching Mars. The agency has been saying it wanted to land a human on the red planet in the 2030s, but now that’s looking less likely. William Gerstenmaier, NASA’s associate administrator for human exploration, has noted that NASA simply doesn’t have the money to make a Mars landing happen. The technology to reach Mars isn’t cheap, and no one seems willing to pay for it.

NASA’s budget has been essentially flat for the last decade. Gerstenmaier’s statements came at a meeting of the American Institute for Aeronautics and Astronautics, where he explained the problem with Mars missions. Landing on Mars is extremely challenging, and it will cost a lot of money to create systems reliable enough to send humans there. Well, if you want them to come home, that is. Even the expensive robotic missions have been hit or miss. Of the 16 attempted landings on Mars since 1970, only seven of them have been successful. The most recent failure was the ExoMars lander, which crashed because atmospheric turbulence caused it to tumble much faster than expected.

Mars has basically the worst possible set of traits for successful landings. Sending people to the moon was no easy feat, but the moon had very little gravity and no atmosphere. Thus, propulsive landing was feasible and the craft didn’t need heatshields. Mars has more gravity, but the atmosphere is too thin for parachutes to do all the work. At the same time, the atmosphere is just thick enough to make landings unpredictable and require the use of heatshields. That’s why Curiosity used a wacky skycrane contraption to land on the surface.

NASA has looked into a skycrane system to land larger craft, but the numbers are tough. Curiosity had a total mass of two tons, but a manned lander would probably clock in at 10 or 15 tons. It’s unclear if it would be possible to land something like that on Mars with our current technology.

So, what about SpaceX? The firm founded by Elon Musk has gotten extremely good at landing rockets on Earth, and it aims to launch an experimental Mars mission as soon as 2020. It took SpaceX a while to figure out how to land in Earth’s atmosphere, so it might take a few tries to get the kinks worked out on Mars. At that point, the process needs to be tested and confirmed safe for humans. It’s impossible to know if that would be possible by the 2030s.

In the meantime, we can at least look forward to the 2020 rover mission, which will tell us more about the potential for life on Mars.