Elon Musk wants humans to live on Mars. NASA wants, at the very least, to send some astronauts over to the Red Planet to check it out. But the Trump administration has recently signaled that it’s more interested in sending astronauts back to the moon.

As a space science professor, I believe this is the perfect opportunity to revive an important debate: If humans needed to beat a retreat from planet Earth, would it be better to live on Mars, or on the moon?

First, let’s consider the advantages of Mars.

Length of the day

Mars spins at a pretty reasonable rate. Its day is just over 24 hours long, so a day on Mars would be roughly equivalent to a day on Earth. A day on the moon lasts as long as 28 Earth days, which would take a fair bit of adjustment.

Also, because Mars’ day is relatively short, the day-to-night temperature difference is not too dramatic. On the moon, the day is very hot and the night is very cold. There is an almost 300 degrees Celsius, or 572 degrees Fahrenheit, temperature difference between the day and night temperatures on the moon. Such a large day-to-night temperature difference can make it really difficult to engineer the right living systems, such as habitats and cars for moving around and space suits for going outside. Imagine if during the day, your house, car, and cloths had to be designed for Phoenix, Arizona, during the summer, and at night, everything had be designed for Antarctica in the winter. That would be tough!

Atmosphere

Mars has an atmosphere. It’s not really a great atmosphere, but at least it is one. It is mostly carbon dioxide, which is great for plants, but really sucks for us humans. The atmosphere allows wind to blow, which helps to equalize the day-to-night temperature differences—but causes a lot of dust to move around, too. The atmosphere also means that we can pressurize domes and structures using air from outside.

The moon, on the other hand, has almost no atmosphere. NASA’s website explains, “At sea level on Earth, we breathe in an atmosphere where each cubic centimeter contains 10,000,000,000,000,000,000 molecules; by comparison the lunar atmosphere has less than 1,000,000 molecules in the same volume.” Since the moon has so little atmosphere, any wind can’t really move heat from the hot day side to the cold night side.

Mars’s atmosphere could also stop a bunch of harmful EUV and energetic particles from the sun. This would prevent you from frying if you went outside. The moon doesn’t really have anything to block you from getting zapped (technical term). Really, you would have to definitely live in caves on the moon, but on Mars, it might be possible to live above-ground. You would still get a fair bit of radiation exposure, though. Really, neither of them is great from a radiation standpoint, but if one had to choose, Mars is better.

An interesting historical fact is that during 1972, when there were a couple of moon landings, there were a lot of solar radiation events. One was bad enough that it could have killed all of the astronauts on the moon if there had been any. Luckily, it happened when everyone was safe on the Earth! Space is dangerous.

Water

Evidence suggests that water may exist in the subsurface all over Mars. This is not definitive, but it is a relatively strong possibility. Ready access to water would be important not only for our ability to stay hydrated and grow things, but because we would be able to use it to create oxygen to breathe. The moon has some surface water, as well as interior water, but we would probably have to live near the poles to access it.

Gravity

Gravity on Mars is more powerful than the gravity on the moon. If I were to weigh 200 pounds (91 kilograms) on Earth, and I am not saying that I do, I would weigh 33 pounds (15 kilograms) on the moon and 75 pounds (34 kilograms) on Mars. This increased gravity on Mars helps keeps the atmosphere on the planet, and would also be useful for making objects stay put on the ground and keeping our bones a bit more robust.

Scientific value

Evidence suggests that the moon is a big chunk of the Earth that split off a long, long time ago. Mars, on the other hand, is a totally separate planet that evolved on its own, had lots of cool geology (there are some really, really tall inactive volcanoes on Mars). Some people theorize that there could actually be life there. Not like human life, but there could be small bacteria or other tiny organisms there. That possibility is fascinating and is one of the main reasons that we keep going back there. So, scientifically, Mars is a much better destination than the moon.

Coolness

Mars is really cool. It is red. It’s named after the Roman god of war. Mars is also capitalized, while the moon is not.

Now let’s turn to the advantages of living on the moon.

Distance

The moon is really close to Earth. This saves a huge amount of fuel (money). We have rockets right now that can land a lot of stuff on the moon. We could start dumping supplies on the moon in months if we really wanted to. To start sending water and rocket fuel for the journey home, along with other supplies to Mars, we’d have to do years of research first.

Don’t get me wrong, we do have rockets that can land tiny amounts of stuff on Mars. But it’s really hard to get big objects to Mars. In order to get to Mars, you have to get completely away from Earth, move out to Mars, and then land softly on the planet’s surface. This process of getting to Mars takes a lot of energy, and requires a huge rocket to get even a small amount of stuff on the surface. Take a look at this post to read about why rockets have to be so big.

Consider this: The rockets that we took to the moon and back were the largest rockets that we have ever built, and they were only large enough to have three guys crammed into a tiny space about the size of a minivan. This is fine for a three-day journey to the moon, but would really not work for the six-month journey to Mars.

Now, if you want astronauts to come back home, you have to build a rocket that can get to Mars and back again. Those don’t exist yet.

Travel and communication

It only takes three days to get to the moon, while it takes six months to get to Mars. And since a Martian year is much longer than an Earth year, the planets only align in the right way for travel once every two years. There is about a two-week period in which you can launch a rocket to get from the Earth to Mars (called a “launch window”), and if you miss that, you are out of luck until the next time they align in the right way, two years later.

The moon, on the other hand, is easy to get to, since it is much closer. In fact, since the moon is orbiting the Earth, we can go there pretty much whenever we want.

This has big safety implications: Rescuing people from the moon would only take a few days, while on Mars, people could be stranded for years. (Remember the book and movie The Martian?) Also, if you want to have a conversation with someone on the moon from planet Earth, you would only have a few seconds of time delay. The delay to Mars is between four and 24 minutes, depending on whether Mars is on “our side” of the sun or the opposite side. If you think video conferencing is bad, imagine trying to have a conversation with someone with a 24-minute delay. Ugh.

Economic benefits

Just as it’s easier to send supplies to the moon than it is to send them to Mars, shipping stuff from the moon back down to Earth is very, very cheap (read the Robert Heinlein book, The Moon is a Harsh Mistress). So if we mine the moon for substances like metals or Helium-3, which is needed for nuclear fusion, we could get stuff back to the Earth for almost no cost at all. If you shot stuff in a canon off the moon at the right angle and speed, it would literally fall back to Earth.

If we mined Mars for these types of raw materials, it would be incredibly expensive to get them back to Earth. Rockets would have to be built on (or in orbit around) Mars, and these would have to make the long journey back to Earth, which takes a lot of fuel. So, from a purely commerce-oriented standpoint, a moon colony is a much better economical venture.

Solar events

Remember that six-month trip to Mars? Astronauts would be out on a tiny ship in the vastness of space. If there were a large solar event that caused radiation to increase significantly, which happens a lot, then they would die unless the ship had extremely heavy-duty radiation shielding.

A solar event could happen on the way to the moon, too. But since it takes only three days to get there, it is easier to avoid times in which there may be radiation flare-ups.

Earth-rise

Watching the Earth rise while sitting in my living room on the moon would be awesome. Unless we’d fled Earth because it had become a nuclear wasteland. Then it would just be a reminder of everything we’d lost.

The upshot

In the long term, Mars wins. The moon is too harsh to be anything but a stepping stone toward life in space. Because of the possibility for life and the very interesting geology that took place a long time ago, Mars provides a much better opportunity for scientific exploration. And if something horrific happened on Earth and it could no longer sustain life, Mars would be a much better place to live because of the possibility of finding water, its higher gravity, and its atmosphere.

That said, the moon is a much more economically feasible location for a colony. A Mars colony will continue to be far off in the future until the funding and political winds shift.

This article was adapted from an original post on The Rocket Science Blog. Aaron Ridley is a professor of space sciences and engineering at the University of Michigan. His podcast, X and Why, talks about science and technology.