I’ve seen lots of posting about how it will be more efficient to build refueling plants on the moon to send Mars missions there, because “It’s easier to reach Mars from the Moon because of it’s lower gravity” and other similar nonsense. This is all dead wrong for the following reasons.
1) It likely takes less fuel to get to Mars from Earth, than it does to get to the Moon from Earth
Here is a deltaV map of the Earth, Moon and Mars. If you add up the amount of deltaV to get to the surface of the moon and the surface of mars you get the following (using 10 km/s for earth to LEO)
Earth Surface to Moon Surface: 14.7 km/s
Earth Surface to Mars Surface: 20.2 km/s
Kind of what you expect, Mars is much farther away and takes much more fuel, right? Not necessarily. Now let’s look at what it takes to get to Mars transfer orbit, the amount of deltaV it takes to actually fly right past Mars.
Earth to Mars Intercept: 13.8 km/s
The difference between landing and intercept is the fuel needed to reduce speed so we can orbit Mars, then land on the surface. But, Mars has something the Moon doesn’t, Mars has an atmosphere.
Instead of using fuel to slow to match Mars orbit, we can use aerobraking. That means our ship skips through Mars’ upper atmosphere to burn off velocity to match orbit. Then we can use that same atmosphere to burn off more velocity to actually land, using relatively little fuel to help out.
Sure Mars takes far longer to reach and return from (and can only be visited once every two years), but using aerobraking means we can get there with much less fuel.
2) Ok, so aerobraking is great, still why not use it for launches from the Moon to Mars too?
Lets see how much deltaV we need to get from the Earth and Moon to Mars transfer orbit.
Moon to Mars Transfer: 2.9 km/sec.
Holy gee whillikers! We should start launching from the moon right way!
Wait a second, it’s not that easy. What are we going to launch from the moon? Even if we can make fuel from lunar crater ice, every part of our space-ship, every piece of cargo, all our food and drinking water, and every astronaut, is going to have to be lifted from the Earth to the Moon first, then to Mars. How much deltaV will that take?
Earth To Moon to Mars Intercept: 17.6 km/sec
That’s way more than directly flying to Mars! And the truth is right now there isn’t any magical way to make fuel on the moon. We don’t know where and how much water ice is easily available, how to efficiently process it in an automated way (separating nasty regolith from actual water) to make millions of pounds of fuel, and how many millions of pounds of equipment and fuel need to land on the moon to run the fuel processors. We may need a full nuclear reactor to do it! We are decades away from solving the problems. So right now we’d also have to take all the fuel on this roundabout journey adding massive amounts of cost and significant time to every mission.
And guess what, fuel is cheap. A Falcon 9 only uses a few hundred thousand dollars of fuel per launch. The massive BFR/Starship will take less than a half million dollars of fuel to reach orbit. It’s a huge waste to use hundreds of launches to send the components for massive fuel making plants to the moon when you can use those same launches to send cargo and people direct to Mars.
3) Mars has far more useful resources than the Moon has and is far more hospitable.
I’m not going to argue exploring the moon doesn’t have scientific merit, we absolutely should explore it. But Mars once had lakes, probably oceans, and possibly life, none of which the moon had. Actual cargo, long term habitats and fuel making equipment should go to Mars.
Atmosphere: Mars has a carbon dioxide atmosphere, which makes it easy to make methane, water ice limits the moon to H2 LOX engines.. We may be able to use helicopters and airplanes even in the thin martian atmosphere to get around (or at least fly drones).
Temperature & Day length: The moon flips between subfreezing (-280 Fahrenheit) and oven hot (+ 260 Fahrenheit), each for two weeks at a time because it has a 4 week long “days”. While Mars is farther from the sun and gets down to -195 Fahrenheit during winter nights, it actually gets up to 70 degrees Fahrenheit during the summer and averages -80 Fahrenheit. And Mars days are barely longer than earth days.
Regolith: Lunar regolith is nasty, sharp and dangerous. Mars dust is likely to be far more easier to handle and work in..
Return Trips: We are decades away from making useful amounts of fuel on the moon, but the first visitors to Mars should be able to make fuel for the return trip. We already know where massive stores of water ice on Mars are already, and so we know where to land. The Sabatier reaction is a known method that can produce methane from the martian atmosphere.
4) The SpaceX Plan
The SpaceX plan is basic engineering that requires little new inventions. The first Mars cycle (Mars trips can only be done every two years due to planetary alignments) Cargo Starships are refueled in low earth orbit and sent to the Mars landing areas full of food, supplies and equipment. The second cycle more Cargo Starships are sent along with the first manned Starships with the first explorers. They live in the Starships until they can dig habitats under the soil, and setup the fuel making equipment so on the third cycle they have enough fuel to return, while new explorers land. By not sending fuel to Mars, it drops costs at least ten-fold over a NASA style mission that requires shipping fuel to mars.
If there are problems making the fuel, more supplies and new equipment are landing every cycle to help solve the problems. Meanwhile the martian explorers will have tons of food, water and other supplies to get them through many cycles if necessary before their return.
In summary, we should explore the moon, but our main focus should be mars. Mars missions can actually cost little more than lunar missions using the SpaceX approach. And Mars is a far better place to actually build long term habitats and manufacturing.