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Comment by maccam94 | Hacker News Reader

The moon has some additional practical hurdles compared to Mars:

1. No atmosphere means no weathering, so all of the moon dust is incredibly sharp and easily coats equipment when it gets kicked up. This dust quickly wears out joints and contact surfaces.

2. The Moon's low gravity and lack of atmosphere mean that you have to carry extra propellant to land.

3. Lack of easily extractable resources. It is comparatively easier to extract water and CO2 on Mars to generate methane for a return trip. A trip to the moon requires carrying all of the fuel for a return trip as well.

Nuclear-powered ships built in orbit would drop transit times significantly but I wonder if they'd significantly reduce Mars <-> Earth costs (you'd still need Starships to shuttle payloads through the atmosphere). They are probably required for mining the asteroid belt though...

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2. The Moon's low gravity and lack of atmosphere mean that you have to carry extra propellant to land.

Lack of atmosphere I get, but how does low gravity require more propellant to land?

(edit: maybe you meant lack of atmosphere due to low gravity, and I got the other meaning contrary to your intent)

You have to use the engines to slow down rather than aerobraking in the atmosphere.

Sorry, my first thought was that it requires more delta-v to enter a lunar capture orbit since the gravity is weak, but I just double checked the delta-v roadmap and it's actually not that high.

However, it is a bit problematic that landing a rocket on the lunar surface can kick rocks around the planet (thanks to the low gravity), possibly into orbit or beyond (which could pelt other equipment). So you need to switch to an entirely different propulsion system for descent, which carries a payload penalty.

https://upload.wikimedia.org/wikipedia/commons/9/93/Solar_sy...

Mars is considered to be the hardest planet to land on in the entire solar system, you have to carry a heatshield or you will burn to a crisp, but it's so thin you still have to land propulsively using rockets. You definitely need to sacrifice less spacecraft mass to land on the moon

Also there is some water on the moon so you can make rocket fuel, though how much exactly remains an open question

Water yes, but I haven't seen anything about CO2 so I think you're stuck with hydrogen-oxygen engines (which have different tradeoffs than methane-oxygen engines).

https://everydayastronaut.com/raptor-engine/ (scroll down to the text immediately above the RP1/Methane/Hydrogen chart)

There is a lot of oxygen on the Moon (combined with silicon and aluminum), but carbon is not readily available, so it'd be more favorable to hydrolox engines.

> Mars is considered to be the hardest planet to land on in the entire solar system

Questionable.

> but it's so thin you still have to land propulsively using rockets

You have to do the same on earth. Starship sized vehicles have to land on earth propulsively too.

Sure, you can do somewhat larger payloads to Orbit without engines but for large things you need to have real engines.

> You definitely need to sacrifice less spacecraft mass to land on the moon

On the moon you can't aerobrake, that is far worse.

> Also there is some water on the moon so you can make rocket fuel, though how much exactly remains an open question

There is a huge difference. Water on the moon has to be mined from permanently shadowed craters. The water is frozen and incredibly hard, its more like mining stone on earth.

On Mars there are places where you can literally land, drill down and make yourself an underground lake pretty easily. Or land basically right on an ice sheet.

On the moon you need to figure out how to get energy down into the crater, you need sophisticated mining equipment, all that equipment needs to handle incredibly low temperatures.

Mars comparatively is not so far away from earth in terms of temperature for your equipment.

In summation, in-situ for rocket fuel/water is far, far easier on Mars.

Did you just make up the entire passage about ice strength? Why do you feel the need to do that?

Ice is 0.5 Mpa stong in the freezer and 2 Mpa at absolute zero.

Thats tensile strenght of chalk and limestone - you can get that with a shovel. Tensile strength of rock is measured in Hundreds of MPA.

The smallest, wimpiest digger you can find can handle it.

Tests have shown then depending on the situation lunar regolith can go up 80MPa. And the more water is contained, the harder it gets. You get low yield from that, around 10%. So you need to mine a pretty large amount and process it.

So you need to create excavator that operate inside a creator that can operate at extrem temperatures.

On Mars you can literally do what we already do at the South Pole, drill a hole and make it warm.

Its multiple orders of magnitude easier.

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