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Comment by sandworm101 | Hacker News Reader

Not as simple as you would think. Mars has atmosphere for capture braking, for parachute landings. The moon requires lots of powered deltav to get onto the surface from low earth orbit, more so than mars iirc.

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That's almost exactly backwards. For any decent-sized payload there's too much atmosphere to ignore (so you need heat shielding) yet not enough to exploit (so you need heavy rockets), parachutes rip off of big payloads... the skycrane was the least ridiculous device JPL could come up with for their rover.

The Moon is much easier, it's just a rock and some delta-V.

Lol, ignore atmosphere. EVERY mars lander has used massive aerobraking with a combination of shields and chutes. 99+% of thier speed was shed passively, without rockets. That Final touchdown is a drop in the deltav bucket. Check any deltav map.

Earth-moon surface = 2.66km/s

Earth-mars surface, with aerobraking = 1.06km/s +0.1 to .5 for final hover/touchdown depending on payload.

The Martian atmosphere is extremely thin — per wikipedia, "the atmospheric pressure on the Martian surface averages … about 0.6% of Earth's mean sea level pressure."

It is not really sufficient for capture braking / parachute landings with any efficiency or safety.

Also, mars takes more delta v than the moon.

Moon is less fuel and risk.

Are you suggesting that going to Mars is easier than going to the moon?

Going to? If you are sending a robot, yes mars can be in some ways easier (less deltaV). Getting back is a totally different equation. Musks tesla, with a heat shield and parachute, could have made it to the surface of mars. Putting it on the moon would require more energy, a bigger rocket.

Hmmm... Mars' atmosphere is <1% Earth's. So far, everything landing there has needed either retro rockets or (if light enough) elaborate airbags. I don't think a parachute is enough to keep the Tesla intact after "landing".

The atmosphere is not thick enough for a soft landing of heavy rovers, but it helps a lot with slowing spacecraft down into an orbit around the planet.

A spacecraft attempting that would have to scream past the surface at extremely high velocity and low altitude and get really lucky not to intersect with terrain (mars is not a perfect sphere). It would also require many orbital passes to lower apoapsis to merely circular orbit (i.e., a lot of time). It is probably not practical.

You're right, it takes a long time and is probably unsuited for a manned mission, but it has been done before and can be useful for getting supplies to a Mars base. https://mars.nasa.gov/mro/mission/timeline/mtaerobraking/

> It lasted about six months [to circularize to a ~450 km orbit].

There's still a lot of orbital energy left at 450km orbit around mars that you need to get rid of some how, and a parachute probably isn't going to do it.

It worked for me in Kerbal Space Program! Don't ask how. Please don't ask.

I always found soft landing on Duna via aerobraking somewhat challenging :-D.

That is how it is done. Mars landers dont do orbital insertions. They crash strait into the atmosphere. 1% is more than enough. The retro rockets for landing only cover a few hundred m/s, all vertical. The atmoshpere is what bleeds off the several thousand m/s of horizonal velocity neede for capture.

Some probes used no rockets. Parachutes and airbags can land a small probe on mars. That isnt an option on the moon.

In terms of delta-V, Deimos is about as far as the surface of the moon. In terms of travel time, it's a hundred times farther away. For humans that is a crippling difference, but for rovers not so much.

Mars is not Deimos of course. My point is merely that from a delta-V perspective, the solar system suddenly looks very different.

Downvoted by people who know nothing about space physics. Deimos is indeed just about the same distance as the moon in terms of energy required. With one exception. When approaching a multi-body system there are gravity-boost options for slowing a spacecraft. Not an option when aproaching the moon.

Yeah, I'm a bit surprised to get downvoted for posting an interesting and unexpected truth. I'd love to hear what kind of reasoning the downvoters have for this.

I'm personally quite fascinated by Deimos. It's far away and yet relatively easy to reach. Some people believe there might be quite a bit of water on Deimos. Put together, that might make it an interesting option for a base, for refuelling, if nothing else. Especially once Mars gets a colony and we start mining asteroids, I wouldn't be surprised if Deimos turns out to be a nice central point between the three.

Or not. I'm not a rocket scientist, but I find this a fascinating subject.

It is a tuesday. HN is full of millenials who think they know everything. Id bet most of the haters in this thread had to google "deltaV". The smart people show up on the weekends. Im only here because im visiting family and work doesnt have my cell number.

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