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Comment by rory096 | Hacker News Reader

>The 50% figure is from memory, and upon further reflection is probably not quite right, so take it with a grain of salt.

You're actually underestimating. Mars has an atmosphere, so spacecraft can use aerobraking to significantly reduce the delta-v required to get there. The moon has no such benefit — propulsion is required all the way to the surface.

The advantage of the moon, of course, is that it's not limited to transfer windows every two years. If there's an emergency or resupply is needed (or you just want to expand the mission), another lander can be sent at any time. (With some restrictions, like hitting your desired landing spot and making sure the sun wasn't in the Apollo crews' eyes at landing. But regardless, no longer than 28 days between windows.)

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You need a far bigger (heavier) craft to get to Mars. You'll need years worth of food, water, air, you'll need radiation shielding, much more space per crewmember, more redundant systems and backups, etc.

Look at the size of the Apollo.

Years? Absolutely not. Transfer can be done in about 90 days, less (30 days) if you pursue aggressive entry, high orbit refueling, and nuclear thermal propulsion. Assuming you're settling the place and not just doing flags and footprints, transfer time is the important part.

That's astoundingly quick. 30-90 days is in the same range as crossing the Atlantic via tall ship. With transfer time that low, it really does seem feasible. The two year gaps are a bit rough, but there are plenty of ways to plan for it.

Nowadays ships cross the Atlantic in less than a week.

Well, if any of these silly EmDrives ever work out, maybe we can get Mars down to that time too.

For the journey, yes. But for permanent settlement, in both cases you'll need of the order of years-worth of supplies while you become self-sufficient.

Back on-topic, the discovery of water on the moon makes this more achievable there; otherwise Mars's resources might have actually meant less resources were required.

You don't have to send all the supplies at once though (you shouldn't actually). You can send many small missions, each providing some parts of a Mars base before sending any humans. See for example the Mars Direct plan by Zubrin: https://en.wikipedia.org/wiki/Mars_Direct

But the presence of water on the Moon is more limited than the presence of CO2 and H2O on Mars. That could be significant for self-sufficiency. Especially if we could find a way to make plastics from CO2 and H2O in a reasonably small mobile device. Just grow your colony out of the stuff you suck out for the atmosphere...

You don't need radiation shielding to go to or stay on Mars. The health risk from the excess radiation is moderate, should be a very low priority for an adventurer.

Shared this video[1] about the same topic above[2]

[1] https://www.youtube.com/watch?v=h2nqgKL2JQU [2] https://news.ycombinator.com/item?id=15093353

The aerobraking advantage only works getting there, not coming back.

ACKSHULLY - aerobraking on Mars is a chore. The atmosphere is dense enough to burn you to a crisp, thin enough it won't help you slow down - much.

Look at the marvel that is the Skycrane. Not needed for the Moon.

For a dramatic coverage of this exact subject, I highly recommend 'Curiosity's Seven Minutes of Terror' [1] from NASA JPL.

[1] https://www.jpl.nasa.gov/video/details.php?id=1090; or on Youtube: https://www.youtube.com/watch?v=h2I8AoB1xgU

Note this is before SpaceX proved supersonic retropropulsion works just fine. Atmosphere is now a huge asset for Mars EDL.

>thin enough it won't help you slow down - much.

No. It's thin enough you can't land with parachutes alone. It's still enough that Mars Global Surveyor used it to make up a 1,250 m/s delta-v deficit.

Is landing on Mars more complex than a thick atmospheric landing on parachutes or an exoatmospheric landing on propulsion only? Yes. But the atmosphere still saves you a lot of delta-v.

[0] https://mars.jpl.nasa.gov/mgs/sci/aerobrake/SFMech.html

> It's still enough that Mars Global Surveyor used it to make up a 1,250 m/s delta-v deficit.

It took MGS 15 months (not counting delays) of aerobraking to do that. MGS only weighed 1000kg too. Maybe it could work for shipping large, pre-assembled, empty hab modules to Mars before people get there. (Low density, higher drag, no rush.) But not practical for actual human bodies or even resupply since the Hohmann transfer takes a mere 8.5 months. Adding an aerobraking maneuver could easily triple the transit time.

It only took that long because MGS was aerobraking into orbit and wasn't designed to survive the stress of lower atmosphere turbulence, so only used the uppermost layer of the atmosphere for braking. If your going to land, obviously you get to use all the atmosphere and also can use parachutes. Something a problem braking into orbit can't do.

Surface probes do direct descent trajectories. There is no extended series of repeated braking passes. You do it in one go.

> If your going to land, obviously you get to use all the atmosphere and also can use parachutes. Something a problem braking into orbit can't do.

Also, you can use lithobraking.

7 minutes of terror is a fantastic video, but that line in it always bugged me. Atmosphere is an enormous asset for Sending large payloads to Mars now that we know supersonic retropropulsion works just fine.

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