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Water Found Deep Inside the Moon

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Re: Water Found Deep Inside the Moon

#171

Post by dragonwriter »

simonh wrote: post #169
keenerd wrote: post #152

Earlier quoted context omitted.

> 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 transf…

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…

> 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.

Re: Water Found Deep Inside the Moon

#172

Post by Robotbeat »

tambourine_man wrote: post #7

I've always been more excited about a Moon colony than a Mars one. I mean, compared to Mars, the Moon is ridiculously close. Many of the same benefits Musk seeks (backup for Civilization, excitement) for a fraction of the cost. Low gravity and atmosphere on a dome would be more complicated, but again, compared to Mars, it's right there . It would be a more humble and realistic first step, IMO.

It's questionable that it's really much lower cost. The Moon, as a vacuum body, doesn't give the option of aerobraking or atmospheric entry, which dramatically reduces delta-V. You can get a payload to Mars' surface with less propellant than getting it to the Moon's surface. And because atmosphere is everywhere (which includes oxygen in the form of CO2 and also water vapor), propellant is way easier to make, so it's easier to get your ship back.

Transit time is lower, but the currency of space travel is delta-V, and by that measure, Mars is just as close or closer.

I would guess that the Moon still slightly wins as far as transport costs, but the advantage is much smaller than you'd think, and the far greater resources available on Mars (as well as effective immunity to solar particle events and micrometeorites that Mars' atmosphere provides) tilts the situation strongly in favor of Mars for actual settlement. Joy trips may be cheaper to the Moon, however.

Re: Water Found Deep Inside the Moon

#173

Post by Robotbeat »

andars wrote: post #36
munificent wrote: post #25

Earlier quoted context omitted.

People way underestimate how far away Mars is. To give you a sense of scale, the settlers of Jamestown came from London. As the crow flies, that's about 3,790 miles. If we treat that as equivalent to settling on Mars, then for them to settle on "the moon" would have been a mere 26 miles from London. Perhaps Canvey Island on the River Thames. If we can't manage settlements that close, we really aren't ready yet for th…

In terms of distance, sure. In terms of delta-v, the budget for the moon is upwards of 50% that for Mars. EDIT: The 50% figure is from memory, and upon further reflection is probably not quite right, so take it with a grain of salt. I just wanted to point out that distance is not the only relevant metric.

Nope. Mars is actually lower than the Moon due to atmosphere.

Re: Water Found Deep Inside the Moon

#174

Post by Robotbeat »

jacobush wrote: post #129
rory096 wrote: post #56

Earlier quoted context omitted.

>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…

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.

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.

Re: Water Found Deep Inside the Moon

#175

Post by Robotbeat »

WalterBright wrote: post #68
rory096 wrote: post #56

Earlier quoted context omitted.

>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…

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.

Re: Water Found Deep Inside the Moon

#176

Post by Robotbeat »

niftich wrote: post #132
jacobush wrote: post #129

Earlier quoted context omitted.

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.

Re: Water Found Deep Inside the Moon

#177

Post by drostie »

keenerd wrote: post #149
drostie wrote: post #108

Earlier quoted context omitted.

I'm glad you asked! So there's an important physics reason why the equator is warm and the North and South poles are cold on this planet, and it has to do with how you point a surface. If you ever find yourself in a dark office with a bright desk lamp (or maybe your home office or kid's bedroom might be suitable) you might even try this experiment: take a flat surface, a notepad or steno pad, and hold it up facing th…

Mercury won't be under any serious consideration for habitation for a long time. It is down at the bottom of the biggest gravity well within a few light years. Too much delta-V to come or go. It is the least explored inner planet for a reason. I think you are drastically overstating the difficulty of heating a lunar polar base. You are surrounded by vacuum, one of the best insulators around. It can be used. Of course…

A solar panel tower at the North Pole of the Moon might be a really good idea for solving these problems, yes.

Vacuum is only one of the best insulators around if you're near other warm things, I'm afraid. Out in space the dominant form of heat exchange is the Stefan-Boltzmann law, which says that if you have a half-acre of total surface area on your moon-base, and it's at 300 kelvin, then you need to supply about a megawatt to keep it warm.

It's not an insurmountable problem, of course -- Tesla just announced a battery for Australia at 129 MW-hrs, which would sustain the thing for 5 days, which is not enough but it's only a half-order of magnitude off. (I'm also not considering something like nuclear heating; it's not uncommon to have a 1000-megawatt nuclear reactor in the US and that's just the power output, not the heat -- so it's not a terribly bad way to go if you can shield the rest of the base from it. The only hazard with that is one recently discussed by John Oliver on his show, firing nuclear things into space is heavily, heavily complicated by the nowhere-near-close-enough-to-zero failure rate of launch rockets and the insanely-scary-cost of distributing a bunch of reactor-grade nuclear fuel into the atmosphere.)

Re: Water Found Deep Inside the Moon

#178

Post by sunflowerfly »

Robotbeat wrote: post #172
tambourine_man wrote: post #7

I've always been more excited about a Moon colony than a Mars one. I mean, compared to Mars, the Moon is ridiculously close. Many of the same benefits Musk seeks (backup for Civilization, excitement) for a fraction of the cost. Low gravity and atmosphere on a dome would be more complicated, but again, compared to Mars, it's right there . It would be a more humble and realistic first step, IMO.

It's questionable that it's really much lower cost. The Moon, as a vacuum body, doesn't give the option of aerobraking or atmospheric entry, which dramatically reduces delta-V. You can get a payload to Mars' surface with less propellant than getting it to the Moon's surface. And because atmosphere is everywhere (which includes oxygen in the form of CO2 and also water vapor), propellant is way easier to make, so it's…

For me the real advantage of a moon base first is safety. If something goes horribly wrong while we are learning to live on other planets, a return to earth or a rescue mission takes far less time. I am not a space expert though, so perhaps this is wrong?

Re: Water Found Deep Inside the Moon

#179

Post by ars »

khedoros1 wrote: post #155
ars wrote: post #112

Earlier quoted context omitted.

> The universe doesn't care. And that's exactly why it's important. > Survival of humanity isn't important universally. Unless there is something there that cares about it, there is little reason for the universe to exist - nothing would know the difference. Which is why it actually is important universally. > But it's the most important thing for humans. No, it's more important for the universe to have humans, than…

That seems like a circular argument. It's not important for the universe to have a purpose, or to "care"...except to us. Things don't need a reason to exist.

What is the difference between a universe that doesn't exist, and one that no one notices exists? There is no difference.

Re: Water Found Deep Inside the Moon

#180

Post by passivepinetree »

Robotbeat wrote: post #172
tambourine_man wrote: post #7

I've always been more excited about a Moon colony than a Mars one. I mean, compared to Mars, the Moon is ridiculously close. Many of the same benefits Musk seeks (backup for Civilization, excitement) for a fraction of the cost. Low gravity and atmosphere on a dome would be more complicated, but again, compared to Mars, it's right there . It would be a more humble and realistic first step, IMO.

It's questionable that it's really much lower cost. The Moon, as a vacuum body, doesn't give the option of aerobraking or atmospheric entry, which dramatically reduces delta-V. You can get a payload to Mars' surface with less propellant than getting it to the Moon's surface. And because atmosphere is everywhere (which includes oxygen in the form of CO2 and also water vapor), propellant is way easier to make, so it's…

Your assertion about getting to Mars with less propellant is super interesting. Can you provide a source for me?
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