“Above every square centimeter of surface is a kilogram of air. It takes a vertical column of about 70 grams about 1/14 the distance through the atmosphere, achieved at an altitude of 20 to 25 kilometers (60,000 to 80,000 feet) before the average incoming proton hits the nucleus of an atom in the air. The rest of the atmosphere serves to absorb the shrapnel of this initial collision.”

“Mars astronauts would receive a dose of more than 80 rems a year. By
comparison, the legal dose limit for nuclear power plant workers in the U.S. is five rems a year. One in 10 male astronauts would eventually die from cancer, and one in six women (because of their greater vulnerability to breast cancer). What is more, the heavy nuclei could cause cataracts and brain damage.”

“To match the protection offered by Earth’s atmosphere takes the same one kilogram of shielding material per square centimeter, although astronauts could comfortably make do with 500 grams, which is equivalent to the air mass above an altitude of 5,500 meters. Any less would begin to be counterproductive, because the shielding material would fail to absorb the shrapnel. If the material is water, it has to be five meters deep. So a spherical water tank encasing a small capsule would have a mass of about 500 tons.”

My mistake, I thought it was 400 tons. This is obviously different than the information Marcel is using.

The briefest round trip scenario during the 2030’s (more than 500 days) would probably just barely exceed 100 Rem of radiation exposure during solar minimum conditions with no radiation shielding. Of course, you’d still have to protect her and the other passengers from the long term of effects of potentially brain damaging heavy nuclei and major solar events. Just 20 centimeters of water shielding would appear to be enough to protect astronauts from heavy nuclei and major solar events.

But, philosophically, I don’t believe that any– single interplanetary mission–should be a career-ender for an astronaut because of excessive radiation exposure, especially for a person still in their 20’s. (Could you imagine traveling to Mars in your 20s and then being banned from traveling into space again for the rest of your life!)

So I would recommend that crewed interplanetary vehicles have trans-habs with at least 50 centimeters of water shielding to reduce annual cosmic radiation exposure to approximately 25 Rem per year during solar minimum conditions.

Permanent rotating artificial gravity habitats in orbit within the solar system, however, should be much more heavily shielded, IMO, probably with at least 50 cm of dense iron ore. Fortunately, there are plenty of iron ore resources on the Moon and in the asteroids and meteoroids.

Happy New Year!

Marcel

As far as tethers go, I have absolutely no problem with them or any other method that works. I do know that large rotating dynamic systems with only tension elements are probably quite complex and may have induced harmonics and other problems that are not evident. It’s hard to dissipate energy in unwanted systems oscillations with only tension elements and nothing to work against.

As I would hope to be able to say regarding my own contribution to this discussion (see comment below), I think the materials you’ve linked to represent some quite enlightening background reading for this post by Dr. Spudis. And who did the wonderful illustrations for your article on the SLS-derived habitat?!?

Still on my wish list: a brief refresher course on the physics of centrifugally-induced gravity.

Regards,
G. W. (Glenn) Smith

Of course, the word “colonize” is loaded with different interpretations, but in this case, I take it to mean the establishment of permanent human settlements on either world. As is so often the case, the discussion at Reddit quickly turns toward comparing the two objects in terms of their resources and surface environments. While some of the comments are well informed, many misconceptions about the properties of both objects are readily evident – both confusing the casual reader and inhibiting the discussion. **

Venus is the most Earth like planet. Venus lacks water- though is has Hydrogen and Oxygen in chemical compounds- the acid clouds have hydrogen and the air is CO2 from which one can get oxygen. So it has a lot of water which can made, but doesn’t have much water which relatively easy to obtain. Though general idea with settlement on Venus is one cities in it’s sky.

But there is no shortage of places to live on Earth- 70% of earth is ocean, and it would easier to have settlement on Earth oceans than Venus or anywhere is space. And Venus has same “problem” as Earth- it’s difficult to get out of it’s gravity well.

One thing we would want from Space is cheap electrical power. The US and the world has spent hundreds of billions dollar “trying” to get affordable electrical energy from solar energy. It has been failure due to the nature of Earth itself. Getting 12 hours of solar energy per day is not very
useful for our needs and we don’t even get 12 hours of solar energy on Earth. Germany is one of worst places to get solar energy. One could say if you make solar energy work in Germany, one could make solar energy work in the rest of the world [because few places where people are living are worst locations]. And Germany’s effort to be the solar capital of the world is driving the country toward poverty. Space exploration [though some may think is wasteful use of money] has never driven any country towards poverty- one argue that’s quite the opposite. No one can argue that Germany’s obsession with it’s solar energy governmental program, has been helpful to it’s society- unless driving up the price of electricity conforms with one confused ideas of being “helpful”.Nor has it lowered Germany’s CO2 emission and causing more coal and wood use, which per unit of energy make the most CO2.
The region of Germany gets yearly average per day of about 2000 watt hour of useable solar flux- if got 20% as electrical power that is 400 watt hours per square meter per day. The rovers on Mars are getting more electrical power than this. Anywhere on Mars is better location than Germany. And with Mars if you do something to inhibit global dust storm, it would be better place to harvest solar energy than compare to where most people live on Earth [not many people live in deserts and nor vast ocean area which less cloudy].
One could argue that like Earth one would need nuclear energy on Mars.

Anyhow the best place to harvest solar energy for people living on Earth is from Earth high orbit- where one can get a constant source of solar energy.
And the main problem with getting solar energy from high earth orbit is the cost of getting to high earth orbit.
People commonly are concerned about to issue of transmitting the power from Space. And would say that this involves another benefit from harvesting solar power from space- it’s “giving you” a global network that provide electrical power anywhere and at any time on Earth.
Or it’s providing way that people could live in the open ocean or anywhere not near current electrical power grids. Or it’s cheaper and better electrical power grid.
Or if one could get solar energy in space, one could want a better power grid. Or we already have a communication grid which space based. If it was cheap to get to high orbit, we would establish a
electrical power grid simply to move electrical power generated on Earth to other locations on Earth. And in addition to that you get abundant, clean, and cheap electrical power to entire earth for as long as the Sun burns.
So harvesting solar power from Space is dependent on much cheaper access to high earth orbit.

But we can’t go directly to getting cheap solar energy from space, in same way we can’t get automobile made 2000 years ago. Or factor in having airplanes was having powerful enough and lighter enough engine. So we could not have gotten airplane [regardless of how smart people were] 200 years ago- as engine had not technological advanced enough. But there was also other factor involved, such as economic development of wealth- lots people who could buy airplane tickets [and wanted to].

The Moon is the first destination because it has low gravity well and because it might have minable water. It’s minable because it’s in a low gravity well and it’s in high earth orbit, and it’s too expensive to get things to high earth orbit.
For lunar water to be directly related to getting solar energy from space to earth requires water and or electrical energy to be very cheap and available on lunar surface. And cheap is somewhere around $1 per lb of water and $1 per kw hour of electrical power.
If one put blinders on and only think of the Moon- such a cheap price is not really possible- not in the “foreseeable” future [or not within a century]. Or in order to get this in foreseeable future one has see the moon for what it is, which is the gateway to the solar system.
Or if focus on colonization the moon, it’s possible it work against getting to such low price.
It depends upon what the people do, but just saying it’s possible people could prevent it.
Because their is shortage of water on the moon in terms of human consumption of water- and people going to want swimming pools and farm things- the Moon doesn’t have enough water for this purpose. The Moon has endless supply of water for the purpose of rocket fuel. The Moon’s water could help send more than million people to Mars.

So the Moon needs to be exporter of stuff rather domestic consumer of stuff. And if it does this fast it would soon be able to import all the water it needs in the future [sooner]- because this solar system has earth oceans of water. It’s possible that water is dumped on the Moon, simply for the purpose of generating “hydro power”. Though long before one gets to point importing massive amount of water on the moon, you have earthling getting electrical power harvested from Space.

So one should think of how to get the moon in the export business. And starts with commercial mining of water.
Now Mars also has relative to the Moon, cheap water. Mars could even have cheaper water compared to Earth. Mars is not a good location to export rocket fuel, but could be a good location for the consumption of water for farming [and human use in general- Martians could “afford” swimming pools]. So what NASA should do is look for cheap water on mars- and cheap water will drive human settlements on Mars.
So obviously such cheap water would be below the mars surface, and probably mean water in liquid form which can be pumped to the surface. Mining permafrost is not I what mean by cheap water. So we know Mars has trillion of tonnes of water in polar cap and frozen ground, I think there could trillions tonnes of water available below the surface as liquid. So Mars could only have
a few spots there there is water which easy to pump from the ground, or it could have many locations- or none. And that one thing NASA should explore for.

http://earthweb.ess.washington.edu/space/M2P2/