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Photo of Donna Fender Journey to Mars:
We're On Our Way

Donna Fender

q It seemed that when you gave Alan Alda a tour of the inflatable habitat, you just peeked into one part of it (with the velcro "bed.") I'm wondering how large the whole habitat is designed to be. How many bedrooms and how many common rooms will it have?

A What you saw was one typical bedroom of six that would make up the crew quarters area near the center or the habitat. All told, the habitat could be approximated by a large, fat cylinder about 23 feet in length and 25 feet in diameter. Besides the crew quarters area near the center, there is a "mechanical" room off to one side of this, a lower level that is a very large crew cooking/eating/recreation and storage area, and an upper level which is the crew exercise, shower, health care area, and more storage.

q What materials are used to make up the layered shield around the inflatable habitat?

A 1) The outermost layer is, as it is on all the space station modules (and also what you see lining the inside of the space shuttle payload bay), a thermal blanket which is a white material called "beta cloth" on the outside, and about 19 to 20 layers of aluminized mylar (kind of like what you see mylar party balloons made out of) on the inside.

2) Below that, are the layers of material that are the orbital debris protection that you saw Eric Christianson shoot an aluminum ball at. This orbital debris protection is made of alternating layers of Nextel fabric (a material commonly used on the inside of car hoods because of its insulating properties), and foam. That's right, common, everyday foam . All it's there for is to space the layers of Nextel.

3) Next is the what we call the restraint, which is the structural layer that holds the 14.7 psi (pounds per square inch) air pressure on the inside of the module. It acts like the pig skin of a football to hold the pressure of the bladder inside. The restraint is made out of a very strong fabric called Kevlar - the same material used to make bullet-proof vests.

4) Finally, the innermost layer is made of Combitherm, a material that is commonly used in the meat packing industry. But it's very good for us because it lets very little air seep through, and that's very important when you don't have much air to spare! There are 3 layers of Combitherm to make extra sure we don't get a leak.

q Will the habitats be heated and cooled to keep the astronauts comfortable and allow them to live inside the habitats without a spacesuit?

A Yes, the plan is for the habitat to be what we call a "shirt sleeve" environment, so they can be comfortable while they work, eat, and sleep. The temperature will be probably be kept around 75 degrees Fahrenheit.

q Are you designing the inflatable habitats to be purely practical - or are you also considering details like interior color, etc. that may affect the astronauts' psychological well-being?

A One of the things we have learned from the comments we got back from the Skylab crews, and the crews that have lived aboard the Russian MIR space station is that attention to the psychological effects of long duration space flight is very important. We have two architects working with us on the layout of equipment, worrying about noise levels, making sure there's enough light to work by, and for doing personal activities in their crew quarters (we're even looking at the "color temperature" of the light). We've also included a window in front of their exercise equipment because crews have said how wonderful it is to be able to watch the earth go by while they exercise (something they will have to do for 2-3 hours a day!). And you're right about the importance of color -- in fact we have a group of architectural design students in Rhode Island studying exactly that for us!

q My science class watched Journey to Mars at school and we had a few questions about it. My first question is what is the temperature (in degrees fareinheit and in celcius) on the surface of Mars? The temperature would obviously be much colder than that of earth so would the habitats have to have heating systems? How would they work? Thank you for your time.

A Mars is indeed colder than earth. The temperature on Mars varies widely, depending on time of day, season of the year, and where you are on the planet's surface. But in the summertime, near the equator, daytime temperatures can reach as high as 0 C (32 F), a cold day on earth. But in the winter, near the poles, the nighttime temperature can drop below -130 C (-202 F), cold enough to snow dry ice (carbon dioxide)! The first astronauts to go to Mars will probably land near the equator, though, where the winters should be no colder than about -80 C (-112 F). Astronauts will probably keep warm in their habitats and space suits through a combination of insulation and electric heaters.

q I'm designing a Mars colony in science class. I believe animals are a vital resource to keep people happy. Have you thought about that? What adaptations would you have to make in the inflatable habitat to support animals?

A We have not yet thought about animals on a Mars colony. But you're right, if we do try to establish human civilization on Mars like we know it here on earth, animals will probably come into the picture.

q If space debris hits more than once in the same spot, how is the fourth layer of the protective shield going to protect it?

A The short answer is -- it probably won't; the fourth layer by itself cannot provide a lot of protection without the other 3 bumper layers. But it is so, so, so unlikely to happen (see NOTE ON PROBABILITIES below) that we don't worry about it. We are far more concerned about a single particle impacting and going through the shield because that is so much more likely than two impacts hitting close to each other and causing a problem.

Even if the impacts occur on the same spot (on the outer bumper), it is very unlikely that they will impact at the same angle (these particles come from all directions); i.e., it is far more likely they will not be going exactly along the same flight path and so the second impact will not hit the fourth layer on the same spot where the first impact did.

Say for instance a good size particle impacts at 45 deg ... say 6mm or so ... and it goes through the 3 bumper layers & 12 inches of foam on the Mars TransHab, and damages the fourth Kevlar layer (this is not very likely to happen, but say it does happen). A large oblique impact on the TransHab shield will make a series of holes in each bumper that will line up along the original flight path, i.e., it will form a kind of "tunnel" at 45 deg to the surface. Now the next impact that comes along will impact at another angle or in a different direction, so the next tunnel will at best intersect at one layer {as an aside: it is very unlikely that a large impact happens at all, and even more unlikely that it impacts near the first one, but it is just really unlikely that it will also impact at the same angle & flight path}. Because of this, the Mars TransHab shield can sustain two impacts of good size even if they impact almost on top of each other, because the impacts will be at an angle and most of bumpers along each flight line will still be there! This means the Multi-Shock shield used on TransHab is far superior than conventional shields used on other spacecraft that have only a single bumper.

I don't remember the numbers exactly, but for the Mars mission, the TransHab had about a 99.5% chance of not being penetrated through all 3 bumper layers & also through the 4th layer & through the redundant bladders during the mission phase after leaving Earth Orbit (from meteoroids) ... this was for about a 7mm particle to penetrate. So assuming an impact from a slightly smaller 6mm particle that goes through all 3 bumper layers & damages but does not perforate the 4th layer, there is a lower probability, say about a ~99% of NOT impacting during the mission; i.e., the Probability of No Impact (PNI) = 0.99. Then getting two 6mm particles to impact would be PNI=1-(1-0.99)^2=0.9999. Since we need these two to impact within ~6" of each other (i.e., the second impact would have to impact in a 1 square foot region surrounding the first impact), the chance of that would be much less:
PNP = 1-(1-0.99)*(1-0.99)*1/3600 = 0.999999972 ... which is 1 chance in 36,000,000. {1/3600 is ~ratio of areas of 1 square foot to total TransHab surface area} This is so low compared to the probably of penetration from a single large impact that exceeds the capability of the shielding, that we can neglect the effect of two nearby impacts in the PNP calculations.

q How would computers and electrical aids work on a home in space? I am very interested in people living on Mars. And this one thing has been bothering me for a while. Thank you for your time!

A Computers in space work pretty much the same as they do on Earth. In fact, several standard laptop computers (with slight modifications for voltage levels and safety) are used on every space shuttle mission that run the same software that you and I use on our own home computers. On the International Space Station, computers and other electrical aids will plug into electrical outlets that will be powered by the energy from the space station's Solar Panels. When we go to Mars, we think that computers and electrical devices can be designed so that users get about the same kind of performance that we get here on Earth.

The Martian environment is relatively unknown to us, but we know that it is harsher than here on earth. We are very concerned about dust storms and radiation there. Numerous studies are being conducted in both areas, as they will have a significant impact on life in the Martian environment. Of particular interest is the effect of radiation on, not only humans, but the computers and other mechanical and electrical devices.

The Martian atmosphere is thinner than the Earth's atmosphere, but many scientists feel that radiation exposure on Mars will be similar to that in the Earth's environment and may pose no greater risks. The danger may come, however, if the astronauts and their equipment encounter any solar particle events (i.e., galactic cosmic rays or solar flares) while they are traveling in deep space. NASA will take precautions to minimize the radiation risks during deep space travel and long-term stays on Mars. The habitats for humans will be made of materials that will shield them and reduce the hazards from radiation (the TransHab incorporates such materials). Similarly, the equipment will be shielded and/or redundant (backup) components will be used in the devices (especially computers) to be able withstand severe radiation.

q I am designing a Mars colony for 28,000 people for science class. What are the advantages and disadvantages of having one big dome covering the whole colony versus separate domes?

A Though NASA is currently focusing on much smaller efforts to develop habitats on other planets such as Mars, several important factors apply whether one is designing habitable structures for six people or six thousand people. One key point to understand is that any sort of habitatable structure on the surface of Mars must be designed to withstand pressurization of the internal atmosphere to a level which humans can tolerate. Sea level pressure on earth is 14.7 pounds per square inch, though many people live at elevations above sea level where the atmospheric pressure is less. Assuming the dome geometry is hemispherical (to best hold in the pressurized atmosphere), the larger that the diameter of the base is, the higher the vertical height of the top of the dome is (by a factor of one half). In other words, if you designed a hemispherical dome to be 1000 feet in diameter, then it would be 500 feet high, or about 50 stories tall.

So to make best use of the interior volume of the dome, you have to build your colony up as well as across the base of the dome. Building vertically tends to be more of a challenge than building at ground level, whether on earth or Mars. Also, the larger your dome is, the greater the volume of pressurized atmosphere it has, thus the more stress is imposed on the shell structure of the dome and the anchoring points to which it is attached on the surface of Mars. An additional consideration is that if a single large dome were subjected to a significant catastrophic event, such as an explosion, the entire colony could perish from loss of pressure. A series of much smaller domes, connected via transport tunnels with pressure-retaining bulkheads and airlocks would theoretically be significantly easier to emplace and would be resistant to any single event causing a catastrophic failure. Privacy would also be increased (but so would isolation!) Lastly, smaller domes impose more constraints on how a colony can be configured than a larger dome would -- but maybe it would be worth it!

q The protective layer on the inflatable habitat was only tested for smaller meteorites the size of a marble going at the average speed. I would like to know the probability of running into a piece of debris as large as, say, a baseball. What would be the effects of a large piece such as that?

A A "baseball" is about 500 times less likely to hit than a "marble". So if the chance of a "marble" hitting is 0.006% or 1 in 17,000 going to Mars (after we get out of Earth orbit), then the chance of a "baseball" hitting is 1 in 8,500,000, or 0.000012%. In other words, it's very, very, very unlikely to ever happen.

What would be the effects of a large piece such as that?
Because of TransHab's unique woven construction of independent structural members, this size of a hole would not be enough to cause the shell to burst, so the only effect would be that it would begin to lose pressure. If the TransHab is still transferring from low Earth orbit to high Earth orbit, it is unoccupied and it is possible that a repair mission could be done (i.e., patch the TransHab and re-inflate it). If the impact occurred while the TransHab was in route to or from Mars, then the crew would have about 12 minutes to patch the hole.

q Were the images of the projectile penetrating the shield a sequence of x-ray images? If so, what equipment was used to expose and record the images?

A No, the images were made by an ultra high-speed laser shadowgraph framing camera made by the Cordin company. The equipment includes a pulsed laser, mirrors/optics, and a camera using infra-red film & a gas-driven turbine that spins a mirror that exposes discrete parts of the film.

q The program showed four astronauts who had lived in a prototype habitat module for several weeks. What is the duration record for this sort of experiment? Are there any plans to run an experiment as long as a trip to Mars (six months) or a complete Mars mission (2 1/2 years)?

A The record for the U.S. is 91-days. That was the Phase III test we completed in December 1997 with a crew of four in a sealed chamber here at JSC where we recycled all their air and water during the 91 days.

The Russians have had people in a sealed chamber for up to 2 years back in the 1970's recycling their air and part of their water and growing some of their food.

We are building a new test bed now where we will be testing technologies to close the air, water, food and solid waste processing loops. This has never been done before. Our tests will begin with 30 day evaluation runs with a crew of 4 and progress through more and more closure during 120-day and 240-day tests leading to one lasting 425 days where we will have nearly complete mass closure. This is where we will evaluate technologies for eventual use on a planetary surface.

Advanced life support systems for use on board vehicles traveling to and from the planets will be evaluated in ground test beds and on the Space Shuttle and International Space Station in order to verify their performance in a zero-gravity environment.


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