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Photo of Malcolm Cohen Journey to Mars:
Getting There

Malcolm Cohen

q On your story, Alan Alda tried out three different kinds of exercise machines. I am wondering which one seems most effective and what type of exercise machine(s) you think will eventually be used for astronauts who go to Mars.

A As of now, we really do not know exactly what kind of exercise machines will be most effective in space to prevent the deconditioning effects of microgravity in a long-duration mission - such as a mission to Mars. The three kinds of exercise machines shown on the program were
(1) the lower body negative pressure treadmill,

(2) the upper body positive pressure treadmill, and

(3) the human powered (bicycle) centrifuge.

All three machines provide a way to exercise the large antigravity muscles of the lower limbs and to stimulate the cardiovascular system. Both treadmill machines also provide contact and impact forces when the foot leaves and contacts the treadmill, thereby possibly providing additional stimulation to the bones that normally would support the weight of the person under normal gravity conditions. The human powered centrifuge generates a centrifugal force that can simulate the pull of gravity on the internal organs and the heart-brain blood column. This stimulation may be extremely effective in maintaining normal cardiovascular functioning for astronauts in space, but we do not yet have any data on this. We expect that some of the machines that were shown on the Scientific American Frontiers program will be tested on the International Space Station when it becomes operational as a test bed, and that the techniques that prove to be the most effective over a long mission duration will be the ones that will be used on a mission to Mars.

q Has there ever been any evidence of permanent muscle, bone or other loss/damage after space flight? Or does everyone "bounce back" to normal after a period of exercise on Earth?

A People are amazingly resilient. Although bone is slowest to "bounce back," even the bone changes appear to be fully reversible. There has been no indication -- so far -- that any of the physiological changes that result from space flight are permanent. The only item that is of really major concern at this time is the possibility of radiation damage, but that is not likely in Earth orbit because of the protection provided by the Earth's electromagnetic field. On a mission to Mars, however, radiation shielding would be considerably more critical.

q My question is about the new type of space suit being tested for astronauts to use on Mars. If Mars has somewhat of an atmosphere, can't humans walk on Mars with just breathing equipment and dress appropriately for the cold environment? Do you need a pressurized space suit?

A You need a pressurized suit! Although Mars has an atmosphere, its composition is very different from that of the Earth. It's mostly Carbon Dioxide with only 0.13% Oxygen, compared to about 20% Oxygen on Earth. Also, the pressure of the Mars atmosphere is EXTREMELY low, about 1/100th of the atmospheric pressure on the Earth. For humans to survive, the partial pressure of Oxygen in the blood should be maintained within a range of 170 to 760 mmHg (760 mmHg is about 14.7 pounds per square inch), and a minimum pressure of Oxygen at about 150 mmHg is needed to maintain Oxygen saturation in the blood if no other gas other than Oxygen is present. Because of the extreme difference in pressure between gases dissolved in the blood and the atmospheric pressure surrounding Mars, the dissolved gases in the blood would dissipate if a person did not wear a pressurized space suit. A pressurized space suit is REALLY necessary on Mars. Also, since Mars does not have much of an atmosphere, radiation exposure at the surface can be extremely high, and a pressurized space suit could also provide radiation protection.

q How about putting a little spin on the spacecraft to create artificial gravity?

A Yes, it's an interesting possibility. Spinning the spacecraft itself to provide artificial gravity is an idea that has seriously been considered for many years. The toroidal design (like a spinning inner tube), such as the one portrayed in the "2001 Space Odyssey" movie, is typical of such an idea. Wernher von Braun suggested such an approach about fifty years ago. Unfortunately, there are complex issues that have not been adequately addressed for implementing this approach, and we need to conduct further research to evaluate it. Also, there may be some drawbacks: for example additional energy is needed to spin and to de-spin the spacecraft; maintaining the axis of spin on the vehicle can also require additional energy; departing or docking with a rotating vehicle can be very tricky; spinning the spacecraft causes Coriolis and cross-coupled accelerations whenever the occupants of the craft move -- these accelerations can be disorienting and can cause severe motion sickness. Nevertheless, the full merits of the idea are still being considered by NASA, and there are some studies underway within NASA to examine some of the possible trade-offs.

q When Alan is in the centrifuge and he feels pressure on his chest and heavy weight on his arms, is this about the same number of Gs the astronauts experience when they take off and land?

A During launches in the Shuttle, the astronauts experience up to 3.2 Gx (G sub x), i.e., 3.2 times the Earth's force of gravity, which is applied from chest to spine -- because the astronauts are lying on their backs during the launch. During re-entry, the astronauts experience between 1.6 and 2.2 Gz (G sub z), about the same force and direction as that experienced by Alan in the human centrifuge. However, the astronauts usually are adapted to microgravity (weightlessness), and are somewhat deconditioned from living in microgravity for about two weeks or more, so the force probably seems to be even greater for them than it did for Alan.

q I really enjoyed your show last night! It was really interesting to learn about the new technologies you will be using for your trip to Mars. After watching the "simulated operation red walkthrough," I wondered if any astronaut ever had heart problems from the changing gravity in space?

A The simple answer is "no." Although their G-tolerance is temporarily reduced from being in space, no astronauts have ever had any real heart problems that were cause by the changes in gravity.

q We thought the exercises were "cool" and we want to be future astronauts. Our question is about difficulties that astronauts might encounter while in space and living on Mars. What do you think are the major mental and physical stresses that may happen?

A I think that the major stresses will ultimately be mental rather than physical. The physical stresses generally involve adapting to lower levels of stimulation and activities than the levels encountered on Earth. The astronauts are generally well protected, and -- if anything -- they are usually less active in space than they were on Earth. On a Mars mission, the major physical problems probably will be encountered when the astronauts land and have to adapt to the Martian gravity after having been weightless for about six months in transit. Although it may take some time, they probably will do quite well, since we know that they can adapt to Earth's gravity after more than a year of being in space -- and Earth's gravity is more than 2.6 times the gravity on Mars (1.00 vs. 0.38 G).

However, try to imagine the isolation of being far away from everyone and everything you have known all of your life. The entire range of astronauts' activities can only be within their space suits or within their habitats and space vehicles. Once the mission begins, the Earth recedes from view, and everything that they have known becomes a little blue dot among the stars.

q Since astronauts are expected to lose large amounts of muscle and bone during a mission to Mars, will they have a special diet rich in protein and calcium to help compensate for this loss?

A I feel that something should be clarified at the outset of my answer to this question. People will lose muscle mass and bone strength in microgravity if nothing is done to prevent the losses, but NASA is working on how to prevent the losses. Astronauts are NOT expected to lose large amounts of muscle and bone during a mission to Mars. NASA is conducting a countermeasures program to PREVENT these losses from ever occurring. The specific kinds of exercises that will be used for the Mars mission will be evaluated on the International Space Station, and the kinds of exercises that most effectively PREVENT the muscle and bone losses will be used on the Mars mission. This is why NASA wants to get the Space Station up and running as soon as practical -- so that we can conduct the research in space that will evaluate the efficacy of different countermeasures that we may want to use. If diet, drugs, or any other techniques can help keep the astronauts healthy, these techniques will also be validated on the Space Station, and they, too, will be used on the Mars mission.

q What special programs do the astronauts go through to regain bone and muscle and cardiovascular strength when they're back on Earth?

A Currently, NASA astronauts exercise, undergo frequent physical examinations, and live a pretty much normal life after they're back on Earth. No "special" programs are needed; although it can take some time, the physiological systems return to normal on their own.

q The exercise equipment tested on earth includes vacuums and pressurized air to add weight to the astronauts to simulate gravity. Does this add a fixed amount of weight, or add a percent of the weight? They said they have not tested it in space. If it only adds a certain percent of your body weight, like %25 or %50, they should realize that in zero-g, you have no weight. Therefore, 0 times %25 or %50 of 0 equals a large waste of equipment space and an expense for something that may not do the intended job. How do they know it will work?

A Your question reflects a misunderstanding of the problem. The exercise equipment that uses vacuums and pressurized chambers produce forces because the person's body is loaded like liquid inside of a straw, or like a piston in an engine. The two devices shown on the program both use a flexible diaphragm to surround the body near the waist and to separate the pressures that are applied above and below it. When the pressure of the air above the diaphragm is greater than the pressure below it, the body is forced down; when the pressure below the diaphragm is reduced (as in a partial vacuum) the body again is forced down. It's not by adding to or multiplying the person's weight. The scale was used with Alan just to show how big the effect was; the effect does not depend on gravity at all. The amount by which the person's body is forced against the treadmill depends on the difference in pressure above vs. below the diaphragm, and on the cross-sectional area of the diaphragm over which the difference in pressure is applied. Thus, the force, although it can be expressed as a proportion of a person's body weight, is actually due to a difference in air pressure. It's not from "adding to" or "multiplying" a person's weight.

We know that the force from pressure differences will "work" because when Alan was walking on the vertical treadmill (while he was on his back) the contact forces with the treadmill were almost as large as what would have been produced by gravity acting on his body. Even though we know that we can use pressure differences like these to increase contact forces, we do not know how effective these forces will be in maintaining normal muscle, bone, and cardiovascular fitness in space. That is why NASA eventually plans to evaluate the devices on the Space Station.

q Given the extended length of the mission (2.5 years, I believe), what research and/or studies are or have been conducted regarding sexuality and possible reproduction, assuming such will be "permitted" aboard, in not only a weightless environment but also within the confines of both ship and habitat, as well as the mission parameters?

A At this point NASA has not conducted experiments on human sexuality in space, although several NASA researchers have given the problem serious thought. People have spent considerably longer than 2.5 years in other environments without participating in normal sexual behavior, and for missions such as one to Mars, human sexuality does not appear to present a major problem. For prolonged habitation on Mars, for a Mars colony, etc., we will have much to learn.

q Since there's less gravity on Mars, would you live longer there?

A We do not yet have an answer to this question. Perhaps after people have colonized Mars for a few generations we will have some reliable data with which to answer your very interesting question.

Malcolm Cohen is also featured in Cool Careers in Science. Check it out!


Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.