The Case for Mars (22 page)

But in fact we don’t need to fly to Mars in a zero-gravity mode at all. A Mars-bound spacecraft can be provided with artificial gravity. This can be done by spinning the spacecraft, using essentially the same “centrifugal force” physics that allows a small child to swing a bucket round and round without losing a drop of water. The equation that governs this effect can be written:

F = (0.001 1)W
²
R

where F is the
centrifugal force measured in Earth gravities; W is the spin rate in revolutions per minute (rpm); and R is the length of the spin arm in meters. I offer this equation because by looking at it you can see that for a given level of gravity produced, the larger W is, the smaller R can be. For example, if Mars’ normal gravity is desired (F = 0.38), then if W is 1 rpm, R is 345 meters. But if W is 2 rpm, then R is 86 meters; if W is 4 rpm, R is 22 meters, and if W is 6 rpm, R is 10 meters. Thus, there are two ways to produce artificial gravity; you can either go with fast spin rates and short spin arms, or slow spin rates and long spin arms. By “spin arm” I mean the distance between the location of the crew and the center of gravity of the spacecraft about which they are being rotated. If the spacecraft is to be a single rigid structure, it can easily be made to spin by having small rocket thrusters at each of its ends fire sideways in opposite directions. However if significant amounts of artificial gravity are desired the only viable option is to go for the fast-spin/short-arm technique. In the 1960s, NASA conducted experiments with humans on rotating structures and found that, after some initial disorientation, humans could adapt to living, functioning, and moving about on structures with rotation rates as high as 6 rpm.
¹⁵

Rapid-spin/short-arm artificial gravity systems are the easiest for the engineer to design and implement, but they also have some disadvantages. For example, if R is 10 meters, then a two-meter-tall person standing in such a gravity field will have his head at R = 8 meters, and thus experience only 80 percent as much gravity at his head as at his feet. This much difference is tangible and can be disconcerting, at least at first. On the other hand, if the spin arm was 100 meters long, the two-meter-tall person would feel 98 percent as much gravity at his head as at his toes, and probably would not notice any difference. In addition, if the crew member were to walk rapidly about the ship, he would experience Coriolis forces due to the interaction between his attempt to move in a straight line and the fact that the ship (the floor he is walking on) is not only moving but changing direction rapidly. Once again, at 6 rpm these effects are quite noticeable but at 2 rpm they are negligible. Thus, if you want the artificial gravity to feel like dry land on Earth (desirable, but not necessarily a requirement—sailors adapt quite well to very erratic gravity/Coriolis-force environments experienced on pitching ships at sea), the best way to do it is to employ a slow spin rate together with a long spin arm. Such very long spin arms can be provided if the spacecraft is split up into several parts that can be connected to each other across long distances (hundreds to thousands of meters) by cables or “tethers.”

While excellent in principle, in the past such tethered artificial gravity systems were generally frowned upon because in traditional “Battlestar Galactica”-type spacecraft designs, the only thing massive enough to serve as a useful counterweight on the other end of the tether from one functional part of the spacecraft would be another functional part of the spacecraft. In other words, if you wanted to provide artificial gravity to the crew habitat on one end of the tether, you probably would have to split your ship in half, and put a good portion of your propellant tanks on the other end. Such a configuration might work fine on paper, but would be an invitation to disaster in practice. If the tether should snag when you reeled it in, a large fraction of your mission critical hardware, such as your Earth-return propellant, would be permanently inaccessible, and, as a consequence, your mission would be lost. In the Mars Direct plan, however, this is not a problem. Because the crew is flying to Mars in a relatively lightweight habitat and not in an interplanetary Battlestar, their spacecraft is light enough to be counterbalanced on the tether’s other end by the burnt-out upper stage booster that threw them toward Mars (
Figure 5.2
). This item is not mission critical—it’s just junk and never needs to be reeled in. A similar tether scheme can be employed between the burnt-out upper stage of the ERV propulsion system and the ERV cabin during the flight home. Thus, except for some brief phases just before trans-Mars and trans-Earth injection, just before aeroentry at Earth and Mars, and just after aerocapture at Mars, the crew of a piloted Mars mission never need be subjected to a zero-gravity regime.

The tether used should be of a hefty interconnected multistrand variety, designed to remain intact even if several of the strands should be individually cut in numerous different places by micrometeorites or other space debris. Such “fail-safe” tethers have been designed and demonstrated by ae
rospace engineers Robert Forward and Bob Hoyt. The tether should also not be used as a wire for the transmission of large quantities of electric power. In the failed tethered satellite mission flown by the Space Shuttle in February of 1996, a power surge in the multikilowatt tether/power system caused the tether to melt itself and break.

FIGURE 5.2
Tethered artificial gravity system requires two objects swinging around a mutual center of gravity. For Mars Direct, the hab (on the right) is counterbalanced by the spent upper stage (on the left)
.

I’ve been asked how a rotating spacecraft will perform necessary maneuvers, such as the midcourse correction ΔVs of 20 meters per second or so that are typically necessary on interplanetary missions. Actually, it’s not that hard. Maneuvers have been performed on spinning spacecraft before.
Pioneer Venus Orbiter
and the
Pioneer Venus Probe Carrier
were spinning, interplanetary spacecraft with precise targeting requirements at Venus. They used repeated, timed thruster firings to create a net ΔV in any direction needed.

The Mars Direct mission tethered assembly would do much the same. For example, if you wantto create a ΔV in any direction that is within the spin plane of the spacecraft, you fire a thruster repeatedly along the line of the tether while the tether is pointing in the direction desired. Since the tether is taut, thruster firings that push the habitat toward the upper stage have the effect of reducing tether tension. As long as the thruster push is less than the centrifugal force, the tether will stay taut, so easy does it. Since the tether-spacecraft system rotates in a fixed plane, maneuvers in the rotation plane are accomplished by timing when the thrusters fire. Conversely, maneuvers out of the plane are accomplished with continuous, very low-thrust burning perpendicular to the plane of rotation.

A piloted Mars spacecraft has so much power (several kilowatts at least) that effective voice and e
ssential flight telemetry data communication with Earth can be achieved with an omnidirectional antenna. So, while the ship would use a high-gain antenna that could actively track the Earth as the ship rotates for high data rate video transmission, this item is not really mission critical. If the spin plane of the assembly is oriented such that it always faces the Sun, then any power-generating solar arrays used by the spacecraft need not be controlled by active gimbals either. Navigation scanning sensors are available that can operate just fine at rotation rates much higher than even 6 rpm, so these can be fixed to the habitat as well. In other words, none of these instruments requires a counterrotating platform to operate successfully on a tethered spacecraft.

In short, using artificial gravity on the Mars Direct spacecraft is thoroughly practical, and completely kills the zero-gravity dragon. At a conference a few years back I quizzed a NASA official who advocated a multidecade program investigating zero-gravity health effects on humans prior to a piloted Mars mission. “Why not just employ artificial gravity?” I asked. “We can’t do that,” he said—“all our data is going to be for zero gravity.” Get the picture?

HUMAN FACTORS

One of the more bizarre dragons that mar the charts of Mars navigators goes by the name of “The Human Factors Problem.” Some people assert that the psychological problems associated with a round-trip piloted Mars mission are unique and probably a show stopper. Either very fast ships that reduce the round-trip time to weeks, or else very large and luxurious ships that can accommodate large crews with ample social and physical space, must be used for the mission, they claim. Unless such concessions to the modern American suburban life-style are provided, they declare, the crew will surely “go crazy.” Unfortunately, since neither the ultra-fast space-warper nor the Club Med interplanetary cruise ship options are in fact feasible, these concerned parties recommend that any Mars mission be postponed until substantial sums have been spent in areas of “psychological research” to solve “The Human Factors Problem.” (Once again we hear the chorus of the now familia
r song, “Oh, you can’t go to Mars till you give us dough …”)

Let’s consider this argument. In the type of human Mars mission that we propose, a crew of four will spend six months on an outbound mission leg more or less confined to the interior of a two-deck hab containing a private room for each crew member as well as some common social areas (recreational space walks or “EVAs” are possible, especially if the mission is conducted in zero gravity, but we’ll put that option aside for now). The total interior floor space is about 101 square meters (1,083 square feet), somewhat small living space for a four-person apart by American standards, but rather large compared to the accommodations available to a middle-income apartment dweller in Tokyo. After the six-month voyage, the crew will land with their hab on Mars, and live there for a year and a half, during which time they will have extra living quarters available to them in the Earth return vehicle cabin on-site as well as in the pressurized rover. Moreover, during the long surface stay, the crew will be heavily occupied outside the hab conducting broad-ranging field exploration. Finally, during the last six months of the trip, the crew will be confined to the ERV’s cabin, which has about half the living space of the hab. During the entire voyage, normal telephone conversations with people on Earth will not be possible because of the time lag in radio signal transmission. Instead, voice, video, or text and still-image messages will have to be sent, with round-trip delays for reply ranging from seconds all the way up to forty minutes.

Well, it’s true that the above mission plan imposes psychological rigors on the crew not experienced by most civilians in the course of today’s daily life. But let’s compare it to the stresses that many ordinary people have overcome in the past.

The space shrinks talk a lot about the trauma of Mars mission crew members “being away from home for three years.” Well, my father, and my uncles, and several million other GIs were “away from home for three years” during World War II under vastly tougher conditions than those that will face the crew of Mars 1 (a dugout at the Anzio beachhead was a much more stressful environment than a habitat on the surface of Mars). In addition to the constant threat of death by enemy action, front line soldiers also
had to endure hard labor, low pay, cold, heat, insects, diseases, lice infestations, terrible rations, and sleeping on cold, wet ground in the snow or rain, sometimes for months on end. In addition, the vast majority of soldiers were enlisted men who had to endure the constant, brutal insults of the military discipline system, day and night being treated like dirt by 90-Day Wonders and other officers whose collective conceit was that rank made them superior beings. In contrast to these conditions, the crew of the first Mars mission may face risks, but not armies and fleets of people and war machines doing everything in their power to kill them.

The Mars crew will not have to endure extended hard physical labor. Insects, lice, and diseases are not part of their program. Their food will be good, and they will sleep in dry clothes in nice warm beds. During the interplanetary cruise phases of their mission, they may share some of the GI’s boredom, but this burden will be greatly lightened by a hefty on board supply of books, games, writing supplies, and other materials to support various hobbies or amusements, as well as by the knowledge shared by all members of the crew that when they get back to Earth their fortunes are made. Compared to the constantly degraded GI, the psychological boost enjoyed by Mars-bound astronauts of knowing that they are “golden people,” celebrated as heroes by millions on Earth cannot be overstated. During the war, the standard method of communication with those back home for the GI was by V-mail, with a round-trip communication time of several weeks. By comparison, the fact that the astronauts might have to wait up to forty minutes to hear back from their folks hardly seems cause for tears.