The first of these "other places'' that the space agencies of the world have their eyes fixedly focused on at the moment is the planet Mars. Compared to other possible landing sites in the solar system, Mars is a relatively hospitable planet, with an atmosphere and a reasonable temperature. However, the emphasis here is on the word relatively, as future Martian surface explorers will still require the protection of space suits. The atmosphere is composed of mainly CO2 (carbon dioxide), and the surface air pressure is less than 1% of that on Earth. Despite the tenuous nature of the atmosphere, winds often whip up dust storms that cover large areas of the Martian surface for several weeks at a time. The approximate average surface temperature is a frigid -50°C, and there is also concern that the Martian dust itself may be toxic to humans. And to top it all, due to the fact that Mars's magnetic field is very weak, the surface is pervaded by a flux of solar particle radiation that is attenuated only by the thin atmosphere.
Put in this way, it does make you wonder why people want to invest huge amounts of time, effort, and money to reach Mars! But it is unquestionably the next obvious step in the enterprise of manned exploration of the solar system, with destinations such as Venus and Jupiter ruling themselves out on the basis that they are even more inhospitable. The other main spur for a manned mission to Mars, from the science point of view, is the quest to find evidence of life there. We are not talking about little green men, but more likely the discovery of microbial life. The scientific community is wildly excited about this prospect, since it would tell us something about the occurrence and nature of life in places other than Earth. Again, the anti-manned spaceflight lobby is active in pointing out that this can be done equally well by robotic explorers on the Martian surface, so I guess at the end of the day it is going to be difficult to justify a manned Mars landing on this basis.
This brings us full circle once again to the issue of cost. The cost of a manned landing on Mars is difficult to estimate at this time, but incredible numbers like $1 trillion have been suggested—an order of magnitude increase in spending compared to the ISS or the return-to-the-Moon programs! Clearly, it is difficult to justify this magnitude of expenditure, other than to say things like "it is our destiny.'' And I think whether we finally go to Mars will hinge on the willingness of the international space-faring community to make this kind of financial commitment.
So how can it be done? Despite the fact that a manned Mars landing may be 30 years away, surprisingly a significant amount of work has been done by space agencies to answer this question. For example, both NASA and ESA (and other space agencies) have developed so-called reference missions, to define a Mars landing strategy, and to identify the technologies that will be required to enable the strategy to succeed. Although the reference missions differ in detail, there is nevertheless something of a consensus about the overall approach needed to land people on Mars. Surprisingly, the technology needed for this trip is available now, although some new technologies have been identified that could possibly allow the objective to be achieved at lower cost. The strategy discussed below is a mix of ideas from the various reference missions, but it gives a good idea of how people are thinking about tackling the job.
The basic strategy hinges on the idea of separating the transportation of crew from that of cargo. One day, perhaps 30 years hence, the momentous journey will begin with the unmanned launch of cargo into Earth orbit. At least two such launches will be needed—one to carry an Earth Return Spacecraft, and the other to carry the Surface Cargo Module. Each of these payloads will be of significant mass, on the order of around 150 metric tonnes. After checkout, each spacecraft will be boosted independently out of Earth orbit into a trajectory to take them on their way to Mars. Both of these unmanned elements will use a slow trajectory to Mars, effectively the Hohmann transfer that we talked about in the propulsion section of Chapter 9. The Hohmann transfer to Mars is shown as the slow trajectory in Figure 10.6. The main attribute of this type of transfer, you may recall, is that the amount of rocket fuel required is minimized, reducing overall costs. Nevertheless, to boost each cargo spacecraft out of Earth orbit requires the rocket engines to provide a A V (a change in speed) of about 3.6 km/sec (2.2 miles/sec). If a high-performance, but conventional chemical propulsion system is used, this still means that about 80 metric tonnes of the initial 150 metric tonnes will be rocket fuel.
After a trip of 259 days, each of the unmanned spacecraft finally approach Mars on a hyperbolic trajectory (see Chapter 4). To prevent them from just swinging by the planet, their speed needs to be reduced so that they can be captured in an orbit around Mars. The obvious thing to do at this point is to fire rocket engines to nudge each spacecraft into orbit, but this again would cost a significant mass of rocket fuel. To save this fuel mass, some reference missions propose the use of aerobraking to achieve Mars orbit. The key to this is the Martian atmosphere. On arrival, each spacecraft dips into the atmosphere and uses the resultant aerodynamic drag to slow down into Mars orbit. Of course, there is a price to pay for this: each spacecraft will require some kind of shield to protect it from frictional heating caused by the rapid passage through the atmosphere. However, calculations show that the mass of this thermal shield is less than the amount of propellant that would be required if the orbit were achieved through firing rockets.
At this point, the paths of the two cargo vehicles diverge. The Earth Return Spacecraft will stay in orbit for a period of years until the astronauts—who at this point are still on Earth—return from their surface exploration. Its job is to take the astronauts home once the mission is completed. On the other hand, the Surface Cargo Module is destined for the Martian surface, where it will wait for the human crew to arrive. As the name implies, this will carry all sorts of equipment that will be useful for the human crew, such as gear for research and exploration, an electrical power plant probably in the form of a nuclear reactor, materials for extending living and laboratory space, surface rovers, and an ascent vehicle to allow the crew to return to Mars orbit at the end of the surface mission. There is also the rather speculative idea of including a fuel production plant to manufacture methane and liquid oxygen from local resources on the surface to fuel the ascent vehicle. The mathematics suggests that an overall saving in mass can be achieved by doing this, but it may not do much for the peace of mind of the crew! So far, so good, but the manned mission has yet to begin!
Once the unmanned spacecraft are in their proper places on and around Mars, and have been checked out to make sure they are in working order, the manned part of the mission can begin. However, to await a suitable planetary alignment, this part of the project will not begin until about 3 years after the launch of the unmanned elements. The Crew Transit and Habitation Module, with a mass of about 150 metric tonnes, will be lofted to low Earth orbit without a crew by a heavy lift launch vehicle. This will be followed within a few days by the crew, probably in an Orion capsule. Once in orbit, the Orion spacecraft will rendezvous with the Crew Transit and Habitation Module to allow the crew to transfer ready for departure. To reduce the travel time to Mars, the Crew Transit and Habitation Module will be boosted into a fast trajectory, as shown in Figure 10.6, at the expense of increased AVand fuel mass. This price is considered to be worth paying, in order to shorten the voyage to about 130 to 150 days, so that the harmful effects of microgravity and radiation exposure on the crew can be reduced.
Another way of decreasing the physical effects of weightlessness is to use artificial gravity onboard the vehicle. This is the idea of using rotation to produce the sensation of weight. You may have seen films of astronauts being tested for the effects of high launch accelerations by sitting in a big centrifuge. As the speed of rotation of the centrifuge increases, the unfortunate occupants sense a steady increase in their effective weight. In the early days of spaceflight, when astronauts were made of "The Right Stuff'' (to quote Tom Wolfe's title of his book about the early astronauts), these machines were used to subject astronauts to levels of acceleration of 8g (and beyond), when the subject effectively weighs eight times their normal weight. Engineers have considered the idea of installing centrifuge-type devices on manned spacecraft destined for the planets so as to combat the physical effects of long-term weightlessness. Another variant is to design the spacecraft so that it consists of two modules attached by a tether system. The two parts are then set in rotation about each other so that the astronauts in each module experience artificial gravity, the level of which is dependent on the rate of rotation. This technology, however, was considered to be inappropriate for the relatively short voyage to Mars, principally on the grounds of increased complexity, mass, and cost.
On arrival at Mars, the Crew Transit and Habitation Module will aerobrake into orbit and descend to the surface, landing within easy walking distance of the waiting Surface Cargo Module so that the surface exploration mission can begin. Figure 10.7 shows an artist's impressions of Martian surface exploration. Hopefully, one day the artist's brush will be replaced by the actuality of photographic images! When the surface stay is over, the crew returns to Mars orbit to dock with the waiting Earth Return Spacecraft. The ascent vehicle is then jettisoned, before the Earth Return Spacecraft is boosted out of Mars orbit for the cruise home. The final act of the mission is a direct entry into Earth's atmosphere, and a parachute descent to a safe landing.
Even in this brief outline, the equipment list for the Mars mission is more extensive than that proposed for Moon missions. This is because the length of stay on the surface of Mars is necessarily much longer, resulting in the need to establish a semipermanent manned outpost during the first landing. This length of stay is dictated by the physics of the motion of the planets around the Sun. To return to Earth, the crew will have to wait for a particular planetary alignment between Mars and Earth, so that the first stay on Mars will probably be many months in duration.
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