The Case for Mars (16 page)

The question then is how many people are really needed on a piloted Mars mission. Put another way, whom do we really need? If the mission were to fail, far and away the most likely cause would be failure of one or more of the mission-critical mechanical and electrical systems employed (propulsion, control, life support). The most important member of the crew then, the one on whom all others depend for their lives, is the
mechanic.
Call this person a flight engineer if you wish (he or she is an engineer in the sense of an old-time railroad locomotive or steamship engineer), but the mission needs an ace mechanic who can sniff out problems before they occur and fix anything that can be fixed. This job is so critical that, despite the need for small crews, I recommend carrying two people capable of handling it.

The next most important job on the mission is that of
field scientist.
Remember, the exploration of Mars is the raison d’être of a human mission to Mars. After those needed to get the crew to Mars and back, the next most important personnel are those essential to competently carry out the mission’s exploration goals. Since no science return would effectively be a form of mission failure, once again I recommend carrying two
people for the job. One of the field scientists should be a geologist, oriented toward exploring the resources and understanding the geologic history of Mars. The other should be a biogeochemist, directed toward exploring those aspects of Mars upon which hinge the question of past or present life. The biogeochemist would also conduct experiments to determine the chemical and biological toxicity of Martian substances to terrestrial plants and animals, and the suitability of local soils to support greenhouse agriculture.

And that’s it. With two mechanics and two field scintists, we have the ability to split the crew into two groups in which no one is alone (one out in the field with a ground rover, say, while the other remains at the base camp) and someone expert at fixing malfunctioning equipment and someone capable of doing scientific work are present at all times. There is no need for people whose dedicated function is “mission commander,” “pilot,” or “doctor.” True, the mission will need someone who is in command, and a second in command for that matter, because in dangerous circumstances it is necessary to have someone who can make quick decisions for all without electioneering or debate. But there is no room for someone whose sole function is to manage others to get the job done. Similarly, there can be no one on board whose job description is “pilot.” The spacecraft will be capable of fully automated landing, and piloting skills would at most be useful as a contingency backup to the automated flight system during a few minutes of the two-and-one-half-year mission. If such a manual flight control backup is desired, then one or more members of the crew could be given cross training as a pilot (it’s much easier to train a geologist to be a pilot than a pilot to be a geologist). Finally, no ship’s doctor as such. The great Norwegian explorer Roald Amundsen always refused to carry doctors on his expeditions, noting that they were injurious to morale and that the large majority of medical emergencies that occur on expeditions can be handled just as well by experienced explorers. And, truth be known, behind their public relations facade, nearly all astronauts hate space doctors. You would too in their shoes—just think about trying to get a hard job done while somebody constantly jabs you with needles, wires, and thermometers. In place of a physician, all crew members will be trained in first aid, and expert systems on boa
rd and medical consultation from Earth will be available to diagnose readily treatable conditions (ear infections and the like). Such diagnoses could be assisted by having a crew member be someone who had either practiced general medicine at some point earlier in his or her career or who had been crossed-trained to a medical assistant’s level of knowledge, and equipping this individual with a country doctor’s black bag and a stock of broad-spectrum anti-biotics. The biogeochemist would be a natural candidate for such a cross-trained role. However, the idea of having a dedicated top-notch doctor on board who spends his or her time reading medical texts and honing skills by practicing surgery with virtual reality gear, or worse, being a pest by subjecting the rest of the crew to an in-depth medical study, is cumbersome and unnecessary.

To summarize in
Star Trek
terminology, what a piloted Mars mission needs are two “Scottys” and two “Spocks.” No “Kirks,” Sulus,” or “McCoys” are needed, and more importantly, neither are the berths and rations to accommodate them.

We can do the mission with a crew of four.

DIRECT LAUNCH

All interplanetary missions flown to date have been flown by “direct launch”—a launch vehicle lifts a spacecraft to LEO, and then uses its upper stage to throw the spacecraft on a trajectory to its planetary destination. It’s the way the
Mariner
and
Viking
missions reached Mars, and it’s also the way the Apollo lunar missions reached the Moon. No missions beyond LEO have ever been flown by lifting the payload to an orbiting spaceport and transferring it to a freshly fueled interplanetary cruiser just back from Saturn. No missions beyond LEO have ever been flown on an interplanetary spaceship constructed in space. The association in many people’s s of humans-to-Mars missions with such futuristic spaceship/spaceport scenarios has caused human Mars exploration to be relegated out of today’s world and into the world of “The Future.” But, if a manned Mars mission can be done by
direct launch
, then we can do it. Get rid of the spaceships and spaceports, and a human mission to Mars moves from the “parallel universe” of The Future into
our
universe. If we can do it
by direct launch, then 90 percent of everything we need to send humans to Mars is available now.

We’ve chosen the trajectory and the crew size. Now, can a realistic heavy-lift launch vehicle deliver, in no more than two tandem launches per mission, everything that’s needed to conduct a four-person Mars mission in accord with the flight plan we’ve chosen? Let’s see.

There is nothing magical about a heavy-lift vehicle—the United States built and operated one thirty years ago. The Saturn V booster that sent the Apollo astronauts to the Moon went into operation in 1967 after a five-year development program, and operated without a single failure for an eight-year period until 1973 when the last working unit launched the Skylab space station. The Saturn V could lift 140 tonnes to LEO. If we wanted equivalent capability today, one foolproof way of doing it would be to reengineer the dies and start producing Saturn V’s. There are other ways to get the job done, though. For example, using Space Shuttle hardware it’s possible to produce a heavy-lift booster in the same class by attaching a pod of four Space Shuttle main engines (SSME) to the bottom of a Space Shuttle external tank (ET), attaching two Space Shuttle solid rocket boosters (SRBs) to either side of the ET, and positioning a hydrogen/oxygen upper stage on top of the ET. This is the Ares booster design David Baker developed for Mars Direct. Depending on the thrust of the upper-stage motor used, an Ares could deliver between 121 tonnes (with a 250,000 lb upper-stage thrust) and 135 tonnes (with a 500,000 lb thrust SSME on the upper stage) to LEO. The Russians have a heavy-lift vehicle right now called the Energia. The existing model can only lift 100 tonnes to LEO, but an upgraded design, the Energia-B, boasts a 200 tonne capability. During the Space Exploration Initiative’s short life, NASA developed dozens of heavy-lift booster designs of all sorts with capacities between 80 and 250 tonnes. In short, if the United States wants a heavy-lift booster, we can certainly get one.

While on paper a booster can be designed to any size desired, reality is different. Some super boosters have been designed with 1,000-tonnes-to-LEO capability. Sounds great, but they would probably blow away Orlando when they took off (or at least the Kennedy Space Center). So let’s be exceptionally conservative and assume that the United States—
today—can build a heavy-lift booster with a capability no greater than the one we fielded in the 1960s. Let’s baseline our booster at 140-tonnes-to-LEO capability, exactly the same as a Saturn V. Would such a launch system be good enough to launch the Mars Direct mission by direct throw?

Part of the answer to this question is given in
Table 4.3
, which shows the amount of payload that a single launch of our 140-tonne-to-LEO booster can deliver to the Martian surface after a preliminary aerocapture at Mars. The table shows variants for both the cargo and piloted outbound trajectory, and for the assumption that the third stage of this vehicle is either a state-of-the-art hydrogen/oxygen chemical stage with a specific impulse of 450 seconds, or a near-term nuclear thermal rocket (NTR) with a specific impulse of 900 seconds.

TABLE 4.3
Payload Delivery to Martian Surface from 140 Tonne to LEO HLV

The payload delivery capabilities shown in
Table 4.3
assume that aerobraking is employed to capture the spacecraft into Mars orbit. This is clearly the optimal way to perform Mars orbital capture (MOC) in the Mars Direct missions because all the payload is destined for the Martian surface, and so it must carry an aeroshield in any case. Using aerocapture in the Mars Direct mission thus eliminates a significant propulsive ΔV essentially “for free.” If rocket propulsion had to be employed for this maneuver instead of aerobraking, the payloads delivered would be about 25 percent less. In mission plans such as that in the NASA 90-Day Report, aerocapture faced many technical difficulties. Aerobraking the enormous “Battlestar Galactica” spacecraft the plan called for would require huge aeroshields that could only be built in orbit, and that, as I’
ve noted, is really not a credible proposition. Furthermore, the opposition-class trajectories employed by those missions hit Mars really hard, thereby increasing the heating and mechanical loads on the aerobrakes during atmospheric entry. Mars Direct uses lower energy conjunction-class trajectories that have lower entry velocities and thus lower heating rates and experience much lower aerodynamic deceleration forces. More decisively, the spacecraft that need to be decelerated in the Mars Direct plan are relatively small, so that aeroshields big enough to protect them can easily be made to fit inside the launch vehicle’s payload fairing. This can be done in one of two ways; either by using flexible fabric umbrella-shaped aerobrakes that fold around the bottom of the payloads, as in the original Mars Direct design, or by replacing the fairing of the launch vehicle with a rigid, bullet-shaped shell that fits over the payload from on top. Both are feasible, and when used with Mars Direct-sized payloads, either can be launched “all-up” without any need for on-orbit assembly. In addition, the guidance, navigation, and control requirements on Mars Direct aerocapture are less than in those plans where a subsequent Mars orbit rendezvous is anticipated, because it does not really matter exactly what orbit the vehicle captures into (since the orbit will be “erased” after the vehicle lands), so long as its orbital inclination is within the broad tolerances that will allow access to the designated landing site.

To deliver payloads, we can also employ an approach known as direct entry. As with aerocapture, aerodynamic drag against a planet’s atmosphere, not rocket propulsion, decelerates the payload. There is a difference, though. With aerocapture, the spacecraft dips into the planet’s atmosphere just enough to slow down and then reemerge from the atmosphere to place itself in orbit. In the case of direct entry, the entering spacecraft plunges deep into the atmosphere until all of its velocity is shed and then proceeds directly to landing. Most people consider aerocapture the better flight plan for a piloted Mars mission, because if the weather is bad, it allows the crew to assume a station on orbit until conditions improve for a landing. With direct entry, the vehicle is completely committed for a landing immediately after Mars arrival. Nevertheless, direct entry will be used on both the
Mars Pathfinder
and
Mars Surveyor ’98
(unmanned) missions, currently scheduled for launch in 1996 and 1998, respectively. If these missions are successful
, a data base will exist that may encourage mission designers to employ direct entry on the piloted Mars mission as well.

The bottom line in allof this, however, is the payload delivered to the surface. If chemical propulsion is used, then the unmanned cargo flight launched by a single 140-tonne-to-LEO booster can deliver 28.6 tonnes to the Martian surface, while the faster piloted flight can deliver 25.2 tonnes. Can a manned Mars mission be designed within these mass limits? If it can’t, we could always design a bigger booster or go ahead and develop the NTR stage. But let’s see if we can make it with nothing better than a Saturn V and chemical propulsion. If we can, then more advanced technologies or propulsion capabilities and their associated benefits are icing on the cake.