The Case for Mars (14 page)

It was a small conference—just one hundred or so people eventually attended—but to the organizers, these were legions. Before the conference, the Colorado group felt more or less alone in the wilderness. There were not many, they thought, who had the interest and know-how to undertake a serious study of humans-to-Mars missions. But now here they were, in the midst of their conference with presentations on resource utilization, life support on the Martian surface, propulsion options. It was heartening, thrilling, indeed liberating to know that there were others who shared their passion. Leonard David had arrived from Washington with a bundle of red buttons. Imprinted on the buttons, below a Case for Mars logo (depicting a DaVinci-style human figure inside the ancient astrological symbol for Mars), were the words “Mars Underground.” A short note accompanied each button, stating that the wearer was now a member of the “Mars Underground,” an ad hoc collective of Mars aficionados (“tightly knit but loosely woven”), and that the button should be worn discreetly, under the lapel, or perhaps inside the coat. Over the course of four days, numerous workshops, and a slew of presentations, the Mars Underground formulated plans for the human exploration of Mars: the whys and wherefores of the program; precursor missions to the human missions; mission profiles; and rosters of surface activities for explorers. Not a bad result from a conference conceived and organized by a bunch of graduate students.

The conferences have continued, one every three years, each building on what preceded and reflecting the character of th
e times. The second conference, in 1984, resulted in a complete end-to-end design for a Mars mission that d.& members used as the basis for a two-hour presentation on Mars exploration they delivered at NASA headquarters and other NASA centers. The 1984 conference was also notable in that it drew to the group people of greater political influence, such as former NASA administrator Thomas Paine. In 1985, President Reagan appointed Paine to head the blue-ribbon National Commission on Space, who then proceeded to guide it to a recommendation that the United States make the establishment of a human outpost on Mars the thirty-year goal of the space program. The White House responded to the report by setting up the “Code Z” organization and “Pathfinder” programs at NASA headquarters, to respectively plan mission strategies and develop the key technologies required for human expansion to the Moon and Mars. It was these organizations that formed the insider network which provided the policy input for Bush’s call for a Space Exploration Initiative in July 1989.

The third Case for Mars conference accelerated the trend, with Carl Sagan giving the keynote address to an audience of over a thousand people, including a substantial representation of the international press. I had first heard about the Mars Underground after Case for Mars II, and along with more than four hundred other out-of-town technical types, went to Case for Mars III to participate in some of the nearly two hundred presentations and sixteen workshops. The two-volume set of papers arising out of Case for Mars III outlines strategies for Mars exploration that touch upon both the technical requirements as well as public policy and political requirements for transforming the vision into a reality. By the fourth conference in 1990 (and held, as always, in Boulder) what had a decade earlier been nearly
verboten
to speak of in NASA—humans to Mars—had become the current president’s stated long-term goal in space. Carol Stoker, who was in charge of the conference scheduling, had attended a private Mars Direct pitch at NASA’s Ames Research Center in California, and liked the plan. She gave the bully pulpit at the opening plenary session to David Baker and me to present Mars Direct to the assembled Mars Underground. The next day, news that a low-cost manned Mars mission plan was now on the table appeared in the
Boston Globe
and dozens of syndicated papers.

Mapping the trajectory of a spacecraft is a relatively straightforward business, bounded only by the laws of physics. Mapping the trajectory of an idea through a political system, on the other hand, can be a dicey business. There were many reasons why George Bush stood on the Air and Space Museum’s steps in 1989 and declared Mars to be a necessary destination for human exploration, but I have no doubt that the Case for Mars conferences and the small group of individuals who form the core of the Mars Underground were instrumental in positioning a human journey to Mars as an attainable, realistic goal for the United States space program. The conferences provided the cauldron for a brew of ideas, all of which served to heighten the profile of human missions to Mars and all of which energized the community of Mars researchers and enthusiasts. For an organization where membership is defined by enthusiasm and effort rather than a spot on a membership roll or a card carried in a wallet, the Mars Underground and the Case for Mars conferences have to be credited with having influence far beyond their modest size.

It is in honor of their efforts that I have chosen the title of this book.

4: GETTING THERE

FAST MISSIONS AND GOOD MISSIONS

In planning a long journey, you firs choose a route and a mode of transportation. So it is with Mars.

Many believe a voyage to Mars is impossible because the Red Planet is so far away from Earth. Until such time as radically more advanced types of space propulsion become available, they argue, the trip will simply take too long. Let’s take a look at this objection.

Mars is indeed far away. At its closest approach, when it stands directly on the opposite side of the Earth from the Sun (a condition that ancient astrologers, with their geocentric world view, described as an “opposition,” of which more anon) it never gets nearer than 56 million kilometers, or 38 million miles. At its farthest, when it stands behind the Sun as seen from the Earth (what the ancient astrologers called a “conjunction”), it lies about 400 million kilometers distant. (See
Figure 4.1
.) Now, no propulsion system is even on the drawing boards that can push directly away from the Sun and perform the transit between Earth and Mars in a straight line when the two are in opposition. This is because a spacecraft leaving Earth possesses the velocity of the Earth, some 30 kilometers per second (km/s), and thus, unless massive amounts of propellant are expended t
o alter course, the spacecraft will continue to circle the Sun in the same direction as the Earth. In fact, as the German mathematician W. Hohmann discovered in 1925, if you want to go easy on the gas, the best time to travel from Earth to Mars occurs when the two planets are in conjunction; at their
maximum
distance from each other on opposite sides of the Sun. (See
Figure 4.2
.) This is the easiest way to go, because if you take this path you can travel along an ellipse which is tangent to the Earth’s orbit at one end, and tangent to Mars’ orbit at the other, thus minimizing the course change which is required for the spacecraft to depart or rendezvous with each. You can deviate from such a flight plan if you wish, but the more you do, the harder your propulsion job, and the costlier your mission. But even if you do decide to pour on some extra gas to cut corners and avoid the full Hohmann transfer, roughly speaking you’ll likely need to traverse at least 400 million kilometers along some curving arc to get from Earth to Mars. Four hundred million kilometers. That’s a lot. In contrast, the Earth’s Moon is “only” 400,000 kilometers away. So to get to Mars, you’ll have to travel a
thousand times
farther than the Apollo astronauts did when they voyaged to the Moon. It took three days for the Apollo spacecraft to make a one-way lunar transit. Will it take 3,000 days,
eight years
, to reach Mars?

FIGURE 4.1
Opposition and conjunction. At opposition, Mars stands directly on the opposite side of the Earth from the Sun. At conjunction, Mars stands behind the Sun as seen from the Earth
.

FIGURE 4.2
Trajectory Options to Mars: (A) Hohmann transfer orbit; (B) Fact Conjunction mission; (C) Opposition mission
.

Fortunately, no. Apollo astronauts traveled between the Earth and Moon with an average speed of about 1.5 km/s. This speed limit was set not by the limits of the propulsion technology of the time—the third stage of the Saturn V could have rocketed the Apollo spacecraft toward the Moon at double or even triple this velocity—but by the nature of the mission geometry. The Apollo astronauts could have been fired off to the Moon at 4mit wd reached it in a single day, but ther
e would have been a huge price to pay: they would have been unable to stop. Because of weak Lunar gravity, a spacecraft’s propulsion system has to do nearly all the work required to capture a trans-lunar spacecraft into lunar orbit. The Apollo command module simply could not have decelerated the spacecraft if it had been tearing by the Moon at a speed much greater than 1.5 km/s.

Mars, on the other hand, has substantial gravity and an atmosphere, both of which can assist in facilitating a deceleration maneuver. So, a spacecraft can reach Mars with a much greater approach velocity and still manage to capture itself into orbit. More importantly, a spacecraft leaving Earth with a departure velocity (technically known as “hyperbolic velocity”) of 3 km/s does not fly across the solar system with a mere 3 km/s speed. Rather, in leaving the Earth, the spacecraft is leaping off a very fast moving platform, and since it is moving in the same direction, picks up an extra 30 km/s of velocity from the Earth as it races its way around the Sun. The spacecraft voyages across the solar system with an initial velocity not of 3 km/s, but 33 km/s, more than
twenty times
the speed of an Apollo command module. (You can’t use this “moving platform” effect to help you reach the Moon, because the Moon is moving about the Sun in company with the Earth.) As it climbs out of the Sun’s gravity well to move outward from the orbit of Earth to that of Mars, it trades some of the kinetic energy associated with this velocity into potential energy, and so slows down a bit, but it’s still moving very fast. Fortunately, Mars will be cruising along its orbit with a velocity of 24 km/s in roughly the same direction as the spacecraft. When the spacecraft reaches Mars’ orbit, its velocity relative to Mars will be only 3 km/s (since it’s traveling along at about 21 km/s), and that’s slow enough to allow orbit capture. By the time the spacecraft reaches Mars, it’s traveled a thousand times farther than the Apollo astronauts, but, on average, about twenty times faster. One thousand times farther divided by twenty times faster gives us a travel time that is a factor of fifty greater than the three-day transits for the Apollo astronauts—150 days, Earth to Mars. This then is a rough estimate of the travel time for a one-way transit to Mars using Apollo-era or present-day technology for propulsion. It’s not a bad estimate either; while a Hohmann transfer actually takes 258 days, a 150-day transi
t is certainly achievable at the cost of some extra propellant.

But getting to Mars is only half the problem—you also have to get back. Earth and Mars are constantly moving around the Sun, and, since they travel at different speeds, they are constantly changing their positions relative to one another. Because only certain Earth-Mars relative positions are appropriate for launching a return flight, the trajectory you pick not only determines how long you will have to travel,
it also determines when you can leave.
Determining flight plans for complete round-trip missions thus starts to get pretty complicated, but when all is said and done, you are left with basically two options for a piloted Mars mission round-trip flight plan. These two options are known as
conjunction-
and
opposition
-class missions. Typical parameters for each of these mission types are given in
Table 4.1
.

TABLE 4.1.
Flight Times and Stay Times of Mars Missions