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Solar Sailing and Comet Rendezvous. In ordinary interplanetary missions, spacecraft are obliged to follow trajectories that require a minimum expenditure of energy. The rockets burn for short periods of time in the vicinity of Earth, and the spacecraft mainly coast for the rest of the journey. We have done as well as we have not because of enormous booster capability, but because of great skill with severely constrained systems. As a result, we must accept small payloads, long mission times and little choice of departure or arrival dates. But just as on Earth we are considering moving from fossil fuels to solar power, so it is in space. Sunlight exerts a small but palpable force called radiation pressure. A sail-like structure with a very large area for its mass can use radiation pressure for propulsion. By positioning the sail properly, we can be carried by sunlight both inwards toward and outwards away from the Sun. With a square sail about half a mile on each side, but thinner than the thinnest Mylar, interplanetary missions can be accomplished more efficiently than with conventional rocket propulsion. The sail would be launched into Earth orbit by the manned Shuttle craft, unfurled and strutted. It would be an extraordinary sight, easily visible to the naked eye as a bright point of light. With a pair of binoculars, detail on such a sail could be made out-perhaps even what on seventeenth-century sailing ships was called the “device,” some appropriate graphic symbol, perhaps a representation of the planet Earth. Attached to the sail would be a scientific spacecraft designed for a particular application.

One of the first and most exciting applications being discussed is a comet-rendezvous mission, perhaps a rendezvous with Halley’s comet in 1986. Comets spend most of their time in interstellar space and should provide major clues on the early history of the solar system and the nature of the matter between the stars. Solar sailing to Halley’s comet might not only provide close-up pictures of the interior of a comet-about which we now know close to nothing-but also, astonishingly, return a piece of a comet to the planet Earth. The practical advantages and the romance of solar sailing are both evident in this example, and it is clear that it represents not just a new mission but a new interplanetary technology. Because the development of solar-sailing technology is behind that of ion propulsion, it is the latter that may propel us on our first missions to the comets. Both propulsion mechanisms have their place in future interplanetary travel. But in the long term I believe solar sailing will make the greater impact. Perhaps by the early twenty-first century there will be interplanetary regattas competing for the fastest time from Earth to Mars.

Mars Rovers. Before the Viking mission, no terrestrial spacecraft had successfully landed on Mars. There had been several Soviet failures, including at least one which was quite mysterious and possibly attributable to the hazardous nature of the Martian landscape. Thus, both Viking 1 and Viking 2 were, after painstaking efforts, successfully landed in two of the dullest places we could find on the Martian surface. The lander stereo cameras showed distant valleys and other inaccessible vistas. The orbital cameras showed an extraordinarily varied and geologically exuberant landscape which we could not examine close up with the stationary Viking lander. Further Martian exploration, both geological and biological, cries out for roving vehicles capable of landing in the safe but dull places and wandering hundreds or thousands of kilometers to the exciting places. Such a rover would be able to wander to its own horizon every day and produce a continuous stream of photographs of new landscapes, new phenomena and very likely major surprises on Mars. Its importance would be improved still further if it operated in tandem with a Mars polar orbiter which would geochemically map the planet, or with an unmanned Martian aircraft which would photograph the surface from very low altitudes.

Titan Lander. Titan is the largest moon of Saturn and the largest satellite in the solar system (see Chapter 13). It is remarkable for having an atmosphere denser than that of Mars and is probably covered with a layer of brownish clouds composed of organic molecules. Unlike Jupiter and Saturn, it has a surface on which we can land, and its deep atmosphere is not so hot as to destroy the organic molecules. A Titan entry-probe and lander mission would probably be part of a Saturn orbital mission, which might also include a Saturn entry probe.

Venus Orbital Imaging Radar. The Soviet Venera 9 and 10 missions have returned the first close-up photographs of the surface of Venus. Because of the permanent cloud pall, the surface features of Venus are not visible through Earth-bound optical telescopes. However, Earth-based radar and the radar system aboard the small Pioneer Venus orbiter have now begun to map Venus surface features, and have revealed mountains and craters and volcanoes as well as stranger morphology. A proposed Venus orbital imaging radar would provide pole-to-pole radar pictures of Venus with much higher detail than can be achieved from the surface of the Earth, and would permit a preliminary reconnaissance of the Venus surface comparable to that achieved for Mars in 1971-72 by Mariner 9.

Solar Probe. The Sun is the nearest star, the only one we are likely to be able to examine close up, at least for many decades. A near approach to the Sun would be of great interest, would help in understanding its influence on Earth, and would also provide vital additional tests of such theories of gravitation as Einstein’s General Theory of Relativity. A solar probe mission is difficult for two reasons: the energy required to undo the Earth’s (and the probe’s) motion around the Sun so it can fall into the Sun, and the intolerable heating as the probe approaches the Sun. The first problem can be solved by launching the spacecraft out to Jupiter and then using Jupiter’s gravitation to fling it into the Sun. Since there are many asteroids interior to Jupiter’s orbit, this might possibly be a useful mission for studying asteroids as well. An approach to the second problem, at first sight remarkable for its naïveté, is to fly into the Sun at night. On Earth, nighttime is of course merely the interposition of the solid body of the Earth between us and the Sun. Likewise for a solar probe. There are some asteroids that come rather close to the Sun. A solar probe would approach the Sun in the shadow of a Sun-grazing asteroid (meanwhile making observations of the asteroid as well). Near the point of closest approach of the asteroid to the Sun, the probe would emerge from the asteroidal shadow and plunge, filled with a fluid that resists heating, as deeply into the atmosphere of the Sun as it could until it melted and vaporized-atoms from the Earth added to the nearest star.

Manned Missions. As a rule of thumb, a manned mission costs from fifty to a hundred times more than a comparable unmanned mission. Thus, for scientific exploration alone, unmanned missions, employing machine intelligence, are preferred. However, there may well be reasons other than scientific for exploring space-social, economic, political, cultural or historical. The manned missions most frequently talked about are space stations orbiting the Earth (and perhaps devoted to harvesting sunlight and transmitting it in microwave beams down to an energy-starved Earth), and a permanent lunar base. Also being discussed are rather grand schemes for the construction of permanent space cities in Earth orbit, constructed from lunar or asteroidal materials. The cost of transporting materials from such low-gravity worlds as the Moon or an asteroid to Earth orbit is much less than transporting the same materials from our high-gravity planet. Such space cities might ultimately be self-propagating-new ones constructed by older ones. The costs of these large manned stations have not yet been estimated reliably, but it seems likely that all of them-as well as a manned mission to Mars-would cost in the $100 billion to $200 billion range. Perhaps such schemes will one day be implemented; there is much that is far-reaching and historically significant in them. But those of us who have fought for years to organize space ventures costing less than one percent as much may be forgiven for wondering whether the required funds will be allocated, and whether such expenditures are socially responsible.