drift away, the greater force of gravity on it would pull that end back toward the earth again. Observatory-size spacecraft were also large enough to carry additional rockets which, when fired against the direction of the satellite's motion, could return the spacecraft to earth, where its equipment and records could be recovered. Once on its way down, the spacecraft would have to be protected against heat generated by friction of the atmosphere, for which purpose retrorockets—that is, rockets fired in the direction of motion to slow the spacecraft-or parachutes, or a combination of the two, could be used. Finally, there were the manned spacecraft (figs. 19-22). These were even larger than the unmanned observatories, since, in addition to housekeeping equipment and instruments, the spacecraft had to be completely maneuverable to return the astronaut to earth after the mission and also had to afford a suitable environment for the crew. All three classes of satellites had their uses. The manned spacecraft introduced the element of exploration-of personal, on-the-spot investigation-into the program. With men aboard it became possible to extend laboratory research into the environment of space, as was done dramatically in Skylab and the Apollo-Soyuz Test Project.26 When Space Shuttle development was begun in the early 1970s, one motivation was to make it relatively easy for researchers to perform experiments in space laboratories. At the other end of the spectrum, Explorer-class satellites permitted scientists to perform a wide range of space science experiments, as shown in table 5. To the scientists, the lower costs meant that more of the funds available could be put into the scientific research itself. Also, the effort required to put an experiment into an Explorer was considerably less than that required for an observatory-one or two years as opposed to many years. When a discovery was made, an experimenter could more quickly follow up with new experiments on sounding rockets and Explorers than he could using observatories. Moreover, not having to fit his instruments alongside those of many other experimenters-with all the problems of electrical, radio, and other kinds of interference-the experimenter could exercise greater control over his experiment. These advantages account for the unvarying insistence of the scientific community that NASA continue to provide sounding rockets and small satellites. Whenever larger projects appeared to threaten the funding of the smaller ones, the scientific community rose in defense of the smaller. Over this issue the scientists came as near to unanimity as they ever did. On the other hand, some investigations required a greater capability than the Explorers afforded. Astronomy experiments that required large telescopes and precision pointing are one example. Bioscience experiments that required the recovery of specimens after exposure to the environment of space-as in the Biosatellite-are another.27 The Orbiting Geophysical Observatory afforded the means of conducting 20 or more experiments on |