The radio science investigations are divided into two groups. The first deals with the atmosphere of Jupiter. During the Voyager flybys, the spacecraft passed behind the planet as seen from Earth, and the radio signal was dimmed by the atmosphere before it was finally extinguished. During an occultation, the propagation of the radio waves is slowed down by passage through the neutral atmosphere and is speeded up by passage through the electrically charged ionosphere. Because of the extreme stability of the ground-based and spacecraft radio transmitters, it is possible to measure these shifts in the signal with high precision. The shifts are proportional to electron density for the ionosphere, and to gas density for the atmosphere. From a careful study of the interactions of the transmitted beam with the Jovian atmosphere, Eshleman and his colleagues can reconstruct a temperature-pressure profile of the ionosphere and the upper atmosphere of Jupiter. The same approach can be used to search for tenuous atmospheres on the satellites. The second area of study is in the field of celestial mechanics. The frequency stability of the communications system permits measurements of the speed of the spacecraft, relative to Earth, to a precision of one part in several million. By careful tracking, gravitational perturbations on the spacecraft can be detected and used to measure the gravitational fields, and hence the masses, of Jupiter and its satellites. [Illustration: Rochus E. Vogt, cosmic ray Principal Investigator] [Illustration: Von R. Eshleman, radio science Team Leader] These scientific instruments and their objectives were selected many years before the first Jupiter encounter in March 1977. Because Voyager was an exploratory mission, every effort was made to fly versatile instruments that could yield valuable results no matter what the nature of the Jovian system. In addition, the Voyager spacecraft control system permitted the instruments to receive commands from Earth to adjust their sensitivities and observing sequences in response to new information. By the spring of 1977, all the instruments were completed, ready to be installed in the Voyager spacecraft for testing and launch. [Illustration: The first picture to capture crescent Earth and crescent Moon in the same frame was taken by Voyager 1, the second-launched spacecraft, on September 18, 1977, at a distance of 12 million kilometers from Earth. On the Earth eastern Asia, the western Pacific, and part of the Arctic can be seen. Since the Moon is much less reflective than the Earth, JPL image processors brightened the lunar image by a factor of three to ensure that both Earth and Moon were visible on this print. [P-19891C]] CHAPTER 5 THE VOYAGE TO JUPITER—GETTING THERE Launch