Sky & Telescope April 1987 [antikvár]

Project Galileo: The Phoenix RisesMichael W. Carroll, San Diego, CaliforniaBy the summer of 1979, four Pioneer and Voyager spacecraft had flown past Jupiter, opening a portal into the splendid worlds of the Jovian system. But in the years since, the king of planets has hosted no more emissaries from Earth. Its remaining secrets have waited patiently to be revealed.That was to change in 1982. One of the most ambitious space ventures yet attempted Project Galileo was to be launched that year. The sophisticated spacecraft would be the first to orbit Jupiter and drop an instrumented probe into the gas giant's dynamic atmosphere. But problems with the intended launchers delayed the mission. Instead of arriving at Jupiter in 1985, Galileo was rescheduled for launch on May 20, 1986, using the Space Shuttle Atlantis and a powerful Centaur upper stage.Galileo was nearly ready to begin its interplanetary voyage when tragedy struck Challenger in January, 1986. In the accident's aftermath, NASA managers have deemed the liquid-fueled Centaur too risky to carry in the orbiter's cargo bay, leaving Galileo with no choice but to fly with a less powerful upper stage.But, like the mythical phoenix, Galileo has risen from the ashes of trimmed budgets and cancelled projects to become a new, stronger mission. Since a lower-thrust Inertial Upper Stage (lUS) will now be used to boost the spacecraft out of Earth orbit, mission planners have had to choose a new flight path. To conserve fuel, Galileo will use the gravity of several planets lo speed its journey and ultimately direct its course toward Jupiter. This revised plan gives the spacecraft unique exploratory opportunities not available before.VISIT TO VENUSAccording to NASA's current scenario, Galileo will thunder into Earth orbit in November, 1989, on the Space Shuttle's 39th mission. The Jupiter-bound craft's interplanetary jaunt seemingly begins in the wrong direction. When the upper stage ignites, it will send Galileo toward the inner solar system.The first target is Venus. After three months in space, the planet-hopping traveler will glide past the white clouds of ournearest planetary neighbor at an altitude of 19,000 kilometers. This flyby will provide the first bonus in the modified mission: Galileo will use many of its sophisticated science instruments to study Venus and its environs as it swings past.But Galileo won't be able to muster its full scientific strength to examine our sister world. A necessarily weak radio link between the craft and Earth will severely limit the amount of data that can be relayed home. During its interplanetary cruise, Galileo's powerful main antenna, 4.8 meters across, remains folded up like an umbrella for protection from the Sun's heat. Moreover, it will usually be pointed away from Earth since the craft's orientation in space will be chosen to make the best use of its sunshades.Therefore, communication will be limited to a small omnidirectional antenna on one of the spacecraft's booms.A week after its Venus encounter, Galileo will pass its closest to the Sun at a distance of just over 106 million km.Lingering within the inner solar system poses some problems for the spacecraft. Galileo was originally designed specifically to operate in the frigid outer solar system. Yet, when nearest the Sun, it will bake in sunlight twice as strong as that found in the Earth's vicinity.For this reason, project engineers have had to develop some new hardware to make possible Galileo's revised mission (the paintings reproduced with this article reflect the craft's updated configuration). Once basic black, Galileo now glistensJ" f Ü ^ Ai^mGiven tlie opportunity, Galileo will carry out scientific research on one or more asteroids en route to Jupiter. Potential targets and encounter distances have yet to be determined. This view is probably optimistic in terms of proximity- to the object, but depicts accurately the spacecraft's configuration at the time of the second and final trip through tlie asteroid belt. Painting by tlie autlior.