Not only Jupiter, but Saturn and even Uranus and Neptune, were considered as possible targets. However, the distances between the outer planets are so vast that many years of flight would be required for a spacecraft to reach them, even using the most powerful rocket boosters then contemplated. If a cautious exploration program were followed, investigating one planet at a time before designing the next mission, it would be well into the twenty-first century before even a first reconnaissance of the solar system could be achieved. A way to bridge the space between planets in a more efficient, economical manner was needed.
In the late 1960s celestial mechanicians—scientists who study the motions of planets and spacecraft—began to solve problems posed by the immensity of the outer solar system. If a spacecraft is aimed to fly close to a planet in just the right way, it can be accelerated by the gravity of the planet to higher speeds than could ever be obtained by direct launch from Earth. If a second, more distant planet is in the correct alignment, the gravity boost given by the first encounter can speed the craft on to the second. Jupiter, with its huge size and strong gravitational pull, could be used as the fulcrum for a series of missions to Saturn, Uranus, Neptune, and even distant Pluto. In addition, the early 1980s would offer an exceptional opportunity, one repeated only about once every two centuries. At that time, all four giant planets would be in approximate alignment, so that gravity-assist maneuvers could be done sequentially. A single spacecraft, after being boosted from Jupiter to Saturn, could use the acceleration of Saturn to continue to Uranus, and in turn could be accelerated all the way out to Neptune. Such an ambitious, multiplanet mission was named the Grand Tour.
The first essential step in the Grand Tour was a flyby of Jupiter. However, this planet is ten times farther away from Earth than Venus or Mars. In addition, there were two potentially lethal hazards that had not been faced before in interplanetary flights: the asteroid belt and the Jovian magnetosphere.
The first danger was presented by the many thousands of asteroids that occupy a belt between the orbits of Mars and Jupiter. The largest asteroid, Ceres, was discovered in 1801 and was initially thought to be the “missing planet” sometimes hypothesized as lying between Jupiter and Mars. However, Ceres is only 1000 kilometers in diameter, too small to deserve the title of planet. Hundreds more of these minor planets were discovered during the nineteenth century, and by the 1960s more than 3000 had well-determined orbits. Most were only a few tens of kilometers in diameter, and astronomers estimated that 50 000 existed that were 1 kilometer or more in diameter. Any spacecraft to Jupiter would have to cross this congested region of space.
Even 50 000 minor bodies spread through the volume of space occupied by the asteroid belt would present little direct danger, although a chance collision with an uncatalogued object was always possible. Much more serious was the possibility that these larger objects were accompanied by large amounts of debris, from the size of boulders down to microscopic dust, that were undetectable from Earth. Collisions with pebble-sized stones could easily destroy a spacecraft. The only way to evaluate this danger was to go there and find out how much small debris was present.
A second danger was posed by Jupiter itself. In order to use the gravity boost of Jupiter to speed on to another planet, a spacecraft would have to fly rather close to the giant. But this would mean passing right through the regions of energetic charged particles surrounding the planet. Some estimates of the number and energy of these particles indicated that the delicate electronic brains of a spacecraft would be damaged before it could penetrate this region. Again, only by going there could the danger be evaluated properly.
The Pioneer Jupiter Mission
In 1969 the U.S. Congress approved the Pioneer Jupiter Mission to provide a reconnaissance of interplanetary space between Earth and Jupiter and a first close look at the giant planet itself. The Project was assigned by NASA to the Ames Research Center in Mountain View, California. The primary objectives were defined by NASA:
Explore the interplanetary medium beyond the orbit of Mars.
Investigate the nature of the asteroid belt, assessing possible hazards to missions to the outer planets.
Explore the environment of Jupiter, including its inner magnetosphere.
The Pioneer spacecraft was designed for economy and reliability, based on previous experience at Ames with Pioneers 6 through 9, all of which had proven themselves by years of successful measurement of the interplanetary medium near the Earth. Unlike the Mariner class of spacecraft being used to investigate Venus and Mars, the Pioneer craft rotated continuously around an axis pointed toward the Earth. This spinning design was extremely stable, like the wheels of a fast-moving bicycle, and required less elaborate guidance than a nonspinning craft. In addition, the spin provided an ideal base for measurements of energetic particles and magnetic fields in space, since the motion of the spacecraft itself swept the viewing direction around the sky and allowed data to be acquired rapidly from many different directions. The only major disadvantage of a spinning spacecraft is that it does not allow a stabilized platform on which to mount cameras or other instruments that require exact pointing. Thus the spacecraft design was optimized for measurements of particles and fields in interplanetary space and in the Jovian magnetosphere, but had limited capability for observations of the planet and its satellites.