The Galilean satellites in orbit around Jupiter, along with the outer satellites, constitute a miniature solar system. Here they are shown relative to the size of Mercury and that of the Moon. The portrayal of their internal and external composition is based on theoretical models that preceded the Voyager flybys. [PC-17054AC]
This image of Io’s extended sodium cloud was taken February 19, 1977, at the Jet Propulsion Laboratory’s Table Mountain Observatory. A picture of Jupiter, drawings of the orbital geometry, and Io’s disk (the small circle on the left) are included for perspective. The sodium cloud image has been processed for removal of sky background, instrumental effects, and the like. This photograph demonstrates that the cloud is highly elongated and that more sodium precedes Io in its orbit than trails it. [P-20047]
This picture of the satellites was developed just as the first space probe reached the Jovian system. In the next chapter we describe the Pioneer program by which scientists reached out across nearly a million kilometers of space to explore Jupiter, its magnetosphere, and its system of satellites.
Pioneer 10 was launched on March 2, 1972, at 8:49 p.m. from Cape Canaveral, Florida. A powerful Atlas-Centaur rocket served as the launch vehicle, which propelled the space probe to its goal nearly a billion kilometers away. The beauty of the night launch was enhanced by the rumbling thunder and flashing lightning of a nearby storm.
CHAPTER 2
PIONEERS TO JUPITER
Reaching for the Outer Planets
Since the beginning of the Space Age, scientists had dreamed of sending probes to Jupiter and its family of satellites. Initially, robot spacecraft were limited to studying the Earth and its Moon. In 1962, however, the first true interplanetary explorer, Mariner 2, succeeded in escaping the Earth-Moon system and crossing 100 million kilometers of space to encounter Venus, studying Earth’s sister planet at close range using half a dozen scientific instruments. By the mid 1960s a U.S. planetary spacecraft had also flown to Mars, there had been a second flyby of Venus, and an ambitious program was under way for two more flybys of Mars in 1969, followed by a Mars orbiter in 1971. Based on this success with the inner planets, NASA scientists and engineers began to plan seriously to meet the challenge of the outer solar system.