Just as in the 15th and 16th century, when the great geographic discoveries were made by only a few countries, the exploration of space was initially dominated by the United Sates of America and the Soviet Union. More recently Europe and Japan have played a significant part and as time passes more and more countries (China, India and Brazil, for example) are becoming interested. The political influence on national budgets has always been important. In the late 1950s and the early 1960s the Moon was the main target. The USSR’s spacecraft Luna 3 imaged the far side of the Moon for the first time in October 1959. Astronomers were surprised to realise that the face that pointed away from Earth was not the same as the face that pointed towards us. On the other side of the Moon there were very few lava-filled basins and volcanic activity had been negligible. Both of these observations indicated that the surface solid rock layers must be thicker than they are on the Earth facing side. Luna 9 soft landed on the Moon in January 1966, followed by the USA’s Surveyor 3 in April 1967. Everyone was relieved to learn that the soil-strewn lunar surface was not a ‘quick sand’ but could support weight and was safe to walk on. Lunar 10 orbited the Moon in March 1966 followed by the USA’s Lunar Orbiter in August of the same year. There were five Lunar Orbiters in total, three passing around the equatorial regions and two over the poles. With the Moon spinning slowly below, the narrow-angle and wide-angle cameras produced detailed maps of the whole surface, and the techniques learnt at the Moon paved the way for the detailed mapping of other planets.

The 1970s saw a considerable expansion of effort. Amongst the major milestones was the USSR Venera spacecraft that parachuted gently down through the dense atmosphere of Venus obtaining about one hour’s data from the surface. After an hour they were eaten away by the sulphuric acid rain drops picked up from the all-encompassing high cloud layers. Then there was the Mariner 10 flyby of Mercury in 1974 which imaged half the surface of this bakingly hot planet. The Viking orbiters and landers went to Mars in 1975 and showed the surface to be a dead, dry, rusty dessert. Pioneer 10 and 11 were launched in 1972 and 1973 and passed Jupiter and Saturn 21 and 75 months later.

In the 1980s the sophistication of the spacecraft increased, the instruments becoming more plentiful and more sensitive. The two Voyager spacecraft took full advantage of the changes that could be induced in their orbits by the gravitational fields of the planets they passed. Voyager 2 left Earth in August 1977 and its gravity-assisted flight path sent in on a Grand Tour, passing Jupiter in July 1979, Saturn in August 1981, Uranus in January 1986 and Neptune in August 1989. The data sent back was impressive. For example, 17500 images of Saturn and its moons Phoebe, Iapetus, Hyperion, Tethys and Enceladus were returned as well as 6000 images of Uranus and its moons Miranda, Ariel, Umbriel, Titania and Oberon.

Voyager is a telling example of the duration and difficulty of a modern space mission. It took about six to eight years to plan, build and test the Voyager spacecrafts. After launch it took a further twelve years to reach Neptune, during which time the instrumentation had to work efficiently and reliably and the science team had to remain motivated. Ground-based radio receivers had to be sensitive enough to pick up signals from a spacecraft that was about 4500 million kilometres away from Earth. The radio dish on the spacecraft was 3.7 m across. The spacecraft had to be powered continually, and the Voyager crafts did not used solar cells but carried with them small plutonium systems, the heat from the radioactive decay being converted into electricity. The surfaces of the planets and satellites that were passed were investigated using narrow and wide angle high resolution solid state cameras, mounted on a tracking platform. The polarisation and intensity of the light was carefully monitored. Spectra were taken in the ultraviolet and the infrared. The suite of instruments provided information about the physical and chemical form of the planetary and satellite surfaces and atmospheres. The surroundings of the planets were investigated using instruments that measured the strength and direction of the magnetic fields and the energy, velocity and composition of the charged particles that occupied the planetary magnetospheres. But we still only got snapshots. The spacecraft were only close to the planets for a day or so. The way in which characteristics varied with time could not be discerned

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