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The Dog, and the Plough, and the Hunter, and all,

And the star of the sailor, and Mars, These shone in the sky, and the pail by the wall Would be half full of water and stars.

Robert Louis Stevenson, from Escape at Bedtime

SHORTLY after the beginning of the Space Age, in the late 1950s, rocketeers in the USA and the USSR realized that their primitive, military-derived rockets could escape the gravitational field of the Earth, navigate by the stars to cruise the depths of interplanetary space, and begin the preliminary reconnaissance of the solar system. This was a very heady time for space scientists—war rockets, launched in a superpower competition, would actually be used to study, at close range, properties of distant worlds.

The training ground for these interplanetary Argonauts was our planet's large Moon. At a mean distance of around 384,000 kilometers, Luna would be the easiest destination for space robots. Although the glory would go to the Space Race winning astronauts of the Apollo Program who would walk on our nearest solar-system neighbor, their missions would have been impossible without robot reconnaissance. And much of the scientific harvest of lunar knowledge would be gathered by these precursors to the human exploration teams.

Robots to the Moon!

Although they could not sustain the pace to win the race to the Moon, Russian space engineers garnered many of the early milestones in lunar exploration. Russia achieved the first lunar flyby with Luna 1 in 1959. This craft's notable scientific accomplishment was confirmation of the solar wind—the energetic stream of charged particles emanating from the Sun. Even in their failures to achieve Earth-escape velocity, America's Pioneers 1 and 3, both launched in 1958, reached distances sufficient to map the outermost fringes of Earth's Van Allen radiation belts.

On September 12, 1959, Luna 2 achieved a significant geopolitical "first" of little scientific value, when it became the first probe from Earth to impact the Moon. This event was followed in October 1959 with Luna 2, which flew by the Moon and returned the first photographs of the lunar far side. As a result of this mission, some large far-side features have Russian names.

Starting in July 1964, American probes of the Ranger series began their lunar reconnaissance missions. Rangers 7-9 returned excellent, highresolution photos of the lunar surface as they screamed in toward impact.

America continued robotic lunar exploration with the highly successful Lunar Orbiters, which provided extensive photographic surveys of the lunar topography that would be invaluable to later Apollo astronauts. Successful landings by the Surveyor spacecraft demonstrated in 19661968 that Earth ships would not sink into the lunar topsoil. One of these, Surveyor 6, used its rockets to take off and fly a short distance from its initial lunar landing site, demonstrating that the Moon's crust could support the stress of a rocket's exhaust. Parts of another Surveyor were later retrieved by the crew of Apollo 12 and returned to Earth. Analysis revealed that bacterial spores from Earth had survived on board Surveyor for many months in the harsh lunar environment.

After America's victory in the Moon Race, Russia continued a series of scientific Moon missions. In September 1970, Luna 16 performed the first robotic sample-return mission from another celestial body. The quantity of lunar rock and soil returned to Earth by Luna was a tiny fraction of that returned by Apollo astronauts, but this mission paved the way for planned sample-return probes to more distant celestial destinations.

Although America's astronaut-bearing Lunar Roving Vehicles would receive much glory after their deployment from Apollo's Lunar Modules, Russia's Luna 17 succeeded in 1970 in placing the first successful robotic roving vehicle, Lunokhod, on the surface of the Moon. The later Mars-roving robots owe a great deal to this early lunar experiment (Figure 5.1).

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FIGURE 5.1 An ancestor to NASA's Mars Pathfinder and Mars Exploration rovers, the Soviet Luna 17 rover explored the lunar surface under remote control. (Courtesy www.mentallandscape.com/ C_CatalogMoon.htm)

After the conclusion of NASA's Apollo Program, there was a long hiatus in both robotic and manned lunar exploration. This was broken by two Soviet successes—the delivery of a second Lunakhod in 1972 and Luna 24's sample return mission in 1976.

Robotic lunar exploration recommenced in 1989 when the NASA Galileo Jupiter probe photographed the Moon during one of its Earth gravitational swing-bys en route to its gas giant destination.

A third space power, Japan, got into the lunar exploration act in 1990 with the launch of Hiten, also called Muses-A. This craft is most significant for its translunar trajectory. Instead of flying a Hohmann-

transfer orbit requiring three days or so to reach the lunar vicinity, Hiten was injected into a long-duration, but fuel-conservative weak-stability-boundary lunar transfer orbit. Months were required to reach lunar orbit.

In the hope of finding signs of comet-deposited ice in craters near the lunar poles, America launched Clementine and Lunar Prospector during the 1990s. So far, the hunt for lunar water-ice in Sun-shielded craters has not yielded conclusive results.

In 2003, the European Space Agency commenced its program of robotic lunar exploration with the launch of SMART-1. A technology demonstrator, this robot used solar-electric propulsion to spiral from Earth orbit to the vicinity of the Moon. More than a year was required to complete the lunar transfer.

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