In this third of five articles, space.com's Andrew Chaikin continues a decade-by-decade look at the 20^th century's most significant developments in space.

For NASA, the sixties had ended in triumph: Humans had walked on the moon and NASA had put them there. For the space agency, the success of the first lunar landing was an invitation to dream even bigger dreams. NASA administrator Tom Paine and his deputies planned a stunning array of space activities so extensive that they would live up to the vision presented in Collier's magazine in the 1950s. There would be space stations in Earth orbit, a base on the moon and human missions to Mars.

Meanwhile, the Apollo moon missions continued to rack up extraordinary accomplishments. Simply getting to the moon and back had been the goal of the first Apollo landing crew. But beginning with Apollo 12, the mission began to emphasize scientific exploration. In November 1969 Apollo 12 astronauts Pete Conrad and Alan Bean had made history's first pinpoint lunar landing by touching down within walking distance of the robotic Surveyor 3 probe, which had rested on the moon's Ocean of Storms since early 1967. In the process, they had demonstrated that future Apollo crews could set down at some of the most geologically enticing places on the moon.

Scientists' hopes were high for the Apollo 13 mission, which left Earth on April 11, 1970, bound for the moon's Fra Mauro highlands. But those hopes evaporated some 55 hours into the mission, when an oxygen tank exploded aboard the Apollo-13 service module, aborting the flight and plunging NASA into the worst crisis it had ever faced on a piloted space mission.

Aboard the crippled spacecraft, lunar veteran Jim Lovell and his rookie crew, Jack Swigert and Fred Haise, were forced to abort their mission and begin an emergency trip home. To survive, they used their lunar lander as a lifeboat, utilizing its oxygen, radio and rocket engines. By the time they reached Earth, four days later, the men were exhausted and battling bone-chilling conditions aboard their spacecraft. But the combined efforts of the astronauts and hundreds of flight controllers and engineers on Earth paid off: Lovell's crew splashed down safely on April 17, crowning a recovery effort that many called NASA's finest hour.

Apollo 13's damaged service module, seen shortly before reentry.

Picking up the torch from Jim Lovell's crew, Apollo 14 was led by America's first space traveler, Alan Shepard, who had been grounded since his 1961 Mercury mission by an inner-ear disorder, but restored to health after an experimental surgery. Shepard's footsteps on the Fra Mauro highlands represented not only a success for NASA, but a personal triumph.

In the summer of 1971, Apollo's scientific phase got into high gear, as Apollo 15's Dave Scott and Jim Irwin became the first astronauts to explore the mountains of the moon. They brought along a new innovation: a battery-powered car called the Lunar Rover that allowed them to range for miles across the landscape on geologic "treasure hunts."

Apollo 15's Jim Irwin poses with the lunar module and lunar rover (right).

For three days, Scott and Irwin lived and worked among the moon's Apennine mountains. Among their finds was a rock that dated back 4.5 billion years, almost as old as the moon itself, which became known as the "Genesis rock." Meanwhile, circling the moon, crewmate Al Worden operated a battery of scientific cameras and sensors in an intensive orbital reconnaissance.

Even as Apollo 15 demonstrated the new heights Apollo had reached, budget cuts were bringing the program to a premature end. In 1970 three lunar missions were canceled, leaving only Apollo 16 and 17 to write the final chapters in the Apollo saga.

In December 1972, Apollo 17 saw Gene Cernan and geologist-astronaut Jack Schmitt -- the first professional scientist to visit another world -- explore the spectacular Taurus Littrow valley, while Ron Evans surveyed the moon from orbit. When Apollo 17 splashed down in the Pacific, the 20th century's brief era of human exploration of the moon was over.

By the early 1970s, the Soviet Union was forced to abandon its efforts to send people to the moon. Even before Apollo 11's lunar landing, Soviet space planners had realized they would likely lose the moon race. They had begun their lunar program relatively late, a few years after the Americans.

But the most severe blow came from the failure of Sergei Korolev's giant N-1 moon rocket: It exploded during each of its four test launches between 1969 and 1972. Some still dreamed of catching up with the U.S. program, or even surpassing it, by establishing a lunar base. But the repeated failures of the N-1 put an end to such plans. Korolev wasn't around to see his dream abandoned; he had died during surgery in 1966.

As an alternative, the Soviets staged a series of robotic missions in an attempt to show the world that they could duplicate Apollo's scientific harvest, for a fraction of the cost. The first tries at an automated lunar sample-return came before Apollo 11. But success did not come until September, 1970: Luna 16 touched down on the Sea of Fertility, drilled into the soil and launched a return capsule to Earth bearing a small vial of lunar dust and rock fragments. Luna 20 repeated the feat in 1972 and Luna 24 retrieved a sample from the Sea of Crisis in 1976.

The Soviets also deployed a pair of automated rovers, which roamed the moon under remote control by engineers on Earth. Named Lunakhod, the rovers were equipped with television cameras and scientific instruments to investigate the properties of lunar soil. Lunakhod 1 reached the Sea of Rains in 1970; Lunakhod 2 traveled the Sea of Serenity in 1973.

Beginning in the early 1970s, the Soviets' human spaceflight program charted a new course -- into Earth orbit. In 1971, two years before NASA's initial Skylab mission, they launched the world's first space station, Salyut 1. Three cosmonauts spent 21 days aboard the station, but their mission ended tragically: All three men died after a sudden loss of cabin pressure immediately before their spacecraft made an automatic reentry. Recovery teams arrived at the craft only to find the crew dead inside.

By September, 1973, the Soviets had recovered from the tragedy and were back to launching Salyuts for scientific missions. They also inaugurated a series of Almaz space stations for military reconnaissance, further evidence that the Cold War was being waged in space as well as on Earth.

Cutaway view of the Salyut 4 space station, with Soyuz ferry attached (left).

The Salyut missions put the Soviet Union at the frontier of long-duration spaceflight. With a four-month residence on Salyut 6 in 1978, the crew of Soyuz 29 broke the U.S. space endurance record set aboard Skylab. The following year Soyuz 32 cosmonauts Vladimir Lyakhov and Valeri Ryumin logged an extraordinary stay of six months on the station. Even that record would be surpassed as the Soviet space marathons continued into the 1980s.

International space activities blossomed in the 1970s. Beginning in 1973, ten European nations joined forces to form the European Space Agency, or ESA. ESA's members embarked on a variety of space projects, including development of a new satellite launcher called Ariane. The first successful Ariane launch in 1979 ushered in a new era of commercial space activities.

Also in 1975, détente found its way into space, if only for a moment, as the U.S. and Soviet Union staged a joint space mission called the Apollo-Soyuz Test project. The dual mission, which commanded by Apollo veteran Tom Stafford and Soviet spacewalker Alexei Leonov, featured the first international space docking.

The 1970s also saw a growing number of nations launching satellites. China's first, lofted in 1971, broadcast a melody entitled "East Is Red." India, whose first satellite was launched by the Soviets in 1975, achieved its own satellite launch in 1979.

In the 1970s, planetary exploration flourished as U.S. and Soviet robotic missions sent back a phenomenal haul of data. Mars was the target of the Mariner 9 spacecraft, which began mapping the planet from orbit in 1971. Mariner's images revealed towering volcanoes, giant canyons and winding valleys that appeared to be dry riverbeds. In 1976, the twin Viking landers made the first successful touchdowns on the martian surface, sending back images and data on the planet's atmosphere and soil. The Vikings' most celebrated experiment -- a search for signs of microbial life -- failed to find any conclusive evidence.

The Mariner 10 spacecraft was the first to take advantage of a technique called gravity assist: During a flyby of Venus, the probe used the planet's gravity to redirect it toward Mercury. Thanks to a lucky coincidence between Mercury's orbital period and the spacecraft's trajectory, Mariner 10 made not one, but three flybys of the innermost planet.

The Soviets, who had little success in several Mars landing attempts, succeeded with an even more challenging task: Their Venera landers actually survived a descent to the hellish surface of Venus. Beneath the planet's dense, opaque atmosphere, the Venera's found surface pressures 90 times that on Earth and temperatures as high as 900 degrees Fahrenheit (482 degrees Celsius) -- hot enough to melt lead. The Venera landings gave scientists their first images and in situ data on the planet's surface composition. But in the 1970s, Venus was the only truly successful target for Soviet planetary missions.

Rocks on the surface of Venus, photographed by Venera 9 in 1975.

The United States, meanwhile, turned its attention to the outer solar system with the Pioneer 10 and 11 spacecraft. These twin probes made the first flybys of Jupiter and Saturn, sending back valuable glimpses of these planets' cloudy atmospheres, along with valuable data on their powerful magnetic fields. The more sophisticated Voyager 1 and 2 probes, launched in 1977, began their own "grand tour" of the outer solar system, a mission that would ultimately stretch to the end of the 1980s, and even beyond.