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

For America, the 1960s begin on an anxious note. Many in the U.S. feared the nation was lagging dangerously behind the Soviet Union in development of intercontinental ballistic missiles (ICBMs). In reality, secret photos from American spy satellites were about to confirm what high-flying aircraft had already shown: the so-called missile gap was not real.

But the Eisenhower administration could not reveal this knowledge to the public, and in 1960 John Kennedy won the presidency over Eisenhower's vice president, Richard Nixon, partly on the strength of his stance on the missile gap.

For the young American president, Gagarin's flight came as a serious blow.

In Kennedy's mind, competition with the Soviets in space had become vital to U.S. international prestige. On May 5, a former Navy test-pilot named Alan Shepard -- judged by many to be the best pilot among the Original Seven astronauts -- became the first American in space.

Inside his tiny Mercury spacecraft, which he named Freedom 7, Shepard rode a Redstone booster on a 15-minute suborbital flight. The nation reacted to Shepard's feat with wild enthusiasm, and Kennedy took notice.

Alan Shepard inside his Mercury capsule, ready for launch

Kennedy had already been thinking about how to pull ahead of the Soviets in space. He'd asked his advisors to come up with a project that would give the U.S. a clear victory.

Less than three weeks after Shepard's flight, speaking before a joint session of Congress, Kennedy made an announcement that would have seemed unthinkable just years before: "I believe this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the Earth."

Many who heard these words -- including some at NASA -- wondered if Kennedy's challenge was realistic. (A few even wondered if Kennedy had lost his senses.) But it didn't take long for the space agency to begin figuring out how to achieve it.

Meanwhile, the space race sped onward with ever more ambitious flights.

John Glenn -- the astronaut who seemed to step most easily into the role of American hero -- became the first American to orbit Earth on February 12, 1962. Inside his Friendship 7 spacecraft Glenn circled the globe three times, marveling at the beauty of orbital sunrises and sunsets before sweating through a fiery reentry into Earth's atmosphere.

Three more astronauts followed Glenn into orbit; in May 1963 the sixth and last piloted Mercury mission saw Gordon Cooper spending more than a day in space.

As important as these missions were for the U.S. program, they were overshadowed by the Soviet Vostok flights. Cosmonaut Gherman Titov made the first daylong flight in 1962. Andriyan Nikolayev in Vostok 3 and Pavel Popovich in Vostok 4 staged the first dual spaceflight in 1963. Also in 1963, a former cotton mill worker and parachute jumper named Valentina Tereskhkova became the first woman in space, logging almost three days in Vostok 6.

Valentina Tereskhkova, first woman in space, in 1963.

And the Soviet firsts didn't end there. Under pressure from Soviet premier Nikita Khrushchev, chief space designer Sergei Korolev staged another orbital "spectacular." The Americans were planning their two-man Gemini flights, but Korolev upstaged Gemini's planned debut by launching three cosmonauts in a "new" spacecraft called Voskhod ("sunrise").

In reality, Voskhod 1 was nothing more than a converted Vostok. Only by taking the dangerous step of denying the cosmonauts ejection seats and spacesuits was Korolev able to achieve the feat. Fortunately, Voskhod 1 flew without mishap.

But that wasn't true for the Voskhod 2 team of Pavel Belyayev and Alexei Leonov, who made their day-long mission in March, 1965.

Early in the flight, a spacesuited Leonov wriggled into a narrow, inflatable airlock attached to the Voskhod's cabin, leaving Belyayev to pilot the ship. Leonov then emerged into the void and spent several minutes floating free in history's first spacewalk.

Leonov almost didn't live to tell the tale: In the vacuum of space his suit ballooned dangerously, making it almost impossible for him to get back inside. Only by releasing some of his suit's air -- an almost desperate measure, considering the risk of decompression sickness -- was the exhausted cosmonaut able to reenter the cabin.

Once again, the world was not told of these difficulties, and Leonov's feat seemed to leave the U.S. program in the dust. But it would not be long before the Americans caught up.

Even as the Soviets racked up one space first after another, NASA was getting closer to the first piloted Gemini missions. Launched by a converted Titan 2 missile, Gemini was the most sophisticated spacecraft yet created. Gemini astronauts would utilize an on-board computer. And they would be able to change their orbit -- something no Soviet crew had yet accomplished.

For NASA, Gemini would serve as a bridge between the relatively simple Mercury flights and the awesome challenge of the Apollo moon program.

In just 20 short months, between March 1965 and November 1966, 10 Gemini crews pioneered the techniques necessary for a lunar mission.

They made spacewalks, some lasting more than two hours. They spent a record-breaking 14 days in space -- the expected duration of a lunar-landing flight -- in a cabin no bigger than the front seat of a Volkswagen. (One astronaut later called the two-week Gemini 7 flight "the most heroic mission of all time.")

They mastered the arcane complexities of orbital mechanics to achieve the first rendezvous between two spacecraft in orbit, and the first space docking. And they made the first controlled reentries into Earth's atmosphere.

Floating outside Gemini 4, Ed White holds a nitrogen-powered maneuvering gun.

To be sure, the Gemini missions had their harrowing moments, none more so than when Gemini 8 astronauts Neil Armstrong and Dave Scott barely escaped disaster when one of their maneuvering thrusters malfunctioned, causing their spacecraft to tumble wildly through space.

And several spacewalkers had their own difficulties -- working in weightlessness was trickier than NASA expected, and more than one sortie had to be cut short when an astronaut became exhausted. Despite these problems, Gemini was considered a tremendous success. It gave the United States the lead in the space race, which was about to become a moon race.

Meanwhile, the Americans and Soviets were extending humanity's reach beyond Earth orbit by means of ever more sophisticated robotic probes. The U.S. Mariner 2 became the first interplanetary spacecraft when it flew by Venus in 1962 and sent back data about this cloud-shrouded world. Another American craft, Mariner 4, took the first closeup pictures of Mars in 1965.

Mariner 4's first closeup image of Mars in 1965.

Closer to home, in 1966, the Soviet Union achieved the first soft landing of a spacecraft on another world when Luna 9 came to rest on the moon's Ocean of Storms and sent back images of its dusty surface.

Also in 1966, U.S. Surveyor landers began exploring the lunar surface, and a series of Lunar Orbiter spacecraft began a detailed photoreconnaissance of the moon from orbit. These missions not only advanced scientific understanding of Earth's nearest neighbor; they helped pave the way for the piloted missions that would follow.

By 1967, both the United States and the Soviet Union were ready to test the spacecraft they would use to send humans to the moon. In the process, both countries suffered devastating failures.

On January 27, 1967 the crew of the first piloted Apollo mission -- veterans Gus Grissom and Ed White, along with rookie Roger Chaffee -- perished when a flash fire swept through the sealed cabin of their Apollo 1 command module. NASA's investigation of the tragedy revealed numerous technical flaws in the craft's design, including the need for a quick-opening hatch and fireproof materials in the cabin. The fire would ultimately delay the Apollo program for more than 20 months.

Disaster struck the Soviets in April 1967, when cosmonaut Vladimir Komarov piloted Soyuz 1 ("union"), an Earth-orbit precursor of a planned lunar-orbit vehicle. When Komarov's flight was plagued by malfunctions, controllers ordered him to come home early. But the craft's parachute did not deploy properly and Soyuz 1 slammed into the Earth's surface at tremendous speed, killing Komarov. The Soviets too had found that winning the moon race could exact a terrible price.

For the Americans, at least, 1967 ended on a triumphant note with the debut of the giant Saturn 5 moon rocket. Towering 363 feet (110 meters) above its launch pad, the Saturn's three stages contained as much chemical energy as an atomic bomb.

When it lifted off on November 9, powered by 7.5 million pounds of thrust, the Saturn's fire and thunder were truly awesome to behold. For NASA, the Saturn 5's flawless test flight marked a key milestone on the road to the moon.

Americans returned to space on October 11, 1968, when the crew of Apollo 7 made an 11-day Earth-orbit test of the Apollo command and service modules, which had been redesigned in the wake of the fire.

The flight went so well -- one mission controller dubbed it "101-percent successful" -- that NASA decided to take a stunningly bold step with Apollo 8 -- its crew would orbit the moon.

There was a note of urgency in the plan: Intelligence reports showed that the Soviets, who had recovered from the loss of Soyuz 1, were planning to send two cosmonauts on a circumlunar flight before the end of the year.

But after two pilotless circumlunar test flights experienced malfunctions in the fall of 1968, Soviet officials refused to give the go-ahead for a piloted mission.

The way was clear for the Apollo 8 crew -- Frank Borman, Jim Lovell and Bill Anders -- to make history.

On December 24, 1968, after a 66-hour journey across 230,000 miles (370,140 kilometers) of space, the three men fired their spacecraft's main engine to go into lunar orbit. They remained there for 20 hours, making navigation sightings, taking photographs and beaming live television pictures back to Earth, before returning home.

After a reentry at 25,000 m.p.h. (40,230 kilometers per hour) -- faster than humans had ever traveled -- Borman's crew splashed down safely in the waters of the Pacific.

Apollo 8 was more than a technical triumph, more even than a milestone in exploration: It was a mountaintop experience for the entire human species. A single photograph from Apollo 8, showing Earth rising beyond the moon's barren horizon, became one of the century's most famous and inspiring images.

For the Soviets, Apollo 8's success was a stinging defeat that seemed to take the wind out of their own moon effort, at least temporarily. For NASA, it had the opposite effect. Now the way was clear to attempt the lunar landing. If all went well on Apollos 9 and 10, Apollo 11 would try for a landing the next summer. But that was a big "if;" each mission ranked as one of the most complex and difficult space missions ever attempted.

Amazingly, both flights -- Apollo 9, an Earth-orbit test of the entire Apollo spacecraft and Apollo 10, a "dress rehearsal" for the landing in lunar orbit -- were almost flawless.

When Apollo 10 splashed down on May 26, Neil Armstrong and his Apollo 11 crew had less than two months left to prepare for the ultimate test flight.

July 16, 1969 dawned clear and hot for the spectators (estimated at a million people) who flocked to Cape Kennedy for the Apollo 11 launch.

They were not disappointed.

At 9:32 a.m. (13:32 GMT), the Saturn 5 came to life, its fire akin to a second sun, its roar shattering the morning stillness as it sent Armstrong and crew mates Buzz Aldrin and Mike Collins on history's third lunar voyage. Three days later the men arrived in lunar orbit, knowing that their real mission -- the landing attempt -- was about to begin.

On July 20, Armstrong and Aldrin, clad in their spacesuits, took their places in the tiny cabin of the lunar module, Eagle, leaving Collins to pilot the command ship, Columbia. The two ships separated, and with a blast from their lander's descent engine, Armstrong and Aldrin began their trip down to the moon's Sea of Tranquillity.

At 50,000 feet (15,240 meters) they ignited Eagle's engine once more, beginning the landing's final phase, called the powered descent. Everyone knew there could be problems, and there were: On the way down, an overloaded computer threatened to abort the mission; only quick thinking by experts in Mission Control allowed Armstrong and Aldrin to continue.

A thousand feet (305 meters) above the lunar surface, Armstrong saw that the craft was heading for a crater the size of a football field that was rimmed with boulders as big as automobiles.

Taking control, he steered Eagle to a clear spot and brought the craft into a vertical descent, while Aldrin called out the diminishing altitude. With his fuel supply running low, Armstrong struggled to see his landing spot through a storm of moon dust kicked up by the descent engine.

Finally, a blue light on the instrument panel signaled that three metal probes on Eagle's footpads had touched the moon.

"Contact light," announced Aldrin. Eagle settled gently onto the dusty lunar ground and Armstrong shut down the engine. The two men turned to each other and shook hands in a brief moment of celebration.

Then Armstrong radioed to a waiting Earth, "Houston, Tranquillity Base here. The Eagle has landed."

Almost seven hours later, Armstrong emerged from Eagle. After descending the ladder on the craft's front landing leg, he planted his left foot on the ancient dust of the Sea of Tranquillity and declared: "That's one small step for [a] man, one giant leap for mankind."

Minutes later, Aldrin joined him on the surface, and for a bit less than two hours, the two men collected rocks, planted the American flag and took pictures.

They also experienced the delights of moving in the moon's one-sixth gravity and marveled at the beauty of the utterly pristine, utterly ancient lunar landscape. Then it was time for history's first moonwalk to end, as the astronauts climbed back into their lander for a fitful rest.

On July 21, the moment of truth for Armstrong and Aldrin was at hand: the firing of Eagle's ascent rocket to return them to lunar orbit, and a reunion with Collins. Everyone, on Earth and in space, knew that the engine had to work, or Armstrong and Aldrin would face a lonely death on the moon.

When the prescribed moment came, Aldrin pushed a button on the on-board computer and, after a brief moment, the engine ignited with an invisible flame. Amid a spray of insulation, Eagle ascended like a superfast, silent elevator, heading for a rendezvous with Columbia. Apollo 11's safe return on July 24 marked the beginning of a new age, one in which human beings could truly be called a spacefaring species.

For NASA, the age of lunar exploration was only beginning: More landings were ahead, including Apollo 12's pinpoint lunar touchdown in November.

The United States had won the moon race. But the 1970s would bring a change of fortunes for the space agency, while the Soviet Union blazed a new trail, as pioneers of long-duration space missions.