Nearly 50 years have passed since NASA's Apollo 8 mission orbited the moon for the first time in history.

On December 21, 1968, astronauts Frank Borman, William Anders, and James Lovell left the Kennedy Space Center to fly around the moon. They spent 20 hours in lunar orbit, then returned home after more than six days in space.

The Apollo 8 mission was a critical step toward achieving President John F. Kennedy's goal of landing a man on the moon.

Nine other lunar missions followed Apollo 8, bringing a dozen men to the moon and gathering hundreds of pounds of rock and soil samples for analysis.

In almost five decades since then, however, no US spacecraft has landed on the lunar surface.

That may change in the next few years. In November, NASA announced that it was offering up to $2.6 billion in contracts to nine American companies that could land probes on the moon by 2022. NASA does not want to buy the lunar landers or take responsibility for launching, landing, or controlling them. Instead, the space agency wants the private sector to deal with those challenges and bid on the opportunity to take NASA's experiments to the moon.

In the meantime, take a look back at all of NASA's Apollo missions, which flew between 1968 and 1972 and succeeded in putting the first human on the moon.

The Apollo 1 mission was designed to launch a spacecraft into low-Earth orbit. But it ended in tragedy when a fire killed three astronauts in their spaceship during a routine pre-launch test.

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Thick smoke filled the crew module of the Apollo 1 capsule on January 27, 1967. Three NASA astronauts Virgil "Gus" Grissom, Roger Chaffee, and Edward White were inside performing a routine test, but they were unable to open a hatch in time to escape the explosion.

Emergency rescue teams rushed to the launchpad (located where the Cape Canaveral Air Force Station is today), but they were too late.

An investigation revealed several issues with the capsule's design, including an electrical wiring problem and flammable materials inside the crew cabin.

On the 50th anniversary of Apollo 1's fatal fire, NASA displayed the hatch at the Kennedy Space Center Visitor Complex.

The deadly fire led NASA to postpone other planned crewed launches, and no flights or missions were labeled Apollo 2 or 3.

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In the spring of 1967, NASA announced it would keep the designation of Apollo 1 for the mission that never occurred.

The rocket meant for Apollo 1 was later reassembled and used to launch Apollo 5.

The Apollo 4, 5, and 6 missions were unmanned. They occurred between November 1967 and April 1968.

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Apollo 4, which launched on November 9, 1967, was the first unmanned test flight of NASA's Saturn V rocket, which was developed to bring astronauts to the moon.

The mission was the first-ever launch from the Kennedy Space Center. It was a success for NASA, as it proved that Saturn V worked. At the time, the 363-foot-tall vehicle was the largest spacecraft to ever attempt flight.

Apollo 5 launched a few months later, on January 22, 1968. The mission successfully tested the ability of the Apollo Lunar Module the spacecraft designed to land on the moon's surface to ascend and descend.

The Apollo 6 launch followed on April 4, 1968. The mission aimed to show that the Saturn V rocket was capable of trans-lunar injection, which puts a spacecraft on its path to the moon. But the system quickly ran into problems : Two of the five engines shut down unexpectedly, and the spacecraft could not be propelled into orbit.

Despite the issues with Apollo 6, NASA pushed ahead with plans for its first manned launch.

Apollo 7 was the first manned test of the spaceship that was built to orbit the moon. The mission launched on October 11, 1968 and was the first live TV broadcast of Americans in space.

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The Apollo 7 crew, comprised of astronauts Walter Schirra, Donn Eisele, and Walter Cunningham, achieved the original goal of Apollo 1: testing a manned spacecraft in low-Earth orbit.

Schirra, Eisele, and Cunningham spent more than 10 days in space, orbiting Earth 163 times. That was more time in space than all of the previous Soviet missions combined, according to the Smithsonian National Air and Space Museum ,

To lower the risk of a fire during liftoff, NASA designed the command module's cabin atmosphere to have 60% oxygen and 40% nitrogen. (A higher percentage of oxygen would have increased the risk of fire, and NASA was trying to avoid another fatal fire after the Apollo 1 accident.) The cabin atmosphere gradually adjusted to pure oxygen shortly after liftoff.

On Christmas Eve of 1968, Apollo 8 astronauts Jim Lovell, William Anders, and Frank Borman became the first people to orbit the moon.

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The purpose of the Apollo 8 mission was to study and take pictures of the moon's surface.

In addition to achieving a historic and important space-travel milestone, Apollo 8 also became known for the famous "Earthrise" photo that the astronauts captured.

It was the first time humans saw what our planet looks like from space.

Earthrise has become one of the most reproduced space photos in history, appearing on posters, US postage stamps, and even Time magazine's cover in 1969.

The Apollo 9 mission stayed in low-Earth orbit and tested all the major components that would be essential for a lunar landing. It featured the first crewed test of the spacecraft designed to land on the moon.

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Apollo 9 launched on March 3, 1969, carrying astronauts James McDivitt, David Scott, and Russell Schweickart.

After a successful 10-day mission, the astronauts splashed down into the Atlantic Ocean.

Apollo 10, the first of three manned missions to the moon that took place in 1969, was described as a "dress rehearsal" for the first lunar landing.

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Astronauts John Young, Thomas Stafford, and Eugene Cernan launched atop a Saturn V rocket on May 18, 1969. The three men came closer to the lunar surface than any astronaut before them.

Young, Stafford, and Cernan also got farther from Earth than anybody before.

The Apollo 10 mission included a test of a lunar lander that was similar to the one later used for the first moon landing. The lander, named Snoopy, was designed to travel most of the way down to the surface (but not all the way) so the astronauts could test its performance and use it to survey the future landing site.

The astronauts successfully piloted Snoopy to about 50,000 feet above the moon's surface before returning to the main spaceship.

An estimated 530 million people around the world watched as Apollo 11 astronaut Neil Armstrong became the first person to step foot on the moon on July 20, 1969.

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Armstrong famously called the historic achievement a "giant leap for mankind." Buzz Aldrin followed him onto the lunar surface, while their crewmate Michael Collins stayed on the main spacecraft in orbit around the moon.

After the three astronauts returned to Earth, they were quarantined for 21 days to make sure they did not bring home any lunar contagions. Armstrong turned 39 during the confinement.

Until the Apollo 11 mission, Russian cosmonauts had been ahead of the US at almost every turn in the Cold War space race. At the time, many Americans did not believe spending $24.5 billion on the Apollo missions was worth it, and some people protested NASA's eight-year effort to land on the moon.

Armstrong, meanwhile, had a few near-death experiences in the years leading up to the moon landing. In March 1966, he and co-pilot David Scott were almost lost in space during the Gemini 8 mission. This was the first attempt to dock one spacecraft with another while in orbit, an essential step in a moon landing. But soon after takeoff, a thruster malfunctioned, which sent Armstrong and Scott spinning out of control. Luckily, they found a way to regain control of the spacecraft by powering thrusters.

A few months after Armstrong and Aldrin walked on the moon, NASA sent another spacecraft to the lunar surface in the Apollo 12 mission.

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The Apollo 12 mission was neither as historic as its predecessor nor as scary as the near-disaster of Apollo 13.

But Apollo 12 was not without drama. The mission, which launched on November 14, 1969, was almost aborted minutes of takeoff because lightning struck the spacecraft and scrambled the rocket's instruments. Many of the instruments were disabled completely after a second lightning strike.

At the time, NASA was unsure whether the mission could safely continue. But the mission wound up being successful astronauts Charles Conrad and Alan Bean landed on the lunar surface while Richard Gordon circled the moon .

The Apollo 13 mission blasted off from the Kennedy Space Center on April 11, 1970, but something went terribly wrong about 56 hours into the trip to the moon.

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After an oxygen tank exploded and damaged the cabin of the spaceship that housed the crew, astronauts Fred Haise, Jack Swigert, and Jim Lovell were in serious danger. The spacecraft lost the ability to generate water and power within three hours of this malfunction, and the astronauts' oxygen stores were lost, too.

The lunar module that was supposed to be used to land on the moon became the astronauts' "lifeboat" as they abandoned the main spaceship. But that small spacecraft was only built for two people, so the lithium hydroxide canisters that absorbed gas from the air were used up quickly.

The astronauts were at risk of dying from high levels of carbon dioxide, but managed to re-design the main ship's gas-absorbing canisters to fit into openings on the lunar module. They then turned around and landed safely in the South Pacific on April 17, 1970. NASA called the mission a "successful failure ."

Read more: The transcript from the Apollo 13 disaster will give you chills

NASA made another successful lunar landing the following year. Apollo 14 astronauts Alan Shepard, Edgar Mitchell, and Stuart Roosa launched from the Kennedy Space Center on January 31, 1971.

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The spacecraft's destination was the same as the aborted Apollo 13 mission's: the moon's Fra Mauro highlands.

Apollo 14 collected more lunar material and data than originally planned to make up for Apollo 13's failure to reach the moon.

Astronauts used a wheeled Lunar Roving Vehicle for the first time during the Apollo 15 mission to study the moon's geology.

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NASA astronauts David Scott, James Irwin, and Alfred Worden made up the Apollo 15 crew.

Using a vehicle allowed astronauts to travel farther from the lunar lander than others before them. The samples that the Apollo 15 astronauts brought back included a rock estimated to be 4 billion years old.

Apollo 15, along with the two missions that followed it, featured a television camera on the lunar rover, an updated lunar module that let crews stay on the moon for longer than before, and redesigned backpacks that let astronauts spend more time on the lunar surface.

NASA used the Apollo 16 mission to explore the moon's highlands for the first time.

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Astronauts John Young, Thomas Mattingly, and Charles Duke comprised the crew.

On April 20, 1972, 36-year-old Duke became the youngest human in history to walk on the lunar surface. Duke also made headlines for leaving a photo of him, his two sons, and his wife on the moon.

"I'd always planned to leave it on the moon," Duke previously told Business Insider . "So when I dropped it, it was just to show the kids that I really did leave it on the moon."

After more than 20 hours of experiments on the lunar surface, the astronauts collected roughly 210 pounds of samples.

Apollo 17 was the last mission to bring people to the moon.

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Apollo 17 commander Eugene Cernan is still the last man to walk on the lunar surface. Compared to previous missions, this trip collected the most rock and soil samples from the moon.

During the Apollo 15 and 17 missions, astronauts also installed heat-flow experiments to gather data on the moon's temperature. Earlier this year, a study published in the Journal of Geophysical Research analyzed that data and concluded that NASA astronauts likely warmed up the moon's surface temperature by as much as 6 degrees Fahrenheit.

According to the study, walking on the moon and driving rovers around caused dark moon dust called regolith to be exposed . This likely prompted the moon's surface to heat up, the scientists said, because darker materials absorb more light.

Nearly half a century has passed since the Apollo 17 mission, but NASA says it could return to the lunar surface as early as 2019.

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In NASA's recent announcement in which the space agency offered up to $2.6 billion in contracts to nine companies the agency suggested the first missions could fly as soon as 2019 (though as late as 2022).

NASA administrator Jim Bridenstine also vowed that unlike previous failed efforts by NASA to get back to the lunar surface, this "Commercial Lunar Payload Services" program will succeed.

"Everybody is ready to go back to the moon," Bridenstine said.

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