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Questioning the reason for our existence and exploring our universe are the key reasons why humans have traveled to space. Since the first efforts to get humans into space and on the Moon, there have been numerous space expeditions and technological advances. But not all have been successful, and many astronauts caught up in space experiments did not live to tell the tale. See the most tragic space missions in history.

▲A famous speech given by John F. Kennedy in the early 1960s promised that, by the end of the decade, American astronauts would set foot on the moon.

▲What seemed a far off dream became reality in 1969: man had already reached space and one of the greatest feats of the space age was about to happen.

▲It was the Apollo mission which took a man to the moon for the very first time. The images of the spaceship flying over the surface of the moon became iconic.

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But before arriving at this historic moment, the Apollo mission went through a terrible tragedy.

▲During a launch test for Apollo 1, the cockpit caught fire, killing Edward White, Virgil Grissom, and Roger Chaffee.

▲There is a conspiracy theory which questions whether humans ever really made it to the moon, or if the whole scene was filmed in a studio. One thing is certain: the tragedy went down in history.

▲Another terrible incident occurred in 1967 with a Soviet spaceship.

▲Soyuz 1 took off from a Soviet base. Its mission was to circle the Earth and change crews with another ship that was already in Space: Soyuz 2. Cosmonaut Vladimir Komarov, however, said that there were several technical problems and so the mission was aborted.

▲On returning to Earth, Vladimir Komarov tried to release the parachutes to slow down the vessel's return to the ground, but the attempt failed.

▲The spaceship hit the ground at a speed of approximately 140 km per hour, killing its pilot.

▲The tragedy earned Komarov numerous posthumous tributes.

▲Another tragedy struck Soviet Space project in 1971, with the spaceship Soyuz 11.

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The crew of Soyuz 11 was made up of Georgi Dobrovolski, Viktor Patsayev, and Vladislav Volkov, who had been in space for about 23 days and were returning to Earth. The crew began the procedure to re-enter the atmosphere, and appeared to have done it perfectly. However, once the ship had landed and been opened, the crew were found mysteriously dead.

▲Their deaths were explained by a fault in the ventilation valve, which made their capsule completely depressurize while in Space.

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The ship had only managed to re-enter the Earth's atmosphere thanks to the automatic pilot. The deaths of the three cosmonauts caused much grieving in the Soviet Union.

▲The Challenger accident of 1986 is one of the most famous incidents of all.

▲The crew who prepared to set off on the mission were: Greg Jarvis, Ronald McNair, Ellison Onizuka, Judith Resnik, Michael J. Smith, D i c k Scobee, and Christa McAuliffe.

▲This NASA mission was unique as it was the first time that a civilian had the chance to travel to space. Teacher Christa McAuliffe was part of the team of astronauts.

▲In January 1986 all eyes were on the launch of this space shuttle, which was broadcast live on TV.

▲About 72 seconds after the launch, a tragedy occurred as viewers watched.

▲The shuttle exploded due to an instability in the fuel tank insulation after it was exposed to colder temperatures.

▲The entire crew were killed in the explosion.

▲NASA paused its space program for two years while it searched for ways to prevent tragedies like Challenger from happening again.

▲Another fatal accident occurred in 2003 with spaceship Columbia.

▲After 16 days in orbit, the space shuttle disintegrated on its return to Earth.

▲There were seven crew members: Rick D. Husband, William McCool, Michael P. Anderson, David M. Brown, Kalpana Chawla, Laurel B. Clark, and Ilan Ramon. None of them survived the accident.

▲An investigation later discovered that the cause of the accident was a rupture in the heat insulation foam on one of the wings during launch.

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These tragic incidents remind us of the fragility of human life in our pursuit for the meaning of the world and the origins of the universe.