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The Fallen Heroes of Human Spaceflight
The prime crew members of NASA's first manned Apollo Space Flight are pictured during training in Florida on March 21, 1966. From left to right are astronauts Gus Grissom, Edward White and Roger Chaffee.
Credit: NASA

Since the age of human spaceflight began 50 years ago, space missions have led in the accidental deaths of 21 explorers, some American astronauts and others Russian cosmonauts, while inside their spacecraft.

The most recent human spaceflight disaster, which occurred in 2003 at the end the space shuttle Columbia's STS-107 mission, caused NASA to ground its shuttle program for more than two years to make safety improvements.

Here's a look at the men and women who have lost their lives on space missions:

Mission: Apollo 1

Date: Jan. 27, 1967

Fatalities: Gus Grissom, Edward White II, Roger Chaffee

What happened: During a launch-sequence rehearsal for NASA's Apollo 1 mission, the cabin was filled with pure oxygen as part of its environmental control system. An electrical fault sparked a flash fire in the cabin.

The fire spread quickly in the pure oxygen atmosphere, suffocating all three Apollo 1 crew members through smoke inhalation. The launch pad test site was renamed Apollo 1 in honor of the crew, and the accident led to major design and engineering modifications as well as revisions to test planning operations and manufacturing procedures. [The Dangers of Human Spaceflight (Infographic)]
 

Mission: Soyuz 1

Yuri Gargarin (left) with Vladimir Komarov
Yuri Gargarin (left) with Vladimir Komarov
Credit: Soviet Academy of Sciences photo via NASA

Date: April 24, 1967

Fatalities: Vladimir Komarov

What happened: Soyuz 1, the Soviet space program's one-day mission, launched on April 23, 1967, but soon began experiencing various mechanical issues – the solar panels did not unfold, and the vessel experienced stability problems.

After the Soyuz module re-entered the atmosphere April 24, its parachute did not open properly, causing it to crash to Earth at almost full speed. Cosmonaut Vladimir Komarov died on impact.

Soyuz 11 cosmonauts Viktor Patsayev, Georgi Dobrovolsky, and Vladislav Volkov are shown in a flight simulator in this photo.
Soyuz 11 cosmonauts Viktor Patsayev, Georgi Dobrovolsky, and Vladislav Volkov are shown in a flight simulator in this photo.
Credit: NASA

Mission: Soyuz 11

Date: June 30, 1971

Fatalities: Georgi Dobrovolski, Viktor Patsayev, Vladislav Volkov

What happened: Soyuz 11 launched on June 6, 1971, and docked with the space station Salyut 1 for a three-week stay.

While the actual station mission was a success, when the Soyuz 11 vehicle undocked, a valve on accidentally opened, causing a pressure leak in the cabin.

The three cosmonauts were killed as the capsule depressurized during preparations for atmospheric re-entry on June 30. The malfunctioning valve was discovered only when the module was opened by a recovery team.

Mission: STS-51-L

On Jan. 28, 1986, NASA faced its first shuttle disaster, the loss of the Challenger orbiter and its seven-astronaut crew. Here, Challenger's last crew – members of the STS-51L mission – stand in the White Room at Pad 39B following the end of a launch dress rehearsal. They are (L to R) Teacher in Space Participant, Sharon "Christa" McAuliffe, Payload Specialist, Gregory Jarvis, Mission Specialist, Judy Resnik, Commander Dick Scobee. Mission Specialist, Ronald McNair, Pilot, Michael Smith and Mission Specialist, Ellison Onizuka.
On Jan. 28, 1986, NASA faced its first shuttle disaster, the loss of the Challenger orbiter and its seven-astronaut crew. Here, Challenger's last crew – members of the STS-51L mission – stand in the White Room at Pad 39B following the end of a launch dress rehearsal. They are (L to R) Teacher in Space Participant, Sharon "Christa" McAuliffe, Payload Specialist, Gregory Jarvis, Mission Specialist, Judy Resnik, Commander Dick Scobee. Mission Specialist, Ronald McNair, Pilot, Michael Smith and Mission Specialist, Ellison Onizuka.
Credit: NASA

Date: Jan. 28, 1986

Fatalities: Greg Jarvis, Christa McAuliffe, Ronald McNair, Ellison Onizuka, Judith Resnik, Michael J. Smith, Dick Scobee

What happened: During the space shuttle Challenger's 10th mission, STS-51-L, the spacecraft broke apart 73 seconds after launching from the Kennedy Space Center in Florida. All seven astronauts aboard were killed, including Christa McAuliffe, a New Hampshire high school teacher tapped to be the first teacher in space.

President Ronald Reagan formed the Rogers Commission to investigate the accident and space shuttle missions were grounded, pending the review. The commission's analysis concluded the Challenger accident had been caused by the failure of an O-ring seal on one of the solid rocket boosters due to extremely cold weather before launch. [Photos: Challenger Shuttle Accident]

The Challenger disaster forced NASA to halt all space shuttle launches for 32 months to recover from the accident.

This image of the STS-107 shuttle Columbia crew in orbit was recovered from wreckage inside an undeveloped film canister. The shirt colors indicate their mission shifts. From left (bottom row): Kalpana Chawla, mission specialist; Rick Husband, commander; Laurel Clark, mission specialist; and Ilan Ramon, payload specialist. From left (top row) are astronauts David Brown, mission specialist; William McCool, pilot; and Michael Anderson, payload commander. Ramon represents the Israeli Space Agency.
This image of the STS-107 shuttle Columbia crew in orbit was recovered from wreckage inside an undeveloped film canister. The shirt colors indicate their mission shifts. From left (bottom row): Kalpana Chawla, mission specialist; Rick Husband, commander; Laurel Clark, mission specialist; and Ilan Ramon, payload specialist. From left (top row) are astronauts David Brown, mission specialist; William McCool, pilot; and Michael Anderson, payload commander. Ramon represents the Israeli Space Agency.
Credit: NASA/JSC

Mission: STS-107

Date: Feb. 1, 2003

Fatalities: Rick D. Husband, William McCool, Michael P. Anderson, David M. Brown, Kalpana Chawla, Laurel B. Clark, Ilan Ramon

What happened: At the end of its 16-day mission, the space shuttle Columbia disintegrated as it re-entered the Earth's atmosphere over west Texas, killing the seven-person crew.

The accident was later determined to have been caused by damage that had occurred during liftoff, when a chunk of insulating foam broke off from the external tank and hit the orbiter's left wing. The structural failure of the shuttle's leading wing ultimately resulted in the spacecraft breaking apart.

All of NASA's space shuttle program was grounded for 29 months following the disaster.

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This article was provided by Life’s Little Mysteries, a sister site to Space.com.