On January 28, 1986, Space Shuttle Challenger exploded 73 seconds after takeoff. It was then the most severe accident in NASA's space program. Challenger was the third of what would eventually become five space shuttles. Before it exploded, the shuttle had completed nine flights.

Space Shuttle Challenger takes off from the Kennedy Space Center in Cape Canaveral. Spectators are applauding. Only seconds later enthusiasm turns into horror. Numerous family members and colleagues of the astronauts are among the viewers.

These seven astronauts were killed in the accident (from left to right): Aerospace engineer Ellison Onizuka, pilot Mike Smith, elementary school teacher Christa McAuliffe, test pilot and aerospace engineer Dick Scobee, payload specialist Greg Jarvis, physicist Ron McNair and electrical engineer and specialist for telemetry Judith Resnik.

McAuliffe was supposed to become the first teacher in space - and an inspiration for many other students and teachers. In subsequent years, many astronauts carried on with the idea of bringing science and technology back from space into the classroom. Here, McAuliffe is talking to Barbara Morgan during astronaut training. Morgan stayed behind as a backup for McAuliffe.

One or more o-ring seals failed after a very cold night with freezing temperatures and then high temperatures during takeoff. Exhaust gas came out at the side of the rocket, rather than through the engine. Here the head of the presidential investigation commission William Rogers is testifying at a Senate committee. His deputy, astronaut Neil Armstrong, is listening.

Germany grieved for the dead astronauts as well. Only three month before the accident, the two first West German astronauts Reinhard Furrer and Ernst Messerschmid (first and fifth from left) flew with the same space shuttle. They launched the German-built Spacelab D-1, the first big space exploration project of the Federal Republic of Germany.

Two and a half years after the tragedy, NASA resumed shuttle flights. On December 29, 1988, Space Shuttle Discovery took off from Cape Canaveral. Again there were glitches: Parts of a tank's insulation broke off. Nevertheless, Discovery became one of the most successful space shuttles, and the most used one, too - between 1984 and 2011, it took off 39 times.

17 years and 87 space shuttle flights after the Challenger tragedy, criticism of the program had nearly died down. Then a second catastrophe happened: Space Shuttle Columbia broke apart during reentry into Earth's atmosphere on February 1, 2003. All seven crew members died. The likely cause was damage a wing had suffered during takeoff, when it was hit by a piece of fuel-tank insulation foam.

Despite the two accidents, NASA's shuttle program can look back on a proud history: 133 flights were concluded successfully. Here, Space Shuttle Atlantis is approaching the International Space Station (ISS). Without the shuttles, it would have been almost impossible to build the ISS. The Russian Soyuz capsule in the foreground is today's standard vehicle for human space travel to the ISS.

In comparison to the ISS, Space Shuttle Endeavour looks almost tiny on this picture. It is one of very few photos ever taken of a docked space shuttle at the station. Italian astronaut Paolo Nespoli had the opportunity to take the shot in 2011 from inside a Soyuz space capsule on the way home from the ISS.

Space shuttle Atlantis is docking on to the ISS for the second to last time in May 2010. Its last flight took place one year later. By then, space shuttles had delivered uncountable telecommunication-, navigation- and research satellites into their orbit, including space telescope Hubble. They retrieved and repaired satellites and even visited the Russian space station Mir.

Space Shuttle Endeavour flies for the last time - mounted on a special Boeing 747 Jumbo Jet. The plane took the shuttle to its retirement home, the California Science Center in Los Angeles. They took a detour to do a final flyby over the Golden Gate Bridge and several cities along the West Coast. Endeavour had seen its last space flight in May 2011, Atlantis' was in July of the same year.

The future of human space travel looks somewhat like the past: The newest NASA spaceship will be Orion. Its design recalls the first Apollo spaceships. The European Space Agency (ESA) will deliver important components for Orion. The new spaceship will be bigger than the Soyuz spacecrafts and also travel farther - to the moon or maybe even to Mars one day.