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In 1986, tragedy struck the American space program. On a cold winter morning, the space shuttle Challenger blasted off. A few seconds after launch, while the world watched on television, the shuttle exploded, killing seven astronauts: Francis Scobee, Judith Resnick, Ronald McNair, Michael Smith, Sharon Christa McAuliffe, Ellison Onizuka, and Gregory Jarvis. It was the worst disaster in the history of the American space program.

It threatened to bring the entire American astronaut program to an end. All of the shuttles were grounded for more than a year while investigators studied the evidence.

The cause turned out to be in the solid rocket boosters. When a space shuttle takes off, it uses both solid and liquid fuel rockets. The main part of the shuttle -- the orbiter with the crew -- uses liquid oxygen and liquid hydrogen as fuel. These liquids are contained in the enormous tank strapped on the bottom of the orbiter.

But for an extra boost, solid rocket boosters (SRBs) are strapped on the outside of the shuttle. The two SRBs are similar to rockets used in fireworks. They contain a solid fuel. And the walls of these rockets are cylinders stacked on top of one another like long pipes. The two cylinders are joined by a circular rubber ring, called an O-ring. The O-ring is supposed to prevent the rocket fuel from burning through.

But on the morning of the Challenger launch, the temperature was below freezing. The rubber O-rings were hard, and did not flex during the stresses of the lift-off. Ordinarily, the boosters would fire for two minutes, and then be released, parachuting down to the ocean to be recovered and reused. Tragically, before the shuttle reached the point where the boosters would be let go, one of the rockets burned through its O-ring. The flames shot out like a blow torch and touched the spacecraft. This caused the shuttle to explode.

Investigators found that on previous flights, flame damage had occurred to the O-rings. This was a warning sign that a serious problem could occur, but the astronauts were not informed and the problem was ignored.

As a result of the Challenger investigation, a major shake-up took place within NASA. Parts of the shuttle were redesigned and safety procedures were greatly improved. If any problems were seen by the ground crew, they were required to tell the astronauts. Any engineer or technician who found a serious problem was encouraged to report it to NASA managers.

NASA had been through a similar tragedy before. In the early days of the Apollo program, a fire broke out during an Apollo ground test. Three astronauts died in the Apollo fire.

This threatened to seriously delay the Apollo program, but as a result safety procedures were greatly improved, and not only did Apollo get to the Moon on schedule, but there has never been a loss of life in space.

Since the space shuttle started flying again after the Challenger disaster, there has never been another serious accident. But it is important to remember that it is always dangerous to explore a new frontier. All of the astronauts know the risks, and accept them.

Read more space facts on my blog.