<p><br /> Nasa has marked the 25th anniversary of the space shuttle Challenger explosion with a wreath-laying ceremony in front of the Astronauts Memorial at the Kennedy Space Center in Florida.<br /> </p><p><br /> Seven astronauts were killed on January 28, 1986, when a booster engine failed just 73 seconds after launch, causing Challenger to explode.<br /> </p><p><br /> Before Challenger's launch, crew member Christa McAuliffe became famous when she was selected in a nationwide search for a teacher who would become Nasa's first citizen passenger to travel into space.<br /> </p><p><br /> McAuliffe's parents were among a crowd of spectators at the Kennedy Space Center in Florida who watched the shuttle climb into the sky and then explode.<br /> </p><p><br /> Nasa is shutting down the 30-year-old shuttle program due to high operating costs and to free up funds to start developing a new space launch system that can carry people and cargo to asteroids and other destinations.<br /> </p>
