Will space travel ever be safe?
Nasa marks 30th anniversary of the Challenger shuttle tragedy
Nasa will today honour the people who have lost their lives pushing back the frontiers of space travel on its annual day of remembrance.
But 30 years on from the launch of the ill-fated Challenger shuttle, will space travel ever be safe?
How many people have died?
Nasa lost its first astronauts in 1967, when a fire during a ground test killed three people preparing for the Apollo I mission.
Disaster struck again on 28 January 1986, 30 years ago today, when a booster engine failed on the space shuttle Challenger and it broke apart within minutes of its launch. All seven astronauts on board were killed.
Seven more Nasa astronauts were killed in 2003, when the Colombia shuttle disintegrated over Texas as it re-entered the Earth's atmosphere.
Why is space travel so dangerous?
One of the main reasons is the staggeringly high cost. "It's so expensive that we tend to start flying our missions very, very early in the development phase," Nasa's Bryan O'Connor has said. Space travel also requires dangerously large amounts of energy, while re-entry generates temperatures so extreme they can burn through the craft's exterior. "Launching humans into space is not easy. We've learned that over time, through some very hard lessons in the shuttle programme," says Bob Doremus, safety manager for Nasa's shuttle programme.
Can it ever be safe?
With growing plans for commercial space travel, safety is an increasing concern. Some predict space flight will become as routine as air travel, but experts warn it will never be without risk - there is a one in 65 chance of something going badly wrong, according to the BBC's Tim Bowler. "This may be acceptable for astronauts and cosmonauts, who are often ex-test pilots used to risks, but not for rich thrill-seekers," he adds.