Challenger disaster's 25th anniversary
28 January 2011 Last updated at 07:02 GMT
The US space agency Nasa has held a day of remembrance for astronauts who have died in the line of duty, particularly the seven who perished in the 1986 Space Shuttle Challenger disaster.
It is 25 years since the shuttle exploded 73 seconds after launch at an altitude of 14,000 metres (46,000ft), killing all on board.
In a ceremony at Arlington National Cemetery in Virginia, a wreath was laid in tribute to the seven people who lost their lives.
The BBC's Andrew North reports.