It has been forty-five years since the Apollo 1 disaster. On the 27th January 1967, a fire killed the crew during a launch pad test. One of the reasons the Apollo 1 fire burned so fast was that the cabin was filled with pure oxygen, which fed the flames. NASA has used a mixture of gases in its spacecraft since then.
One of the astronauts who died, Ed White, had been the first American to do a space-walk, on 3rd June 1965. He enjoyed his 21-minute space-walk so much that NASA had to order him to return to his capsule. When he went back inside, he said “this is the saddest moment of my life.”
It has also been 26 years since the Challenger disaster. On 28th January 1986, just 73 seconds into its flight, the Space Shuttle exploded on its way into orbit, killing everyone on the ship.
On 1st February 2003 – ten years ago tomorrow – the Space Shuttle Columbia broke apart as it re-entered the atmosphere. The last Shuttle, Atlantis, flew on 8th July 2011. Now, they are all in museums – but the fleet has been grounded for less than two years.
There have been humans just like us for about 200,000 years now. There have been astronauts for only 51 years. Yuri Gagarin was the first, on 12th April 1961. America followed with Alan Shepard on 5th May the same year, but NASA did not put a man into orbit until John Glenn on 20th February 1962.
It is dangerous to be an astronaut: 22 have died on their ships. But they were doing the most important thing humans have done in their 200,000 years. Their deaths are not reasons to stop. They are reasons to continue.
[Edit: added a bit more detail]
This is what the Bad Astronomy blog said:
So, for Grissom, Chaffee, and White; for Scobee, Smith, McAuliffe, Onizuka, Resnick, McNair, and Jarvis; for Brown, Husband, Clark, Chawla, Anderson, McCool, and Ramon, and for all the others who gave their lives for this great adventure:
I hope that we have learned from your experience, I hope that we have become better through your experience, and that, while we will never forget what happened to you, we will also remember what you were trying to do, and what you did do.
Per ardua ad astra.