Wednesday marks the 14th anniversary of the Columbia Space Shuttle disaster, in which seven astronauts lost their lives as the craft broke up during reentry on February 1, 2003.

Mark Kelly, a retired astronaut and the brother of astronaut Scott Kelly— both of whom are known for NASA’s Twin Study— took to Twitter to commemorate the tragic anniversary.

“Remembering the 7 astronauts lost aboard Columbia 14 years ago today,” he wrote. “They were brave explorers and really good people.”

The Columbia Shuttle was the second time the space agency lost a vehicle in a catastrophic accident: the first, of course, was Challenger, in 1986. NASA recently marked another very sad occasion, with the 50th anniversary of the Apollo 1 capsule fire, in which three astronauts died.

Remembering the 7 astronauts lost aboard Columbia 14 years ago today. They were brave explorers and really good people. pic.twitter.com/2yIQPGcuNG — Mark Kelly (@ShuttleCDRKelly) February 1, 2017

Kelly was joined by others in paying their respects for the loss of Columbia on Wednesday.

Congressman Don Beyer of Virginia took to Twitter as well, to share three tweets on the tragedies, which, stitched together, read:

“The 3rd of 3 tragedies whose anniversaries fall in 1 tough week for @NASA - today is the anniversary of #Columbia 1/ … Take some thought today for seven Americans lost in the space shuttle #Columbia disaster, for those lost on Challenger and Apollo I, and 2/ … for men and women from around the world who have made the ultimate sacrifice for science and exploration 3/3”

The 3rd of 3 tragedies whose anniversaries fall in 1 tough week for @NASA - today is the anniversary of #Columbia 1/ https://t.co/YUmyaJz94x — Rep. Don Beyer (@RepDonBeyer) February 1, 2017

NASA ultimately determined the loss of the vehicle stemmed from a foam strike on the Shuttle’s wing during takeoff— that impact created a hole and fatally compromised Columbia while it was reentering the Earth’s atmosphere.