CAPE CANAVERAL, Fla. – Space station commander Scott Kelly led NASA in a moment of silence Monday as he struggled with the senseless shooting of his sister-in-law.

It was part of the national observance for all the victims of Saturday's shooting in Arizona.

His identical twin brother, Mark Kelly, is married to Rep. Gabrielle Giffords. Both brothers are astronauts.

Flight controllers in Houston fell silent as Scott Kelly spoke via radio from space.

"We have a unique vantage point here aboard the International Space Station," he said. "As I look out the window, I see a very beautiful planet that seems very inviting and peaceful. Unfortunately, it is not."

"These days, we are constantly reminded of the unspeakable acts of violence and damage we can inflict upon one another, not just with our actions, but also with our irresponsible words," he said.

"We're better than this. We must do better."

Kelly described his sister-in-law as "a caring and dedicated public servant."

Flags flew at half-staff at NASA centers nationwide as part of the observance.

Kelly is midway through a 5 1/2-month space station stay. He's in charge of the six-person crew. His brother is supposed to command a shuttle flight in April; his status now is uncertain.

Mark Kelly, who rushed from Houston to Tucson, Ariz., after the shootings, issued his first public statement Sunday night. He thanked everyone for the "unbelievable outpouring of support."

"Gabby was doing what she loved most — hearing from her constituents — when this tragedy occurred," he said. The stories of heroism that are emerging "are a source of strength for us during this difficult time," he added.

Scott Kelly is staying busy aboard the orbiting lab, exercising and performing maintenance, while keeping abreast of the situation in Tucson via Mission Control, the Internet and an on-board Internet phone.