The International Space Station zooms through orbit, skimming above the Earth's surface so fast that it rounds the planet every 90 minutes - giving crew members dominion to look at just about any wonder of the world.
Navy Capt. Chris Cassidy's eyes were drawn to the sun-scorched dunes of Afghanistan's Helmand province.
Cassidy's platoon of SEALs carved positions to sleep in the sand over Christmas 2001, just a few months after the 9/11 terrorist attacks. His men flushed out hardened al-Qaida fighters through claustrophobic cave routes. The SEALs got so close to some, Cassidy said, that abandoned bedrolls were still warm with body heat.
Then, more than a decade later, as Cassidy orbited 250 miles above the Earth, he knew where to find the familiar mountain foothills melting into sand.
"It made me send a few emails to my former teammates and say, 'Hey, just looking out the window and saw that sea of dunes,' " he said in a phone interview. "It made me think back to those days."
Five current and former astronauts who served in three wars said combat played a significant role in their trajectory to the space program - where they became perhaps unlikely evangelists in the fight to protect a fragile planet.
NASA has long relied on the deep bench of service members and veterans. But less considered are astronauts who have experienced the extreme ends of the humanity spectrum: the life-altering brutality of war, then the peaceful and scientific exploration of space's infinite potential.
Out of 350 astronauts NASA has selected, about 60 current or former NASA astronauts were deployed under combat orders or awarded decorations denoting wartime service, according to an analysis of NASA biographies by The Washington Post.
The vast majority were fighter pilots. A handful were infantry officers and helicopter pilots closer to the fight. At least two received Purple Hearts for combat wounds. Other biographies were unclear about deployments.
War can leave an indelible mark on its combatants and completely upend conceptions of life, death and mankind's place in the world.
The same thing happens in space, though in a more peaceful way. The overview effect is the phenomenon of the emotional tumult and startling realization that Earth is a fragile spaceship, or a "tiny pea, pretty and blue," as Neil Armstrong said after returning from the moon.
"I never was a big crunchy tree-hugger kind of person," Cassidy said. But both epiphanies collided for him when his eyes locked on Afghanistan from behind a telephoto lens in 2013 aboard a six-month mission on the ISS.
Troops on the ground, he said, "are down there are probably still doing the same things we were doing in 2001, and I got to imagine the overall scheme of things hasn't changed that much," Cassidy said he thought at the time.
"But it made me think: When you look down at Earth from above, you don't see borders, you don't see names of countries . . . you just see this big blob of blue and brown and green and white clouds. It made me feel a little bit more introspective about conflict than when I was a sledgehammer-wielding 25-year-old."
Combat prepared Army Lt. Col. Anne McClain, who flew missions over Iraq in an OH-58 Kiowa recon and fire support helicopter, for the dangerous reality of strapping to a rocket and heading to orbit.
Her wartime assignment proved deadly. She agonized over how leaders could better protect soldiers when she attended funerals for comrades killed in action.
"It's not an experience I would wish on anyone, but it's experience we bring to the table," McClain said in an interview days after she returned June 25 from 204 days in space.
The space program has closely tracked the country's wars, which have churned out some of its most notable astronauts. Buzz Aldrin, the second man on the moon, shot down two MiGs in the Korean War, where Armstrong also flew combat missions. John Glenn, the first American in orbit, served in World War II and the Korean War.
Two of the astronauts killed in the 1986 Challenger explosion, Francis "Dick" Scobee and Michael J. Smith, were Vietnam veterans.
Another Vietnam veteran, retired Marine Col. Robert Springer, slipped through dense jungle beyond friendly lines to direct helicopter fire and air support on Viet Cong insurgents in 1968 and 1969. He also flew missions in an F-4 Phantom and medevac missions in a Huey helicopter.
Sometimes he carried the dead.
Saigon fell, but Vietnam stayed with him. He said combat was a formative laboratory to hone a job executed under extraordinary duress.
"It gave me a different perspective," he said. "I've seen the worst of it from combat, and the best of it, and what we're doing in the space program bringing benefits to mankind."
Space has also given combat veterans a different view of their own war.
For most soldiers, the overhead view of a war zone is flattened into drone feeds or laminated maps. When McClain flew gun runs to support troops on the ground, she referred to highways and roads renamed to simple English words.
But from the ISS, McClain had a much different view of the area. She could see the damage of powerful floods that swept through Iraq.
"From space, you can see the water coming in. You can see what people are dealing with down there," she said.
The overview effect for McClain was one of desperate urgency to preserve the planet, she said. "I want to shake people's shoulders and say 'No, listen, we are all in it together.' "
Navy Cmdr. G. Reid Wiseman, who flew combat missions in Iraq and Afghanistan and later served as a flight engineer aboard the ISS, had a similar realization after combing over the borderless globe to find Kandahar.
"It changes you," he said, to see the entire planet. "The Earth is alive and it is powerful. And when it gets tired of us, it will fix that."
But war has prepared other astronauts for a grim task.
Col. James Buchli led infantry Marines in Vietnam and received the Purple Heart for shrapnel wounds. He later entered the astronaut program.
After two shuttle missions, Buchli stopped training for a moment to watch the Challenger launch. The space shuttle disintegrated 73 seconds into its flight.
NASA scrambled to figure out how to handle the families of seven crew members who had watched their loved ones die.
He presented an idea to administrators: He knew casualty notification officers helped families of slain Marines navigate anguish that began with a white-gloved knock on the door.
"The families needed to know more than what they saw on TV," Buchli said.
He spent the next year and a half helping those families. Then he went back to space.