TIME contract photographer Marco Grob’s first memory was of the Moon landing. “Anything that’s space-related is very special for me,” he says. So when TIME asked him to photograph Mark and Scott Kelly, NASA’s famous twin brothers, he jumped on the opportunity, traveling to the Johnson Space Center in Houston, Texas.

“In a weird way, being at NASA and having such access felt too good to be true for a guy like me. It’s one of the most memorable shoots I’ve done,” he says. “Both of them, by all means, are very extraordinary men.”

Grob used a simple lighting setup to photograph the twin brothers. “I wanted the textures to stand out on their own, especially with the space suit. I wanted a lighting that served the purpose instead of overpowering the image.”

The photographer also wanted to highlight the extraordinary nature of his subjects. “People don’t always understand how enormously difficult it is to become an astronaut,” he says. “So to have a family where you have two twin brothers that are astronauts, it’s just tremendous. So I felt that showing them next to each other, without any distraction, would be a really big deal. I like the simplicity of that photo.”

Given Grob’s love of space, it’s no surprise that these portraits of Mark and Scott Kelly will remain among his favorites — yet the photographer was surprised by one thing: how welcoming NASA was. “For them, it was a big deal to have a TIME cover,” he says. “So it’s pretty cool when you go to NASA and they are the ones that think it’s a big deal. Normally, it’s the other way around.”

Marco Grob is a TIME contract photographer.

Olivier Laurent is the editor of TIME LightBox. Follow him on Twitter and Instagram @olivierclaurent