He promised her the Moon — and meant it.

Over 23 years ago, a NASA intern named Thad Roberts devised a bold scheme to steal 17 pounds of moon rocks and a meteorite from the Johnson Space Center in Houston. These samples, gathered from every Apollo mission, were securely housed in a 600-pound safe and assessed by the FBI to be worth $21 million.

At the time, Roberts was 24 years old and juggling a triple major in physics, geology, and geophysics at the University of Utah. He was supporting his wife back in Utah and was under financial strain, which led him to believe his plan was flawless.

The initial step in his scheme was to secure a buyer. With assistance from his friend, Gordon McWhorter, Roberts found a prospective Belgian buyer online who was prepared to offer between $1,000 and $5,000 per gram.

The buyer, however, grew suspicious and alerted the FBI, who instructed him to keep talking while they investigated.

Around this time, he met Tiffany Fowler, a 22-year-old NASA intern conducting stem cell research.

Their friendship quickly became romantic, and after three weeks, they moved in together. When Roberts revealed his plan, Fowler agreed to help.

They recruited another NASA intern, Shae Saur, and one night the trio used their NASA IDs to slip into the Johnson Space Center, making off with the entire safe.

Back at the hotel, they cracked it open with a power saw.

On July 20, 2002—the 33rd anniversary of the first moon landing—Roberts and Fowler drove to Orlando to meet family members of the Belgian buyer.

While waiting, Roberts placed moon rocks beneath the bed covers, later claiming the couple had “sex on the moon” as a symbolic gesture.

“I take some of the moon rocks and I put them underneath the blanket on the bed… I never said anything but I’m sure she could feel it,” he told CBS News in 2012.

“It was more about the symbol of what we were doing, basically having sex on the moon. It’s more uncomfortable than not, but it wasn’t about the comfort at that point. It was about the expression. And no one had ever had sex on the moon before. I think we can safely say that.”

When they met with the supposed buyers, they were instead greeted by undercover FBI agents. The moon rocks were recovered from their hotel room, though the FBI reported they were now “virtually useless to the scientific community” and that the heist destroyed three decades of handwritten research notes by a NASA scientist.

After his arrest, Roberts admitted to also stealing dinosaur bones and fossils from the Natural History Museum in Salt Lake City.

When asked why he stole the moon rocks, Robert told CBS News he “wasn’t” looking at it like stealing at the time.

“We weren’t going to take this money we were getting from it to go buy a yacht or lots of cars or a big house. We were gonna live just the small kind of lifestyle we were, but fund science that might change the world, you know?” he said.

In the end, Roberts, Fowler, and Saur all pleaded guilty to conspiracy to commit theft and interstate transportation of stolen property.

Roberts was sentenced to eight years in federal prison, serving six. Fowler and Saur each received 180 days of house arrest and 150 hours of community service.

McWhorter, convicted at trial, was given six years in prison.

Roberts and Fowler never saw each other again.