While he wasn't sure what to expect, Shatner did not predict this. He had been excited to travel to space, and had thought about it for nearly 60 years, but didn't think he'd be overwhelmed with sadness, or that he'd go through "the strongest feelings of grief" that he's ever experienced.

There's a name for what Shatner felt: it's called the "overview effect." The term was coined by space philosopher Frank White in his 1987 book of the same name.

"The overview effect is a cognitive and emotional shift in a person's awareness, their consciousness and their identity when they see the Earth from space," White told NPR. "They're at a distance and they're seeing the Earth ... in the context of the universe."

This context was what struck Shatner the most.

"It was the death that I saw in space and the lifeforce that I saw coming from the planet — the blue, the beige and the white," he said. "And I realized one was death and the other was life."

According to White, everyone who travels to space experiences an "overview effect" — an emotional or mental reaction strong enough to disrupt that person's previous assumptions about humanity, Earth, and/or the cosmos. Everyone's overview effect is unique to them, but there are reactions that are more common than others.