The Debunker: Is There Really No Gravity In Space?

by Ken Jennings

 

As NASA’s Curiosity rover continues to inch across the red planet’s dusty Gale Crater, America’s interest in space exploration inches upward as well. Jeopardy!’s Ken Jennings is a bit of space nerd himself, and this month he’ll be navigating us through an asteroid belt of misconceptions about the exploration of the cosmos. Even if you’re not one of the 6 percent of Americans who believes that the moon landing was a hoax, you might have been fleeced by one or more of these fallacies about the final frontier.

Space Myth #3: Astronauts Are Weightless in Space Because They’re Beyond Earth’s Gravity.

We’ve all seen the images of astronauts—real-life and movie ones alike—bouncing around the insides of their space capsules like crewcutted Ping-Pong balls. Of course, we think. On Earth, there’s gravity. In space, far from the Earth’s pull, there’s not. But that’s not even close to correct.

Remember that gravity is one of the fundamental forces of the universe, so it’s everywhere. According to Newton’s laws, it diminishes fairly rapidly with distance, but objects in low Earth orbit, like a Space Shuttle, are only 350 kilometers or so above the planet—the Moon is over one thousand times farther away! If you do some rough math, you’ll see that the gravitational force on an astronaut in orbit is still about 650 newtons, about 90% of what it is on Earth. Gravity is almost identical in orbit! So how are spacemen doing those cool tricks with the spinning clipboards and the floating Tang and whatnot?

An astronaut in orbit feels weightless for the same reason a skydiver does: he’s falling. Or, to be precise, his spacecraft is falling, and he’s falling inside it at exactly the same speed. You could get the exact same experience (more briefly) inside a free-falling elevator—and you could do all his cool bouncing-off-the-ceiling space tricks in a falling elevator, like Joseph Gordon-Levitt in Inception. The spacecraft is falling, but it doesn’t fall to Earth. Why? Because it’s moving forward sufficiently fast (17,500 miles per hour, aka “orbital velocity”) that as it falls, it falls around the Earth. And keeps falling, making a near-circular orbit, until it slows down enough (using its engines, in the Space Shuttle’s case) to leave orbit in a controlled way and return to Earth.

Without gravity, none of this would be possible. Of course, it’s also possible to imagine an astronaut somewhere in deep space, too far from any celestial body for anything to pull noticeably on him. Now that guy has the exact same weightless feeling he would in orbit, but in his case it is due to a lack of gravitation. Unfortunately, he’s probably dead.

Quick Quiz: What musician’s hit song “Living in America,” featured on the Rocky IV soundtrack, appeared on his 1986 album Gravity?

Ken Jennings is the author of Because I Said So!, Brainiac, Ken Jennings's Trivia Almanac, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at ken-jennings.com or on Twitter as @KenJennings.