This week’s Cosmic Quiz question comes from Daniel Wallace:
Why are astronauts aboard the International Space Station weightless while they are still so close to the Earth?
The strength of Earth’s gravitational field as felt by astronauts aboard the Space Shuttle is almost exactly the same as the gravity we feel here on the ground. So why are the astronauts weightless?
The answer goes back to Sir Isaac Newton in 1687 and a “thought experiment” he performed using just his imagination and his newly-formed Theory of Gravitation.
Newton imagined what would happen if you had a cannon located on a mountain top and aimed at the horizon.
If you fire a cannon parallel to the ground the ball moves horizontally away from the cannon due to its energy from the gunpowder. At the same time the cannon ball also begins to accelerate vertically toward the ground because of gravity.
The higher the speed of the cannon ball, the farther it travels before it hits the ground.
If you fire the cannon ball fast enough, the ball travels so far from you that the curvature of the Earth begins to curve the ground away from the falling cannon ball.
If you fire the cannon ball at just the right speed – about 17,500 mph, the cannon ball never hits the ground because the speed of the cannon ball’s fall towards the ground is exactly cancelled by the speed of the ground curving away out from under the falling cannon ball.
Newton realized that in the vacuum of space no force would act to slow the cannon ball down, and so a cannon ball that once achieved enough velocity to put it into orbit would remain in orbit indefinitely. It would be in perpetual free-fall around the Earth.
So even though you are “weightless,” in orbit around Earth, you’re still experiencing almost exactly the same gravity you are right now as you read this. The difference is that as you’re orbiting the planet at roughly five miles per second, the ground curves away beneath you at exactly the same speed that you’re falling toward the ground.