Expert: Ed - 10/18/2009

Question
Hi. I'm confused about gravity and orbits. Basically my question is if the Earth's gravity is strong enough to hold a space craft in orbit, how are the astronauts weightless inside? If the gravity is holding the spacecraft in orbit shouldn't it be kind of like a plane where they would be standing on the floor inside the craft?

Answer
Josh, I am very sorry I haven't answered this yet.  I should have told AllExperts I was on vacation.  If you rate this response, you need to give me a ZERO for timeliness, unless they let you give negative numbers for those things... I'm not sure if that's an option.

Astronauts are "weightless" inside spacecrafts because they are FALLING, relative to the Earth, at the same rate of speed as the spacecraft.  Earth's gravity does affect them, but their entire surrounding frame of reference (the spacecraft) is affected EXACTLY EQUALLY, so it seems to them that they are weightless.

Google "David Scott", "Apollo 15", moon walk, hammer feather to get an interesting example.  In this context, the astronaut and the spaceship are equivalent to the hammer and the feather, and the Earth's gravity is to be compared to the gravity of the moon.  If a miniature spacecaft and miniature astronaut were dropped by David Scott on the moon, together with the hammer and feather that he was also dropping, they would all fall at the EXACT same speed.  Read that sentence again!!

That's why they call it "free fall."  It's not really weightlessness, it's just that the astronauts are "falling."  The odd thing is, they "fall" AROUND the Earth, because their orbital velocity allows them to do this.  But that doesn't change the fact that they are FALLING.

Keep Looking Up,
-Ed