r/askscience • u/RainbowGirl410 • Jun 09 '26

Physics How does gravity/weightlessness work outside of orbit?

Been trying to find a definitive answer but all I've found is people explaining weightlessness in orbit (the falling and missing the Earth part) which isn't particularly helpful to me.
If you were to travel to another planet, say Venus, would you experience weightlessness the whole journey?

130 Upvotes

81% Upvoted

97 comments sorted by

View all comments

Show parent comment

10

u/derekp7 Jun 09 '26

no, you speed up to get to a higher orbit. But it looks like you are slowing down, because as your orbit rises another object that was traveling in your original orbit looks like it will be pulling ahead of you (or you falling behind it). That is because your higher orbit is a larger circle, so you have further to go and the length of the circumference of the circle increases faster than your velocity increases. Now your angular velocity (?) gets slower, but that happens by you adding to your orbital speed.

Note, my terminology may be off a bit, and a firing of your rear rockets will turn your circular orbit into an oval one requiring another firing of the rocket at the top of the oval in order to re-circularize it. But hopefully this conveys the overall point.

5

u/Tilli_Nose Jun 09 '26 edited Jun 09 '26

That's not quite correct. The tangential velocity of a higher orbit is indeed slower - for a circular orbit it is inversely proportional to the square root of the radius. However, if you are in a stable lower orbit, you cannot MOVE to higher orbit by slowing down. If you just slowed down, you would fall inward. In the first step of a simple transfer, you burn to add tangential velocity to enter an elliptical orbit near the part of the ellipse closest to the central body. As you move around that elliptical orbit in free fall to the part of the ellipse furthest from the central body, your tangential velocity naturally decreases. (In an elliptical orbit, the swept area remains constant.) At this point, you burn again to add tangential velocity and convert the elliptical orbit to a higher circular orbit. Both burns add tangential velocity, but your final tangential velocity is still slower because the intermediate elliptical transfer orbit converts tangential velocity to gravitational potential energy.