e_j_white t1_j26ocwx wrote on December 30, 2022 at 1:13 AM

Reply to comment by swampshark19 in How fast does the Milky Way spin? How far does Earth move through space in a year? by Sabre-Tooth-Monkey

Yes, it's true.

The outer stars are moving too rapidly to stay in orbit, and under normal calculations they should be thrown out of the galaxy. Additional gravitational forces must be keeping them in orbit at those faster speeds, and that's where the theory of dark matter comes in.

For example, Venus is moving at 78K mph, while Saturn is moving at 22K mph (because it is much farther away from the sun). If Saturn were moving at the same speed as Venus, it would be thrown farther out from its current orbit.

bitwaba t1_j26vh0u wrote on December 30, 2022 at 2:05 AM

I thought you have to speed up to go to a higher orbit? I remember seeing something that said a mission to the sun is really difficult because you have to essentially slow your orbit down to 0 to move to an orbit that brings you closer to the center of the solar system.

Aseyhe t1_j26wt7e wrote on December 30, 2022 at 2:14 AM

Yes, you have to speed up to get to a higher orbit -- and paradoxically, that still results in you moving slower, on average! This is an extremely interesting feature of gravitational systems; for example, it means they have a negative heat capacity (adding energy cools them).

desepticon t1_j271xta wrote on December 30, 2022 at 2:51 AM

Learning this in Kerbal space program was a big "aha". You aren't so much controlling your speed as you are just altering your orbit on the opposite side of the planet.

swampshark19 t1_j2brjyx wrote on December 31, 2022 at 2:03 AM

Is this related to gravitational potential being understood as negative energy?

e_j_white t1_j26wo0k wrote on December 30, 2022 at 2:13 AM

Nope, orbital speed goes down as you get farther away. The equation is:

v = sqrt(G * M / R)

Larger R, smaller v.

In order to REACH a higher orbit, you need to do work to move the mass to a higher gravitation potential. That type of work requires thrust, but once you're at the larger orbit, the speed is slower.

Conversely, to move CLOSER to the sun, you need "anti-thrust" to move lower in the gravitational potential.

bitwaba t1_j26xukm wrote on December 30, 2022 at 2:21 AM

Ah, thank you. That is much clearer.

adm_akbar t1_j279juf wrote on December 30, 2022 at 3:48 AM

Yes you have to speed up to get to a higher orbit but then your orbital speed is slower.