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

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

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.