## On Orbital Decay

A forum set up for physics questions from students in the courses PHYS1121, 1131, 1221 and 1331 at the University of New South Wales. It is intended for questions that cannot readily be answered in class,
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angle.alpha
Posts: 21
Joined: Tue Mar 08, 2011 8:28 pm

### On Orbital Decay

LEOs experience orbital decay when exposed to the fringes of the earth's atmosphere, causing friction. Friction implies a decrease in Kinetic energy(?) which is converted to heat. LEOs however drop to a lower orbit meaning they increase in KE while decreasing in GPE and then due to greater KE experience greater drag and so decay... But how does the process occur between decreasing KE due to friction and dropping to a lower orbit (increase in KE)? I understand that the total mechanical energy decreases (due to conversion of KE to heat), but how does this explain why GPE should also decrease? I did some googling and one partial explanation offered on a forum was that in the process of dropping to a lower circular orbit, the satellite is briefly in an elliptical path which requires less KE than a circular orbit at the same altitude? I have a feeling I'm overlooking/misunderstanding some very basic physics
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: On Orbital Decay

Interesting: the last part of a question in Phys1131 this year involved something like this. And there's a relevant animation in the last section of
http://www.animations.physics.unsw.edu. ... avity.html

Friction does not, in general, imply a decrease in K. Just think of a runner accelerating from rest.

Consider a spacecraft in LEO (or using rockets). Atmospheric drag (or firing the engines forwards) both create a force in a direction opposite the velocity of the spacecraft.

dW = F.ds
so these do negative work on the spacecraft, and so reduce its mechanical energy. So it falls from a high-energy high-altitude orbit (which is slow) down to a low-energy low-altitude orbit (which is fast). So it gets faster. See the last section of
http://www.animations.physics.unsw.edu. ... avity.html

Joe
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: On Orbital Decay

Another interesting thing about atmospheric drag and orbital decay.

The ISS has a non-zero drag due to atmospheric friction, but the astronauts don't. So, while they are in free fall around the earth, the ISS isn't (quite). So they would appear to accelerate (very, very slowly) towards the side of the ISS that is pushing against the atmosphere. In the movies from the ISS, I cannot notice this.

Joe