Deorbiting: Possibility and Explanation

[deliveryman]

Is it possible for an object to deorbit something it's revolving around? Today my Cosmology teacher said that it was possible, and I just want to know how, she didn't give a very good explanation. I always thought because the Conservation of Anguler Momentum it wouldn't be possible. Explaination, anyone?

 

[Nereid]

If the 'something' has an atmosphere, no matter how tenuous, it will result in the other something deorbiting, eventually. Just like low-orbit satellites.

If the two aren't totally isolated, interactions with third objects could disrupt the orbit, and may result in a 'deorbit'. Something like this happens to some binary stars in globular clusters.

Left alone for long enough, the system will lose energy due to gravitational radiation, and de-orbiting will result. This is happening with a well-observed pair of neutron stars.

If each object is rotating and not perfectly rigid, angular momentum will be exchanged (think of the tides), and deorbiting may happen, depending on the relative sizes, masses, etc. One day the Moon will deorbit due to this mechanism.

Then there is the Poynting-Robertson Effect and the Yarkovsky Effect. Depending on what EM radiation the two somethings are bathed in, and the smaller something's size (and density, albedo, etc), their rotation rates, etc, etc, deorbiting is possible.

I'm sure there are more ways still; e.g. what about outgassing, e.g. a comet in orbit around Mercury?

 

[LURCH]

Originally posted by deliveryman
Is it possible for an object to deorbit something it's revolving around? Today my Cosmology teacher said that it was possible, and I just want to know how, she didn't give a very good explanation. I always thought because the Conservation of Anguler Momentum it wouldn't be possible. Explaination, anyone?

Not quite clear on the question. Are you talking about a sattelite deorbitting into its host? That seems to be Nereid's take on things. Or are you asking about a sattelite causing its host to deorbit into something, like the Moon causing the Earth to fall into the Sun?

 

[deliveryman]

Thank's for that reply, though I still have a question

If the 'something' has an atmosphere, no matter how tenuous, it will result in the other something deorbiting, eventually. Just like low-orbit satellites.

How would having an atmosphere cause something to deorbit?

 

[enigma]

Originally posted by deliveryman
How would having an atmosphere cause something to deorbit?

For the same reason your hand flys back when you stick it out of a car window.

For something to remain in orbit, it must be going fast... very fast.
If you have an atmosphere, the satellite gets slowed down by flying through it, even if the atmosphere is very thin. That slowing eventually gets large enough that the sat can't stay in orbit any longer.

 

[Loren Booda]

Galaxies may deorbit in an outward sense due to dark energy.

 

[deliveryman]

Gravitational Radiation will cause the Earth's Moon for example to slow down eventually to deorbit, because it's converting some of it's orbital kinetic energy into Gravitational Radiation?

Is that the reason why Gravitational Radiation causes a loss in energy with Pulsars and stuff? Please correct me if I'm wrong.

 

[Phobos]

deorbiting - - basically, the orbiter either crashes into the orbitee or the orbiter finds a new orbitee. The cause? Changes in mass/velocity/transfer of orbital energy.