What causes orbits to change over time?

[Algren]

There are two examples: A planet rotating around a star and a spacecraft maneuvering through space. Let's assume that solar radiation has no affect on the spacecraft and the spacecraft is not bumping into any stuff, and niether is the planet; and no other gravitational sources are involved. What are the other things which could cause the orbit to change its shape and energy?

 

[rootone]

In the case of the rocket it only has to fire up it's engine to get a change of orbit.
For the planet though, changes of orbit are largely due to gravitational interactions with other large bodies, and I can't think of anything else that would have a similar result.

 

[Drakkith]

There are two examples: A planet rotating around a star and a spacecraft maneuvering through space. Let's assume that solar radiation has no affect on the spacecraft and the spacecraft is not bumping into any stuff, and niether is the planet; and no other gravitational sources are involved. What are the other things which could cause the orbit to change its shape and energy?

Gravitational interaction of the planet with the star can change its orbit over time. Other than that, I can't think of anything. You've essentially said that no other forces are involved, and without other forces there can't be a change in the orbit.

 

[Algren]

Gravitational interaction of the planet with the star can change its orbit over time.

What is this called?

 

[Bandersnatch]

What is this called?

Tidal acceleration.

 

[Algren]

Tidal acceleration.

Ahh, so it seems all orbits have a tendency to proceed to an orbit which is tidally locked. Alright, thanks.

 

[Alltimegreat1]

What force is pushing the Moon's orbit away from the Earth? Will the Earth eventually lose the Moon?

 

[Ophiolite]

The mass of the star decreases over time through fusion reactions and so the planet's orbit must necessarily change.

 

[russ_watters]

Tidal acceleration.

My guess is Drakkith was referring to precession via General Relativity.

 

[tony873004]

Gravity waves will radiate energy away.
An oblate spheroid shape to the star will cause an orbit that is inclined to the star's rotational axis to precess.
Frame dragging.

If solar radiation counts:
Yarkovsky effect -- re-radiate solar radiation, but in a preferred direction.
Poynting-Robertson effect -- planet bashing into photons similar to driving in rain. More raindrops hit the windshield than the back window. (although this is from solar radiation.)

 

[Janus]

What force is pushing the Moon's orbit away from the Earth? Will the Earth eventually lose the Moon?

It is caused by a transfer of angular momentum from the Earth to the Moon. The Moon raises tidal bulges on the Earth. Due to friction, the Earth's rotation drags these bulges out of alignment with the Moon. Think of this way, the Earth tries to make the bulges rotate with it, while the Moon tries to keep them in line with itself and you end up with a compromise where the bulges don't quite line up with the Moon. The the resulting gravitational pull of the bulges now tend to pull the Moon forward in its orbit, which in turn causes the Moon to lift to a higher orbit. At the same time, Earth's rotation slows down a bit.

The Earth can't lose the Moon by this process because eventually a time would come when the Earth's rotational period and the Moon's orbital period will be the same and once that happens the mechanism that increases the Moon's orbit will go away. (Also most estimates indicate that before even this happens, our Sun will have expanded into a Red Giant and could envelop both the Earth and Moon.

This process can work the other way too. If the Moon either orbited faster than the Earth rotated or orbited in the opposite direction, then it's orbit would shrink rather than grow. The Mars moon Phobos is an example of this, its orbits Mars in less time than it takes for Mars to rotate and is slowly being pulled into a lower and lower orbit.

 

[Alltimegreat1]

Will this send Phobos crashing into Mars?

 

[rootone]

Yes I think that is what is generally concluded, unless something external to the Mars and it's moon system radically shakes thing up gravitationally.
Nearby passage of a large asteroid?

 

[Janus]

Yes I think that is what is generally concluded, unless something external to the Mars and it's moon system radically shakes thing up gravitationally.
Nearby passage of a large asteroid?

The other possible result is that it will be torn apart and form a ring. It all depends on its structural strength. Some models suggest that it not a solid object, but more like a pile of rubble held together by a thin crust. In which case tidal forces could pull it apart when it gets close enough to Mars.

 

[Alltimegreat1]

And could that impact send Mars off its orbit and out of the solar system?

 

[Janus]

And could that impact send Mars off its orbit and out of the solar system?

No. For one, the mass of Phobos is only 1/60,000,000 that of Mars. So even if it impacted Mars at escape velocity (5 km/sec) and you could treat this strictly as a collision, there is no way that it impart anywhere enough momentum to seriously change Mars' orbit.

But the main reason is that Mar's orbit around the Sun is really the orbit of the Mars-Phobos-Deimos barycenter, and nothing that is a result of the mutual gravitational interaction between these bodies will shift the orbit of the barycenter.