Maintaining orbit around the moon

[vjk2]

http://news.discovery.com/space/ast...s-put-an-asteroid-into-lunar-orbit-130111.htm

Basically, wouldn't any object that orbited around the moon gradually see its orbit be made more and more eliptical until it crashed into the moon itself?

Also, it actually takes longer for a satellite to orbit around the moon than the Earth itself, due to the Earth being more dense despite having a much larger diameter, correct?

 

[mfb]

Basically, wouldn't any object that orbited around the moon gradually see its orbit be made more and more eliptical until it crashed into the moon itself?

Why do you expect this?

Low orbit times are proportional to $\sqrt{\rho}$ with the average density ρ only. This gives ~105 minutes for very low lunar orbits.

 

[vjk2]

Why do you expect this?

Low orbit times are proportional to $\sqrt{\rho}$ with the average density ρ only. This gives ~105 minutes for very low lunar orbits.

Earth gravity should gradually affect any object orbiting. I wonder for how long thought.

It takes 90 minutes for the space shuttle to orbit the Earth.

 

[cepheid]

Earth gravity should gradually affect any object orbiting. I wonder for how long thought.

It takes 90 minutes for the space shuttle to orbit the Earth.

I think the spacecraft is well within the distance that the effect of the moon's gravity dominates over the effect of the Earth's. But another way to think about it is that the object is in a stable orbit around Earth just like the moon is.

 

[mfb]

I took 82 minutes, which would correspond to a height of 0 km (not possible as Earth has an atmosphere, of course).

Earth affects the orbit, but orbits which are not too high can be stable for a long time - a satellite around the moon should be closer than 20 000 to 30 000km.
Consider the earth-moon-system as interesting example: The gravitational force between moon and sun exceeds the gravitational force between moon and earth! But still, the lunar orbit has been stable for some billion years.

 

[vjk2]

I think the spacecraft is well within the distance that the effect of the moon's gravity dominates over the effect of the Earth's. But another way to think about it is that the object is in a stable orbit around Earth just like the moon is.

It should be akin to how the moon affects tides on the earth. As a thought experiment...if the moon had oceans, and if it rotated instead of being tidally locked, would the "tides" be greater than the Earth's tides, due to the mass of the Earth being so great compared to the mass of the moon vs the earth?

 

[mfb]

With an earth-like ocean and 24-hours rotation period, moon would have larger tides.
This has nothing to do with an orbit, however.

 

[willem2]

I took 82 minutes, which would correspond to a height of 0 km (not possible as Earth has an atmosphere, of course).

Earth affects the orbit, but orbits which are not too high can be stable for a long time - a satellite around the moon should be closer than 20 000 to 30 000km.

The maximum altitude seems to be much lower for circular orbits

http://science.nasa.gov/science-news/science-at-nasa/2006/30nov_highorbit/

And low orbits can also be unstable, because of mass concentrations in the moon

http://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/

 

[mfb]

Inclination is the problem here:

Stable circular lunar orbits do exist below an inclination of 39.6º, says Ely, but they spend so much time near the equator that "they are terrible orbits for covering the poles."
[...]
The instability of polar orbits poses a real problem for exploration.