The bandwidth to the orbiters is up to 2 Mbps, but because they're in low orbit they're only in sight 8 minutes per day.

http://mars.jpl.nasa.gov/msl/mission/communicationwithearth/...

I think it may be time to put a dedicated communications satellite around Mars. These rovers do seem to last a long time so it probably would be worth it.

We would need at least three for constant contact, Arthur C. Clarke style.

There is, that's the role MSO is performing, no?

What you'd need is a whole series of them to increase coverage.

A bit of clarification, I think the orbiters are in sight of Earth for most of the day.

It is Curiosity that is not in sight of the Orbiters, again because of their low orbit.

I'm not sure I understand. Gale Crater is pretty near the equator, so any equatorial satellite would pass quickly over it, but many many times per day.

You answered your own question: both satellites are in polar orbits. They pass the equator many times a day, but at a different longitude each time.

> But I haven't seen an explanation why bandwidth is so low -- is it lack of spectrum? Interference issues?

Transmit power.

According to wikipedia article, the power source is designed to create 125 watts of electrical power from 2000 watts of thermal power. (It might look inefficient, but they are using the heat in all parts of the rover.)

For comparison, that might be how much your graphics card might be consuming while you are reading this article.

I guess that also means it is going to move very slow.