This probably has more to do with the massive antenna (also known as a radio telescope) here on earth that are used to pick up the signal. Fortunately a few such antenna exist around the globe, so Mars should be in view of one of these antennas at all times. According to Wikipedia (http://en.wikipedia.org/wiki/Deep_Space_Network), radio telescope complexes used by NASA are located at: Goldstone Deep Space Communications Complex near Barstow, California, USA Robledo de Chavela near Madrid, Spain Canberra Deep Space Communications Complex near Canberra, Australia

You'd be amazed what you can do just with a watt of radio power.

If a transmitter and a receiver is in space, what's the furthest distance the receiver can go away from and have a 1 watt signal still above the noise floor? Assume the transmitter is a perfect omnidirectional.

About 9200 miles.

9200 miles is not very much in space. That's like 0.006% of the distance between Earth and Mars.

I well realize that.

I was assuming absolute worst conditions, at 1 watt PEP.

With hardware noise reduction, we can drop easily to .1 watt and still keep everything else consistent.

Next, if we choose a directional antenna, we can also greatly magnify directed output. No sense in directing energy in the ground, is there?

We can also have receiver systems (dish arrays) that can provide great gains in reception and transmitting.