Sending information cheap
To successfully transmit one bit of information, enough of the
transmitted power must be received to be able to identify the bit.
If the coding scheme used were Morse, that means we've got to
receive enough power to tell the difference between a dot and a dash.
If lots of bits of information are being transmitted, it will
take lots of power to carry it. For a spacecraft orbiting Mars, power
is very limited and it makes sense to talk about the 'cost per bit'
of information transmitted.
When radiation is transmitted, it spreads out through space and
not all of it goes to the receiver. The more tightly the radiation
can be focussed towards the receiver, the less is wasted by being
sent off in other directions.
There is a physical limit to how tightly radiation can be focussed.
It depends on the wavelength (big waves can't be focussed as tightly
as little ones) and on the size of the transmitter (a big transmitter
can focus radiation more tightly than a small one).
So to focus as much as possible of the transmitted power onto the
receiver, we want a big transmitter and small wavelengths. A laser
transmitter could readily be imagined to measure a few metres in diameter.
To match that focussing ability, a microwave transmitter would need to
be a few kilometres across.
If a 2-metre diameter microwave transmitter were to send data from
Mars to Earth, the best focus it could achieve would spread the energy
over a spot hundreds of thousands of kilometres in size. A 2-metre
laser transmitter could focus its energy into a spot only a few hundred
With energy focussed this much better, a million times more power
gets to the receiver. The transmitter doesn't need to use so much power
to be detected, and the 'cost per bit' of running the communications