It depends on the size of the transmitting antenna, the power available on the spacecraft, and its distance among other parameters. Data rates of hundreds of kilobits/sec are typical for deep space missions to the outer solar system. In the near-earth environment, the rates can be in the megabits/sec. Galileo, without its main antenna, is forced to transmit at a few hundred bits per second, which is about 1000 times slower than intended. The low bit rates insure that each bit gets enough 'signal - to - noise' that it can be detected back on Earth with 100 percent reliability. Too fast a bit rate means that the power per bit can fall below the transmitter/receiver noise level and not be detected, so data rates are low and transmission power is high when you want the maximum reliability on the receiving end.
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