How Many Bits per Second Can Cross Interplanetary Distance? The Physics of a Deep-Space Channel

We compute, from stated first principles and stated hardware, how many bits per second a deep-space link can carry, and what sets that limit. For a reference X-band downlink between Mars and Earth (100 W spacecraft transmitter, 3.0 m spacecraft antenna, 34 m ground antenna, 25 K receiver system temperature at 8.42 GHz) the power-limited Shannon capacity is 42.2 Mbit/s-class at finite bandwidth at closest approach and 15.6 Mbit/s in the power-limited asymptote at the mean distance, falling as the inverse square of distance to about 41.8 kbit/s at Neptune distance for the same hardware. In the photon-starved regime the Holevo capacity gives the maximum bits per photon as a function of mean photons per mode, compared with uncoded pulse-position modulation at a stated frame error rate and with the best coded laboratory demonstration. A flown Mars orbiter operates within one order of magnitude of the computed capacity for the same geometry. Model calculations are SIMULATED, NOT MEASURED. Computations and drafting were performed with Dave, an AI research system.