Why does data transfer rate decrease with distance ?
If the vehicle is solar powered, then it is possible that it will also reduce the transmitted power as it goes farther from the Sun. However it is more likely to reduce the transmission duration by drawing on the batteries, since you’d like the best signal to noise ratio possible. Either way, the effective result is that if all the power is going to the radio, then the average data rate integrated over the time spent transmitting and not transmitting will go down as the fourth power of distance (assuming that the distance from the Sun and the distance from the Earth are about the same, which it is for a probe at Pluto). Readmore..