<font color="yellow">"Why don't we design them for the ability to relay from one satelite to another?"</font><br /><br />There's several issues.<br /><br />First, over interplanetary distances, communication requires either huge antennas, extremely high-power transmissions, or incredible focus (or some combination thereof). NASA's Deep Space Network (DSN) provides all three. I've seen several articles that detail the minute levels of transmission power that the DSN arrays are able to pick up. It's generally listed as the number of years of continuous broadcasting that would be required to list a 60-watt bulb for a second. It's simply not possible to make probes with anything remotely like the receiving capability of the DSN. Likewise, they can't come anywhere close to the output capability for sending messages, nor can they aim with anything like the same degree of precision. The end result is that probes would not be useful in this capacity even if they were in a straight line.<br /><br />...which leads to the second problem. They won't be in a straight line. Probes are sent to points of interest. Those points of interest aren't necessarily, or even usually, in a straight line. Even if they were at one point, they won't be for long because everything is orbiting the sun.<br /><br />Nothing really *led* me to a third point, but said point would be that space hardware has a definite lifetime. Probes going the distances that your post indicates interest in (presumably outside of Saturn and likely further) take <b>years</b> to get there. New Horizons is the fastest probe we've ever sent, and it will take (going from memory rather than Googling) ten years to get to Pluto. This is a significant fraction of the lifetime of the probe. The plutonium won't provide sufficient heat to power the transmitter forever. By the time another probe is built, launched, and makes it out to where it could provide a relay for New Horizons (if we were to ignore the first