The long form answer: small probe sent it to Proximal Centauri, the purpose of a probe being to gather information. A small probe would not have a large and powerful antenna, nor would a singular probe be part of an swarm forming an array, hence the probe arrives, gathers information, and departs for Earth.
This makes sense to me. If you're sending a single small probe, it would probably preclude a large antenna or a power source that could boost a signal to a strength that could cross interstellar space mostly intact and detectable. Other models featuring larger probes or a probe swarm might be able to forgo a return trip and communicate directly.
Since we want the information in our lifetime, and wish to avoid future colonists from greeting the first launched probe on its late arrival, we need to practice time dilation management, so we want to avoid anything over 50% of c—and I prefer not going that fast. This means a brachistochrone flight path for the trip in and trip out, so I'd say a safe ballpark figure for arrival Proxima Centauri at about 20 years.
Sound reasonable?
However, this argument makes no sense to me. Going faster means the probe arrives at its destination sooner, and the return trip also takes less time. The
actual time the probe travels while at high fractions of lightspeed doesn't change, only the
perceived time
for the probe. While traveling at such speeds, it's going
very fast, not crawling in slow-mo. This is what the rest of the universe sees. Your "time dilation management" description sounds backwards to me.
Now, traveling at high velocity
can pose issues for the probe. As was pointed out previously, the time the probe takes to react to any sort of potential collision or other problem could be quite dangerous. Navigation could also be problematic, since the closer to lightspeed the probe travels, the more distorted the starlight falling on the probe becomes. Indeed, it's believed that close to
c, say at 90% lightspeed or so, the stars will appear to group ahead or behind the probe, severely red-shifted or blue-shifted. That would make the navigation systems used by most probes, triangulating on visible star patterns, difficult to impossible to use. Also, the faster you go, the harder interstellar dust hits your craft, with potentially catastrophic effects.
One final point -- even if this propulsion technology proves to be effective, your probe won't be traveling at high velocity the entire trip. There will be long acceleration and deceleration periods at each end, hitting maximum velocity only for a short time in the middle of the voyage. And it sounds like the acceleration/deceleration rate would be quite small, judging by the amount of thrust potentially produced. No one seems to have taken those slower periods of travel into consideration in this discussion.
