If you have a stream of them going, you can relay the message back through the other ships, until it reaches the origin.

In theory, you can relay messages within any power budget, it's only a matter of reducing the distance between ships.

Perhaps, the sail could modulate the reflected beam? That way, all of the heavy communications gear can stay in our own solar system.

Instead of trying to stop at the destination, you could instead send a stream of these tiny expendable probes past the target. Each one would get, say, a microsecond to take a snapshot, and then it would spend the short remainder of its life relaying the raw data.

To do a chemical analysis, some probes could be sent on relativistic collision course with the subject planet, and other probes could watch the impact. It would be a scale-up of NASA's Deep Impact mission from 2005.

as far as I understood the starshot discussions, they weren't optimistic about managing to pack any interferometry equipment in such a small package, so there'd be little hope of proper chemical analysis... There was some discussion of what could be figured out from color alone, w/o spectra -- which sounds rather desperate.

A big telescope on earth/in orbit could possibly do more actual work on figuring out stuff about a nearby star than such seconds-long flybys of such wimpy payloads, which is all you get even after you manage the non-trivial challenges of building the phased array, surviving the ISM, establishing communications somehow etc.

A relativistic impact sufficient to allow chemical analysis from space is probably sufficient to kill creatures that may live on the planet. I'm not entirely comfortable with this.

I don't think a 0.1g probe would make it through an atmosphere at any speed. The targets I had in mind, were moons, rings, asteroids, much like Deep Impact; I used the term "planet" as a catch-all.

Nevertheless, your point stands, if we consider technologically advanced life a possibility. It would be real shame to clobber someone's moon colony. Perhaps, these impactor missions should not be considered on the first expedition.

Not only would it be rude to pelt someone's colony (or planet) like that, but if they were sufficiently advanced to recognize what's happening, they would likely consider it an attack. Not a great way to make friends.

Anything living on stellar bodies without an atmosphere should be prepared for micrometeorite impacts.

Of course this is a somewhat strange micrometeorite, but it shouldn't be too far from the standard set of space debris.

There's some very interesting work on using the Sun itself as a gravitational lense for sending signals back and forth to the craft, using surprisingly small amounts of power. More writeup here:

http://www.centauri-dreams.org/?p=35486

Really cool! Thanks for sharing.

Seems like there'd need to be a huge amount of effort to set up the transmission point on the other side of alpha centauri, but it sure would be amazing to have stable cheap communications across light years.

Modulate reflections of the light-sail propulsion beam?

at destination? There's no propulsion beam there; all acceleration would have been done at launch, in the vicinity of Earth. You can't point lasers over light-years of distance.