lets say we did this, and we got the tiny probes up to even ~speed of light... then what are they going to do when they get there? what sensors can they carry a

lets say we did this, and we got the tiny probes up to even ~speed of light... then what are they going to do when they get there? what sensors can they carry and report back. And we wouldn't be able to slow them down, so they're going to transit through the target solar system relatively quickly (days even).