It seems to me that the accuracy required to aim an unguided projectile anywhere near the target stars will be impossible. They'll fly by pretty far. At what distance to Alpha Centauri does an iPhone quality camera outmatch the Hubble's photos?
“The task of pointing the array is dominated by the problem of keeping the sail on the beam. This problem is defined by the width of the sail and the distance to it. As an example, for meter-scale sail size the launch distance is on the order of a few million km. The pointing accuracy required for beam stability at this distance is on the order of a milliarcsecond. There are several mitigation approaches that could be used to counter these effects. A model of the atmosphere, calibrated with radar, laser beam, and optical measurements in real time, would enable the required beam precision to be achieved. Targets such as Alpha Centauri are bright star systems that will inform pointing requirements.
“Monitoring the laser beam output provides the information needed to form the beam. The Starshot system would be very different than a conventional telescope, and specialized to its purpose. For example, most ground-based telescopes, such as the Keck telescope, point to within a few arcseconds and can track in a closed loop mode to better than 100 milliarcseconds. For the purposes of Starshot, a significant improvement on this precision is required. However, the beam synthesis inherent in the phased array system provides considerable fine-pointing capability, supplemented by closed loop tracking of the beacon on the spacecraft.”
http://breakthroughinitiatives.org/index.php?controller=Foru...
So the flash can also be used for course corrections? Cool!
It's a serious point though. Perhaps the gravity of the star will be enough to help them right?