Breakthrough Starshot has no intention of colliding any nanocraft with any object in space. Even though an accidental collision between a nanocraft and another object is a remote possibility happens, the resulting effects must still to be examined.
A gram-scale nanocraft traveling at approximately 20% of the speed of light carries with it a kinetic energy equivalent to about 0.5 kilotons of TNT. If the nanocraft were to intercept an Earth-like exoplanet with a thick atmosphere, it would burn up in the atmosphere.
If it were to collide with a body resembling the Earth’s moon, it would create a small crater (on the order of 10m). NASA and others have collected significant amounts of data on the historic rates of asteroid collisions with the Earth, and determined that such collisions are frequent. Asteroids that deposit on the order of 1 kiloton of energy collide with the Earth about once a month.
Nanocraft-scale impact poses a very small chance of causing harm to any planet.
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