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Two prototypes for a NASA mission concept called SWIM (short for Sensing With Independent Micro-swimmers) are arranged beside a much smaller nonfunctioning model representing the final envisioned size of the robot: about 5 inches (12 centimeters) long. The plastic prototypes were built at NASA's Jet Propulsion Laboratory in Southern California to demonstrate the feasibility of the concept, a swarm of dozens of self-propelled, cellphone-size robots exploring the waters of icy moons like Jupiter's Europa and Saturn's Enceladus. Delivered to the subsurface ocean by an ice-melting cryob…