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 cryobot, the tiny robots would zoom away to look for chemical and temperature signals that could point to life.

The prototypes were used in more that 20 rounds of underwater testing in a pair of tanks at JPL and in a competition swimming pool at Caltech in Pasadena. Relying on low-cost, commercially made motors and electronics, the robots are pushed along by two propellers and use two to four flaps for steering.

The prototype in the center of the image weighs 3.7 pounds (1.7 kilograms) and is 14.5 inches (37 centimeters) long, 6 inches (15 centimeters) wide, and 2.5 inches (6.5 centimeters) tall, with a volume of 104 cubic inches (1.7 liters). The upgraded prototype at left is slightly bigger: 16.5 inches (42 centimeters) long, 3 inches (7.5 centimeters) tall, with a weight of 5 pounds (2.3 kilograms) and a volume of 140 cubic inches (2.3 liters).

In pool tests, the prototype at left demonstrated controlled maneuvering, the ability to stay on and correct its course, and a back-and-forth "lawnmower" exploration pattern. It managed all of this autonomously, without the team's direct intervention. The robot even spelled out "J-P-L."

As conceived for spaceflight and represented by the model at right, the robots would have dimensions about three times smaller than these prototypes – tiny compared to existing remotely operated and autonomous underwater scientific vehicles. The swimmers would feature miniaturized, purpose-built parts and employ a novel wireless underwater acoustic communication system for transmitting data and triangulating their positions.

Several years more of work would be needed to make such an advanced concept ready for spaceflight.

Led by JPL, SWIM development took place from spring 2021 to fall 2024. The project was supported by Phase I and II funding from NASA's Innovative Advanced Concepts program under the agency's Space Technology Mission Directorate. JPL is managed for NASA by Caltech in Pasadena, California.