Just as NASA’s Mars rovers rely on sturdy wheels to roam the Red Planet and conduct scientific research, some orbiters also rely on wheels — in this case, reaction wheels — to stay pointed in the right direction. Engineers and technicians at NASA’s Jet Propulsion Laboratory in Southern California recently installed four reaction wheels on Europa Clipper, which will rely on them on its journey to Jupiter’s frigid moon Europa.
When NASA’s spacecraft heads into deep space, orbits Jupiter and collects science observations as it flies by Europa dozens of times, the orbiter’s wheels rotate so its antennas can communicate with Earth and its scientific instruments, including television cameras, can remain oriented .
Two feet wide and made of steel, aluminum, and titanium, the wheels spin rapidly to create torque that spins the orbiter in the opposite direction. Isaac Newton’s third law of motion also applies in deep space and explains the underlying phenomenon: for every action, there is an equal and opposite reaction. Reaction wheels cause the spacecraft to react to the spinning action of the wheels.
Here’s one way to visualize how reaction wheels work: Imagine you’re sitting in a swivel chair and you lift your feet off the floor so you’re free to spin. If you move your torso in one direction, the chair and legs will rotate in the opposite direction. Reaction wheels work the same way: as the reaction wheel motor accelerates the metal wheel in one direction, the spacecraft experiences acceleration in the opposite direction.
Without those reaction wheels, Europa Clipper would be unable to carry out its scientific investigations when it arrives in the Jupiter system in the 2030s following its launch in 2024. Scientists believe that Europa carries a vast internal ocean that could have conditions suitable to sustain the life. The spacecraft will collect data about the moon’s atmosphere, surface and interior, information that will help scientists learn more about the ocean, ice crust and potential plumes that could vent groundwater into space.
During its orbits around Jupiter, Europa Clipper will rely on the reaction wheels to help it make thousands of revolutions, or “spins”. While the spacecraft can perform some of these maneuvers with thrusters, its thrusters require fuel, a limited resource aboard the orbiter. The reaction wheels will run on electricity supplied by the spacecraft’s vast solar arrays.
The trade-off is that the reaction wheels run slowly. It will take the Europa Clipper’s reaction wheels about 90 minutes to rotate the aircraft 180 degrees, a movement so gradual that, from a distance, it would be imperceptible to the human eye. The rotation of the spacecraft will be three times slower than the minute hand of a clock.
Also, they can wear out over time. It happened on NASA’s Dawn spacecraft, requiring engineers to figure out how to rotate using thrusters with available fuel. To solve this problem, the engineers installed four wheels on the Europa Clipper even though only three are needed for maneuvering. They alternate which three wheels are in operation to even out the wear. That leaves them with a “spare” tire if one of the others fails.
The installation of the wheels was one of the most recent phases of the phase known as the assembly, test and launch operations. Scientific instruments continue to arrive at JPL to be added to the spacecraft. Next, a series of tests will be conducted as the spacecraft moves into its October 2024 launch period. After traveling more than 1.8 billion miles (2.9 billion kilometers), Europa Clipper will begin unveiling the secrets of this frozen world.
Read more about the mission
Missions like Europa Clipper contribute to the field of astrobiology, the interdisciplinary research field that studies conditions on distant worlds that could harbor life as we know it. While Europa Clipper is not a life-sensing mission, it will conduct a detailed exploration of Europa and investigate whether the icy moon, with its subsurface ocean, has the ability to support life. Understanding Europa’s habitability will help scientists better understand how life developed on Earth and the potential for finding life beyond our planet.
Managed by Caltech in Pasadena, California, JPL leads the development of the Europa Clipper mission in collaboration with the Johns Hopkins Applied Physics Laboratory (APL), in Laurel, Maryland, for NASA’s Science Mission Directorate in Washington. APL designed the main body of the spacecraft in collaboration with JPL and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The Planetary Missions Program Office at NASA’s Marshall Space Flight Center in Huntsville, Alabama performs program management of the Europa Clipper mission.
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