Just as rovers rely on wheels to navigate lunar or Martian surfaces, some probes also use wheels (in this case, reaction wheels) to orient themselves in space without using thrusters. Engineers and technicians at NASA’s Jet Propulsion Laboratory recently installed four of these wheels on Europa Clipper which is being prepared for its journey through the Jovian system.
Europa, one of Jupiter’s satellites, contains under its thick layer of ice an enormous underground global ocean containing at least twice as much water as all the Earth’s oceans combined. Is this water capable of sustaining life? To try to find out, NASA is preparing a mission called Europa Clipper, which is scheduled to be launched in two years. Understanding the habitability of Europa will help scientists better understand how life developed on Earth and the potential for finding life beyond our planet.
Once in the Jovian system, Europa Clipper will perform several low-altitude flybys of Europa in an effort to gather as much data as possible. The probe, which was initially to end its days in the bowels of Jupiter, could then focus on the moons Ganymede or Callisto. In addition, to help it orient itself, Europa Clipper will rely on a four-wheel reaction system. How does it work?
Slow but economical movements
Isaac Newton’s third law of motion tells us that for every action there is an equal and opposite reaction. This law also applies in space. These four wheels about sixty centimeters wide, made of steel, aluminum and titanium, will have a fast rotational speed to spin the orbiter in the opposite direction.
Here’s one way to visualize the process: imagine you’re sitting in a swivel chair. You lift your feet off the ground so you can spin freely. Then shake your torso in one direction, and the chair and your legs will rotate in the opposite direction, details NASA. The reaction wheels work the same way: when the motor of these wheels accelerates in one direction, the ship experiences an acceleration in the opposite direction.
During its orbits, Europa Clipper will therefore rely on reaction wheels to help it perform thousands of turns or “rotations”. These movements will be necessary to orient the antennas towards the Earth or direct the scientific instruments towards Jupiter or Europa. However, the effect generated by these wheels will be relatively slow to be felt. It will indeed take about 90 minutes to rotate the machine 180 degrees.
A spare wheel, just in case
The probe will obviously be able to perform some of these maneuvers with thrusters, but these thrusters need fuel. However, it will only be available in limited quantities. The reaction wheels, on the other hand, will run on electricity supplied by the spacecraft’s solar panels.
These wheels were recently installed as part of the assembly operations. On paper, Europa Clipper will only need three wheels. However, these may wear out over time and the unforgiving environment of space. This notably happened on the Dawn spacecraft. That is why the engineers installed four wheels. The idea will be to alternate the three wheels in operation to equalize wear and to keep a “spare wheel” in case of problems.