Space telescopes and other satellites function in a way very similar to a tightrope walker. When Earth's gravity pulls on a satellite, a series of wheels - called "reaction wheels" - spin to produce an opposing force that maintains the satellite's pointing direction, or orientation. Reaction wheels also turn and point space telescopes.
Because Webb is so far from Earth, gravity isn't much of a problem. Instead, the wheels help the telescope maintain its orientation against the gentle pressures created by sunlight.
How a Reaction Wheel Works
A reaction wheel is a motorized, spinning wheel within a housing, or "shell." When the wheel spins in one direction, the housing spins in the other.
If the housing is attached to something else, like a satellite, it makes the entire object spin. Operators control different reaction wheels to rotate the satellite and aim it in different directions.
This is how most satellites, including the Hubble and Webb space telescopes, are aimed.
We don't often think of sunlight creating pressure. On Earth, there are so many other, stronger forces - like gravity - that we never notice it. But in the vacuum of space, when light particles streaming from the Sun strike Webb's immense sunshield, they push it like wind against a sail.
Uncountered, this gentle pressure could cripple Webb in two ways.
The shove could cause the telescope to pivot about its center of mass, flipping it. The shield would no longer be positioned to protect Webb's sensitive components from heat and light. The solar arrays would stop facing the Sun, allowing Webb's batteries to die. The telescope could no longer function and would be damaged by the heat.
But the trouble wouldn't stop there. The constant push from the light would also move the telescope out of its precise, gravitationally balanced orbit. It would drift off into space and, within a couple of months, move beyond the point of ever returning.
Webb's sunshield under pressure
Particles of light travel through space. When they encounter Webb's large, reflective sunshield, most of them bounce off. In doing so, they push the sunshield. This is where most of the pressure on Webb's sunshield originates.
Webb also gets a small shove when other light particles are absorbed by the sunshield. These particles either give their energy to the sunshield, or are absorbed and then radiated away. Both cases give the Webb Telescope's sunshield a push.
Keeping Webb's position in space is a constant feat of force and counterforce, with nature and technology combining to keep Webb poised delicately in place.
Sunlight applies force to Webb's sunshield, and operators use a combination of reaction wheels and jets to counter its pressure.
Keeping Webb Steady
1Sunlight's Dangerous Push
Light from the Sun pushes on Webb's sunshield. Unchecked, this pressure would eventually cause Webb to drift out of orbit. But the problems would start earlier than that. Because the sunshield is both tilted at an angle and not uniform, the pressure would be uneven, causing Webb to begin to spin. Eventually, the telescope could flip, exposing its sensitive "cold side" instruments to the heat of the Sun and ruining them.
2Reaction Wheels to the Rescue
When light applies pressure to the sunshield, the reaction wheels spin in the same direction as the Sun's push, causing the housing and telescope to spin in the opposite direction.
The reaction wheels continue to spin, both to counteract the pressure of the sunlight and to help point the telescope. Eventually, the telescope needs to slow the wheels down before their increasing speed causes the telescope to vibrate. But it can't just shut the wheels off - it has to maintain the telescope's balance.
Pencil-sized jets attached to the telescope ignite. At the same time, the wheels begin to slow down. Because the jets' push is balanced by the loss of force from the slowing wheels, the telescope stays balanced. This is called a "momentum dump."
Every 22 days, operators make a greater adjustment. They activate a small burst from the engine that helped propel Webb to its location, in order to keep it in its orbit around the second Lagrange Point.