On Monday, Jan. 24, engineers plan to instruct NASA's James Webb space telescope to complete a final modified boost, sending it into an ideal orbit of nearly 1 million miles from Earth, the second sun-Earth Lagrange point (L2).
Mathematically, the Lagrange point is the solution to the so-called "restricted three-body problem." In space, any two massive, gravitationally significant objects produce five specific locations—lagrange points—whose gravitational pull is in equilibrium. Lagrange points are labeled L1 through L5, preceded by the names of the two gravitational bodies that produce them (the first is larger).

Source: NASA/WMAP Scientific Group.
Although all Lagrange points are gravitational equilibrium points, not all points are completely stable. L1, L2, and L3 are "meta-stable" positions with saddle-shaped gravitational gradients, like the point in the middle of the ridgeline between two slightly higher peaks, where there is a lower stability point between the two peaks, but it remains a very high, unstable point relative to the valleys on either side of the ridge. L4 and L5 are stable because each location is like a shallow depression or bowl, located in the middle of a long, high ridge or hill.
So why send Weber into the Sun-Earth L2 orbit? Because it is the ideal location for an infrared observatory. In the Sun-Earth L2, the Sun and earth (or moon) are always on one side of space, allowing Weber to keep its telescope optics and instruments forever in the shadow position of the earth. This allows them to be more sensitive to infrared, though. Observe at any point at an angle close to half the sky. Observe any point in the sky and just have to wait a few months to uncover the secrets "behind the sun."
In addition, at L2, the Earth is far enough away that heat radiation at room temperature does not affect Weber at all. Because L2 is the gravitational equilibrium position, Webb is easy to stay there steadily. Note that running around L2 is simpler, easier and more efficient than precisely stopping at L2. In addition, by orbiting rather than in L2 orbit, Webb will never be replaced by Earth, which is necessary for both Webb's thermal stability and power generation. In fact, Weber's orbit around L2 is larger than the Moon's orbit around Earth! L2 can also easily stay in touch with mission operations centers on Earth. Deep space networks. Other space observatories include WMAP, Herschel, and Planck orbiting Sun-Earth L2 for the same reason.
In general, getting a spacecraft to reach the Sun-Earth L2 is fairly straightforward, but Weber's design was with special considerations in mind. Karen Richon, Weber's flight dynamics principal engineer, describes how to take Webb to L2 and keep it there:
"Think about it, throwing a ball straight into the air, the harder the better; it starts fast, but when gravity pulls the ball back to Earth, it slows down and eventually stops at the top and then returns to the ground." Just as your arm gives the ball the energy to rise a few meters from the Earth's surface, the Ariane 5 rocket gives Weber 1.1 million kilometers of energy, but not enough to escape Earth's gravity. Like a ball, Webb is slowing down and will eventually stop and fall toward Earth if we allow it. Unlike the sphere, Weber does not return to the Earth's surface, but is in an extremely elliptical orbit with a perigee altitude of 300 km and an apogee altitude of 1.3 million km. Using the thrust of a small rocket engine on the Webb about once every three weeks or so, it will remain in L2 orbit, orbiting the sun every six months.
"So why didn't Ariane give Webber more thrust, and why did Webber need correction work?" If Ariane gives Weber a little more energy than it needs to send it to L2, it will run too fast when it gets there and will exceed the science track it wants. Weber will have to make an important braking move, pushing toward the sun to slow down. This huge combustion not only costs a lot of propellant, but it is also impossible, because it requires Webb to turn a 180-degree return to the sun, and the sun will directly expose the telescope's optics and instruments to the sun, thus overheating their parts, and the glue that binds them together really melts. Installing a thruster on a telescope as a way to directly brake thrust is not feasible for many reasons and is never a design option.
"So Webb asked the Ariane rocket to provide deliberate thrust to make sure we never needed a brake boost, but still needed Weber's booster to make up for the difference precisely and put it in the ideal orbit." Ariane 5 gave Weber the last thrust so precisely that our first and most important burn was smaller than we had planned and designed, while providing more remaining fuel for extending Weber's successor mission!"
- Karen Richon, Principal Engineer in Flight Dynamics at Webb, NASA's Goddard Space Flight Center