Inside Science) — At 7:20 a.m. EST on Dec. 22, the largest and most powerful space telescope ever built is scheduled to hurtle into space from a launch point near Kourou, French Guiana. It will spend a month traveling roughly a million miles from Earth to a special spot called the second Lagrange point, or L2.
L2 is just 1% farther away from the sun than Earth is, forming a straight line with the star and planet. As Earth orbits the sun, so does L2 at the same speed, as if they were both attached to the sun by the same string. The telescope will travel in an ovular orbit around L2 — from the perspective of the sun, Webb’s orbit would look like a halo behind Earth. From this position, the telescope will observe the faint, distant light traveling through space from the earliest galaxies to form in the universe around 13.5 billion years ago. Webb will also learn more about the chemistry of the atmospheres of planets outside the solar system, the surface composition and evolution of bodies inside the solar system, and much, much more.
Part of the reason the telescope will be able to capture such distant signals and observe closer objects with a high level of detail is because of its destination. “At L2, we can meet all our challenging, top-level science requirements that require this thing to be supersensitive,” said Paul Geithner, the deputy project manager for Webb. Other places in space don’t have the same helpful alignment with the sun and Earth. So, what is it about L2 and its alignment that make for an ideal telescope location?
Lagrange points and the three-body problem
L2 is named after Joseph Louis-Lagrange, an Italian-French mathematician who published a paper in 1772 detailing a particular solution to a simplified version of the otherwise unsolvable “three-body problem.” The problem Lagrange studied was how do two large bodies (such as the sun and Earth) and a third smaller body move around each other in space?
There are five such Lagrange points in space where, relative to the two larger bodies, the smaller body would remain in the same place. For Earth and the sun, Earth and the third body would always be the same distance from each other and orbit the sun at the same speed.
Small bodies remain relatively motionless at these points because the gravitational forces of the sun and Earth and the centripetal force pulling the third body inward as it orbits the sun balance out. So, Webb won’t have to use as much energy to orbit L2 as it would in other orbits. Geithner said they’ll only have to burn some fuel every three weeks to maintain orbit and that it’s easier and more efficient to orbit L2 than to stay exactly at the point. “It’s just a conspiracy of the sun and Earth’s gravity that [L2] is a happy place to go,” said Geithner.
L2 is one of three Lagrange points on a straight line with Earth and the sun. All three points are semistable, meaning that objects orbiting these points gradually drift away. The fourth and fifth Lagrange points are along Earth’s orbit of the sun and are equidistant from the sun and Earth. These points are fully stable, so they attract things like asteroids and space dust. Stéphanie Lizy-Destrez, a professor of space systems at the Higher Institute of Aeronautics and Space in Toulouse, France, said that L4 and L5 in the Earth-moon system “are very crowded, they have a lot of objects there. So, it means if you want to send a satellite there, you will have to maneuver it a lot.”
A great location for making measurements
But scientists didn’t just pick L2 for fuel efficiency’s sake. Webb’s instruments need to stay at a steady, cold temperature to make accurate measurements. The telescope will be detecting very faint infrared light, and any other source of light or heat (like the sun, moon and Earth) would interfere with the measurements.
Webb is equipped with a shield the size of a tennis court that will keep the telescope and instruments (which are facing away from the sun) close to minus 400 degrees Fahrenheit, while the side facing the sun will be close to 200 F. Since L2 orbits the sun at the same speed as Earth, the shield can protect the instruments from the sun, Earth and the moon at the same time. “We can always keep everything on one side of the sunshield and just have deep space on the other for the telescope and the instruments to look at,” said Geithner.
Escaping Earth’s background noise
Beyond the specific benefits L2 offers for fuel efficiency and sunshield positioning, its location far from Earth is generally helpful for Webb. One reason is that the telescope’s measurements won’t get interrupted by large objects passing by. The Hubble Space Telescope, for example, orbits Earth every 90 minutes, losing sight of what it’s observing every 45 minutes when Earth blocks its view. “By getting away from Earth, you’ve got this big area you can point the telescope to and stare at things for a long time to record a really faint object,” said Geithner.
You can make infrared observations from Earth’s surface, but Earth’s atmosphere can get in the way. “If you want to look at the stars, you need to be in the void to be sure there’s no light that disturbs your measurements,” said Lizy-Destrez. The heat in the atmosphere “totally dominates your background — it’s like trying to see a firefly in front of a searchlight,” said Geithner.
Sending a satellite to L2 also isn’t as difficult as it is to send one to some other places. While L2 is a million miles from Earth, that’s a relatively small distance on the scale of the solar system — it’s only 3% of the minimum distance between Earth and Mars. To get further into space would require a larger rocket at liftoff, or a smaller telescope. Plus, unlike sending a satellite to another planet, which requires a very specific launch window, Webb can launch any morning, except for a few days a month when escaping the moon’s gravitational tug would require extra fuel.
Geithner said that L2 is actually easier to get to than the moon, which is only a quarter of the distance to L2. Webb can mostly coast to L2 once it escapes Earth’s gravity, whereas a satellite headed to the moon would need to brake as it gets pulled in by the moon’s gravity.
L2 is relatively easy to get to, requires little energy to orbit, and allows the instruments to make uninterrupted, high-quality measurements of the early universe. “It was just the most efficient and sensible place to put something like this,” said Geithner. “L2 is kind of this Goldilocks place.”