What Are LaGrange Points and What Do They Have to Do with The James Webb Space Telescope?
Several months have already passed since the James Webb Space Telescope reached its destination 1.5 million kilometers from Earth [1], at the second Lagrange point, better known as L2.
But what exactly does that "L2" mean and why is the world's largest space telescope located at that point and not somewhere else?
Everything has its explanation and now that the telescope is there and your scientific curiosity has been aroused, this is the perfect time to learn a little more about Lagrange points.
What are LaGrange points?
Lagrange points are specific points located in the orbital systems of two massive objects such as the Sun and the Earth (Sun-Earth system) or even the Moon and the Earth (Earth-Moon system), where the gravitational forces are balanced with the centrifugal force of a much smaller object, such as a spacecraft or a telescope.
At Lagrange points, there is an equilibrium that prevents "small" objects - such as a telescope - from being thrown towards massive bodies like planets, attracted by their gravitational forces.
These points were discovered by the French mathematician Louis Lagrange in 1772 while trying to solve a particular case of the three-body problem, using a third very small body (m) in orbit around the orbits of two more massive bodies M1 and M2.
Where are LaGrange points?
Let's imagine a simplified version of the Sun-Earth system.
We could place a satellite at a specific point on the imaginary segment or line that joins - and at the same time separates - both bodies so that it'd be in perfect gravitational balance.
The force of attraction of both bodies would be equal because both depend on distance and mass.
Since the Sun is larger, the satellite must be farther away from the Sun, while the distance of the satellite from the Earth can be smaller since the Earth is smaller. This is the L1 point.
Point L2 is on the same line but beyond the Earth's orbit. In this case, as it rotates in its orbit around the Sun, the satellite would tend to escape, but would be retained by the Earth's gravitational force.
The point L3 is located on the opposite side, on the same line, but somewhat farther away than the distance to L2. The satellite would move around the center of mass of both bodies, away from the Sun.
Did you know?
At this point, it'd be very easy to hide an invading alien spacecraft, since it'd remain stationary and always hidden from the view of telescopes on Earth.
Finally, L4 and L5 are located at points with an angle of 60 degrees and are also much more stable than the other points since, even if the satellite deviates a little from its trajectory, it tends to correct on its own.
Here is an image courtesy of Wikimedia commons for you to see it more clearly.
Now you know that Lagrange points are spaces in which the gravitational forces of two closely orbiting bodies cancel each other out so that a third, much smaller object can remain stabilized.
This is perfectly applicable with respect to the L4 and L5 points and that's why, in the course of time, asteroids orbiting between the L4 and L5 points of the Sun-Earth system have accumulated there.
However, at the L1, L2, and L3 points, the position varies slightly with time, so if nothing is done about it, the objects in them end up being ejected outward.
In the case of the James Webb Space Telescope, it could start to move a little bit towards the Earth, getting closer and closer to it, but it could also happen that it starts to move away.
Fortunately, these deviations can be easily corrected with the engines on board the telescope; since there is very little gravity, very little fuel is spent in the correction maneuvers.
Now, to answer the question at the beginning of the article: Why is the James Webb at L2 and not at another point?
Let's analyze each Lagrange point further.
Point L1
Due to the powerful gravitational force exerted by the Sun, objects that are closer to it tend to move faster.
However, if a small object is placed right between the Sun and the Earth, the Earth's gravity in the opposite direction to the Sun cancels out some of the Sun's gravitational force, causing the satellite to orbit at a slower speed.
If the distance is correct, the satellite will travel slowly enough to maintain its position between the Earth and the Sun.
In this way, the L1 point is used to monitor the surface of the Sun, since the flux of particles from there arrives at L1 before reaching the Earth.
In this point is where the European Space Agency's SOHO is located.
Point L2
If a satellite is placed at this point, it'd be farther away from the Sun than we are, so it'd end up lagging behind.
However, at a distance of 1.5 million kilometers, the gravitational influence of the Sun in conjunction with that of the Earth causes the satellite to orbit at the same speed as our planet.
This is a perfect place to observe the Universe, since it's hidden from the Sun thanks to the Earth and that's why it's the ideal spot for an infrared telescope like the James Webb because the Sun and the Earth are always on one side of the space.
Because of this, the James Webb's instruments are continuously kept cold, specifically at -233 degrees celsius, which is ideal for its infrared sensitivity and gives enough space to access the sky.
In addition, both the Earth and the Sun are far enough away so that the heat that both radiate doesn't cause damage to the telescope.
Point L3
The L3 point is located on the other side of the Sun, slightly behind the Earth's orbit, which makes it impossible to observe objects located there from our planet - hence its use in science fiction novels and movies.
Also, due to the action of planets like Venus, this point is not as stable as L1 or L2 deb, which means that any disturbance can cause a spacecraft, probe or satellite to start moving away from it, so constant use of engines is needed to stay in the proper zone.
Points L4 - L5
Viewed from the Sun, the L4 and L5 points are at 60 degrees just ahead and behind the Earth, close to our planet's orbit.
Unlike the other points, L4 and L5 are extremely stable to any gravitational perturbation, which means that any object located at one of these points, even if its trajectory deviates slightly, will correct its trajectory on its own.
The problem with these points is that they tend to accumulate a lot of dust and asteroid material, so they aren't perfect for locating satellites or spacecrafts there.
Did you know?
A theory about the origin of the Moon relates that at the beginning of the Solar System, there was a planet somewhat smaller than Earth (Theia) that formed at one of these points. After millions of years, the system suffered a strong perturbation that precipitated Theia against our planet, giving origin to the Moon.
Final words
As you have seen, Lagrange points are especially useful for placing artificial satellites, sending space probes, exploration missions, and could even be used to build space bases in areas from which it'd be easier to travel to other points in the Solar System with a minimal energy consumption.
Although today these points don't have many practical applications, maybe in the future, with a possible manned mission to Mars and even to other places in our Solar System, these stability points will end up becoming space colonies that make us more easy the path of space conquest.