9 Key Facts About Lagrange Points
The Phenomena Keeping the James Webb Space Telescope Right Where It Needs to Be and Maintaining Balance in The Force(s)
Marking the first anniversary of arrival at its destination, the James Webb Space Telescope (JWST) has given humans the ability to peer deeper into space and therefore further back in time and closer to the beginning of the universe than ever before. The largest and most advanced optical telescope in space was launched on Christmas of 2021 and reached its home at the Sun-Earth L2 Lagrange point on January 24th of 2022 in a spectacularly successful launch and deployment, (in contrast with its predecessor, the Hubble Space Telescope which could only produce blurry images upon inception, before a fix was implemented in essence giving it “glasses”).
Journalists and bloggers alike have churned out an abundance of stories across the web of the mesmerizing images (such as this composite of the Tarantula Nebula), and meaningful discoveries that the JWST has made already.
Many of these post, article and news segment authors have mentioned in passing the location of this wonder of human engineering being a Lagrange point, and also may have said that a Lagrange point is a location in space where the gravity of the sun and the Earth are in balance, but more than likely, that was the most on the subject that was mentioned.
If you’re not an Astronomer, like most people, this may have been the first that had you heard about these gravitational phenomena, which tends to leave a curious mind with a few questions. If this at all describes you then you’ve come to the right place. In order to better understand our universe, it is important for scientists and enthusiasts alike to become acquainted with Lagrange points. They have a long and fascinating history within astronomy and can provide insight into how the gravitational forces of different bodies interact in space, so let’s dive into nine key questions, the answers to which should give you a solid “in a nutshell” overview of these orbital idiosyncrasies. (I have attempted to arrange the questions as they might arise in a conversation.)
1.) Why are Lagrange points useful in astronomy?
If we look again at the Hubble Space Telescope, it is in low Earth orbit which causes limitations, beyond those of just its mirrors and instruments, by way of different kinds of interference, the biggest of which is the planet itself. Whatever you are trying to observe, be it a celestial body or a section of space, the Earth is going be obstructing your view about half of the time as your remote observatory circles it every hour-and-a-half or so. Add to this various terrestrial radio waves, light reflections, temperature fluctuations and so on and you can see why in many cases it is an advantage to be farther away from the Earth.
At the same time you would like for your hypothetical astronomical observatory to stay in one place in relation to Earth for manageable tracking and communication, so the natural solution is that it should orbit the sun at the same rate as the Earth. The problem is that, if in attempting to do this, you place it just anywhere, the tugging of the Earth and the sun’s gravity is more than likely to make the satellite wander off of that perfect orbit and consequently to need frequent correction, adding weight to the mission in the form of fuel, and limiting its useful lifespan.
At a Lagrange point, a spacecraft can maintain that solar orbit in sync with the Earth for much longer and with far less energy expenditure.
2.) How many Lagrange points are known?
In every two body orbital system, there are five Lagrange points which relate to a potential third body (much smaller, such as a satellite). The Sun-Earth system has five. The Moon-Earth system has five, as well as the Sun-Jupiter system and so on.
3.) Where are these Lagrange points?
We will take a look at the Sun-Earth system, and you should be able to easily extrapolate the Lagrange points’ general positions to other systems as the relative locations are consistent across systems.
The first Lagrange point (L1) is located about 1.5 million kilometers (932,000 miles) from the Earth, directly between the Earth and the sun. The second Lagrange point (L2) is located at the same distance from Earth and on the same line but it is farther away, with the Earth and L1 between it and the sun. The third Lagrange point (L3) is located on the opposite side of the Sun from the Earth, about 5.5 million kilometers (3.4 million miles) away.
The fourth and fifth Lagrange points (L4 and L5) are located 60 degrees ahead of and behind the Earth in its orbit around the Sun, respectively.
4.) How do Lagrange points work?
I won’t go into enormous technical detail here but the but the main idea is that at the Lagrange points, the centripetal and centrifugal forces acting on the third orbiting body are balanced enough that a stable orbit can be achieved without a lot of energy expenditure. Centripetal force pulls the object toward the center (i.e. gravity) and Centrifugal force pulls the object away from the center due the object’s tendency to travel in a straight line instead of a circle (as described in Newton’s first law of motion, or seen in a tilt-a-whirl). The above NASA illustration is useful as it depicts these forces as slopes that the satellite would tend to slide down, with the Lagrange points located at the plateaus or ridges.
5.) Have other spacecraft used Lagrange points in the past?
Yes, those clever space agency scientists have been utilizing Lagrange points for years on numerous missions. In the interest of time I just want to mention a couple of them which happen to have captured compelling images, chosen because I believe that for a lot of people, myself included, those pictures are a big part of the attraction to astronomy and other space related sciences.
First, NASA’s Solar and Heliospheric Observatory (SOHO).
Coronal mass ejections, anyone?
SOHO was the first solar observatory to use L1 instead of orbiting Earth. This has given it an uninterrupted view of the sun since 1995 as it is still in operation.
This photo was making the rounds a few years back as well. You may have seen it:
The DSCOVR satellite (Deep Space Climate Observatory) had its mission cancelled due to the vicissitudes of American politics before it was set to launch in 2001 but was brought out of storage to finally leave Earth in 2015. From its vantage point at Sun-Earth L1, among other things it can study the Earth’s atmosphere over time, send warnings of approaching waves of particles from (awesome) coronal mass ejections up to 60 minutes before they reach the Earth, and take photos like this one of the fully illuminated “dark” side of the moon as it traverses the sunward side of our planet.
Astronomers and enthusiasts shared the above and similar images widely on social media in 2017 to raise awareness as politics were threatening the DSCOVR mission once again.
6.) Can multiple spacecraft occupy the same Lagrange Point simultaneously or will they bump into each other?
Interestingly, there are two special paths that spacecraft use to circle the point itself while maintaining the larger orbital trajectory around—in this case—the sun, called the halo and Lissajous orbits. And, while these orbits are relatively small in relation to their distance from the larger two bodies, they are quite large relative to a satellite (think 160,000 kilometers or 100,000 miles) so multiple spacecraft can use the same Lagrange point simultaneously with an extremely low probability of ever colliding.
7.) When were Lagrange points first used in a space mission?
The first space mission to utilize a Lagrange point was NASA’s International Sun-Earth Explorer 3 (ISEE-3), which launched in 1978. The spacecraft was placed in a halo orbit around the L1 Lagrange point, from where it was able to study solar wind and the magnetic field of Earth.
8.) How long have Lagrange points been theorized?
The mathematics which first indicated the existence of Lagrange points were published in 1772 by Joseph-Louis Lagrange, more than 200 years before ISEE-3 was launched.
9.) Who was Lagrange?
He was born Giuseppe Luigi Lagrangia in Turin, Italy and early on in his scholarly endeavors, he held a disdain for math. It was not until he was 17 and stumbled upon a paper by Edmund Halley from 1693 that he came not only to like mathematics but to be enthralled with the subject. The skill he developed therein would lead him to live, study and teach in Berlin and then Paris, where he became a naturalized French citizen and made it through the French Revolution without being deported or worse, falling victim to the guillotine, as many of his colleagues did.
By the end of his life in April of 1813, he had been married, widowed and married again. He made many contributions and advancements including being instrumental in the development of the metric system.
He was trying to solve what’s known as the three body problem in physics, for which there is no general solution to this day, when he managed to solve for the two “constant patterns” with the third body being much smaller than the first two, and these solutions contained the first identification of what have come to be known as Lagrange points.
*All photo descriptions in quotes are from NASA at the links provided unless otherwise stated.
I have attempted here to give you an easily digestible overview of what Lagrange points are all about. If you would like to take a deeper dive into the subject, here is a list of sources/further reading in addition to the websites linked in photo descriptions:
NASA — What Is a Lagrange Point?
