The best places to park (in the Universe)
Due to gravitational effects and the laws of physics (thank you Isaac Newton), there are great parking spots throughout the Universe. This article will provide a short explanation around these so called “Lagrange points” with emphasis on astronomy and the relatively recent deployment of the James Webb Space Telescope.
In 2021 the James Webb Space Telescope (JWST) launched into space on an Ariane-5 rocket from French Guiana. Two months later after traveling roughly 1.5 million kilometers, JWST arrived at its carefully selected parking spot (L2) — putting it further away from Earth than our Moon. The location was not chosen by chance, it is in one of the five so called Lagrange points around Earth. These special places were named after 18th-century mathematician Joseph-Louis Lagrange who described them in a 1772 paper. In each of the five Lagrange points (labeled L1-L5) surrounding a body, gravitational forces produce enhanced attraction and repulsion. To put that more simply, in these spots the gravity from the Earth and the Sun plus the centrifugal force of orbiting the Sun essentially cancel each other out (not entirely for L1-L3 however, more on that below).
Each of the five Lagrange points has unique advantages due to its relative position and forces. At a very high level:
L1-L3 have slightly unstable orbits (the forces are not perfectly balanced), this means that spacecraft in these locations must make occasional adjustments to remain in their so called ‘parking spots’.
L1 — great location for studying the Sun (and for continuous monitoring of one side of the Earth), closest to the Moon, direct line of communication to Earth
L2 — great location for astrophysics missions, Earth blocks quite a lot of the Suns effects, no Earth atmosphere to contend with, direct line of communication to Earth
L3 — further from the Sun than L2, no direct communication to Earth, not presently used
L4-L5 are ahead of and trailing the moon in their orbits. What makes L4 and L5 unique is that they have stable orbits. This stability makes these locations tempting for things like space colony’s. In terms of conducting science however, For humans their location makes them less desirable for studying the Sun or deep space than L1 or L2. However their stable orbital nature results in dust and asteroids accumulating in these regions — and potentially being there a very long time. For that reason, it is believed they are good places to look for material left over from the formation of our Solar System. It’s also worth mentioning that L5 was the proposed location of a Space Colony in a famous 1974 article published in Physics Today.
Here is a diagram, courtesy of NASA, showing the five Lagrange points in the Earth-Sun system:
James Webb is now ‘parked’ in Earth’s L2 Lagrange point (along with several other spacecraft), to be more specific James Webb is actually orbiting a specific point in space called L2. You’ll notice from the diagram above that this region is fixed on the side of the Earth opposite the Sun and has constant line of sight communication with Earth.
Advantages of L2
L2 is wonderful for astrophysics missions due to the clear view of outer space from this location and the fact that from L2 the Earth permanently eclipses the Sun. As a result, spacecraft orbiting L2 also don’t have to pass in and out of the Earth’s shadow, which would cause regular significant heating and cooling events. Having continuity of temperature, and a relatively cool temperature at that, simplifies the engineering. Because of it’s distance from Earth, L2 also is of course very far away from Earth’s thing atmosphere. We have all probably seen the difference of trying to view the stars from a large city with a lot of light pollution vs a remote location on Earth with very little light pollution. So you can imagine how much better it is when you are in a permanently dark location (in terms of the Earth blocking the Sun) and with no Earth atmosphere in the way. In addition to things like wind and clouds causing problems, the atmosphere has a distortion effect that can be eliminated by viewing from L2.
Disadvantages of L2
In addition to the minor course corrections needed (this is called ‘station keeping’) to maintain a L2 orbit, there are other disadvantages to placing a telescope so far away. The Hubble space telescope, for example, orbited very close to Earth (just 570km above the ground). This enabled the telescope to be serviced by astronauts, which turned out to be very useful when shortly after entering service a key problem was discovered in the mirror (later corrected by astronauts). By contrast the James Webb is essentially unable to be serviced (at least not in the foreseeable future). For JWST to be useful it had to arrive in perfect working condition and if anything breaks in the near future, there is little chance of conducting fixes or replacements.
I’ve always been fascinated by Lagrange points, in particular it’s incredible that they were predicted in 1772, more than 250 years ago.
