A Lagrange point is a point in space where the gravitational forces of two large bodies,
such as the Earth and the sun or the Earth and the moon, equal the centrifugal force
experienced by a considerably smaller third body.
The forces interact to generate a point of equilibrium where a spacecraft can “park”
and conduct observations.
These points are named after Joseph-Louis Lagrange, an 18th-century mathematician, who
wrote about them in 1772 in a paper called "The 3-Body Problem”.
They are also known as libration points and Lagrangian points.
There are five Lagrange points around major bodies like planets and stars.
Three of them are positioned along the line that connects the two large bodies.
For example, in the Earth-sun system, the first point, L1, lies between the Earth and
the sun at approximately one million miles from Earth.
L1 has a direct view of the sun and is currently occupied by the Solar and Heliospheric Observatory
(SOHO) and the Deep Space Climate Observatory.
L2 is also a million miles from Earth but in the opposite direction of the sun.
A spacecraft can have a clear view of outer space at this point, with the Earth, moon,
and sun behind it.
The Wilkinson Microwave Anisotropy Probe (WMAP) of NASA is currently at this point, L2, monitoring
cosmic background radiation left over from the Big Bang.
The James Webb Space Telescope, which was launched recently, is set to move into this
region.
Meanwhile, the third Lagrange point, L3, is located behind the sun, on the other side
of the Earth’s orbit.
Science is yet to find a use for this point, but science fiction has.
L1, L2, and L3 are unstable points with a precarious balance.
Like a barely balanced cart on a steep hill, a spaceship at L3 drifting toward or away
from Earth would plummet irreversibly toward the sun or Earth.
Spacecraft must make minor changes to maintain their orbits.
However, points L4 and L5 are stable like a ball in a huge bowl.
These points are located along Earth’s orbit at 60 degrees ahead of and behind Earth, forming
the apex of two equilateral triangles with the large masses, for example, Earth and the
sun as their vertices.
As we all know, the Earth orbits the sun once a year.
L2, exactly lined up with both the sun and the Earth, the combined gravity of the two
large bodies tugging in the same direction offers a spacecraft an extra boost of energy,
locking it into perfect unison with the Earth’s yearly orbit.
So why is L2 the most preferred location for most advanced space telescopes?
It’s an excellent point from which to view the greater Universe.
A spacecraft would not have to perform regular orbits around the Earth, which would result
in it passing in and out of the Earth’s shadow, and causing it to heat up and cool
down, distorting its view.
L2 provides a much more stable viewpoint because it is free of this constraint and far away
from the heat emitted by Earth.
Also, one advantage of L2 when compared to Earth orbit is that it allows scientists to
have an enormous bandwidth because, over conventional Ka-band radio, the communication speeds are
very high.
Another advantage is that scientists can radiate the heat away.
The L2 point of the Earth-Sun system is home to the WMAP spacecraft, Planck, Herschel,
PLATO, and the recently launched James Webb Space Telescope.
L2 is great for astronomy since a spacecraft is close enough to communicate with Earth
easily, can maintain the Sun, Earth, and Moon behind the spacecraft for solar power, and
provide a clear view of outer space for the telescopes with sufficient shielding.
The number of spacecraft at the L2 point instantly indicates that things could get crowded.
We all know there’s a lot of space out there, but these spacecraft are situated in stable
periodic orbits around the L2 point itself.
The Lagrange points won't begin to get too crowded until many more probes move to these
areas.
As we all know, space is pretty big, and the Lagrange points are not really single points,
but 3D expanses that cover quite a large dynamic region.
So L2 is not crowded, and at the moment, there’s still plenty of room for other spacecraft.