Hey Everyone, Daniel from Spacedock here.
You may have heard the term ‘Lagrange Point’ come up from time to time in certain works
of more grounded science fiction, for example in the seventh Expanse Book ‘Persepolis
Rising’, or in the sci-fi anime ‘Planetes’.
I’d like to take this short video to explain exactly what a Lagrange point is, and why
it can be such an interesting concept for use in science fiction.
Any system of two large objects in space, such as the Earth and our Sun, will produce
five points of stability, where the gravitational pull of both objects cancel eachother out
to create a small area where stations or satellites could be kept at a relative halt, without
the need for control thrusters or orbital correction.
This segment will use the Earth-Sun Lagrange Points as an example, but systems of five
points such as this exist between any two objects, such as the Sun and Jupiter or Earth
and Luna.
On a direct vector between Earth and the Sun and a short distance from our planet lies
Lagrange One, this is the point where the direct gravitational influence of the Earth
and Sun cancel out.
On the same vector, but beyond the Earth’s orbit lies L2, the Earth’s gravity well
maintains the position of L2 in spite of its higher orbit around the Sun, which would normally
cause an increase in its orbital period.
Lagrange point one is particularly useful, as it allows for a satellite placed there
to not only remain equidistant from both the sun and earth, but also provides a position
for a consistent visual feed to the sun and the illuminated surface of the earth.
One such example of a satellite place at L1 is the Solar and Heliospherical Observatory
or “SOHO” satellite, which has provided us with information about complicated gas
currents below the sun’s visible surface, and tracking changes in its magnetic fields.
The third Lagrange point is located on the other side of the system, far beyond the Sun,
this is the point where the Sun and Earth’s gravitational pull align in the opposite direction
to L2, and an object placed there would remain eclipsed by the Sun and completely hidden
from the Earth.
Practically speaking this would mean that it would be difficult to maintain communication
with one such satellite, and as of now, scientists have not yet found a use for L3.
In sci-fi however, it would make an excellent spot to hide, say, an alien base away from
the prying eyes of unsuspecting earthlings.
Lagrange Points 1 through 3 are considered to be ‘Metastable’, meaning that objects
placed there will remain in position so long as they are never influenced by any other
forces or nudged out of alignment, for this reason a Station or Satellite would still
need some manner of control thrusters, in order to correct its position should it be
affected by another object’s gravity, or otherwise misaligned.
This is not the case with Lagrange Points four and five, which are located by charting
equilateral triangles above and below the direct vector to the Sun to find the gravitational
barycentre.
L4 and L5 Points are extremely stable gravity wells, that pull objects into themselves,
certain asteroids have been captured naturally by these wells, and sizeable asteroid fields
known as the Trojan Fields exist in the L4 and L5 points between the Sun and Jupiter.
The Lagrange four or five points between Earth and the Sun would be perfect locations for
large spaceborne colonies or other superstructures, as they would require no delta-v or orbital
correction to maintain their position relative to Earth and the Sun.
This would allow huge stations to be built, with no concern given to their thrust-to-mass
ratio or correctional thrust abilities, with the structure maintaining station-keeping
as a natural side effect of its location.
The distance and travel times between Earth and this facility would always remain the
same, even as both objects continued to orbit the Sun, creating a simple trajectory for
resupply and passenger transport between them.
Systems of five Lagrange points exist between every pair of large objects in space, and
scientists are still developing plans to take advantage of these phenomena.
As soon as 2021, the James Webb Orbital Space Telescope is scheduled to be deployed at Earth-Sun
Lagrange 2, and a number of satellites are already in place at Lagrange 1.
The potential uses of these locations are many, and extremely exciting.
Imagine an orbital shipyard facility constructed at Lunar Lagrange 5, a place where supplies
could be sent along a direct vector to assemble spacecraft in High Earth orbit, at the same
distance as the moon.
Freeing us from ever needing to use costly and inefficient rockets to break earth’s
atmosphere, and allowing for much larger, long-range spacecraft to be assembled and
launched.
This is Daniel from Spacedock, signing off.