We’ve reached the third part of our series on Lagrange Points, those stable spots in
the Solar System, where you can sort of hover with the minimum amount of fuel.
This episode we’re going to look at the L4 and L5 points which share the orbit with
a more massive object.
Again, this is the third part of a series, so if you need an overall explainer on what
the Lagrange points are, I suggest you go back to the beginning and watch episode 1.
All right, let’s talk about L4 and L5.
While the previous L1, L2 and L3 points are located along a line that connects the two
masses, L4 and L5 orbit with the less massive object, maintaining a position 60 degrees
ahead and behind.
And unlike the previous points we talked about, L4 and L5 are meta-stable.
Which means that a spacecraft can loiter in this area without needing to use any fuel
at all.
In fact, it’s a gravity well that a spacecraft would need to fight to escape if it wanted
to.
As with the previous points, it’s important to remember that any object in L4 and L5 needs
to have insignificant mass compared to the two massive objects.
Astronomers call natural objects hanging out in these regions: Trojans.
So, in the case of the Sun and Jupiter, you can have a pretty hefty asteroid hanging out
in these points.
In fact, the most massive Trojan asteroid discovered so far is 624 Hektor, measuring
about 225 kilometers across and has a mass of 9 quintillion kilograms.
Many other smaller asteroids have been detected in this region as well.
For the longest time, astronomers knew that the Earth had trapped dust in its L4/L5 points.
But in 2010, NASA’s Wide-field Infrared Survey Explorer, or WISE discovered the first
actual Trojan asteroid lurking in this region.
Designated as 2010 TK7, the object measures about 300 meters across, and traces an unusual
orbit that moves above and below the Earth’s orbital plane.
In fact, WISE was able to discover the trojan in the first place because it has such an
unusual orbit that carries it quite far afield from the L4 point.
Normally the Sun obscures our view into this region.
Astronomers have also discovered dust in the Earth-Moon L4 and L5 points, known as Kordylewski
clouds, but so far, no actual asteroids.
Astronomers have found Trojan asteroids sharing Neptune’s orbit, Uranus, and even Mars has
its share of Trojans, which astronomers think might have formed out of ejected material
from an ancient collision with Mars.
Astronomers think that the largest Mars Trojan, 5261 Eureka, was blasted out of Mars more
than a billion years ago, and then it was spun up over eons to the point that fragments
were ejected into new, but similar orbits.
So, clearly Nature has figured out how to use the L4 and L5 points, but what about spacecraft
missions?
Actually, so far, no mission has ever been sent to these regions.
No Sun-Earth L4 missions, nothing to Jupiter or Mars.
There, the history portion is over.
So, what about the future?
Are there any clever ideas to use them?
You bet.
At the 46th Lunar and Planetary Science Conference, astronomers proposed that a Cubesat mission
be sent to one of the Sun-Earth L4/L5 regions to search for additional Trojan asteroids
beyond 2010 TK7.
Equipped with an infrared camera, this Cubesat mission would be able to perform a comprehensive
survey of the region, tracking down many of the larger Trojan objects.
From this, astronomers would be able to identify potential candidates for future robotic orbiters
and landers.
Depending on their inclination to the Earth’s orbit, they would require very little change
in velocity to reach from Earth, which means they’d be inexpensive energy wise.
We could get a lot of science done with not a lot of spacecraft.
Another mission that’s coming together is NASA’s Lucy Mission.
Due to launch in 2021, Lucy will spend 12 years visiting seven different asteroids;
one in the main asteroid belt and then 6 in Jupiter’s Trojan regions.
Its flight path will carry it on several slingshot maneuvers past Earth to gain speed, and then
it’ll fly out to Jupiter’s L4 region in 2025, visiting 4 asteroids, including a double
asteroid.
Then it’ll fly back to Earth, slingshot out to the L5 region and visit another asteroid
in 2033, and then fall back down into the inner Solar System again.
Of course, Lucy will be equipped with the standard array of remote sensing instruments
to study the geology, surface composition and physical properties of all the asteroids
it flies past.
In the last episode, I mentioned that there’s no good use for the Sun-Earth L3 point, but
there’s actually one idea that might use L3, L4 and L5 to build a gigantic gravitational
wave interferometer called the Astrodynamical Space Test of Relativity using Optical Devices,
or ASTROD.
Astronomers are already planning the LISA observatory to fly in the 2030s, which will
consist of three spacecraft flying in an equilateral triangle formation 2.5 million kilometers
apart from each other to detect gravitational waves with more sensitivity than the best
Earth-based instruments.
The ASTROD plan would take this to the next level, building a gigantic equilateral triangle
by positioning a spacecraft at L3, L4 and L5.
Each arm of the instrument would measure 260 million kilometers across, and by timing their
movements, they could remain within about 20,000 kilometers of this distance at all
times.
A gravitational wave laser interferometer this big would be 260 times more sensitive
than LISA, able to detect collisions between supermassive black holes, intermediate mass
black holes, compact binaries, and even the gravitational wave vibrations left over from
the Big Bang; a time that we can’t see with optical wavelengths.
A laser interferometer the size of the Earth’s orbit.
That would be pretty cool.
Those are some missions to explore the L4 and L5 regions, but I know what you want to
hear.
You want to know about massive rotating space colonies proposed by Gerard O’Neill in the
1970s.
Okay fine, we’ll get to that in a second, but first I’d like to thank:
Timothy Pifer Bill Munn
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The most science fiction use of the Earth-Moon L4 and L5 Lagrange Points has got to be the
O’Neill Cylinder Space Settlements proposed by Princeton physicist Gerard O’Neill in
the 1970s.
The concept came out of a 10-week program in engineering systems design at Stanford
and Ames Research Center in 1975.
More than two dozen scientists, students and professors came together for ten weeks to
design how a future space colony might work, allowing people to survive and even thrive
in the harsh environment of space.
The O’Neill Cylinder would be a giant, rotating space station measuring 1,800 meters across,
and be a home for 10,000 people.
And to keep it in a stable location, they proposed putting the station at Earth-Moon
L5.
People would live in the outer ring, connected to the central docking area by 6 giant spokes.
The centripetal force from the rotation of the space station would give the inhabitants
artificial gravity, allowing them to carry on regular lives as if they were under Earth’s
gravity.
Because it’s located out beyond the protection of the Earth’s magnetosphere, the space
station needs to protect the inhabitants from cosmic radiation.
Back in the 1970s, the design team decided that an active magnetic shield would be too
complicated and energy intensive.
Actually, it still is today, we’ve done a whole episode on this.
Instead, they proposed mining thousands of tonnes of rock from the surface of the Moon
and surrounding the outer shell of the habitats with protective bricks.
If you want to prevent cosmic radiation, all you need is mass.
A lot of mass.
It would be too expensive to bring it up from Earth, but electromagnetic railguns on the
Moon could be constantly throwing material into space that could be added to the station.
Giant mirrors would reflect sunlight into the station, at an angle that prevents cosmic
rays from coming in too.
Variations on the O’Neill Cylinder were proposed over the next few years, like the
Stanford Torus and the McKendree Cylinder but they all suffered from the problem that
they’re inherently unstable.
If you have a long tube rotating in space, it has two axis of rotation.
Whichever is the longer axis is the preferred direction of rotation.
Which means that you have to fight against your space station wanting to tumble end over
end.
O’Neill and other’s solutions was to build their stations in pairs which rotate in opposite
directions from each other, balancing out the forces.
The amount of raw materials that will need to go into a space station of this size will
help you understand why we’ve never built one yet.
The largest version of the O’Neill Cylinder would require 4.5 billion tonnes of rock,
steel, machinery, shielding and other components.
Even if the SpaceX Starship could bring launch costs down to $100 per kilogram, that would
still cost about $450 trillion.
That would require about 45,000 Starship launches to build one in Low Earth Orbit, not to mention
getting it up to L5.
That’s why it’s so important for us to develop space-based mining and manufacturing
before we have any chance of living in our rotating space stations.
I hope you enjoyed this tour through the Solar System’s Lagrange Points.
We’ve already take advantage of them with our missions and space telescopes, and I’m
sure this is only the beginning.
I can’t wait to see what happens next.
What do you think?
Are you aware of any other ideas to use the L4 and L5 points in the Solar System?
Let me know your thoughts in the comments.
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I’ll put a link in the shownotes.
Here’s a link to the next episode in the series.