Thanks to gravity, there are places across the Solar System which are nicely balanced.
They’re called Lagrange Points and they give us the perfect vantage points for a range
of spacecraft missions, from observing the Sun to studying asteroids, and more.
Various spacecraft have already visited Lagrange Points, used them for some or all of their
missions, and there are fascinating plans in the works that could put new missions and
even space colonies into these balanced places in the Solar System.
Let’s explore the Lagrange Points.
I’d say roughly half the questions I get from viewers are about Lagrange Points.
Okay, maybe that’s a bit of an embellishment, but seriously, this is a topic you’re fascinated
about.
Why do we use them?
Which Lagrange Points work best for which missions?
Could there be planets hiding in Lagrange Points behind the Sun?
So, today, I thought I’d begin a series on the Solar System’s Lagrange Points.
How have they been used practically in the past by various space missions, and what are
the clever ideas that have been proposed to use them in the future.
This series will be a total of three episodes, I think.
Today we’ll focus on L1
But before we get into that, let’s do a quick refresher on the Lagrange Points.
Thanks to gravity and orbital mechanics, there are 5 relatively stable spots in space in
relation to two massive objects.
So, the Sun and the Earth have 5 Lagrange Points, the Earth and the Moon have 5.
The Sun and Jupiter have 5, excetera.
Let’s use the Earth and the Sun for an example.
L1, is at a point in between the Sun and the Earth.
Without the gravity of the Earth, an object at this point would be faster in its orbit
and drift away from the Earth.
But the Earth’s gravity pulls it back a bit, keeping it stable.
L2 is on the far side of the Earth, at a distance of about 1.5 million kilometers.
Same deal, an object there would be in a higher orbit, moving slower than the Earth, but the
gravity of the Earth tugs on it to keep in line.
L3 is on the opposite side of the Sun, at almost the same distance the Earth is from
the Sun.
There are two additional Lagrange Points, L4 and L5, and these lie ahead and behind
the Earth in its orbit by 60-degrees.
The L1, L2 and L3 points are meta-unstable, which means that you can’t just park a spacecraft
there.
Imagine a rock sitting at the top of a hill.
You could keep it balanced with a little bit of effort, but once it started rolling down
the hill, it’s hard to stop it.
Any spacecraft that goes to these points needs to be constantly firing its thrusters to get
back to this stable point.
L4 and L5 are stable.
So, now imagine a rock sitting at the bottom of a valley.
If you push it up the side of the valley, it wants to roll back down.
So if you can park a spacecraft into one of these Lagrange points, it’ll remain there
forever without fuel.
You can only get these stable points if the mass ratio between the two objects is greater
than 24.96.
The Sun is 332,000 times more massive than the Earth, so that qualifies.
And the Sun is 1048 times more massive than Jupiter, so again… that works.
But Pluto and Charon, for example, are too close in mass to generate L4 and L5 points.
There are a couple of other things to note.
First, the Lagrange points only work when the third object has a negligible mass compared
to the mass of the bigger objects.
So, Sun, Earth and spacecraft.
Or Sun, Jupiter and small asteroid.
You can’t have Sun, Earth, and another Earth.
Also, Lagrange Points aren’t actually points.
They’re more like regions.
Consider how planets and moons follow elliptical paths as they orbit.
This means that the exact location of the stable area is changing over the course of
an orbit, and a spacecraft will need to use up even more propellant to keep in a stable
location.
Okay, there’s your explainer, in this episode we’ll consider the L1 Lagrange point, how
it’s been used in the past, and some clever ideas to take advantage of this spot for future
missions.
Let’s start with the Sun-Earth L1 Lagrange point, a point located about 1.5 million kilometers
closer to the Sun than the Earth.
This is the perfect spot to put a spacecraft that needs to constantly observe the Sun without
the pesky Earth or Moon getting in the way.
Or observe the daytime side of the Earth.
Or both.
This is the location of NASA’s Solar and Heliospheric Observatory, or SOHO.
This spacecraft was launched in December, 1995, and was originally only supposed to
observe the Sun for 2 years in its main mission.
Well, here we are, over 24 years later, and SOHO is continuing its observations; although,
it’s down most of its instruments..
SOHO isn’t exactly in Sun-Earth L1, nothing really is.
It actually orbits around the region, completing one circuit every six months.
Other Sun-observing satellites, such as NASA’s Advanced Composition Explorer and the Wind
Satellite are also in L1, making observations of the Sun.
The first spacecraft to actually use L1 was the International Sun-Earth Explorer-3, which
launched to the region in 1978.
Its job was to watch how the solar wind interacts with the Earth’s magnetic field.
Several other spacecraft have used this point as well.
NASA’s Genesis mission collected samples of the Sun’s solar wind by loitering around
in this region before returning back to Earth in 2004.
ESA’s LISA Pathfinder mission was also sent to the L1 Lagrange point in 2015.
The purpose of LISA was to demonstrate the underlying technologies that will go into
a future space-based gravitational wave observatory, with multiple spacecraft flying in formation.
Inside the LISA spacecraft, there were two gold-platinum cubes which were kept in a constant
state of freefall inside the spacecraft, isolated from all the forces except for gravity.
The spacecraft then used thrusters to keep itself centered around the cubes, while a
laser continuously monitored the distance between them.
I’ll talk about other gravitational wave observatory ideas in the next episode.
NASA’s Deep Space Climate Observatory, or DSCOVR, is also located at the Sun-Earth L1
point.
From this vantage, it can continuously monitor the Sun’s solar wind, giving us advance
notice of coronal mass ejections.
And then it can also see the sunlit side of the entire planet Earth, watching changes
in ozone levels, volcanic eruptions and weather systems.
India and China have their own missions planned to travel to L1 to observe the Sun.
These are the missions which have used the L1 Lagrange Points in the past, next I’m
going to talk about some future mission ideas that I hope will blow your mind, and we’ll
get to that in a second, but first I’d like to thank:
Thomas Mouritsen GeoAstroRV
Tim Allen Something
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As we’ve already discussed, the Sun-Earth L1 Lagrange point is the perfect spot to continuously
observe the Earth, and one idea to use this as a place to observe asteroids.
One of the problems with asteroid observation is that astronomers can’t see the spacerocks
flying towards us through the glare of the Sun.
NASA’s NEOCam mission would send a space telescope to the Sun-Earth L1 point to continuously
observe as much of the sky as possible looking for hazardous asteroids.
From this vantage, it would be able to see asteroids as close as 45-degrees to the Sun.
Within about 4 years, it could find 2/3rds of the potentially hazardous asteroids out
there.
If all goes well, the spacecraft could launch in 2021.
There have been some fascinating ideas to use the Sun-Earth L1 Lagrange point to slow
down climate change.
Researchers have proposed using a single large sunshade, or a giant fresnel lens that would
distort the light from the Sun.
Here’s the idea I like the best proposed by Roger Angel from the University of Arizona
- it’s mega-engineering at an absurd scale.
He estimates that it should be possible to launch a cloud of tiny spacecraft into the
L1 point, and use them to to block some of the light from the Sun.
Each spacecraft would measure about 1 meter across and weigh only a gram.
Instead of being a single, enormous sunshade that needs to push back against the light
pressure from the Sun, millions would orbit in a vast cloud about 100,000 kilometers across
in the L1 point.
They wouldn’t need to station keeping, they’d just orbit around the L1 region like a swarm
of bees, slowly falling out of orbit, and getting replenished as necessary with replacements.
It would take a total of 20 million tons of these spacecraft to make a difference, and
they could be constructed by dismantling a metal asteroid, or spending a few trillion
dollars in SpaceX Starship launches.
With this cloud, we could dim the Sun enough to counteract the contribution of carbon dioxide
in the Earth’s atmosphere.
Although trillions of dollars sounds like a lot of money, we might get to the point
where it makes sense to spend that kind of money to get the problem under control.
And that makes me sad.
I’ll put a link to his paper in the show notes, and I highly recommend you give it
a read, it’s absolutely fascinating.
Then chase down anything else Roger Angel has proposed.
Mars sucks, it’s cold, dry and has a thin atmosphere.
But we could use the Sun-Mars L1 Lagrange Point to fix it.
In a proposal from Jim Green, the Chief Scientist at NASA, we could position an artificial magnetosphere
shield at the L1 spot in between the Sun and Mars.
This would block the solar wind which is constantly tearing away at the Martian atmosphere and
blowing it out into space.
This would allow trace gases to build up in the thin Martian atmosphere, thickening it
up to the point that carbon dioxide poles would sublimate, further thickening the atmosphere.
The greenhouse gases would warm the atmosphere further, and even the trapped water would
start melting during the Martian summers, adding even more water vapor into the Martian
atmosphere, warming it up even more.
Over time, you could restore up to 1/7th of Mars’ original oceans, just by blocking
the solar wind.
We’ve talked about this idea more in another episode, which I’ll link to here and in
the show notes.
Another place that could really use a Sun block is Venus.
In fact, it might be the only way to make that planet habitable.
If you could position a sunshade that at the Sun-Venus L1 point that’s 4 times bigger
than Venus, you could block the radiation from the Sun.
Its atmosphere would slowly cool down over centuries to the point that the carbon dioxide
freezes out and falls as snow.
Then you could use various chemicals to bind up the carbon dioxide before you let the planet
warm up again.
Closer to Earth, we’ve got the Earth-Moon L1 point, which is located about 85% the distance
from the Earth to the Moon.
This is a spot that’s balanced above the Moon, and would make the ideal spot to conduct
lunar activities.
Although NASA has chosen a different orbit for the Lunar Gateway, L1 would work well
too, because it’s easy to reach the surface of the Moon from this spot.
Astronauts on board the station would remain over the same spot of the Moon, able to operate
rovers and mining equipment with almost no latency.
But at the same time, they’d have a constant communications link with the Earth.
An L1 station could serve as an off-Earth facility for testing samples of other worlds
without risking any kind of contamination with Earth.
A spacecraft there could also observe the entire planet Earth, but unlike the geostationary
satellites, the Earth would rotate through its point of view, allowing it to observe
the entire planet every day.
But one of the most ambitious ideas would be to have the Earth-Moon L1 point be the
top of a space elevator leading up from the surface of the Moon.
You’ve probably heard of the idea of a space elevator from the Earth, but the problem is
that we don’t have strong enough materials to support the weight of a cable, from the
Earth into space.
But the pull of gravity on the Moon is much lower, and so a lunar elevator could be made
with composite materials we already have today.
Imagine mining robots on the surface of the Moon that transfer lunar regolith and helium
3 to a lunar elevator that carries them up to the L1 Lagrange point.
From this point, it takes very little propellant to transfer the material to another spot in
the Solar System, or maybe to that cloud of satellites you’re constructing to block
the light from the Sun and slow down climate change.
I foolishly thought this would be a single episode, but the Lagrange points are just
too big a topic.
In our next episode, we’ll talk about L2 and L3.
What do you think?
Have you got some clever ideas to use the Lagrange Points?
Any missions I missed?
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.