After a 1.5 million kilometer journey, the James Webb Space Telescope has arrived at
the Sun Earth L2 point.
It did this by executing its final Mid-Course Correction burn called "MCC-2".
It lasted just 297 seconds, or a little less than five minutes, and it only changed Webb's
speed by about one and a half meters per second.
That's a brisk walking pace.
It may not seem like much, but it was enough to place Webb into an orbit around L2.
In fact, it placed it into a very large orbit, one that's more than twice as large as the
Moon's orbit around Earth!
It'll take Webb about six months to complete a single orbit around L2, and this first go-around
is jam packed with mirror alignments, phasing, instrument calibrations, moving target tracking
tests, verifications, station keeping maneuvers and finally, the first science observations.
But what exactly is so special about L2?
I mean, it's just a point in space, and it's an empty point in space at that.
So how can Webb orbit something that isn't there?
Well, it is true that L2 is empty, but orbiting empty points in space is actually very common.
And that's because whenever there are two massive bodies that are orbiting each other,
they both orbit a common center of mass or "barycenter".
If the objects are massive enough, the center of mass lies somewhere in between.
We only say that the Moon orbits Earth because its center of mass is located inside Earth.
But Pluto and Charon have similar masses, so their barycenter is somewhere in the space
between them.
So the fact that something can orbit an empty point space really isn't all that unusual,
but at the same time, that's a two-body system.
It doesn't explain how web can orbit a point that's outside of another two-body system.
Well, the good news is that we can break this problem down and understand just what the
L2 point is and how Webb can orbit around it.
In fact, there are many spacecraft that have orbited L2, such as NASA's WMAP Satellite
and ESA's Plank and Herschel telescopes, and Russia's Spektr-RG and ESA's Gaia spacecraft
are both orbiting there today.
In fact, Gaia is featured in an episode of "Space The New Frontier", which is a series
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L2 is one of the five Lagrangian points between the Sun and Earth.
Actually, these five points exist around any two-body system, so there are Lagrangian points
for the Earth-Moon system, a set of points for the Sun-Jupiter system, and so on.
Now, if a third object is placed at any of these points, the gravitational forces of
the two large bodies combine in such a way that holds the third object stable with respect
to the others.
The practical upshot is that as the system orbits, the third body tags along.
And that's why the Sun-Earth L1 point is a great place to position Sun-facing missions
like the Solar Heliospheric Observatory and Earth-facing missions like DISCOVR.
Likewise, L2 is the place to put astrophysics missions like Gaia and Webb.
At both locations, spacecraft maintain a constant line of sight communication with Earth, even
as Earth orbits the Sun.
But L2 is particularly important for Webb because it can block the heat of the Sun,
Earth, and Moon with a single sunshield.
That's a perfect place for infrared astronomy.
But there are some important caveats.
For one thing, the third object has to have a mass that's really, really, really small
relative to the larger bodies.
Obviously, that's not a problem.
Webb's mass is negligible compared to Earth, let alone the Sun.
But the other caveat is that the Lagrangian points are at best described as metastable,
especially the first three points, To understand what I mean,
imagine a saddle that's perfectly smooth and frictionless.
Now imagine trying to balance a marble in the very center of the saddle.
It's theoretically possible, but as a practical matter, it's extremely hard to do.
The L2 point is a lot like this; an object moving perpendicular to the Sun-Earth line
falls toward the center.
But then it either falls toward or away from Earth.
Now, if Webb falls away from Earth, it's really in trouble because the only way to bring it
back would be to turn it around and thrust back.
But that will cause the telescope to suddenly warm up.
All of the structures would expand glues and adhesives would melt and the mission would
be over.
And that's bad.
Webb has thrusters only on the warm Sun-facing side of the observatory.
And that's because hot thrusters would contaminate the cold telescope's side with heat or with
rocket exhaust that would otherwise condense on the optics.
That would also be bad.
A safer approach is to place Webb almost, but not quite at L2.
And that way, when it falls back toward Earth, a tiny thruster burn pushes it back almost,
but not quite back to L2.
And this means Webb never runs the risk of actually falling beyond L2, where it cannot
be recovered.
And that's why the Ariane five launch was intentionally designed to slightly undershoot
and let the spacecraft do a series of Mid Course Correction burns.
The first two, MCC-1a and MCC-1b, were executed on the way out to L2.
However, arriving even just shy of L2 leaves us with another problem.
At that distance, the Sun is in a permanent annular eclipse by Earth.
Webb is solar powered.
Now it could use nuclear power, but that's expensive and hard to do.
Another complication is that Coriolis forces would cause Webb to liberate around L2.
In other words, Webb would find itself in a tiny orbit around L2 that would need even
more thruster burns to manage.
Oh, and the Moon's gravity on Webb is also still a thing, and it's constantly varying
as it orbits Earth.
It turns out that these real-world complications can be addressed by simply taking up a large
orbit around L2.
But how is it possible to orbit something that isn't there?
I mean, L2 is still an empty point in space.
Well, to help us understand better, let's look at the problem by considering some of
the forces acting on Webb.
If Webb were actually at L2, it would be feeling gravitational forces from the Sun and from
Earth.
So let's draw them separately so that we can keep track of them.
Now, Webb is orbiting the Sun at the same time, so we can also imagine a centrifugal
force pulling Webb in the opposite direction.
Now, the great thing about L2 is that at this distance, the centrifugal force is exactly
balanced by the gravity of the Earth and Sun.
Now, you may have heard that there is no such thing as a centrifugal force, and that is
correct, but we are talking about a rotating non inertial frame here, so centrifugal forces
are still
a useful way of summarizing all of the real effects that are going on.
Oh, and we're not going to worry about the gravitational effects of the Moon or even
the fact that the Sun and the Earth are not point sources.
We're just going to keep everything simple for the time being.
Now let's move Webb up some distance away from L2.
The gravitational force from the Sun changes a tiny amount, but not very much because it's
over 150 million kilometers away.
But Earth is just 1% of that distance, so its gravity changes considerably.
Because Webb is now farther away from Earth, its gravitational pull gets weaker.
It's also no longer parallel with the Sun's direction, either.
So let's break it up into its X and Y components.
Now we can really see the problem.
The centrifugal force is in the X direction of our diagram, but it's actually greater
than the gravitational forces from the Sun and Earth.
That means Webb is going to drift both down and farther away from L2.
So we need to fix this drift in both the X and Y directions.
First, we bring Webb a little closer to Earth so that it's now hovering above a new point
that we'll just call L2-prime.
Earth's gravity gets stronger, and its X component helps to balance the centrifugal force once
again.
But the Y component has gotten stronger as well.
So Webb wants to move downward.
But the closer it gets to the Sun Earth line, the stronger Earth's gravity gets and the
more it moves toward Earth.
We can't let that happen, either, because then the heat load would become too strong
for the sunshield to handle.
However, we can counteract this downward force by giving Webb a sideways push into the screen.
This sets up a new centrifugal force that balances the downward force of Earth's gravity.
When combined with the sideways motion, Webb takes up a new circular halo orbit around
L2-prime,.
And that means there doesn't have to be a physical mass at
L2-Prime in order to attract Webb.
Rather, Webb is simply being pulled by the Y component of Earth's gravitational force.
Now this is an oversimplified discussion of Webb's orbit around L2.
We deliberately ignored real-world complications in order to understand how Webb can orbit
something that isn't there.
But getting into Webb's actual orbit required taking all of these real-world complications
into account.
So to make things easier, Webb wasn't actually launched to L2.
Instead, it was launched to a point along the side of L2.
As it was starting to fall away,
Webb executed its MCC-2 burn to settle into its orbit.
Since part of this initial fall was directed in the X direction toward Earth, the burn
propelled Webb into an orbit that's now tilted by about 33 degrees.
The orbit isn't perfectly circular either, but rather follows an elliptical shape.
As a result, Webb's distance from the L2 point varies between 250,000 kilometers and 832,000
kilometers.
Now, for reference, the Moon's semi-major axis is about 400,000 kilometers, so Webb's
orbit around L2 is enormous!
This large orbit was chosen because it makes getting to L2 a lot easier.
The farther something is from its orbital focus, the slower it moves and the less energy
is required to maneuver.
And that's why it only took a five-minute burn and a 1.5 meters per second
Delta-V.
The larger orbit also means that Webb will never drift into the shadows of the Earth
or the Moon during its mission.
However, larger orbits can permit stray light from the Earth or Moon to get past the sunshield
and reach the primary or secondary mirrors.
In addition, a larger orbit can reduce the number of communication opportunities with
the Deep Space Network.
That's why Webb will have to change its orientation to keep stray light off the mirrors at all
times throughout its orbit and schedule the communication downlinks accordingly.
Webb takes about six months to complete one orbit around L2.
Although it sometimes flies out just a little bit past this point.
The geometric center of the orbit is still centered on the L2-prime point on the Earth
side.
Remember, we don't want that ball right at the very middle of the saddle.
That's why Webb will execute small stationkeeping burns every now and then to maintain its orbit.
Missions like Gaia only require three to four of these burns every year.
But Webb will require much more frequent burns for a couple of reasons.
For one, its sunshield is always feeling a torque from solar radiation pressure.
Webb can counter this rotational force by spinning up its reaction wheels.
But the solar radiation pressure is unrelenting, so the reaction wheels must increase their
spin over time.
Eventually, a station-keeping burn is required to rotate Webb back into place and let the
wheels throttle back.
Webb also changes its orientation as it points from one position to the other.
All of these motions perturb Webb's orbit ever so slightly, requiring more station-keeping
burns to maintain.
And that's why Webb executes a burn once every 21 days.
The exact timing and duration of those burns are calculated by the Flight Dynamics Team
at NASA's Goddard Space Flight Center.
They'll do this by monitoring Webb's telemetry data and using ranging data from the Deep
Space Network to pinpoint Webb's location and its motion at all times.
They even take into account gravitational perturbations from the Moon and even the gravitational
perturbations of the rest of the planets!
So, yeah, the flight dynamics team are really awesome, and they've got their work cut out
for them over the next...20 years?
And speaking of amazing teams, how about these amazing folks are helping to keep Launch Pad
Astronomy going, and I'd like to welcome Marcel Barros, Steve C, Joe Curley, Timothy Fudge,
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