It’s amazing to think there are telescopes
up in space, right now, directing their gaze at distant objects for hours, days and even
weeks. Providing a point of view so stable and accurate
that we can learn details about galaxies, exoplanets and more. And then, when the time is up, the spacecraft
can shift its gaze in another direction. All without the use of fuel. It’s all thanks to the technology of reaction
wheels and gyroscopes. Let’s talk about how they work, how they’re
different, and how their failure has ended missions in the past. Here’s the quick answer. Reaction wheels allow spacecraft to change
their orientation in space, while gyroscopes keep a telescope incredibly stable, so they
can point at a target with high accuracy. If you’ve listened to enough episodes of
Astronomy Cast, you know I always complain about reaction wheels. It always seems to be the point of failure
on missions, ending them prematurely before the science is all in. I’ve probably used the terms reaction wheels
and gyroscopes interchangeably in the past, but they serve slightly different purposes. First, let’s talk about reaction wheels. These are a type of flywheel used to change
the orientation of a spacecraft. Think about a space telescope that needs to
switch from target to target, or a spacecraft that needs to turn itself back to Earth to
communicate data. They’re also known as momentum wheels. There’s no air resistance in space. When a wheel turns in one direction the entire
telescope turns in the opposite direction, thanks to Newton’s Third Law - you know,
for every action, there’s an equal and opposite reaction. With wheels spinning in all three directions,
you can turn the telescope in any direction you like. The wheels are fixed in place and spin between
1,000 and 4,000 revolutions per minute, building up angular momentum in the spacecraft. In order to change the orientation of the
spacecraft, they change the rate at which the wheels are spinning. This creates a torque that causes the spacecraft
to shift its orientation, or precess, in a chosen direction. This technology works with electricity alone,
which means that you don’t need to use up propellant to change the orientation of the
telescope. As long as you’ve got enough rotors spinning,
you can keep on changing your direction, using only the power from the Sun. Reaction wheels are used on pretty much every
spacecraft out there, from tiny Cubesats to the Hubble Space Telescope. With three wheels, you can change your orientation
to any spot in 3-dimensions. But the Planetary Society’s LightSail 2
has only a single momentum wheel to shift the orientation of its solar sail, from edge-on
to the Sun and then broadside to raise its orbit by sunlight alone. Of course, we’re most familiar with reaction
wheels because of the times they’ve failed, taking spacecraft out of commission. Missions like FUSE and JAXA’s Hayabusa. Most famously, NASA’s Kepler Space Telescope,
launched on March 9, 2009 to find planets orbiting other stars. Kepler was equipped with 4 reaction wheels. Three were necessary to keep the telescope
pointed carefully at a region of sky, and then a spare. It was watching for any star in its field
of view to change in brightness by a factor of 1 in 10,000, indicating that a planet could
be passing in front. To save bandwidth, Kepler actually only transmitted
information about the change in brightness of the stars themselves. In July, 2012, one of Kepler’s four reaction
wheels failed. It still had three, which was the minimum
it needed to be able to be stable enough to continue its observations. And then in May, 2013, NASA announced that
Kepler had a failure with another of its wheels. So it was down to two. This brought the main science operations of
Kepler to a halt. With only two wheels operating, it could no
longer maintain its position accurately enough to track star brightness.. Although the mission could have been a failure,
engineers figured out an ingenious strategy, using the light pressure from the Sun to act
as a force in one axis. By perfectly balancing the spacecraft in the
sunlight, they were able to continue using the other two reaction wheels to continue
making observations. But Kepler was forced to look at the tiny
spot in the sky that happened to line up with its new orientation, and shifted its science
mission to looking for planets orbiting red dwarf stars. It used up its onboard propellant turning
back to Earth to transmit data. Kepler finally ran out of fuel on October
30, 2018, and NASA wrapped up its mission. At the same time that Kepler was struggling
with its reaction wheels, NASA’s Dawn mission was having problems with the exact same reaction
wheels. Dawn was launched on September 27, 2007 with
the goal of exploring the two of the largest asteroids in the Solar System: Vesta and Ceres. The spacecraft went into orbit around Vesta
in July, 2011 and spent the next year studying and mapping the world. It was supposed to leave Vesta and head off
to Ceres in August, 2012, but the departure was delayed by more than a month because of
problems with its reaction wheels. Starting in 2010, engineers were detecting
more and more friction in one of its wheels, so the spacecraft switched to the three functioning
wheels. And then in 2012, the second of its wheels
started to gain friction as well, and the spacecraft was left with only two remaining
wheels. Not enough to keep it fully oriented in space
using electricity alone. This meant it had to start using its hydrazine
propellant to maintain its orientation throughout the remainder of its mission. Dawn made it to Ceres, and through careful
use of propellant it was able to map out this world, and its bizarre surface features. Finally, in late 2018, the spacecraft was
out of propellant, and it was no longer able to maintain its orientation, to map Ceres
or to send its signals back to Earth. The spacecraft will continue to orbit Ceres,
tumbling helplessly. There’s a long list of missions whose reaction
wheels have failed. And now scientists think they know why. There was a paper released in 2017 that determined
that the environment of space itself is causing the problem. As geomagnetic storms pass the spacecraft,
they generate charges on the reaction wheels that cause an increase in friction and make
them wear down more quickly. I’ll put a link to a great video by Scott
Manley that goes into more detail. The Hubble Space Telescope is equipped with
reaction wheels to change its overall orientation, rotating the entire telescope about the speed
of a minute hand on a clock - 90 degrees in 15 minutes. But to stay pointed at a single target, it
uses another technology: gyroscopes. There are 6 gyroscopes on Hubble which spin
at 19,200 revolutions per minute. They’re large, massive and spin so fast
that their inertia resists any changes to the telescope’s orientation. It works best with three - matching the three
dimensions of space - but can operate with two, or even one, with less accurate results. In August, 2005, Hubble’s gyroscopes were
wearing down, and NASA shifted into two-gyroscope mode. In 2009, during Servicing Mission 4, NASA
astronauts visited the space telescope and replaced all six of its gyroscopes. This is likely the final time astronauts will
ever visit Hubble, and its future depends on how long these gyroscopes last. As we’re getting closer and closer to the
launch of the James Webb Space Telescope, I’m sure you’re worried about its gyroscopes
and reaction wheels. James Webb has some different technologies
on board. I’ll explain in a second, but first I’d
like to thank: Amir Kochek
Daryl Warner William Wedin
Paul Holloway And the rest of our 795 patrons for their
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and get in on the action. I know the mere mention of the James Webb
Space Telescope makes everyone nervous. More than $8 billion dollars invested so far
and due for launch in about two years from now. It’s going to be flying to the Earth-Sun
L2 Lagrange point, located about 1.5 million kilometers away from Earth. Unlike Hubble, there’s no way to fly out
the James Webb to repair it if anything goes wrong. And seeing how often gyroscopes have failed,
this really does seem like a dangerous weak point. What if James Webb’s gyros fail? How can we replace them. James Webb does have reaction wheels on board. They’re built by Rockwell Collins Deutschland,
and they’re similar to the reactions wheels on board NASA’s Chandra, EOS Aqua and Aura
missions - so a different technology from the failed reaction wheels on Dawn and Kepler. The Aura mission provided a scare in 2016
when one of its reaction wheels spun down, but it was recovered after ten days. James Webb isn’t using mechanical gyroscopes
like Hubble to keep it on target. Instead, it’s using a different technology
called hemispherical resonator gyros, or HRGs. These use a quartz hemisphere that has been
shaped very precisely so that it resonates in a very predictable way. The hemisphere is surrounded by electrodes
that drive the resonance, but also detect any slight changes in its orientation. I know that kind of sounds like gibberish,
like it’s powered by unicorn dreams, but you can experience this for yourself. Hold a wineglass and then flick it with your
finger so that it’s ringing. The ringing is the wineglass flexing back
and forth at its resonance frequency. As you rotate the glass, the flexing back
and forth turns as well, but it lags behind the orientation in a very predictable way. When these oscillations are happening thousands
of times a second in a quartz crystal, it’s possible to detect tiny motions and then account
for them. That’s how James Webb will stay locked on
its targets. This technology has flown on the Cassini mission
at Saturn and worked perfectly. In fact, as of June 2011, NASA had reported
that these instruments had experienced 18 million hours of continuous operation in space
on more than 125 different spacecraft without a single failure. It’s actually very reliable. I hope that clears things up. Reaction or momentum wheels are used to re-orient
spacecraft in space, so they can face in different directions without using propellant. Gyroscopes are used to keep a space telescope
accurately pointed at a target, to provide the best scientific data. They can be mechanical spinning wheels, or
they use the resonance of vibrating crystals to detect changes in inertia. What do you think? Let me know your thoughts in the comments. Once a week I gather up all my space news
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