It’s wonderful to assume there are telescopes up in house, proper now, directing their stare upon distant objects for hours, days and even weeks. Offering a viewpoint so steady and correct that we will be taught particulars about galaxies, exoplanets and extra.
After which, when the time is up, the spacecraft can shift its gaze in one other route. All with out the usage of gas.
It’s all due to the expertise of response wheels and gyroscopes. Let’s discuss how they work, how they’re completely different, and the way their failure has ended missions previously.
Right here’s the short reply. Response wheels enable spacecraft to vary their orientation in house, whereas gyroscopes hold a telescope extremely steady, to allow them to level at a goal with excessive accuracy.
If you happen to’ve listened to sufficient episodes of Astronomy Forged, you understand I at all times complain about response wheels. It at all times appears to be the purpose of failure on missions, ending them prematurely earlier than the science is all in.
I’ve in all probability used the phrases response wheels and gyroscopes interchangeably previously, however they serve barely completely different functions.
One of many excessive efficiency response wheels carried by Kepler, Daybreak and a number of other different NASA missions. Credit score: Ball Aerospace
First, let’s discuss response wheels. These are a sort of flywheel used to vary the orientation of a spacecraft. Take into consideration an area telescope that should swap from goal to focus on, or a spacecraft that should flip itself again to Earth to speak knowledge.
They’re also called momentum wheels.
There’s no air resistance in house. When a wheel turns in a single route the complete telescope turns in the wrong way, due to Newton’s Third Legislation – you understand, for each motion, there’s an equal and reverse response. With wheels spinning in all three instructions, you possibly can flip the telescope in any route you want.
The wheels are mounted in place and spin between 1,000 and four,000 revolutions per minute, build up angular momentum within the spacecraft. With a view to change the orientation of the spacecraft, they alter the speed at which the wheels are spinning.
Structure of the Kepler House Telescope, together with its four response wheels. Credit score: NASA Ames / Ball Aerospace
This creates a torque that causes the spacecraft to shift its orientation, or precess, in a selected route.
This expertise works with electrical energy alone, which signifies that you don’t want to make use of up propellant to vary the orientation of the telescope. So long as you’ve acquired sufficient rotors spinning, you possibly can carry on altering your route, utilizing solely the facility from the Solar.
Response wheels are used on just about each spacecraft on the market, from tiny Cubesats to the Hubble House Telescope.
With three wheels, you possibly can change your orientation to any spot in Three-dimensions. However the Planetary Society’s LightSail 2 has solely a single momentum wheel to shift the orientation of its photo voltaic sail, from edge-on to the Solar after which broadside to lift its orbit by daylight alone.
Picture of LightSail 2’s sail deployment. Credit score: The Planetary Society
After all, we’re most conversant in response wheels due to the occasions they’ve failed, taking spacecraft out of fee. Missions like FUSE and JAXA’s Hayabusa.
Kepler’s Lack of Response Wheels and the Ingenious Answer
Most famously, NASA’s Kepler House Telescope, launched on March 9, 2009 to seek out planets orbiting different stars. Kepler was geared up with four response wheels. Three have been essential to hold the telescope pointed rigorously at a area of sky, after which a spare.
An artist’s illustration of NASA’s Kepler spacecraft. The Kepler mission is nearly over, and the final of its gas is being reserved to verify its knowledge makes it house. Picture: NASA/Kepler
It was looking forward to any star in its subject of view to vary in brightness by an element of 1 in 10,000, indicating that a planet could possibly be passing in entrance. To save lots of bandwidth, Kepler truly solely transmitted details about the change in brightness of the celebrities themselves.
In July, 2012, one in every of Kepler’s 4 response wheels failed. It nonetheless had three, which was the minimal it wanted to have the ability to be steady sufficient to proceed its observations. After which in Could, 2013, NASA introduced that Kepler had a failure with one other of its wheels. So it was down to 2.
This introduced the primary science operations of Kepler to a halt. With solely two wheels working, it might not preserve its place precisely sufficient to trace star brightness..
Though the mission might have been a failure, engineers discovered an ingenious technique, utilizing the sunshine stress from the Solar to behave as a power in a single axis. By completely balancing the spacecraft within the daylight, they have been in a position to proceed utilizing the opposite two response wheels to proceed making observations.
Infographic exhibiting how the Kepler house telescope continued looking for planets regardless of two busted response wheels. Credit score: NASA Ames/W Stenzel
However Kepler was pressured to take a look at the tiny spot within the sky that occurred to line up with its new orientation, and shifted its science mission to in search of planets orbiting crimson dwarf stars. It used up its onboard propellant turning again to Earth to transmit knowledge. Kepler lastly ran out of gas on October 30, 2018, and NASA wrapped up its mission.
On the similar time that Kepler was battling its response wheels, NASA’s Daybreak mission was having issues with the very same response wheels.
Daybreak’s Lack of Response Wheels
Daybreak was launched on September 27, 2007 with the aim of exploring the 2 of the most important asteroids within the Photo voltaic System: Vesta and Ceres. The spacecraft went into orbit round Vesta in July, 2011 and spent the following 12 months learning and mapping the world.
An artist’s illustration of NASA’s Daybreak spacecraft with its ion propulsion system approaching Ceres. Picture: NASA/JPL-Caltech.
It was supposed to go away Vesta and head off to Ceres in August, 2012, however the departure was delayed by greater than a month due to issues with its response wheels. Beginning in 2010, engineers have been detecting increasingly more friction in one in every of its wheels, so the spacecraft switched to the three functioning wheels.
After which in 2012, the second of its wheels began to realize friction as properly, and the spacecraft was left with solely two remaining wheels. Not sufficient to maintain it absolutely oriented in house utilizing electrical energy alone. This meant it needed to begin utilizing its hydrazine propellant to keep up its orientation all through the rest of its mission.
Three views of the Ahuna Mons ice volcano. Prime is a reconstruction of the volcano from topographical knowledge, the picture on the left was captured by Daybreak’s framing digicam, and the false coloration picture on the appropriate reveals the presence of sodium carbonate in crimson and inexperienced. Picture: By NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/ASI/INAF
Daybreak made it to Ceres, and thru cautious use of propellant it was in a position to map out this world, and its weird floor options. Lastly, in late 2018, the spacecraft was out of propellant, and it was not in a position to preserve its orientation, to map Ceres or to ship its indicators again to Earth.
The spacecraft will proceed to orbit Ceres, tumbling helplessly.
There’s an extended record of missions whose response wheels have failed. And now scientists assume they know why. There was a paper launched in 2017 that decided that the surroundings of house itself is inflicting the issue. As geomagnetic storms cross the spacecraft, they generate expenses on the response wheels that trigger a rise in friction and make them put on down extra rapidly.
I’ll put a hyperlink to an amazing video by Scott Manley that goes into extra element.
Hubble House Telescope and its Gyroscopes
The Hubble House Telescope is provided with response wheels to vary its total orientation, rotating the complete telescope concerning the velocity of a minute hand on a clock – 90 levels in 15 minutes.
However to remain pointed at a single goal, it makes use of one other expertise: gyroscopes.
NASA’s Hubble House Telescope. Credit: NASA
There are 6 gyroscopes on Hubble which spin at 19,200 revolutions per minute. They’re massive, large and spin so quick that their inertia resists any modifications to the telescope’s orientation. It really works greatest with three – matching the three dimensions of house – however can function with two, and even one, with much less correct outcomes.
In August, 2005, Hubble’s gyroscopes have been sporting down, and NASA shifted into two-gyroscope mode. In 2009, throughout Servicing Mission four, NASA astronauts visited the house telescope and changed all six of its gyroscopes.
STS61 was the primary servicing mission to the Hubble House Telescope. Credit score: NASA
That is seemingly the ultimate time astronauts will ever go to Hubble, and its future is dependent upon how lengthy these gyroscopes final.
What About James Webb?
I do know the mere point out of the James Webb House Telescope makes everybody nervous. Greater than $eight billion invested thus far and due for launch in about two years from now. It’s going to be flying to the Earth-Solar L2 Lagrange level, positioned about 1.5 million kilometers away from Earth.
Illustration of NASA’s James Webb House Telescope. Credit: NASA
In contrast to Hubble, there’s no approach to fly out the James Webb to restore it if something goes flawed. And seeing how typically gyroscopes have failed, this actually does look like a harmful weak level. What if James Webb’s gyros fail? How can we exchange them.
James Webb does have response wheels on board. They’re constructed by Rockwell Collins Deutschland, they usually’re just like the reactions wheels on board NASA’s Chandra, EOS Aqua and Aura missions – so a special expertise from the failed response wheels on Daybreak and Kepler. The Aura mission offered a scare in 2016 when one in every of its response wheels spun down, nevertheless it was recovered after ten days.
The Hemispherical Resonator Gyroscope (HRG), additionally known as wine-glass gyroscope or mushroom gyro, makes utilizing a skinny solid-state hemispherical shell, anchored by a thick stem. Credit score: Sagem CC BY four.zero
James Webb isn’t utilizing mechanical gyroscopes like Hubble to maintain it on the right track. As a substitute, it’s utilizing a special expertise known as hemispherical resonator gyros, or HRGs.
These use a quartz hemisphere that has been formed very exactly in order that it resonates in a really predictable method. The hemisphere is surrounded by electrodes that drive the resonance, but additionally detect any slight modifications in its orientation.
I do know that form of feels like gibberish, prefer it’s powered by unicorn desires, however you possibly can expertise this for your self.
Maintain a wineglass after which flick it along with your finger in order that it’s ringing. The ringing is the wineglass flexing forwards and backwards at its resonance frequency. As you rotate the glass, the flexing forwards and backwards turns as properly, nevertheless it lags behind the orientation in a really predictable method.
When these oscillations are occurring 1000’s of occasions a second in a quartz crystal, it’s potential to detect tiny motions after which account for them.
That’s how James Webb will keep locked on its targets.
Artist’s impression of the Cassini orbiter getting into Saturn’s ambiance. Credit score: NASA/JPL
This expertise has flown on the Cassini mission at Saturn and labored completely. Actually, as of June 2011, NASA had reported that these devices had skilled 18 million hours of steady operation in house on greater than 125 completely different spacecraft with no single failure. It’s truly very dependable.
I hope that clears issues up. Response or momentum wheels are used to re-orient spacecraft in house, to allow them to face in numerous instructions with out utilizing propellant.
Gyroscopes are used to maintain an area telescope precisely pointed at a goal, to offer the most effective scientific knowledge. They are often mechanical spinning wheels, or they use the resonance of vibrating crystals to detect modifications in inertia.