It’s superb to assume there are telescopes up in area, proper now, directing their stare upon distant objects for hours, days and even weeks. Offering a perspective so steady and correct that we are able to 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 gasoline.
It’s all because of the expertise of response wheels and gyroscopes. Let’s speak about how they work, how they’re totally different, and the way their failure has ended missions prior to now.
Right here’s the fast reply. Response wheels enable spacecraft to vary their orientation in area, whereas gyroscopes preserve a telescope extremely steady, to allow them to level at a goal with excessive accuracy.
Should you’ve listened to sufficient episodes of Astronomy Solid, you recognize I all the time complain about response wheels. It all the time appears to be the purpose of failure on missions, ending them prematurely earlier than the science is all in.
I’ve most likely used the phrases response wheels and gyroscopes interchangeably prior to now, however they serve barely totally different functions.
One of many excessive efficiency response wheels carried by Kepler, Daybreak and several other different NASA missions. Credit score: Ball Aerospace
First, let’s speak about response wheels. These are a sort of flywheel used to vary the orientation of a spacecraft. Take into consideration an area telescope that should change from goal to focus on, or a spacecraft that should flip itself again to Earth to speak information.
They’re often known as momentum wheels.
There’s no air resistance in area. When a wheel turns in a single route the complete telescope turns in the wrong way, because of Newton’s Third Legislation – you recognize, for each motion, there’s an equal and reverse response. With wheels spinning in all three instructions, you’ll be able to flip the telescope in any route you want.
The wheels are fastened in place and spin between 1,000 and four,000 revolutions per minute, increase angular momentum within the spacecraft. To be able to change the orientation of the spacecraft, they alter the speed at which the wheels are spinning.
Format of the Kepler Area 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 obtained sufficient rotors spinning, you’ll be able to 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 Area Telescope.
With three wheels, you’ll be able to 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 boost its orbit by daylight alone.
Picture of LightSail 2’s sail deployment. Credit score: The Planetary Society
In fact, 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 Resolution
Most famously, NASA’s Kepler Area Telescope, launched on March 9, 2009 to seek out planets orbiting different stars. Kepler was outfitted with four response wheels. Three have been essential to preserve the telescope pointed fastidiously 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 gasoline is being reserved to ensure its information makes it residence. Picture: NASA/Kepler
It was looking forward to any star in its area 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 really solely transmitted details about the change in brightness of the celebrities themselves.
In July, 2012, one among 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 now not keep its place precisely sufficient to trace star brightness..
Though the mission might have been a failure, engineers discovered an ingenious technique, utilizing the sunshine strain from the Solar to behave as a drive in a single axis. By completely balancing the spacecraft within the daylight, they have been capable of proceed utilizing the opposite two response wheels to proceed making observations.
Infographic displaying how the Kepler area telescope continued looking for planets regardless of two busted response wheels. Credit score: NASA Ames/W Stenzel
However Kepler was compelled 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 information. Kepler lastly ran out of gasoline 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 biggest asteroids within the Photo voltaic System: Vesta and Ceres. The spacecraft went into orbit round Vesta in July, 2011 and spent the subsequent 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 an increasing number of friction in one among 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 area 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. High is a reconstruction of the volcano from topographical information, the picture on the left was captured by Daybreak’s framing digital camera, and the false colour picture on the proper 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 capable of map out this world, and its weird floor options. Lastly, in late 2018, the spacecraft was out of propellant, and it was now not capable of keep its orientation, to map Ceres or to ship its alerts again to Earth.
The spacecraft will proceed to orbit Ceres, tumbling helplessly.
There’s a protracted checklist 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 area itself is inflicting the issue. As geomagnetic storms move the spacecraft, they generate fees on the response wheels that trigger a rise in friction and make them put on down extra shortly.
I’ll put a hyperlink to a fantastic video by Scott Manley that goes into extra element.
Hubble Area Telescope and its Gyroscopes
The Hubble Area Telescope is supplied 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 Area 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 area – however can function with two, and even one, with much less correct outcomes.
In August, 2005, Hubble’s gyroscopes have been carrying down, and NASA shifted into two-gyroscope mode. In 2009, throughout Servicing Mission four, NASA astronauts visited the area telescope and changed all six of its gyroscopes.
STS61 was the primary servicing mission to the Hubble Area Telescope. Credit score: NASA
That is doubtless the ultimate time astronauts will ever go to Hubble, and its future depends upon how lengthy these gyroscopes final.
What About James Webb?
I do know the mere point out of the James Webb Area Telescope makes everybody nervous. Greater than $eight billion dollars invested to date and due for launch in about two years from now. It’s going to be flying to the Earth-Solar L2 Lagrange level, situated about 1.5 million kilometers away from Earth.
Illustration of NASA’s James Webb Area Telescope. Credit: NASA
Not like Hubble, there’s no solution to fly out the James Webb to restore it if something goes flawed. And seeing how usually gyroscopes have failed, this actually does appear to be a harmful weak level. What if James Webb’s gyros fail? How can we substitute them.
James Webb does have response wheels on board. They’re constructed by Rockwell Collins Deutschland, they usually’re much 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 supplied a scare in 2016 when one among its response wheels spun down, however 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 track. As an alternative, 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 approach. The hemisphere is surrounded by electrodes that drive the resonance, but additionally detect any slight modifications in its orientation.
I do know that type of appears like gibberish, prefer it’s powered by unicorn goals, however you’ll be able to expertise this for your self.
Maintain a wineglass after which flick it together with your finger in order that it’s ringing. The ringing is the wineglass flexing backwards and forwards at its resonance frequency. As you rotate the glass, the flexing backwards and forwards turns as properly, however it lags behind the orientation in a really predictable approach.
When these oscillations are occurring hundreds 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 coming into Saturn’s ambiance. Credit score: NASA/JPL
This expertise has flown on the Cassini mission at Saturn and labored completely. In truth, as of June 2011, NASA had reported that these devices had skilled 18 million hours of steady operation in area on greater than 125 totally different spacecraft with out a single failure. It’s really very dependable.
I hope that clears issues up. Response or momentum wheels are used to re-orient spacecraft in area, 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 supply one of the best scientific information. They are often mechanical spinning wheels, or they use the resonance of vibrating crystals to detect modifications in inertia.