Over the past few decades, numerous space probes sent to the far-flung reaches of the Solar System have fallen silent. These failures weren’t due to communications problems, probes flying into scientifically implausible anomalies, or little green men snatching up the robotic scouts we’ve sent out into the Solar System. No, these space probes have failed simply because engineers on Earth can’t point them. If you lose attitude control, you lose the ability to point a transmitter at Earth. If you’re managing a space telescope, losing the ability to point a spacecraft turns a valuable piece of scientific equipment into a worthless, spinning pile of junk.
The reasons for these failures is difficult to pin down, but now a few people have an idea. Failures of the Kepler, Dawn, Hayabusa, and FUSE space probes were due to failures of the reaction wheels in the spacecraft. These failures, in turn, were caused by space weather. Specifically, coronal mass ejections from the Sun. How did this research come about, and what does it mean for future missions to deep space?
What Is A Reaction Wheel?
A reaction wheel is important to any space mission: it’s the main method nearly every space probe uses to orient itself. It’s how a space telescope points at a star, and it’s how a Martian lander makes sure the heat shield is pointed towards the atmosphere before reentry. But how does it do this?
A reaction wheel is really just a flywheel — a heavy, spinning disk — that normally rotates at a few hundred to a few thousand RPM. This flywheel stores angular momentum, and changing this rotation speed imparts a tiny bit of torque around the axis of the flywheel. Think of it as a very advanced version of the infamous introduction to physics experiment where a volunteer sits on a spinny chair, holding a spinning bicycle wheel. The bicycle wheel stores angular momentum, and by tilting the wheel the volunteer can spin in their chair. No, it’s not a perfect representation of a reaction wheel, because the flywheels in a satellite don’t tilt, but the idea is the same: controlling a flywheel means you can spin. Do it on a satellite in microgravity, and you can spin the entire spacecraft.
Of course, reaction wheels aren’t the only way a spacecraft can orient itself. Satellites flying in Low Earth Orbit (or any magnetosphere, really), can use magnetorquers, or long electromagnets, to orient themselves within a magnetic field. The cubesats from several universities, in fact, use magnetotorquers for all their attitude control requirements. But magnetotorquers don’t work outside of a magnetosphere, and in deep space, space probes will also use thrusters in conjunction with reaction wheels. Using two methods of attitude control is a necessity for any deep space mission. Not only can one method fail, but reaction wheels can become ‘oversaturated’, or have the flywheel spin at the limits of its bearings. If this happens, the flywheel would need to be de-spun, which means thrusters must provide an opposing torque to keep the spacecraft from spinning out of control.
Despite these limitations, reaction wheels really are the best way to orient a small spacecraft. They only require electricity to keep them spinning, which is abundant thanks to solar panels. Using thrusters as the sole means of orientation uses valuable fuel, and the mass of that fuel would be better spent on sensors and experiments anyway.
Failures of Reaction Wheels
Reaction wheels are found in nearly every spacecraft, and over the last few decades there have been a few notable failures. The Hubble space telescope was suffering from failures of multiple reaction wheels before a servicing mission saved the space telescope in 1997. This was vitally important, because without a full suite of reaction wheels, the Hubble could not point at anything; an impressive failure for a telescope. Before the servicing mission, the problem was solved to an extent by using two of the remaining reaction wheels and magnetotorquers. With the replacement of the reaction wheels, Hubble happily continued exploring the cosmos.
Beyond Earth orbit, there have been numerous failures of reaction wheels in space. The Dawn mission to Vesta and Ceres is still continuing, but while this space probe was in orbit of Vesta, it suffered a loss of its reaction wheels. The cause of the failure was excessive friction in the bearings, and while engineers managed to get Dawn to Ceres, it wasn’t easy. A combination of the remaining reaction wheels and ion engines did allow Dawn to travel to Ceres, but it couldn’t even do that with its main antenna pointing at Earth.
In the summer of 2005, the Hayabusa spacecraft was cruising towards an asteroid named Itokawa, when a reaction wheel controlling its X axis failed. As Hayabusa had redundant reaction wheels, the mission continued towards the asteroid until September of 2005 when it assumed a 7 km orbit around its target asteroid. Just days after achieving this orbit, a second reaction wheel failed, this time controlling the Y axis. Despite this, Hayabusa landed on Itokawa momentarily, collected a small sample, and returned to Earth, sending a recovery capsule to land in the Australian desert.
While not launched into deep space, the FUSE spacecraft — the Far Ultraviolet Spectroscopic Explorer — was launched into orbit in 1999, and designed as a telescope for the far ultraviolet portion of the spectrum. This satellite was launched into a 760 km orbit around Earth (still close enough that magnetotorquers were also used in the design). After several years, two of the four reaction wheels showed a sudden increase in friction and stopped spinning. Engineers managed to update the software to use the two remaining wheels and magnetotorquers, allowing the mission to continue for several years but in 2007, the last two wheels failed, bringing an end to the mission.
But perhaps the greatest failure of reaction wheels in space is that of the Kepler spacecraft. This spacecraft was designed to sit in deep space and look at a small speck of the cosmos for planets passing in front of stars. If there was ever a mission that required accurate pointing and redundant reaction wheels, Kepler is it. Launched in 2009, the planet-finding mission was at first expected to last until 2016. This changed in the summer of 2012, when one of the four reaction wheels failed. Less than a year later, a second reaction wheel failed, causing the cancellation of the primary mission.
While the Dawn, Hayabusa, FUSE, and Kepler missions were saved by Apollo 13-level engineering heroics, all of these missions have another thing in common. The reaction wheels were all manufactured by Ithaco Space Systems. Sounds like a great opportunity for a root cause analysis, doesn’t it?
Finding The Failures In Reaction Wheels
Simply due to their nature as space probes, we’ll probably never recover the reaction wheels from Dawn or Kepler. The reaction wheels from Hayabusa were a fine mist over the Australian desert before they weren’t anymore. So, how do we figure out how these reaction wheels failed? That’s exactly what one researcher did, and the evidence is intriguing.
The failures of these reaction wheels can be traced to one problem: friction in the bearings which allow these flywheels to spin at thousands of rotations per minute. When the motor inside the reaction wheel can’t overcome the friction of the bearings, the reaction wheel has failed. But what would cause this in deep space, millions of miles away from Earth?
As it turns out, the failures of reaction wheels, specifically on the FUSE and Kepler missions, was correlated with space weather, specifically coronal mass ejections (CMEs) from the Sun. These CMEs induce a voltage across the bearings and the bearing races in the reaction wheels, and after testing, these researchers discovered it doesn’t take much to generate a small arc from the bearing to the bearing race. This arc causes a small bit of pitting, which over time increases the friction on the reaction wheels, eventually causing it to fail.
Solving Bearing Friction Failure
While the researchers provide the experimental data showing that bearings can fail due to a voltage across a bearing and a race, it’s unlikely we’ll ever be able to prove Kepler, FUSE, or Dawn suffered a reaction wheel failure because of space weather. To recover these space probes for an inspection of the bearing races we need to wait for the development of impulse drives or space pirates on a salvage mission.
The good news is the failures of these reaction wheels may be in the past. Ithaco Space Systems has since changed from metal ball bearings to ceramic bearings in their reaction wheels, greatly reducing the chance of arcing across the bearing races. While we might never know for sure if these reaction wheels failed due to space weather, the problem, at least in these Ithaco reaction wheels, is solved.
Read more: hackaday.com