As has been reported this week, the Kepler planet hunting space telescope, may have to end its mission earlier than hoped, due to the failure of the system that keeps it pointed in the right direction. That system consists of four reaction wheels, which are basically electric motors attached to fly wheels. By speeding up or slowing down, they transfer angular moment to the satellite, rotating it around its center of mass.
Kepler’s mission is find exoplanets by staring, unmoving, at small patches of space and look for periodic dips in the brightness of the stars there. Those dips could mean the presence of planets. But without at least three working reaction wheels—Kepler is down to two—the satellite can’t steer it’s gaze or keep it from gently drifting in the solar wind.
According to David Cooper, CEO of Microsat Systems Canada Inc., in Ottawa, Ont., a provider of reaction wheels for small satellites, there are two main classes of things that can go wrong with reaction wheels—mechanical and electrical. And that means Kepler's pointing system was probably damaged either by the shock of launch or in space by radiation.
But first a bit of background: Reaction wheels are mounted on a satellite to transfer some of their torque, turning the satellite through its center of mass along each of three axes. Kepler and other spacecraft have a fourth wheel, explains Cooper, mounted at an angle to these axes that allows that wheel to partly make up for the loss of one of the others.
The more massive the satellite, the more massive the reaction wheels must be. Kepler weighs in at just over one metric ton. Honeywell and only a few other companies make reaction wheels massive enough for that job, says Cooper. MSCI focuses on small satellites, so its largest wheel can handle a craft only half as massive. “But the principle is the same,” he says. In fact Kepler’s mass can be an asset, because once the satellite is pointing in the right direction it’s harder to make it drift.
Also in its favor is that Kepler is not in Earth orbit. The reaction wheels in most earth-orbiting satellites are fighting to hold the craft’s position against tugs from the Moon’s gravity and differences in the density of the earth below. “We tend to think of the Earth as homogenous, but it’s not,” he says. Kepler, which trails the Earth through the solar system only has to contend with the sun.
So what can cause a reaction wheel (or two) to fail?
Launch trauma can lead to mechanical problems, according to Cooper. Both the wheel and the motor that drives it have bearings that can be damaged by the g-forces and vibrations of being hurled into space. “The most difficult load for the bearings is during the launch itself,” he says. “Once they get into orbit they don’t often fail. It’s possible, depending on the type of bearing, but if they survived launch they should be OK. It all depends on how rough the ride is.”
“Designing a reaction wheel to survive the harsh environment of a launch is very difficult,” he says. Mechanical engineers have to account for the shock, acoustics, and random and sinusoidal vibrations of the rocket. To keep the systems safe, the wheels are usually not spinning during launch. But some systems require a spinning wheel on the ride into space, and those must be protected in other ways, such as a mechanism in the wheel to isolate the shaft from the bearings during launch.
Once a wheel is in space it’s more likely to suffer an electronic ailment than a mechanical one, says Cooper. Radiation is the big problem here. An unusually energetic particle can knock out an individual component (called sudden event burn out), or damage from the steady dribble of lesser-powered particles can accumulate and cause a failure. If Kepler’s having electronic trouble, Cooper would guess it’s from the latter. “It looks to me that since they’ve lost two wheels, they have a component that’s weak” to accumulated radiation, he says. (Recall that a memory chip that couldn’t handle the right dose of rads was blamed for the loss of the Russian Mars probe Phobos-Grunt.)
Cooper’s company prides itself on understanding the effects of radiation on electronics. Apart from supplying reaction wheels (which house their own controllers that have been hardened to radiation), MSCI has been operating Canada’s MOST satellite for nearly ten years. MOST, knicknamed the “Humble Space Telescope”, works similarly to Kepler but at a much smaller scale. Operating it “gives an idea of how electronics degrade over time,” he says.
Some radiation damage is easy to work around, Cooper points out. If the damage is to a memory chip, often rebooting the system will cause the controlling computer to notice the problem and avoid using the bad memory addresses.
But there are potentially worse problems. The wheel’s computer, which can be separate or housed with the machine can check the wheel’s state by measuring a number of parameters, but if there is damage to the system that communicates these parameters to ground controllers, coming up with a fix would be difficult indeed. Controllers and the wheel’s designers would have to figure out an alternate communication pathway to diagnose the wheel.
What are the odds that Kepler will make a comeback? “It really depends on the diagnosis,” says Cooper.
Photo: Detlev van Ravenswaay/Getty Images
Samuel K. Moore is the senior editor at IEEE Spectrum in charge of semiconductors coverage. An IEEE member, he has a bachelor's degree in biomedical engineering from Brown University and a master's degree in journalism from New York University.