Hacked Kepler Continues to Amaze
Sun’s stroke keeps Kepler online, Nature
“Wiemer had fashioned a crutch for Kepler using the only resource available: sunlight. Positioned so that its long side faces the Sun, the spacecraft leans against the pressure created by the onslaught of photons and balances using its two good wheels. With this approach, the team hoped to get within a factor of ten of Kepler’s original performance — but with additional software refinements, NASA’s Kepler project manager Charlie Sobeck says that it is better than that, more like a factor of two or three. Wiemer thinks that further tweaks will close the gap entirely. One limitation of the K2 mission is that Kepler must keep the Sun side-on as it orbits, forcing the telescope to switch its field of view roughly every 80 days. This is not enough time to hunt for Earth-like planets around Sun-like stars, but it does let K2 track other celestial bodies such as clusters of newly-formed stars.”
Sweet! But I hope we someday learn how to make reliable reaction wheels.
Actually, reaction wheels are pretty reliable. I can think of some which have clocked up about five billion rotations over the course of almost two decades, which seems pretty impressive for a moving part which hasn’t seen any maintenance. A quick check on the numbers tells me that five billion rotations on a car’s wheels would amount to driving 6.4 million miles. A car that could do that without any maintenance would be considered extremely reliable.
Or engine rotations. National statistics (e.g., http://nhts.ornl.gov/2009/p… suggest an hour per day of driving. Very roughly figure a mean of 2000 rpm (depends on ratio of idling to driving). That’s only 120,000 revolutions per day. It would take something over a hundred years to accumulate five billion revolutions.
A better example, though, of a device that has to keep rotating, but in a cleaner and more controlled environment, is a hard disc drive. Figure 7200 rpm (mid-grade enterprise drive), running continuously. That 3.8 billion revolutions in a year. We typically expect these sorts of drives to hold up for 3-5 years. So that’s definitely in the same general regime as reaction wheels.
From what I understand, the life span of recent generations of reaction wheels is fairly strongly dependent on details of how they are used. I was surprised to learn about how much engineering discretion is involved in controlling, for instance, the range of rotation speeds that are called for under a spacecraft’s control laws.
That machine is not even as amazing as the guys that got the bird flying again. I only know a few by name, so won’t mention any, but you know who you are. YOU are the amazing ones.
Period.
NASA engineers always perform best in a crisis. Apollo 13, Skylab, Hubble fixes are few examples that come to mind.
This sounds like a solution they would come up with in a Star Trek episode to get around some problem 🙂