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NASA's Kepler Enters Emergency Mode 75 Million Miles From Earth (theverge.com)
Loren Grush, writing for The Verge: NASA engineers have declared a mission emergency for the agency's planet-hunting spacecraft Kepler, which has somehow switched into emergency mode. Now that a mission emergency has been declared, the Kepler team has priority access to NASA's deep space telecommunications system in order to try to get the spacecraft back to normal operations. Emergency mode is the lowest operational mode the spacecraft has. It also requires a lot more fuel than usual, which is why the Kepler mission team is working hard to get the spacecraft back to normal. But communication with Kepler isn't easy. The spacecraft is estimated to be 75 million miles away from Earth right now, according to NASA, so any communications signal traveling at the speed of light will take up to 13 minutes to travel to and from the spacecraft. Kepler has detected nearly 5,000 exoplanets over the years -- of which 1,000 have been confirmed.
You know, if you say "is", you really don't have to say "currently" or "right now", much less both.
Safe mode probably use thrusters instead of reaction wheel to control the attitude. Reaction wheel works with electricity (from solar array, so "unlimited"), thrusters use fuel.
A gyro to rotate you, not sense your rotation - here are the Kepler reaction wheels. (It has already had 2 reaction wheel failures.)
Note: Inertial sensing gyros are generally gimbaled, so they can stay rotating in the same direction. Reaction wheels are generally fix mounted, and are spun up and down as needed to get the desired attitude (or rate of rotation). If the spacecraft is being torqued by something (say, a small gas leak), the wheels will spin faster and faster to soak up the excess angular momentum until they reach their design limit, and have to be despun. This is called a momentum dump, and requires some other system (i.e., thrusters) to finally get rid of the excess angular momentum.
Given that we know that Kepler has a bad batch of reaction wheels, that two of the four have already failed, and that this emergency mode happened while the spacecraft was being repointed to the Galactic Center for a microlensing campaign, which inevitably would mean a lot of reaction wheel use, I very much fear that this means that another reaction wheel has failed and the K2 mission is over.
As of Sunday morning, the Emergency Mode was resolved; and the spacecraft was returned to normal mode. https://www.nasa.gov/feature/m...
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