Dawn Spacecraft 3rd Reaction Wheel Malfunctions
Dawn Observing Ceres; 3rd Reaction Wheel Malfunctions, NASA JPL
“While preparing for this observation, one of Dawn’s two remaining reaction wheels stopped functioning on April 23. By electrically changing the speed at which these gyroscope-like devices spin, Dawn controls its orientation in the zero-gravity, frictionless conditions of space.”
“The team discovered the situation during a scheduled communications session on April 24, diagnosed the problem, and returned the spacecraft to its standard flight configuration, still with hydrazine control, on April 25. The failure occurred after Dawn completed its five-hour segment of ion thrusting on April 22 to adjust its orbit, but before the shorter maneuver scheduled for April 23-24. The orbit will still allow Dawn to perform its opposition measurements. The reaction wheel’s malfunctioning will not significantly impact the rest of the extended mission at Ceres.”
For reference, Deep Space One ran out of fuel for its attitude control thrusters, and managed to continue. They did it by running its electric propulsion system at low thrust and gimbals rotated to the limits, to provide torque for attitude control. Dawn is using more-or-less the same ion propulsion system and has the same chief engineer. I am not worried since I have enough confidence in both the hardware and the people involved.
Well, you would know.
A question: in using the high-gain antenna as a shield when going through the risky spots, is there any type of sensor on the spacecraft that yields usable data about that environment? Maybe about the size, or density, or mass of any encountered particles?
That’s Cassini, but I believe at least the plasma wave instrument provides evidence of impacts. Loss of signal is also suspicious…
“The spacecraft also determines where and how much dust is orbiting between Saturn’s moons and rings, and it does so by touch. “We’re talking dust particles the size of those in cigarette smoke, but there’s enough kinetic energy when that dust hits the spacecraft that it destroys the dust particle and a little part of the spacecraft.” The potent but miniscule collisions produce tiny puffs of plasma, which the instrument detects.”
I assume you are thinking of Cassini. Yes, Cassini (and many other spacecraft) can measure dust impacts, even without an instrument designed to do so. A dust impact produces a small puff of ionized gas, and that causes the spacecraft’s bus voltage (ground) to twitch. Nothing off spec or dangerous to the spacecraft, but enough to be detected. Specifically, a plasma wave instrument (basically a specialized and very sensitive dipole radio antenna) sees a voltage spike. You can count dust impacts by programming the software to count spikes. That’s been done on many spacecraft, starting with Voyager. The most recent I’m aware of is MAVEN at Mars. In the case of Cassini, the spacecraft also has an instrument designed to measure dust particles (the Cosmic Dust Analyzer) and the plasma wave results have been cross-calibrated. The plasma wave measurements aren’t as good in many ways (CDA provides composition, mass and velocity of the dust particles), but it is sensitive to impacts anywhere on the spacecraft and you get data regardless of spacecraft orientation (e.g. in the protective high gain antenna to ram attitude.)
Yes, Cassini of course, thanks for the run down. I wonder what was found; I suppose it will be mentioned in the weekly reports or somewhere else at some point.
You didn’t have to wait long.
https://www.jpl.nasa.gov/ne…
It seems the gap between the D ring and the atmosphere is pretty empty.
Cassini is also supposed to have a dust particle counter made by the Univ. of Chicago. I integrated it’s sister instrument on Stardust. It was basically a stretched membrane whose capacitance changed if a particle punctured the membrane. I believe the Cassini version was similar.
I think it’s three membranes, not one, and one was (ironically) damaged by a particle impact. But technically, that’s not the instrument. The Cosmic Dust Analyzer, like many Cassini instruments, comprises several sensors. You’re describing the High Rate Detector. The instrument overall, and the other and sometimes considered the “main” sensor, are out of Max Planck Institute (fur Kernphysik, if you care) and the PI is currently at the University of Stuttgart, if memory serves. That sensor doesn’t deal well with high impact rates (over one every few seconds) but gives the size, velocity, charge and (sometimes) composition of the particles. Unfortunately, neither can observe impacts in the safe, “high gain antenna to ram” orientation. That takes a more indirect measurement by the plasma wave instrument.
I understand about the CDA, as I also integrated the CIDA on stardust. The PI was Dr. Jochen Kissel and the I&T manager was the late Dr. Hanna Von Hoerner (I used to get Xmas cards from her every year, I forget her cat’s name). The instrument with the membranes on it was called the Dust Flux Monitor or DFM on Stardust. I met Tony Tuzzolino himself while doing the functional testing. He reminded me of “Father Guido Sarduchi” from old-school SNL.