Why would Kepler’s pointing problems be minimized if aimed within the ecliptic?

Question

Having read a few articles on the suggested new uses for the crippled Kepler space observatory with only two out of four functioning reaction wheels, several ideas suggest that Kepler’s pointing problems would be minimized, if it aimed its camera within the ecliptic plane.

   

   ^{What’s Next For Kepler? is an Astrobites article listing most interesting proposals to repurpose the Kepler telescope.}

One of the ideas advocating Kepler's use for the target objects within the ecliptic is this proposal for A Survey of the Ecliptic Plane For Transiting Planets and Star Formation (PDF), that mentions:

This White Paper calls for the use of Kepler to conduct a survey in the ecliptic plane to search for planet transits around stars at high galactic latitudes and to study star forming regions to investigate physics of very young stars not studied by Kepler in its prime mission. Recent analysis by the Kepler project indicates that the spacecraft's best pointing will be possible in the ecliptic plane.

The proposal seems otherwise sound enough. Searching for planet transits was within the Kepler's tasks anyway, with its original goal to determine the frequency of Earth-like planets around Sun-like stars and its wide-angle view, albeit monitoring them for years, instead of what would now be possible perhaps for weeks.

But I couldn't find any other reference as to why Kepler would be easier to control when pointing towards targets within the ecliptic plane. This seems to me rather non intuitive. What are the reasons behind this claim? Have any tests been performed on the current Kepler's attitude control capabilities that would officially support this theory?

Answer 1

Kepler has four reaction wheels positioned in such a way that the attitude control system is tolerant to one wheel failure (i.e. if one wheel fails, all three axes are still controllable). However, two of Kepler's wheels have failed, and so the need for off-nominal operations.

Ball Aerospace, the prime contractor for Kepler, performed a preliminary study on two-wheel operations, the results of which are part of the call for white papers for alternative missions. This supplement does a good job of pulling out the most relevant points:

3) The reaction wheels can maintain pointing for 4 days while absorbing solar torque momentum in the Y & Z axes. This establishes a 4-day cycle for managing wheel momentum, although more frequent management is allowable.

4) Momentum in the X axis must be absorbed through spacecraft roll. Low drift about the boresight limits the sun to the XY-plane.

5) Maximum pointing stability is achieved when the spacecraft is pointed in the velocity- or anti-velocity-vectors, where the sun remains in the XY-plane throughout the 4-day period.

Here is the body axis system they're talking about (from the call for white papers):

So basically, any torque in the X axis will need to be absorbed. Therefore, to minimize the amount of roll, you want to balance the torque in that axis (i.e. keep the sun in the XY-plane, since solar radiation pressure is the main source of attitude disturbance). In order to do that, they recommend that the pointing target is in the ecliptic plane, since the sun essentially remains in that XY-plane for the entire imaging period (up to 4 days). This plot from the supplement shows the drift due to solar radiation pressure over a 4 day period:

As you can see, they expect relatively little drift in the roll axis with the right attitude. The paper has plots like this for other attitude profiles, and they exhibit a much faster drift.

Combine the "sun in the XY-plane" requirement with the fact that you need sun on the solar arrays, and you have your optimal attitude defined as along either the velocity vector or the anti-velocity vector, both of which are in the ecliptic plane.