MISSION PLAN - PDS Small Bodies Node

6.2.5 Nucleus Tracking
Given the delivery uncertainty and associated probabilities described in the previous
section, imaging of Wild-2 could be accommodated adequately until E-3 minutes.
However, in order to capture the highest resolution images, obtainable only after E-3
minutes, some action must be taken. As mentioned above, mirror control, centroiding
and the roll maneuver are used to maintain the nucleus within the camera field-of-view.
Centroiding uses selected images (maximum of once every 10 seconds which is
equivalent to every other image after E-4 minutes) and a 1-D algorithm to compute the
mirror correction required to place the comet back into the center of the field-of-view of
the camera. Designed to provide 180° in-plane tracking of Wild-2, this mirror slewing
will compensate for the in-plane and time-of-flight delivery errors. Mirror control and
centroiding are planned to start at E-50 minutes when the comet nucleus is about 3x3
pixels in size. Centroiding is planned to end at closest approach while mirror control is
planned to end at E+20 minutes to support Post Encounter “fill memory” imaging.
To correct the out-of-plane delivery error, a correction must be made along the spacecraft
y-axis. A roll maneuver about the x-axis is planned only once and implemented only if
and when the nucleus image is expected to escape the camera field-of-view. In order to
maintain a real-time telemetry link as long as possible into the encounter, it is desirable to
have this maneuver not occur until absolutely necessary. Current estimates place the
execution of this maneuver between E-4 to -3 minutes. The maneuver is expected to take
no more than a minute to implement including time for attitude stabilization. Figure 6.2-
4 shows the expected roll angle as a function of out-of-plane error and flyby distance. If
a roll maneuver is performed, once the encounter is over, at about E+3 minutes, a
subsequent roll maneuver is performed in order to re-establish communications as soon
as possible.
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