MISSION PLAN - PDS Small Bodies Node

6.0 Wild-2 Encounter Phase (E-100 to E+50 days)
6.1 Overview
The most important phase of the STARDUST mission nominally starts 100 days prior to
and ends 50 days after comet encounter (L+1790.9 days). DSM-3, completed during the
Cruise-2 (Earth-Wild 2) phase, at encounter minus 180 days (E-180 d), is the first aim at
Wild-2. Knowledge of the orbital state of Wild-2 at this time will still be based on
ground observations which will have an estimated position uncertainty of about 1520 km
(1-σ). A better estimate of the comet ephemeris is expected at about E-50 d after optical
navigation (OPNAV) has been in operation for some time. (See Navigation Plan
document, SD-76000-100, for more details). Independent of launch date, encounter with
the comet Wild-2 occurs at a reference time of 02 January 2004 19:20:00 ET.
The primary goal of obtaining comet coma samples during the encounter flyby is
accomplished by a navigation plan that delivers the spacecraft with the required
accuracy. It is also necessary to assure the survival of the spacecraft at a reasonable level
of confidence. The spacecraft encounters Wild-2 at 98.5 days past perihelion. At this
point of its orbit, Wild-2 is far from its “peak” active period and it should be relatively
safe for a 150 km closest approach. A risk analysis based on a probabilistic model of the
Wild-2 dust environment is required, however, this exercise is regarded as preliminary
until a time when a better model may be constructed upon approach to the comet. The
current model is based on best estimates. Adjustment of the encounter parameters (flyby
date and distance) is possible if required by mission risk issues. Given the current dust
models, these adjustments are possible without penalizing mission performance.
This mission phase proceeds from an initially slow pace (scale of days) and progresses
gradually to an extremely fast pace (minutes/seconds) centered around the closest
encounter. In order to organize this mission phase in a more orderly way, it is divided
into four subphases: Far Encounter, Near Encounter, Close Encounter and Post
Encounter. The Far Encounter subphase focuses on acquisition of comet and coma
science data. The Near Encounter subphase emphasizes terminal navigation and high
resolution coma and nucleus activities. The Close Encounter subphase is the core science
period of STARDUST. It contains comet dust sample collection and high resolution
nucleus imaging. Finally, the Post Encounter subphase is dedicated to mission
performance assessment and return of stored science data. Five TCM’s are performed on
approach to the comet. The final TCM before encounter is performed at 6 hours from
closest approach and is expected to be implemented only in case of a contingency. These
subphases are further defined in Table 6.1-1.
Encounter activity is not scheduled to start until the spacecraft exits from the preencounter
solar conjunction. For planning purposes, ‘exit’ has been defined as when the
Sun-Earth-Probe angle reaches 3 degrees (increasing). On the current trajectory, this
occurs 88 days prior to closest approach. The encounter phase definition, however, is left
at E-100 days as operational experience may allow a reduction of the 3 degree SEP angle
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