MISSION PLAN - PDS Small Bodies Node

The proposed 2 km nucleus radius is based on the yet to be published, eight years of data
on P/Wild-2 (provided by Dr. Karen Meech) which was gathered using telescopes up and
including the world’s largest, the 10m Keck reflector. These data easily show that Wild-
2 is active out to 4.5 AU both pre- and post-perihelion. There are 13 first quality points
(positive identification, no evidence of clouds, proper subtraction of other stars in the
reduction aperture, etc.) taken at heliocentric distances greater than 4.5 AU. Ray
Newburn has reduced these to a radius by using a geometric albedo of 0.04 and a phase
function of 0.035 mag/deg. By grouping the data points (averaging points taken within
30 minutes of each other), he finds the following:
Table 2.4-3 P/Wild-2 Nucleus Radius Observation Reduction
Heliocentric Distance (AU) No. of points Mean Radius (km)
4.688 1 2.384
4.764 2 2.009
4.769 4 2.117
4.974 3 1.898
5.025 3 2.037
Source: Private communication from R. Newburn
Note that the single point nearest to the sun, at 4.688 AU, results in the largest radius.
This could be a rotational or statistical effect, but it may also indicate residual activity.
In any case, the average of the remaining 12 points is 2.015 km. The variation from
1.900 to 2.100 km is probably a rotational effect.
It is anticipated that there will be a separation between the center-of-brightness and the
center-of-mass of the comet. An 800 km separation was seen for Halley at 1 AU, but for
an order of magnitude less active comet such as Wild-2, much less separation (~100 km
or less) is suspected. However, photometric model data appears to indicate that the
difficulty for navigation or imaging may be removed at about 10 million km, by imaging
the comet in continuum.
2.4.2.3 Dust Environment
The current best estimates of the dust flux at comet closest approach and the dust fluence
referred to a solar distance of 1.86 AU at TP+98.5 days are shown in Table 2.4-4. For
comparison, the table also contains the dust model documented in the Flight Systems
Requirements Document. The FRSD model was used early during the development
phase design of the spacecraft whipple shields.
The current dust model incorporates information obtained from observations of Wild-2
taken during 1997. It is an extrapolation from Halley comet data with the following
model parameter assumptions:
• Particle size distribution = McDonnell Four slope particle size distribution = -3.65
• Nucleus Radius = 2.0 km, with 40% surface activity
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