MISSION PLAN - PDS Small Bodies Node

The particle size and number distribution of the current science model shown in Table
2.4-4 incorporates the observations made during the appearance of Wild-2 in 1997.
However, the hazard predictions based on this model is still considered to be speculative.
The mission plan includes a real time hazard assessment, on approach to the comet, and
an adjustment of the flyby distance at E-2 days, if necessary.
2.4.3 Interstellar Dust Particle (ISP) Sizes, Impact Velocity
and Direction
Reports made from 1993 to 1995 by the Ulysses and Galileo dust experiment team
suggested that the upstream speed and direction of interstellar dust particles in ecliptic
latitude and longitude were within a range of (26 km/s, 2.5° ±6°, 252° ±6) to (30
km/s,10°±10°, 280°±30°). The former, an earlier conjecture, indicated the dust stream to
be in the general direction of the interstellar helium gas flow but the latter, a report by
Baguhl, et. al., 1995, suggested a different direction for the dust stream. The
STARDUST project has used both values in the past, but based on a more recent
communications with M. Landgraf (7 Feb 1997), we have adopted a magnitude of 26
km/s and an upstream direction of 7.7° ecliptic latitude and 259° ecliptic longitude. It
does not matter too much which value we adopt because of the magnitude of the
uncertainty. However, it is very important that the ISP collection experiment be designed
in such a manner that the directional information of the collected particles, a key factor in
tracing the particle’s origin, is retained to the extent possible.
The flight paths of ISP’s are modified by the gravity of the sun, the solar pressure and
various other complex processes not well or easily formulated. If one considers only the
simple effects of solar gravity and solar pressure, the velocities of ISP’s of various sizes
can be calculated easily as a function of β, where β is the ratio of the solar pressure to
solar gravity. High β particles (greater than 1) are low density, fluffy ones while low β
particles (less than 1) are dense dark ones.
One to one correspondence between the β group and the particle size cannot be made
without knowledge of individual particle density, shape and radiative parameters. If
some assumptions regarding these parameters are made, it is possible to estimate the
possible range of particle sizes as summarized in Table 2.4-5.
Table 2.4-5 Particle Size Estimate
β Particle Size (µm) β Particle Size (µm)
0.6 0.1 - 0.7 1.4 0.2 - 1.2
1.0 0.1 - 0.9 1.8 0.3 - 2.0
Assumptions: Density = 1-3 g/cc, Reflectivity = 0 - 1
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