Note: From the January 2002 issue of Orbital Debris Quarterly News

The Image Science and Analysis Group at the NASA Johnson Space Center
has just released its latest assessment of small particle damage to the
HST in Survey of the Hubble Space Telescope Micrometeoroid and Orbital
Debris Impacts From Space Shuttle Service Mission 3A Imagery
(JSC-29539). Prepared primarily by David Bretz and the late Leif
Anenson, the report characterizes the results of a special
investigation of HST photographs from the December 1999 mission of
STS-103 to identify high velocity impact features. A similar analysis
was performed following the second HST servicing mission by STS-82
(see Survey of the Hubble Space Telescope Micrometeoroid and Orbital
Debris Impacts from Service Mission 2 Imagery, JSC-28472, 1998).

During the STS-103 mission the Electronic Still Camera (ESC) was used
from inside the Space Shuttle crew cabin to map all visible areas of
the body of HST using a 80-200 mm lens at full zoom (200 mm) and to
image selected areas with the 400 mm telephoto lens. Astronaut Scott
Kelly was the primary survey photographer, who underwent preflight
training for this mission task. A total of 99 images (50 with the
200 mm lens and 49 with the 400 mm lens) were taken from the aft
flight deck. Many of these images were selected for special particle
impact analysis.

In all, 571 impact features (strikes) were selected for characterization
and measurement: 398 in the 200 mm lens images and 173 in the 400 mm
lens images. Where possible, the dimensions of both the central hole
and the outer delamination ring were measured. The most prevalent
hole size seen in the 200 mm lens images was 2-3 mm, but the higher
resolution 400 mm lens images yielded a majority of hole diameters in
the 1-2 mm range. The delamination ring diameters typically are twice
the size of the associated hole.

One of the most important metrics for determining the flux of the
particle environment is the density (number per square meter) of
impacts. Average impact densities of about 45/m2 were found on the
-V3 quadrant of HST. This is a cumulative effect over almost 10 years
in Earth orbit. Unfortunately, micrometeoroid and orbital debris
impacts cannot be differentiated in the images. Therefore, statistical
techniques, taking into account the mean densities of micrometeoroids
and orbital debris and their effects on hypervelocity impact
morphology, will need to be applied to distinguish the probable