Two Space Shuttles, Discovery (OV-103) and Endeavour (OV-105), have
recently been examined for orbital debris and meteoroid impacts following
missions to the International Space Station (ISS) late last year. Both
exhibited numerous impacts on a variety of inspected orbiter surfaces,
covering more than 200 m2.

Discovery visited ISS last year on the STS-92 mission for seven days
of its 13-day flight in October 2000. A total of 38 impacts were
identified on the orbiter window thermal panes from orbital debris (9),
meteoroid (7), and unknown (22) particles. The largest impact feature
with a diameter of nearly 1 cm was apparently caused by collision with
a small paint particle. Three of Discovery’s thermal panes were
subsequently replaced.

Six impacts (3 orbital debris, 1 meteoroid, and 2 unknown) were found on
the radiators with three of these achieving penetration. The largest
radiator impact site was approximately three-quarters of a millimeter in
extent and was caused by a meteoroid strike. Four other impacts were
also discovered: three on the flexible reusable surface insulation (FRSI)
covering the external payload bay doors and one on the vertical stabilizer.
Of these, two were meteoroids, one was orbital debris, and one was of
unknown material.

In December Endeavour conducted the 11-day STS-97 mission, which again
included seven days docked to ISS. Although the number of identified
window impacts decreased to 30, the number of impacts to the radiators
and the FRSI (12 and 6, respectively) actually increased compared to the
longer duration STS-92. A total of two windows were replaced. Of the
12 radiator impacts, only one penetrated the thin aluminum sheet, but two
struck the silver-teflon-aluminum doubler installed recently to protect
the radiator coolant loops. Four additional impact sites were found on
the leading edges of the orbiter wings, two from orbital debris particles
and two from unknown sources.

Overall, the number of identified impactors of all sizes is roughly evenly
divided between orbital debris and meteoroids. However, a significant
number of impactors cannot be identified by type, particularly for the
smaller window strikes. Complete inspection details are provided in
STS-92 Orbiter Meteoroid/Orbital Debris Impact Damage Analysis, JSC-29318,
January 2001, and STS-97 Orbiter Meteoroid/Orbital Debris Impact Damage
Analysis, JSC-29373, March 2001.

*****

A related technical article “International Space Station Debris Avoidance
Operations” is available in the same issue of the newsletter at
http://www.orbitaldebris.jsc.nasa.gov/newsletter/v6i2/v6i2-3.html#project_reviews .