Close inspections of the radiators and other exposed surfaces also show minor punctures and evidence of spallation. The TPS has been struck by meteoroids and orbital debris, but reentry heating and localized deformations around the damage have made it difficult to differentiate damage from meteoroids and orbital debris from damage suffered during ascent. All surface damage adds to the workload during turnaround operations to prepare the vehicle for the next flight.

EXTRAVEHICULAR ACTIVITY

Astronauts performing tasks outside the orbiter are also at risk from meteoroids and orbital debris. The most vulnerable parts of the EVA mobility unit (EMU) are the soft areas of the space suit, the arms, gloves, and lower torso. (NASA calculates that the harder areas of the space suit contribute less than 10 percent of the overall risk.) The soft areas of the suit are constructed of multiple layers of abrasion and thermal protection material and a single pressure bladder.

The secondary oxygen pack on the EMU is sized to provide astronauts with a 30 minute supply of oxygen in case of a 4 mm puncture in the space suit. Presumably, this would be sufficient time for an injured astronaut to be assisted back to the pressurized crew compartment. NASA estimates that a 2 mm diameter particle could cause a 4 mm hole, and a 0.1 mm particle could cause a minute puncture. The degree of damage from these impacts has not yet been assessed in detail, but NASA now has a trauma physician on staff to examine the issue (Heflin, 1997).

NASA’s calculations of the risk to two astronauts on the end of the orbiter mechanical arm during a six-hour EVA (with no shielding by the structure) are summarized in Table 2–3. The predictions for 180 EVAs are also shown as an example of what the risks might be during the years of ISS operations (Heflin, 1997).

A three-phase study is under way to characterize the vulnerability of the EMU to meteoroids and orbital debris better. The last phase of the study, scheduled to be completed in October 1997, is intended to determine improvements in crew safety that can be realized through practical enhancements to the EMU. A comparable study is in progress for the Russian Orlan EVA suit, which will be used by U.S. astronauts during some cooperative space activities.

TABLE 2–3 Risk during EVA

 

6-Hour EVA

180 EVAs

Probability of no penetration

99.98% (1/4,800)

92.7% (1/14)

Probability of no critical penetration (hole >4 mm)

99.997% (1/31,000)

98.9% (1/91)



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