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November 01, 2007

EVA Difficulty Rated at 7.5

At Thursday's press conference, astronaut and veteran spacewalker Dave Wolf rated Saturday's planned EVA a 7.5 on a scale of difficulty from 1 to 10. He cited the fact that this spacewalk to repair the torn P6 solar array involved brand new procedures (which is why the EVA was delayed from Friday to Saturday, and may even yet be postponed to Sunday) in a dangerous environment.

The Danger

The solar array blankets have no shut-off switch. Whenever the solar cells are exposed to the sun, they produce electricity. The space suits have metal fittings, and the astronauts carry metal tools. If any of these should come in contact with the cells: ZAP! the astronaut could be electrocuted. No one would estimate the potential effects because there are so many factors involved. But the number 100 volts was mentioned.

Another danger to Parazynski is that if he should have any damage to, or system failures of, his space suit, he may not be able to get to the safety of the airlock in time to avoid permanent injury. His suit carries a backup supply of oxygen that can make up for leaks for some period of time, but mission planners usually require spacewalkers to be able to execute an emergency return to the airlock within 30 minutes of a failure. Because of his location far out on the array, Parazynski would not be able to meet that time window. This is worrisome after the torn glove experienced earlier in the mission and on a previous flight and that Parazynski may be cutting guide wires whose sharp ends may bounce around and strike him when tension is released. Not to mention the danger of electrocution...

Astronaut Wolf was quick to downplay the danger, however, and warned the media against sensationalizing it. He said that it is the team's job to asses the risk and "mitigate" them. All tools are being covered in three layers of insulating tape, and the crew wrapped up in their best space mummy costumes. Cameras and crewmembers will be watching every move from every angle and be in constant communications.

The Procedure

Veteran spacewalker Scott Parazynski will be the man in the "hot" zone on the end of the robotic arm. The station arm is not long enough to reach out to the section of the array that is damaged, about 50 feet from the mast box. So the arm is being extended using a piece of equipment borrowed from the space shuttle, the orbiter boom sensor system (OBSS).

The OBSS is designed to extend the shuttle arm so it can inspect the tiles under the belly. The station arm will grab the OBSS out of the shuttle bay, then hand it to the shuttle arm for safekeeping while the station arm trundles down to the end of its track on the port side. Then the shuttle arm will hand the boom back to it, and Parazynski will hop on for what is sure to be the ride of a lifetime with Dan Tani at the controls.

Fellow spacewalker Doug "Wheels" Wheelock will position himself at the base of the array as a lookout. His primary task is to make sure that arm does not become a lightning rod and that his partner is warned to get out of the way if the array should bounce up and down or if something comes loose. Those of us who have ridden the air-bearing chair at Space Center Houston know just how easy it is to impart unwanted motions to objects in freefall.

The Goal

The repair will place what engineers are calling a "cufflink" on the array. This will keep the torn section folded and prevent further tearing. With the array repaired, the station will have all the power it needs at least until the spring. By then, some other brave spacewalkers will have tackled the repair of the starboard SARJ.

If the repair is not successful or not completed on Saturday (the suit limit is 7 hours), NASA will decide whether or not to do another spacewalk on this mission. They are really hoping they won't have to face this question because the schedule tradeoffs are very complicated. Assuming the repair is successful Saturday, then the shuttle will land at KSC on Wednesday.

Marianne Dyson
NSS member

Posted by m_dyson at November 1, 2007 04:04 PM



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