Five Questions About the Shuttle Repair Mission

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SPACEWALK: With the Earth in the background, astronaut Soichi Noguchi of Japan waves from the Shuttle payload bay. Noguchi made some repairs to the space station

The troubled space shuttle will get some up-close care tomorrow when astronaut Stephen Robinson ventures outside to trim what’s known as gap filler protruding from tiles at the nose of the ship. The procedure is a straightforward one, but it’s not without risks. What you need to know to follow the operation:

WHAT IS GAP FILLER AND WHY IS IT A PROBLEM? The underside of the shuttle flexes during the stress and heating of reentry. To prevent the rigid tiles from grinding or chipping, heat resistant cloth is fitted between them. Ordinarily, the filler is flush with the ship, all but invisible to the eye. But on Discovery, two pieces are protruding near the nose—one 1.1 inch, the other .06 inch, and need to be snipped away.

WHY? During reentry, the shuttle must slip smoothly through the atmosphere to prevent heat from concentrating in particular spots or turbulence from causing instability. At the fantastic velocities the shuttle achieves—22 times the speed of sound through the upper atmosphere—even such tiny irregularities as the gap filler can lead to both problems.

HOW WILL THE FILLER BE TRIMMED? Robinson will secure his feet to the end of the shuttle’s manipulator arm and be lowered under the nose of the ship—a place where spacewalking astronauts have never ventured. His first attempted fix will be a decidedly low-tech one: He’ll try to pluck the fabric free. If that doesn’t work, he’ll use a blade—a portion of which has been taped over to prevent it from nicking the ship. If that fails, he’ll try a small saw.

IS THIS THE FIRST TIME GAP FILLER HAS CAUSED SUCH A PROBLEM? No. Shuttles have flown more than 100 times and the filler has always been a part of them. In 1995, the shuttle Columbia returned with a protruding piece. On other shuttles, it’s likely some filler worked free but burned away during reentry. The fact that the underside of Discovery was photographed so carefully on this mission to look for chipped tiles is the only reason the filler problem turned up this time.

IS ROBINSON IN ANY DANGER DURING HIS SPACEWALK? No spacewalk is remotely risk-free. Space suits could be damaged by tools; equipment could malfunction; there’s even the risk of being struck by mictometeors or bits of space debris. But NASA astronauts have been venturing outside of their ships for more than four decades and all of them have come safely back inside.