HOUSTON - Working with space caulk guns and
a pair of scrapers, two NASA astronauts put a black, sticky goo through its
paces during a Wednesday spacewalk to test basic shuttle heat shield repair
techniques.
Spacewalkers
Piers
Sellers and Michael
Fossum spent most of their more than seven-hour excursion in the payload
bay of the space shuttle Discovery, where they evaluated methods of applying an
experimental heat-resistant material onto samples of the same type of panels
that line the orbiters wing edges and nose cap.
The astronauts
began their spacewalk at 7:13 a.m. EDT (1113 GMT) as Discovery and the
International Space Station (ISS) passed over Spain, and spent seven hours and 11 minutes working outside their spacecraft.
Sellers
said their primary task - testing a putty-like heat shield repair material
dubbed NOAX - was more akin to a "meticulous lab experiment" rather than the
heavy lifting of Monday's spacewalk to repair the space station's Mobile
Transporter.
The
spacewalkers squirted the heat-resistant material onto intentionally damaged
sample areas using a caulk gun-like tool and smoothed it out with spatulas.
"It bubbles
up nice when it comes out," Fossum said.
NOAX, which
is short for non-oxide adhesive experimental, is a sticky black substance that
carries the initial consistency of peanut butter before it is worked into place
in orbit, NASA officials said. The space agency described the material as a
pre-ceramic polymer that is impregnated with carbon silicon carbide powder.
Engineers
designed NOAX as a coating, crack and gouge filler for the black panels that protect Discovery's
wing leading edges and nose cap. Made of a carbon composite called reinforced
carbon carbon (RCC), the panels endure the hottest temperatures when the
shuttle reenters the Earth's atmosphere.
"It's easy
to work with," Sellers said after applying the material into several samples of
intentionally damaged samples. "It's just like anything we used in the
experiments."
Today's
tests were the second orbital evaluations of NOAX. The material was first used
during Discovery's last flight in July
2005, when STS-114 spacewalkers spent one
hour performing shuttle tile and RCC repair techniques.
In ground
tests, engineers found that NOAX appears to be most effective when applied to
RCC panels that are cooling from 100 to 35 degrees Fahrenheit (38 to 1.6 degrees
Celsius), so Fossum and Sellers used temperature probes to pick and choose when
to smear the material onto their samples. The samples will later be studied on
the ground and some seared in NASA's arcjet facility to determine the repair's
effectiveness
Applying
the NOAX material was easy at first for the astronauts as they used spatulas to
smooth the material. But applying some later coats as a finishing touch was a
bit tough, they later said.
"This is
quite hard work," Sellers said as he applied a finishing coat over an earlier
repair. I'm getting warm just mashing this stuff in."
Discovery's
STS-121 pilot Mark Kelly orchestrated today's spacewalk from inside the
orbiter, while crewmates Stephanie Wilson and Lisa Nowak worked the space
station's robotic arm. The spacewalk, the third and last of the STS-121
mission, was added
to the spaceflight - along with a 13th
flight day - once flight controllers found the shuttle's power supply could
support it, NASA said.
Heat
shield repair
Developing
the ability to repair at least some orbiter heat shield damage has been a
critical concern for NASA since the Columbia accident. In 2003, a 1.67-pound
(0.7-kilogram) chunk of shuttle fuel tank foam insulation pierced the RCC
shielding along Columbia's left wing leading edge during launch, a critical
wound to the spacecraft, leading to the loss of the orbiter and its
seven-astronaut crew during reentry.
An
investigation board later recommended
that NASA develop reliable shuttle heat shield repair methods.
NASA has
since redesigned shuttle fuel tanks to reduce the amount of foam they can shed
during launch. The largest area shed during Discovery's STS-121 ascent weighed
about 0.055 pounds - or less than an ounce, and about one-fourth the largest
allowable - came off in six small pieces from a total area of 12.3 inches by
14.2 inches (31 by 36 centimeters), mission managers have said.
"Our number
one goal is to never have to use this," Tony Ceccacci, NASA's lead shuttle
flight director for Discovery's STS-121 mission, said of the repair method Tuesday.
Sellers and
Fossum also tested a new infrared video camera during today's spacewalk. The
spacewalkers used the camera to record two, 20-second videos of Discovery's
wing leading edges. They then used the camera to study two of the intentionally
damaged RCC samples in the shuttle's payload bay, as well as two others that
had undergone a test repair.
"Their hope
is that this is another sensor that we can use to detect damage," Ceccacci said
of the new camera.
Ceccacci
said the camera could be used by hand or be mounted to the tip of Discovery's
50-foot (15-meter) orbital boom if its images prove useful for heat shield inspections.
Sellers
also used the camera to observe parts of the ISS, including its radiators and
one spacewalkers - Fossum - as he crawled across the station's hull.
"It looks
like a glowing person moving over a mirror," Sellers said.
Space
spatula lost, SAFER taped
Wednesday's
spacewalk was not without incident. At one point, one of the five spatulas
Sellers carried out with him drifted away after it separated from its appointed
tether.
"No sign of
the spatula, guys, it is gone, gone, gone," Sellers said after an initial
search around his Discovery payload bay worksite.
Flight
controllers later caught the spatula on video cameras as it floated out of
Discovery's bay and into open space. It is not expected to pose a debris hazard
to Discovery or the ISS, NASA officials said.
"That was
my favorite spatch...don't tell the other spatulas," Sellers said, then later
took heart when Kelly told him it was not a debris risk. "Thanks for doing all
the work on this, I'm sorry to let it go."
Unlike Monday's
spacewalk, in which the latches connecting Sellers' emergency jetpack -
known as the Simplified
Aid for EVA Rescue (SAFER) - to his spacesuit popped open and had to be tethered
in place, the astronaut left the airlock prepared Wednesday.
His
crewmates covered the two SAFER latches with Kapton tape to prevent the latches
from shaking loose. Even so, Fossum had to help Sellers a few times to prevent
the tape from coming free and to secure the SAFER door, NASA officials said.
Wednesday's
spacewalk marked the third extravehicular activity (EVA) of Fossum's
spaceflight career and the sixth for Sellers. It was the 21st spacewalk
staged from the station's Quest airlock.
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