Window on the World
It's only a small pane in the International Space Station.
- By Leonard David
- Air & Space magazine, May 2000
NASA has certainly taken travel photography to new heights over the years. Beginning with the Mercury missions of the early 1960s, camera-toting astronauts have snapped more than 400,000 pictures of the globe, documenting everything from natural events to the impact humans have had on the environment to, well, the simple beauty of Earth from space.
But while the images made by orbiting astronauts are visually stunning and often scientifically rich, they are usually far from perfect. The problem is that even though the astronauts have excellent photographic equipment, they’re shooting through glass designed to stay intact under the pressures of space travel, rather than to allow photographers to capture fine detail several hundred miles away. It’s a little like buying a top-of-the-line Nikon or Hasselblad and then mounting a piece of thick household window glass to the front of the lens.
But now there’s a new photo opportunity on the horizon. When the U.S. Laboratory Module of the International Space Station (ISS) becomes operational—its launch date depends on that of the Russian-built service module—astronauts will study the planet through the highest quality optical window ever installed in a crewed spacecraft. This 20-inch-diameter circular window, which has been designed and manufactured to eliminate the distortions that plague conventional glass, will be the centerpiece of an active program of Earth observation and photography—and the culmination of a decades-long battle waged over a little bit of glass.
The view from the station will be striking. For starters, its path will give its crew plenty to look at. The ISS follows an orbital track inclined 51.6 degrees from Earth’s equator. That means it is flying over more than 75 percent of the planet’s surface, an area containing 95 percent of the world’s population. Thus, the U.S. Laboratory, along with its nadir window facing Earth, offers a vantage point from which to monitor, with specialized equipment, the global environment. “On the space station, we have people who are highly trained, very curious, and interested in doing something useful out the window,” says longtime window proponent Dean Eppler, a senior scientist in the Houston office of Science Applications International Corporation, a NASA program contractor. “By taking advantage of a good window, along with the interactive capability of the brain—the human computer—we will have an interactive remote sensing laboratory in space, and that’s a unique thing.”
NASA selected Corning, Inc., a high-performance glass veteran—and producer of windows for every manned NASA space program—for the job. “This is the best space window we have manufactured, and it was no small feat,” says Larry Sutton, space products account manager for Corning in Canton, New York. Design and fabrication of the window took Corning a year, with two other firms helping to deliver the final product, a four-piece assembly consisting of a tough, micrometeorite-resistant outside pane, primary and secondary panes that serve as the optical heart of the window, and a protective interior “scratch” pane. The assembly features external and internal shutters to further protect the glass. As scratches and contaminants accumulate, the external pane can be removed by spacewalking astronauts, returned to the ground for repolishing and recoating, and then reinstalled. The interior scratch pane, which protects against bumps from cameras and other instruments, can also be replaced.
The window’s glass is a special stock of high-purity, colorless, synthetic fused silicon dioxide—a material chosen for its resistance to the effects of thermal variations and its exceptional light transmission. Corning took extra care in producing the glass, making sure that tiny imperfections such as bubbles and other blemishes were minimized. The homogeneity of the window is very high—light waves pass through the glass with barely any distortion. A polishing job by Zygo Corporation, a high-performance testing, manufacturing, and measurement firm in Middlefield, Connecticut, made the glass almost perfectly smooth, and coatings by Optical Coating Laboratory, Inc., of Santa Rosa, California, will block out harmful ultraviolet radiation and enhance light transmission to give the space window its unequalled optical rating. “For the astronauts to be able to see without the distortion is going to be amazing,” Sutton says.
To take full advantage of the station’s high-quality port, engineers have designed a work closet to enclose the window. The closet—the Window Observational Research Facility—is a maneuverable open rack that will help position and operate all of the cameras, sensors, telescopes, and other equipment. Fit flush against the lab’s wall, the WORF will be installed after the U.S. Laboratory is itself in orbit and will frame the window. “Everything is going to be aimed at the center of that window, the sweet spot,” explains Scott Britnell, a lead technician working on the enclosure at Boeing Missiles and Space Division in Huntsville, Alabama. The WORF has a black interior to stop stray light from fouling sensitive devices aimed out of the viewing port, and to reduce stray glare from the lab itself, a flexible shroud, similar to a splash skirt on a kayak, fits tightly around the waist of the astronaut working at the window. Depending on the bulkiness of equipment installed within the rack, Britnell says, there may be enough room to squeeze three people inside. But, he adds, they’d better be pretty good friends.
Once in orbit, the lab window will be face to face with Earth’s surface most of the time. Those peering through the nadir window are promised an eyeful—a more color-true and vivid Earth, rich in detail and there for the taking, photographically speaking. Though the images won’t be any better than those from cameras mounted outside spacecraft, they will be far superior to previous hand-held efforts. This is key, as experience has shown that using the astronaut’s eye often produces better results than relying on remotely controlled equipment.