ISS Opens Window to World

The U.S. laboratory, Destiny, was attached and activated during the shuttle mission of the past week. Representative of the many experimental stations which will be available is the Window Observational Research Facility (WORF). Spacewalking astronauts Tom Jones and Bob Curbeam removed a protective thermal cover and then installed a shutter for the new International Space Station (ISS) Destiny lab observational window.

The Window Observational Research Facility (WORF) consists of an optical quality window and a special rack facility surrounding the window. The WORF is available for scientific and commercial payloads. The WORF will also be a resource for public outreach and educational opportunities for Earth Sciences (e.g., the EarthKAM, etc.). Images from space have many applications; they can be used to study global climates, land and sea formations, and map routes for city planners. Special sensors can also provide important data-for example, atmospheric conditions or properties as well as data for new sensor technology development.

The WORF is located on the nadir (Earth facing) side of the U.S. Destiny laboratory module. The window can view 39.5 degrees forward along the axis of the ISS, 32.2 degrees aft, and a total of 79.1 degrees from port to starboard.

A rack system surrounds the laboratory window for attaching sensors (cameras, multispectral scanners, and other instruments). It provides attachment points, power and data transfer capability for instruments to be mounted in the window. Multiple instruments can be mounted at the same time. The rack is designed to allow rapid changes of equipment by the crew. The WORF will have available a bracket for small cameras such as 35 mm, 70 mm and camcorders. Other payoads which are larger, require a nonstandard attachment, or require additional instrument isolation must supply their own brackets or platforms which mount to the WORF using the attachment points. The WORF also provides protection for the interior of the Lab window and can control stray light exchange between the Lab interior and the external Station environment.

The window is 508 cm (20 in) in diameter. The window is made up of four panes pressed together. The outermost pane is a replaceable, "debris" glass a little more than a 8.5 cm (third of an inch) thick. It is designed to protect the rest of window from orbital debris or micrometeoroids that might strike the station. If it breaks, it can be replaced by a space walking astronaut. The two middle panes serve as the primary and secondary pressure windows, ensuring that the laboratory module stays pressurized. Each of the panes is 31.75 cm (1-1/4 in) thick. The innermost pane is a "scratch" pane a little less than a 12.7 cm (half-inch thick). Its primary purpose is to prevent condensation from forming on the pressure panes. The scratch pane has a special anti-scratch coating that stands up to accidental nicks or bumps from any camera lenses that might be mounted to observe Earth. When not in use, the window is protected by the kick pane on the inside, and by an external cover on the outside. The external cover can be rotated in or out of position from within the cabin by an astronaut using a hand crank.

The spectral properties of the window support remote sensing work. The reflective coating on the window absorbs UV radiation, but transmittance rises rapidly after 304 nm to > 90% in the visible and into the near infrared. Transmittance tails off after 800 nm, reaching zero at approximately 2600 nm.

Corning Inc., Corning, N.Y., provided glass capable of supporting viewing with high-resolution telescopes. The glass panes were polished to a fine finish by Zygo Corp., Middlefield, Conn. The panes were coated by Optical Coating Lab Inc., Santa Rosa, Calif., with anti-reflection chemicals that block ultra-violet radiation from the sun. The coating also is electro-conductive to cut down on condensation. The glass is homogenous, free of color, been polished very smoothly and is flat. In addition, for extra protection, the outermost panel has a clamshell-like protective aluminum cover that can be closed from inside the station. 

Originally, general viewing glass, basically shuttle glass derived from X-15 aircraft window requirements and designed primarily to support pilots in landing the shuttle, was to be used. While images through the glass might not appear distorted to the human eye, wave front error (wave front error is the amount of distortion caused by the glass) becomes critical when using large aperture instruments such as cameras or telescopes. They sample more of the wave front than the human eye and, therefore, are more sensitive to error. The result can be degraded images or data. However, scientists were able to persuade NASA of the need for optical-quality glass to permit Earth observations using high-resolution telescopes or the operation of precision remote-sensing instruments.

The window cost about US$800,000.

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