Spacecraft and Instruments

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Spacecraft
 
The WISE spacecraft is situated on a work stand. The WISE science instrument. Image credit: NASA
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The WISE spacecraft is about the height and weight of a big polar bear, only wider. It measures 2.85 meters tall (9.35 feet), 2 meters wide (6.56 feet), 1.73 meters deep (5.68 feet) and weighs 661 kilograms (1,433 pounds). It is composed of two main sections: the instrument and the spacecraft bus.

The Space Dynamics Laboratory in Logan, Utah, designed, fabricated and tested the instrument. They also manufactured the electronics used to control the instrument and perform onboard processing of the detector images.

The spacecraft bus was built by Ball Aerospace & Technologies Corp, Boulder, Colo. Ball was also responsible for integrating the instrument to the spacecraft bus and testing the completed spacecraft.

The instrument includes a 40-centimeter-diameter (16-inch) telescope and four infrared detectors containing one million pixels each, all kept cold inside an outer cylindrical, vacuum-tight tank filled with frozen hydrogen, called a cryostat. Some say the whole assembly looks like a giant Thermos bottle, while others see a resemblance to the Star Wars robot R2-D2. After launch, the hydrogen vents on the cryostat are opened and the instrument cover is ejected. Once these events have occurred, a scan mirror in the telescope will be the only moving instrument part.

At the bottom of the instrument is a three-axis stabilized, eight-sided spacecraft bus that houses the computers, electronics, battery and reaction wheels needed to keep the observatory operating and oriented correctly in space. Two star trackers for precision pointing are mounted on the sides of the spacecraft bus. A fixed solar panel that provides all the spacecraft's power is mounted on one side of the bus, and a fixed high gain antenna for transmitting science images to the ground is mounted on the opposite side. The bus structure is composed of an aluminum skin backed by aluminum honeycomb panels. It has no deployable parts -- the only moving parts are four reaction wheels used to maneuver the satellite.

The base of the spacecraft structure includes a “soft-ride” system of springs to reduce stress from the rocket on the satellite. A metal clamp band attaches the second stage of the rocket to the base of the satellite, and is released to allow the spacecraft to separate from the launch vehicle in orbit.




Science Instrument


Telescope

The WISE telescope has a 40-centimeter-diameter (16-inch) aperture and is designed to continuously image broad swaths of sky at four infrared wavelengths as the satellite wheels around Earth. The four wavelength bands are 3.4, 4.6, 12 and 22 microns. The field of view is 47-arcminutes wide, or about one-and-a-half times the diameter of the moon.

The telescope was built by L-3 SSG-Tinsley in Wilmington, Mass. Its design uses a total of 10 curved and two flat mirrors, all made of aluminum and coated in gold to improve their ability to reflect infrared light. Four of the mirrors form an image from the 40-centimeter primary mirror onto the flat scan mirror. The scan mirror moves at a rate that exactly cancels the changing direction of the spacecraft on the sky, allowing freeze frame images to be taken every 11 seconds. The scan mirror then snaps back to catch up with the craft as it continues to survey the sky.

The remaining mirrors form a focused image of the sky onto the detector arrays. Before reaching the arrays, the light passes through a series of flat "dichroic" filters that reflect some wavelengths and transmit others, allowing WISE to simultaneously take images of the same part of the sky at four different infrared wavelengths.

The image quality, or resolution, of WISE is about six arcseconds in its 3.4, 4.6 and 12 micron bands, meaning that it can distinguish features one six-hundredth of a degree apart. At 22 microns, the resolution is 12 arcseconds, or one three-hundredth of a degree. This means WISE can distinguish features about five times smaller than the Infrared Astronomical Satellite could at 12 and 25 microns, and many hundred times smaller than NASA's Cosmic Background Explorer could at 3.5 and 4.9 microns.

Detectors

Light gathered by WISE's telescope is focused onto what is called a focal plane, which consists of four detector arrays, one for each infrared wavelength observed by WISE. Each of the detector arrays contain about one million pixels (1,032,256 to be exact). This is a giant technology leap over past infrared survey missions. The Infrared Astronomical Satellite's detectors contained only 62 pixels in total.

The 3.4- and 4.6-micron detectors convert light to electrons using an alloy made of mercury, cadmium and tellurium. The electrons from each of the million-plus pixels are measured on the spot every 1.1 seconds, and the result sent to the instrument electronics. These detector arrays, a type known as the HAWAII 1RG, were manufactured by Teledyne Imaging Systems, Camarillo, Calif. They need to be warmer than the rest of the instrument to improve their performance. The 12- and 22-micron detectors sense light using silicon mixed with a tiny amount of arsenic. They have readout electronics specially developed for the low-temperatures of WISE and were manufactured by DRS Sensors & Targeting Systems, Cypress, Calif.

Cryostat

Because WISE is designed to detect infrared radiation from cool objects, the telescope and detectors must be kept at even colder temperatures to avoid picking up their own signal. The WISE telescope is chilled to 12 Kelvin (minus 261 degrees Celsius or minus 438 degrees Fahrenheit) and the detectors for the 12- and 22-micron detectors operate at less than 8 Kelvin (minus 265 degrees Celsius or minus 447 degrees Fahrenheit). The shorter wavelength 3.4- and 4.6-micron detectors operate at a comparatively balmy 32 Kelvin (minus 241 degrees Celsius or minus 402 degrees Fahrenheit). To maintain these temperatures, the telescope and detectors are housed in a cryostat, essentially a giant Thermos bottle.

The WISE cryostat, manufactured by Lockheed Martin Advanced Technology Center, Palo Alto, Calif., has two tanks filled with frozen hydrogen. The colder, or primary cryogen tank, the smaller of the two tanks, cools the 12- and 22-micron detector arrays. To achieve this low operating temperature, a larger 12-Kelvin secondary tank protects the primary tank from nearly all the heat from the outer structure of the cryostat, which is comparatively warm at about 190 Kelvin (minus 83 degrees Celsius or minus 117 degrees Fahrenheit). This secondary tank also cools the telescope and the 3.4- and 4.6-micron detectors. Small heaters are used to warm the 3.4- and 4.6-micron detectors from 12 to 32 Kelvin.

It is important to maintain a vacuum inside the cryostat when it is cold and on the ground; otherwise air would freeze inside it. It would become a giant popsicle. A deployable aperture cover seals the top of the cryostat while on the ground to prevent air from getting in. After WISE is safely in orbit, a signal is sent to eject the aperture cover. Three pyrotechnic separation nuts will fire, and the cover will be pushed away from the spacecraft.

An aperture shade is mounted at the top of the telescope to shield the open cryostat system from the sun and Earth's heat.

The expected lifetime of WISE’s frozen hydrogen supply is 10 months. Since it takes WISE six months to survey the sky, this is enough cryogen to complete one-and-a-half surveys of the entire sky after a one-month checkout period in orbit.