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Focus — A WISE Project
By Jet Propulsion Lab + NASA
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United Launch Alliance capped 2009 with the launch of a Delta II carrying NASA’s Wide-field Infrared Survey Explorer (WISE) spacecraft at 6:09 a.m. PST, on Thursday, December the 17th. Rocketing from Space Launch Complex-2, the launch was the eighth Delta II during 2009 and represents the 37th successful mission launched by ULA in its first 36 months of operation.

WISE will scan the entire sky using an infrared telescope with sensitivity hundreds of times greater than ever before possible, picking up the glow of hundreds of millions of objects and producing millions of images. The mission will uncover objects never seen, including the coolest stars, the universe’s most luminous galaxies and some of the darkest near-Earth asteroids and comets.

wise launch sm 0210 For the WISE mission, the spacecraft was launched on a Delta II 7320-10C configuration vehicle featuring a ULA first stage booster powered by a Pratt & Whitney Rocketdyne RS-27A main engine and three Alliant Techsystems (ATK) strap-on solid rocket motors. An Aerojet AJ10-118K engine powered the second stage. The payload was encased by a 10-foot-diameter composite payload fairing.

Mission Overview
The Wide-field Infrared Survey Explorer, or WISE, will scan the entire sky in infrared light, picking up the glow of hundreds of millions of objects and producing millions of images. The mission will uncover objects never seen before, including the coolest stars, the universe’s most luminous galaxies and some of the darkest near-Earth asteroids and comets. Its vast catalogs will help answer fundamental questions about the origins of planets, stars and galaxies, and provide a mountain of data for astronomers to mine for decades to come.

Thanks to next-generation technology, WISE’s sensitivity is hundreds of times greater than its predecessor, the Infrared Astronomical Satellite, which operated in 1983.

WISE will join two other infrared missions in space — NASA’s Spitzer Space Telescope and the Herschel Space Observatory, a European Space Agency mission with important NASA participation.

WISE is different from these missions in that it will survey the entire sky. It is designed to cast a wide net to catch all sorts of unseen cosmic treasures, including rare oddities.

The closest of WISE’s finds will be near-Earth objects, both asteroids and comets, with orbits that come close to crossing Earth’s path. The mission is expected to find hundreds of these bodies, and hundreds of thousands of additional asteroids in our solar system’s main asteroid belt. By measuring the objects’ infrared light, astronomers will get the first good estimate of the size distribution of the asteroid population. This information will tell us approximately how often Earth can expect an encounter with a potentially hazardous asteroid. WISE data will also reveal new information about the composition of near-Earth objects and asteroids — are they fluffy like snow or hard like rocks, or both?

The next closest targets for WISE are cool “failed” stars called brown dwarfs.These Jupiter-like balls of gas form like stars but fail to gather up enough mass to ignite like stars.The objects are cool and faint, and nearly impossible to see in visible light. WISE should uncover about 1,000 in total, and will double or triple the number of star-like objects known within 25 light-years of Earth. What’s more, if a brown dwarf is lurking closer to us than the closest known star, Proxima Centauri, WISE will find it and the little orb will become famous for being the “closest known star.”

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The most distant objects that will stand out like ripe cherries in WISE’s view are tremendously energetic galaxies. Called ultraluminous infrared galaxies, or ULIRGs, these objects shine with the light of more than a trillion suns.They crowd the distant universe, but appear virtually absent in visible-light surveys. WISE should find millions of ultra-luminous infrared galaxies, and the most luminous of these could be the most luminous galaxy in the universe.

Other WISE finds will include: newborn stars; disks of planetary debris around young stars; a detailed look at the structure of our Milky Way galaxy; clusters of galaxies in the far universe and more. The most interesting discoveries will lay the groundwork for follow-up studies with other missions, such as NASA’s Spitzer Space Telescope, the Herschel Space Observatory, NASA’s Hubble Space Telescope, NASA’s upcoming SOFIA airborne telescope and NASA’s upcoming James Webb Space Telescope. Powerful ground-based telescopes will also follow up on WISE discoveries.

As with past all-sky surveys, surprises are sure to come. For example, one of the most surprising finds to come out of the Infrared Astronomical Satellite mission was the discovery of excess infrared light around familiar stars like Vega and Fomalhaut. Astronomers soon determined that the excess light comes from pulverized rock in disks of planetary debris. The findings implied that rocky planets like Earth could be common. Today hundreds of astronomers study these debris disks, and Hubble recently captured an actual photograph of a planet orbiting Fomalhaut within its disk.

WISE will orbit Earth at an altitude of 525 kilometers (326 miles), circling Earth via the poles about 15 times a day. A scan mirror within the WISE instrument will stabilize the line of sight so that snapshots can be taken every 11 seconds over the entire sky. Each position on the sky will be imaged a minimum of eight times, and some areas near the poles will be imaged more than 1,000 times. About 7,500 images will be taken every day at four different infrared wavelengths.

The mission’s sensitive infrared telescope and detectors are kept chilled inside a Thermos-like tank of solid hydrogen, called a cryostat. This prevents WISE from picking up the heat, or infrared, signature of its own instrument. The solid hydrogen, called a cryogen, is expected to last about 10 months and will keep the WISE telescope a chilly 12 Kelvin (minus 438 degrees Fahrenheit).

After a one-month checkout period, the infrared surveyor will spend six months mapping the whole sky. It will then begin a second scan to uncover even more objects and to look for any changes in the sky that might have occurred since the first survey. This second partial sky survey will end about three months later when the spacecraft’s frozen-hydrogen cryogen runs out. Data from the mission will be released to the astronomical community in two stages: a preliminary release will take place six months after the end of the survey, or about 16 months after launch, and a final release is scheduled for 17 months after the end of the survey, or about 27 months after launch.

Mission Operations
The observatory’s ground segment includes all facilities required to operate the satellite; acquire its telemetry; and process, distribute and archive data products. JPL is responsible for all ground operations that track and control the spacecraft.

integral ad sm feb10 2 WISE will survey the sky from a sun-synchronous polar orbit. It will circle around the poles at the line where day turns to night, called the terminator. The orbit is designed to precess, or shift, with time, so that it stays over this line. This ensures that WISE’s solar panels continuously soak up the sun, while the telescope snaps images of the sky overhead, mapping out a 360-degree strip each orbit. As Earth orbits around the sun, these strips of images sweep across the whole sky in only six months. Adjustments are made every half orbit to keep the telescope pointed at the correct strip of sky, following commands that are planned out a week in advance.

Surveying is interrupted four times a day to transmit images to the ground via the Tracking and Data Relay Satellite System. These satellites first relay WISE data to a ground station at White Sands, New Mexico. From there, spacecraft telemetry is sent to JPL, while image and some telemetry data are transferred to the WISE Science Data Center at the Infrared Processing and Analysis Center at Caltech.

Science Data Processing + Archiving
A specialized hardware and software system combines the raw science data with the spacecraft telemetry and converts them into high-quality images and a catalog of stars and galaxies detected by WISE. These products will be archived and distributed to the user community via the web-based services of the Infrared Science Archive at the Infrared Processing and Analysis Center.

Science Goals + Objectives
The primary goal of WISE is to scan the entire sky at infrared wavelengths with vastly improved sensitivity and resolution over past missions. All-sky surveys are essential for discovering new targets of interest, and, in some instances, have opened up entire fields of study. Many modern surveys have combed the sky using various wavelengths, but a gap remains at the infrared wavelengths WISE will observe. The best existing infrared all-sky survey at wavelengths beyond 10 microns is from the highly successful Infrared Astronomical Satellite, operated in 1983. The only existing all-sky survey between 3 and 10 microns is from the Cosmic Background Explorer, operated in 1989.

WISE will scan the sky with far better sensitivity and resolution. Its millions of images will provide the astronomical community with a vast atlas of the infrared universe, populated with hundreds of millions of space objects. Like scanning the grains of sand on a beach with a metal detector, this infrared telescope will find rare gems buried in the vastness of space. As with past all-sky surveys, the mission’s legacy will endure for decades to come.

In addition, WISE will scan much of the sky a second time. This will reveal even more asteroids, stars and galaxies, and catch objects that have changed brightness or position since they were last observed six months earlier. For example, if a cool star has moved noticeably, astronomers will know it is relatively nearby.
  • The science goals of the mission are:
  • To find the nearest and coolest stars
  • To find the most luminous galaxies in the universe
  • To find and study asteroids in our solar system
  • To better understand the evolution of planets, stars and galaxies
Other Goodies
As WISE is scanning the entire sky, it is going to see all kinds of cosmic wonders, both odd and expected. It will detect many varieties of stars and galaxies, including thousands of dusty, planet-forming disks swirling around stars, factories pumping out newborn stars, clusters of distant galaxies and more. Surprises are also sure to come, as was the case with previous all-sky surveys. In the end, the data will help astronomers piece together the evolution of stars and galaxies, and gain a better understanding of how our own planet, sun and galaxy came to be.

miteq ad sm feb10 WISE has already captured its first look at the starry sky that it will soon begin surveying in infrared light. WISE data will serve as navigation charts for other missions such as NASA’s Hubble and Spitzer Space Telescopes, pointing them to the most interesting targets WISE locates. A new WISE infrared image was taken shortly after the space telescope’s cover was removed, exposing the instrument’s detectors to starlight for the first time. This link will reveal the picture, which shows 3,000 stars in the Carina constellation. The image covers a patch of sky about three times larger than the full moon. The patch was selected because it does not contain any unusually bright objects, which could damage instrument detectors if observed for too long. The picture was taken while the spacecraft was staring at a fixed patch of sky and is being used to calibrate the spacecraft’s pointing system.

The Spacecraft
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 (1,457 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, which 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.

wise craft sm 0210 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 part in the instrument.

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 aluminum honeycomb panels sandwiched between aluminum skin. It has no deployable parts — the only moving parts are four reaction wheels used to turn the satellite.

smi ad sm feb10 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 once in orbit.

Command + Data Handling
The command and data handling system is the spacecraft’s brain, responsible for monitoring and controlling all spacecraft functions. It consists of a single box, called the Spacecraft Control Avionics, which was developed by the Southwest Research Institute, San Antonio, Texas. The box includes a single-board RAD750 computer, memory, a command and telemetry interface, an instrument interface, a flash memory card and spacecraft interface cards. This box can operate the spacecraft either with commands stored in its memory or via “real-time” commands radioed from Earth. It also handles engineering and science data to be sent to Earth.

Electric Power
WISE is powered entirely by a fixed solar panel. The panel is approximately 2 meters (80 inches) wide by 1.6 meters (61 inches) tall. To ensure that sunlight will hit the solar panel properly, the satellite is always oriented with its solar panel facing the sun and the instrument pointed 90 degrees away from the sun. The panel contains 684 solar cells manufactured by Spectrolabs, Sylmar, California. The maximum power produced is 551 Watts.

Attitude Determination + Control
The spacecraft attitude determination and control system is used to adjust the orientation of WISE in space. It consists of a complementary set of four reaction wheels used for maneuvering and three torque rods, which use Earth’s magnetic field to slow down the wheels.

Momentum build-up in the reaction wheels is periodically dumped via the torque rods as WISE flies over the poles. Two star trackers, a fiber-optic gyroscope, a magnetometer and 14 sun sensors provide measurements of the spacecraft’s position.

The star trackers are small telescopes with visible-light electronic cameras called charge-coupled devices (CCDs). They are capable of acquiring, tracking and identifying multiple stars in their fields of view. The positions of the observed stars are compared with an onboard star catalog to help orient the spacecraft. WISE will use both trackers simultaneously during its survey. The two trackers are pointed in different directions, so that, for example, if the moon interferes with measurements from one star tracker, the other one can be used. Ball Aerospace & Technologies Corp. manufactured the CT-633 star trackers.

WISE will communicate with Earth via NASA’s Tracking Data Relay Satellite System, a network of Earth-orbiting satellites. The spacecraft has one fixed high-gain radio antenna, which uses Ku-band frequencies to relay science data, as well as stored spacecraft health and telemetry, down to the ground at a rate of 100 megabits per second. The high-gain antenna is turned toward NASA’s tracking satellites four times per day, an average of 15 minutes each time, while WISE is orbiting over the poles. Because WISE observes the sky over the poles every orbit, the survey can be interrupted there without creating gaps in the all-sky map.

The NASA satellites relay the spacecraft health and science data to a ground facility in White Sands, New Mexico. From there the spacecraft telemetry is sent to JPL, and the science data to the Infrared Processing and Analysis Center at Caltech.

The NASA satellites also relay commands and spacecraft data between JPL and WISE using four omni-directional antennas on the spacecraft, which operate at S-band frequencies. Two of these antennas can receive commands from the ground at a speed of two kilobits per second; the other two transmit spacecraft health and telemetry data down to Earth at speeds of either four or 16 kilobits per second.

Flight Software
The observatory uses stored commands to perform its normal operations and also receives commands and sequences from Earth. The software on the flight computer translates the stored and ground commands into actions for various spacecraft subsystems. The flight software also gathers science data as well as engineering telemetry for all parts of the spacecraft and continuously monitors the health and safety of the observatory. The flight software can perform a number of autonomous functions, such as attitude control and fault protection, which involve frequent internal checks to determine whether a problem has occurred. If the software senses a problem, it will automatically perform a number of preset actions to resolve the problem or put the spacecraft in a safe mode until ground controllers can respond.

Thermal Control System
The spacecraft is protected by an onboard thermal control system that consists primarily of passive elements: multilayer insulation blankets, radiator panels, thermal coatings and finishes. In addition, thermostatically controlled heaters provide precise temperature control where necessary for the instrument camera and scan mirror electronics, for example. Heaters are also used for maintaining electronics above survival temperatures in contingency modes.

Science Instrument
wise telescope sm 0210 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 centered at 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, Massachusetts. 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 of 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 spacecraft 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.

Light gathered by WISE’s telescope is focused onto what are called focal planes, which consist 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 Sensors, Camarillo, Californnia. 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, California.

wise cryosat sm 0210 As 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, which is essentially a giant Thermos bottle. The cryostat is extremely efficient at keeping heat away from the detectors -- its insulating power is equivalent to a home insulation rating of “R-300000.”

The WISE cryostat, manufactured by Lockheed Martin Advanced Technology Center, Palo Alto, California, 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 by a set of springs. 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.

NASA’s Explorer Program
WISE was developed as a medium-class Explorer mission under NASA’s Explorer Program. The Explorer Program is the oldest continuous program within NASA. It has launched more than 90 missions, starting with the Explorer 1 launch in 1958 and including the Nobel Prize-winning Cosmic Background Explorer (COBE) Mission.

The early Explorer missions were managed by JPL for the U.S. Army. The objective of the Explorer Program is to provide frequent flight opportunities for world-class scientific investigations from space. Explorer missions are focused science missions led by a principal investigator and occur over relatively short periods of time. They are selected via a highly competitive announcement of opportunity process. The program currently administers only principal investigator-led heliophysics and astrophysics science investigations; in the past, it covered more fields of science.

The Explorer Program seeks to enhance public awareness of and appreciation for space science and to incorporate educational and public outreach activities as integral parts of space science investigations.

Individual Explorer missions are mutually independent, but share a common funding and NASA oversight management structure. The program is designed to accomplish high-quality scientific investigations using innovative, streamlined and efficient management approaches. It seeks to contain mission cost through commitment to, and control of, design, development and operations costs.

The Explorer Program is directed by the Heliophysics Division within NASA’s Science Mission Directorate. The Explorer Program Office is hosted at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

NASA’s Jet Propulsion Laboratory, Pasadena, California, manages WISE for NASA’s Science Mission Directorate. The mission’s principal investigator, Edward L. (Ned) Wright, is at UCLA. The mission’s education and public outreach office is based at the University of California, Berkeley.

Editor’s Note: Our thanks to JPL and NASA for their invaluable contributions in authoring this article.

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