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Stardust (spacecraft)
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Stardust is an American interplanetary mission of the NASA Jet Propulsion Laboratory, whose primary purpose was to investigate the makeup of the comet Wild 2 and its coma. It was launched on February 7, 1999 by NASA, travelled nearly 3 billion miles (5·109 km), and returned to Earth on January 15, 2006 to release a sample material capsule.
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Stardust is an American interplanetary mission of the NASA Jet Propulsion Laboratory, whose primary purpose was to investigate the makeup of the comet Wild 2 and its coma. It was launched on February 7, 1999 by NASA, travelled nearly 3 billion miles (5·109 km), and returned to Earth on January 15, 2006 to release a sample material capsule. It is the first sample return mission to collect cosmic dust and return the sample to Earth. On July 3, 2007 a second mission was approved to revisit the comet Tempel 1.
Primary mission
NASA began construction of the Stardust spacecraft in 1996. After launch in 1999, the Stardust spacecraft travelled in an initial orbit beyond — but intersecting — Earth's orbit. The Delta II booster did not have enough energy to reach Wild 2 directly. The Stardust spacecraft then approached Earth in January 2001 for a gravity assist maneuver. The encounter with Earth enlarged the spacecraft's orbit to intersect that of Wild 2.
On the second orbit, Stardust flew by the comet Wild 2 on January 2, 2004. During the flyby it collected dust samples from the comet's coma and took detailed pictures of its icy nucleus. Additionally, the spacecraft accomplished several other goals. It passed within 3300 km of the asteroid 5535 Annefrank on November 2, 2002 and took several photographs. The aerogel collector also acquired interstellar dust. In March-May 2000 and July-December 2002, the spacecraft angled itself into a dust stream believed to originate outside the solar system. The reverse side of the aerogel collector then caught a sample of such particles.
The sample material capsule from Stardust returned to Earth at approximately 10:10 UTC on January 15, 2006 in Utah's Great Salt Lake desert, near the U.S. Army Dugway Proving Ground, to deliver the sample material. The landing coordinates were . Winds had blown the capsule a few miles off its ballistic trajectory, but it was within the target area. On arrival, the capsule was travelling in a nearly flat trajectory, at 12.9 km/s (28,900 miles per hour), which is the fastest re-entry speed into Earth's atmosphere ever achieved by a man-made object. As a point of comparison, NASA stated it would be able to travel from Salt Lake City, Utah to New York City, New York in less than six minutes. A large fire ball and sonic boom were observed in western Utah and eastern Nevada.
The Stardust mothership had been put into a "divert maneuver" to keep the hardware from hitting Earth. Under twenty kilograms of fuel remain onboard after the maneuver. On January 29, the craft was put in hibernation mode with only its solar panels and receiver still active in a three-year heliocentric orbit that will return it to Earth's vicinity on January 14, 2009.
Donald Brownlee, from the University of Washington, was the Principal Investigator for the Stardust mission. Ken Atkins of the Jet Propulsion Laboratory managed the project during development. Joe Vellinga was the Program Manager at the spacecraft contractor, Lockheed Martin. The Project Manager during Stardust operations and the current Project Manager for the NExT secondary mission is Tom Duxbury of the Jet Propulsion Laboratory.
Secondary mission: exploration of Tempel 1 (NExT)
On 19 March, 2006, Stardust scientists announced that they were considering the possibility of redirecting the spacecraft on a secondary mission to photograph Tempel 1, the comet that was impacted by the Deep Impact spacecraft in 2005. This possibility is important because Deep Impact did not succeed in capturing a good image of the crater formed on Tempel 1, due to obscuring dust from the impact.
In July 3, 2007, this extended mission was approved, under the designation of New Exploration of Tempel 1 (NExT). This investigation will provide the first look at the changes to a comet nucleus produced after its close approach to the sun. NExT also will extend the mapping of Tempel 1, making it the most mapped comet nucleus to date. This mapping will help address the major questions of comet nucleus "geology" raised by images of areas where it appears material might have flowed like a liquid or powder. NExt is scheduled to fly by Tempel 1 on February 14, 2011.
The craft
The mission spacecraft is derived from the SpaceProbe deep space bus developed by Lockheed Martin Astronautics. This new lightweight spacecraft incorporates components, virtually all of which are either currently operating in space or are flight qualified and manifested to fly on upcoming missions. Several components have heritage from the Cassini mission; some were developed under the Small Spacecraft Technologies Initiative (SSTI).
Being a sample return mission, Stardust is subject to the maximum contamination restrictions, classified under level 5 planetary protection. However, the risk of interplanetary contamination by alien life was judged low, for instance particle impacts at over 1000 miles per hour — even into aerogel — would destroy any known microorganism.
The total weight of the spacecraft, including the hydrazine propellant needed for deep space maneuvers, is 380 kilograms. The overall length of the main bus is 1.7 meters, about the size of a refrigerator or an average office desk. It appears orange-brown due to the blankets of Kapton film.
At one end of the spacecraft is the sample return capsule; the capsule contains the aerogel tray, and an arm to extend the tray. The opposite end of the spacecraft has the main dust shield, and the interface to the launch vehicle. Two sides of the spacecraft body hold solar arrays. Unlike most other missions, the silicon arrays do not articulate to track the sun after their initial deployment. The spacecraft is fairly passive and generates adequate power during the lengthy cruise portions of the mission. The encounter phase, when Stardust must orient the collector and dust shields at Wild 2 regardless of solar illumination, is relatively brief. Each array also has a dust shield. The remaining sides of the spacecraft contain the communications dish and scientific instruments.
Stardust runs VxWorks, an embedded operating system developed by Wind River Systems, on a RAD6000 32-bit processor. There are 128 megabytes for both program space and data collection.
Science payload
Aerogel sample collectors
Comet and interstellar particles are collected in ultra low density aerogel. More than 1,000 square centimeters of collection area is provided for each type of particle (cometary and interstellar). The collector tray contains ninety blocks of aerogel in a metal grid. The appearance of the grid has been likened to an ice cube tray; the round collector is about the size of a tennis racket.
When the spacecraft flew past the comet, the impact velocity of the particles in the coma as they were captured was 6100 metres per second, up to nine times the speed of a bullet fired from a rifle. Although the captured particles were each smaller than a grain of sand, high-speed capture could have altered their shape and chemical composition — or vaporized them entirely.
To collect the particles without damaging them, a silicon-based solid with a porous, sponge-like structure is used in which 99.9 percent of the volume is empty space. Aerogel is 1,000 times less dense than glass, another silicon-based solid. When a particle hits the aerogel, it buries itself in the material, creating a carrot-shaped track up to 200 times its own length, as it slows down and comes to a stop — like an airplane setting down on a runway and braking to reduce its speed gradually. Since aerogel is mostly transparent — a property earning it the nickname "solid smoke" or "blue smoke" — scientists will use these tracks to find the tiny particles.
The aerogel was packed in a Sample Return Capsule (SRC) which was released from the spacecraft just before reentry, for a separate landing on a parachute, while the rest of the spacecraft fired its engines, putting it into orbit around the sun.
While there was some concern about this landing, as the capsule shares a parachute design with Genesis, a solar probe whose parachute did not deploy properly in 2004 due to an assembly and integration error, the Utah landing saw the spacecraft arrive intact and within a minute of estimates.
To analyse the aerogel for interstellar dust, about one million photographs will be taken, each one of a very small section of the gel. These will be distributed to home computer users who will be credited for any particles found, in a program called Stardust@home modeled after SETI@home and Mars Clickworkers.
Comet and Interstellar Dust Analyzer (CIDA)
The CIDA instrument is a time-of-flight mass spectrometer that determines the composition of individual dust grains which collide with a silver impact plate.
The purpose of the Cometary and Interstellar Dust Analyzer (CIDA) instrument on Stardust is to intercept and perform real-time compositional analysis of dust as it is encountered by the spacecraft for transmission back to Earth.
The CIDA separates ions' masses by comparing differences in their flight times. The operating principle of the instrument is the following: when a dust particle hits the target of the instrument, ions are extracted from it by the electrostatic grid. Depending on the polarity of the target positive or negative ions can be extracted. The extracted ions move through the instrument, are reflected in the reflector, and detected in the detector. Heavier ions take more time to travel through the instrument than lighter ones, so the flight times of the ions are then used to calculate their masses.
The CIDA is the same instrument design that flew on Giotto and two Vega program spacecraft where it obtained unique data on the chemical composition of individual particulates in Halley's coma. It consists of an inlet, a target, an ion extractor, a time-of-flight (TOF) mass spectrometer (MS) and an ion detector.
The co-investigator in charge of the CIDA is Jochen Kissel of Max-Planck-Institut für extraterrestrische Physik in Garching bei München, Germany where the instrument was developed. Electronics hardware was built by von Hoerner & Sulger GmbH in Schwetzingen Germany. Software for the CIDA instrument is developed by The Finnish Meteorological Institute.
Navigation camera (NavCam)
The Navigation camera is used for targeting the flyby of the Wild 2 nucleus, but also provides high-resolution science images of the comet.
The Navigation Camera (NC), an engineering subsystem, was used to optically navigate the spacecraft upon approach to the comet. This allowed the spacecraft to achieve the proper flyby distance, near enough to the nucleus, to assure adequate dust collection. The camera also served as an imaging camera to collect scientific data. The data includes high-resolution color images of the comet's nucleus, on approach and on departure, and broadband images at various phase angles while nearby. These images were used to construct a 3-D map of the nucleus in order to better understand its origin, morphology, to search for mineralogical inhomogeneities on the nucleus, and potentially to supply information on the nucleus rotation state. The camera will provide images, taken through different filters, that gave information on the gas and dust coma during approach and departure phases of the mission. These images are providing information on gas composition, gas and dust dynamics, and jet phenomena (if they exist).
The camera peers out of a "periscope." An initial fold mirror looks past the dust shield, and keeps the body of the camera out of the path of damaging dust particles. A scan mirror then gives the camera some panning capability, independent of the spacecraft orientation. This dual-mirror design also provides robustness. Upon approach to the nucleus, both mirrors are used to navigate and take images. Then, when the spacecraft is retreating from Wild 2, the camera looks "backward" by turning the scan mirror, bypassing the fold mirror. If comet dust has etched the fold mirror on approach, the mission can still take images with the clean scan mirror. Etching from Wild 2 did not appear to be severe; the spacecraft can still image future objects with either method.
Early in the mission, contamination threatened the camera's performance. Volatile substances from elsewhere on the spacecraft escaped in the vacuum of space ("outgassing"), and some redeposited on the camera, resulting in cloudy images. Although this did not impact the primary mission goal (the aerogel collectors), it would reduce the science return from Wild 2. Electric heaters, used to maintain the camera at a moderate temperature, were overdriven to "boil" off the contamination. The majority of deposits were eliminated, and test images were deemed acceptable. A similar problem appeared on the Cassini mission, with similar techniques and results.
Dust shield and monitors
Whipple shield
The Whipple shield is designed to protect the spacecraft during its flyby of comet Wild 2. It consists of three sections, two protecting the solar panels and one protecting the main spacecraft body. The first layer is made of composite panels. The panels are augmented by blankets of Nextel ceramic cloth. The shield is designed to protect Stardust from particles as large as 1 cm in diameter.
Dust Flux Monitors (DFM)
The DFM instrument, mounted on the front of the Whipple shield, monitors the flux and size distribution of particles in the environment.
Developed under the direction of Tony Tuzzalino at the University of Chicago, the DFMI is a highly sensitive instrument designed to detect particles as small as a few micrometres. It is based on a very special polarized plastic (PVDF) that generates electrical pulses when impacted or penetrated by small high speed particles.
The Dust Flux Monitor Instrument (DFMI) consists of a Sensor Unit (SU), Electronics Box (EB), and the two acoustic sensors mounted to the Stardust spacecraft. The SU is mounted to the Whipple shield, and the EB is mounted internally to the spacecraft enclosure.
Sample processing
The samples returned by the spacecraft were flown by military transport from Utah to Ellington Air Force Base in Houston, Texas, then transferred by road to the Johnson Space Center in Webster, Texas. NASA officials said "prudence" dictated that the materials be transferred in secrecy, though the agency said they had received no specific security threats. According to the Houston Chronicle, the sample container was taken to a clean room facility which has "a cleanliness factor 100 times that of a hospital operating room to ensure the star and comet dust is not contaminated by earthly grime." Johnson Space Center is also the home of most of the moon rock samples brought back by the Apollo missions.
NASA made a preliminary estimation of a million microscopic specks of dust embedded in Stardust's aerogel collector. There are about 10 particles of 100 micrometers in size. The largest is around a millimeter. Johnson Space Center is the curator of the samples collected, as well as the interstellar dust particles, while as many as 150 scientists worldwide are analyzing those samples.
There is also an estimated 45 interstellar dust impacts on the Stardust Interstellar Dust Collector (SIDC), which resides on the flip side of the cometary dust collector. The search for these grains is being done by a volunteer team through the distributed computing project called Stardust@Home.
Sample analysis
Seven papers in Science magazine (December 2006) discuss details of the sample analysis. Among their findings are discoveries of a wide range of organic compounds, including two that contain biologically usable nitrogen. Indigenous aliphatic hydrocarbons were found with longer chain lengths than those observed in the diffuse interstellar medium. The Stardust samples contain abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene. The presence of crystalline silicates in Wild 2 is consistent with mixing of solar system and interstellar matter, something which had been deduced spectroscopically from previous astronomical observations. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust. Very few pure carbon (CHON) particles were found in the samples returned. However, the organic compounds methylamine and ethylamine derived from the comet were found in aerogel not associated with specific particles.
See also
External links
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