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INTEGRAL

INTEGRAL

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The European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) is an operational Earth satellite, launched in 2002 for detecting some of the most energetic radiation that comes from space. It is the most sensitive gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...

 observatory ever launched.

INTEGRAL is an ESA mission in cooperation with the Russian Space Agency
Russian Federal Space Agency
The Russian Federal Space Agency , commonly called Roscosmos and abbreviated as FKA and RKA , is the government agency responsible for the Russian space science program and general aerospace research. It was previously the Russian Aviation and Space Agency .Headquarters of Roscosmos are located...

 and NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

. It has had some notable successes, for example in detecting a mysterious 'iron quasar'. It has also had great success in investigating gamma-ray bursters and evidence for black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

s.

Mission


Because gamma rays and X-rays cannot penetrate Earth's atmosphere, direct observations must be made from space. INTEGRAL was launched from Baikonur
Baikonur
Baikonur , formerly known as Leninsk, is a city in Kyzylorda Province of Kazakhstan, rented and administered by the Russian Federation. It was constructed to service the Baikonur Cosmodrome and was officially renamed Baikonur by Russian president Boris Yeltsin on December 20, 1995.The shape of the...

 spaceport, in Kazakhstan
Kazakhstan
Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...

. The 2002 launch aboard a Proton
Proton rocket
Proton is an expendable launch system used for both commercial and Russian government space launches. The first Proton rocket was launched in 1965 and the launch system is still in use as of 2011, which makes it one of the most successful heavy boosters in the history of spaceflight...

-DM2 rocket achieved a 700 km perigee
Perigee
Perigee is the point at which an object makes its closest approach to the Earth.. Often the term is used in a broader sense to define the point in an orbit where the orbiting body is closest to the body it orbits. The opposite is the apogee, the farthest or highest point.The Greek prefix "peri"...

. The onboard thrusters then raised the perigee out of the residual atmosphere, and the worst regions of the radiation belts. The apogee was trimmed with the thrusters to synchronize with Earth's rotation, and thus, the satellite's ground stations.

INTEGRAL's operational orbit has a period of 72 hours, and has a high eccentricity, with perigee
Perigee
Perigee is the point at which an object makes its closest approach to the Earth.. Often the term is used in a broader sense to define the point in an orbit where the orbiting body is closest to the body it orbits. The opposite is the apogee, the farthest or highest point.The Greek prefix "peri"...

 close to the Earth at 10,000 km, within the magnetospheric radiation belt. However, most of each orbit is spent outside this region, where scientific observations may take place. It reaches a furthest distance from Earth (apogee) of 153,000 km. The apogee was placed in the northern hemisphere, to reduce time spent in damaging eclipses, and maximize contact time over the ground stations in the northern hemisphere.

It is controlled from ESOC in Darmstadt
Darmstadt
Darmstadt is a city in the Bundesland of Hesse in Germany, located in the southern part of the Rhine Main Area.The sandy soils in the Darmstadt area, ill-suited for agriculture in times before industrial fertilisation, prevented any larger settlement from developing, until the city became the seat...

, Germany, ESA's control centre, through ground stations in Belgium (Redu) and California (Goldstone
Goldstone Deep Space Communications Complex
The Goldstone Deep Space Communications Complex  — commonly called the Goldstone Observatory — is located in California's Mojave Desert. Operated by ITT Corporation for the Jet Propulsion Laboratory, its main purpose is to track and communicate with space missions. It includes the Pioneer...

).

Fuel usage is within predictions. INTEGRAL has already exceeded its 2.2-year planned lifetime; barring mechanical failures, it should continue to function for six years or more.

Spacecraft


The spacecraft body ("service module") is a copy of the XMM-Newton
XMM-Newton
The XMM-Newton is an orbiting X-ray observatory launched by ESA in December 1999 on a Ariane 5 rocket...

 body. This saved development costs and simplified integration with infrastructure and ground facilities. (An adapter was necessary to mate with the different booster, though.) However, the denser instruments used for gamma rays and hard X-rays make INTEGRAL the heaviest scientific payload ever flown by ESA.

The body is constructed largely of composites. Propulsion is by a hydrazine
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

 monopropellant
Monopropellant
Monopropellants are propellants composed of chemicals or mixtures of chemicals which can be stored in a single container with some degree of safety. While stable under defined storage conditions, they react very rapidly under certain other conditions to produce a large volume of energetic gases...

 system, containing 544 kg of fuel in four exposed tanks. The titanium tanks were charged with gas to 24 bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

 (2.4 MPa) at 30 °C, and have tank diaphragms. Attitude control is via a star tracker, multiple Sun sensors, and multiple momentum wheels. The dual solar arrays, spanning 16 meters when deployed and producing 2.4 kW BoL, are backed up by dual nickel-cadmium battery sets.

The instrument structure ("payload module") is also composite. A rigid base supports the detector assemblies, and an H-shaped structure holds the coded masks approximately 4 meters above their detectors. The payload module can be built and tested independently from the service module, reducing cost.

Alenia Spazio was the spacecraft prime contractor.

Instruments


Four instruments are coaligned to study a target across several ranges. The coded masks were led by the University of Valencia, Spain.

The INTEGRAL imager, IBIS (Imager on-Board the INTEGRAL Satellite) observes from 15 keV
Kev
Kev can refer to:*Kev Hawkins, a fictional character.*Kevin, a given name occasionally shortened to "Kev".*Kiloelectronvolt, a unit of energy who symbol is "KeV".* Krefelder Eislauf-VereinKEV can refer to:...

 (hard X-rays) to 10 MeV
MEV
MeV and meV are multiples and submultiples of the electron volt unit referring to 1,000,000 eV and 0.001 eV, respectively.Mev or MEV may refer to:In entertainment:* Musica Elettronica Viva, an Italian musical group...

 (gamma rays). Angular resolution is 12 arcmin, enabling a bright source to be located to better than 1 arcmin. A 95 x 95 mask of rectangular tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

 tiles sits 3.2 meters above the detectors. The detector system contains a forward plane of 128 x 128 Cadmium-Telluride tiles (ISGRI- Integral Soft Gamma-Ray Imager), backed by a 64 x 64 plane of Caesium-Iodide tiles (PICsIT- Pixellated Caesium-Iodide Telescope). ISGRI is sensitive up to 1 MeV, while PICsIT extends to 10 MeV. Both are surrounded by passive shields of tungsten and lead.

The primary spectrometer aboard INTEGRAL is SPI, the SPectrometer for INTEGRAL. It was conceived and assembled by the French Space Agency CNES
CNES
The is the French government space agency . Established under President Charles de Gaulle in 1961, its headquarters are located in central Paris and it is under the supervision of the French Ministries of Defence and Research...

. It observes radiation between 20 keV
Kev
Kev can refer to:*Kev Hawkins, a fictional character.*Kevin, a given name occasionally shortened to "Kev".*Kiloelectronvolt, a unit of energy who symbol is "KeV".* Krefelder Eislauf-VereinKEV can refer to:...

 and 8 MeV
MEV
MeV and meV are multiples and submultiples of the electron volt unit referring to 1,000,000 eV and 0.001 eV, respectively.Mev or MEV may refer to:In entertainment:* Musica Elettronica Viva, an Italian musical group...

. SPI consists of a coded mask of hexagonal tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

 tiles, above a detector plane of 19 germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....

 crystals (also packed hexagonally). The Ge crystals are actively cooled with a mechanical system, and give an energy resolution of 2 keV at 1 MeV.

IBIS and SPI need a method to stop background radiation. The SPI ACS (AntiCoincidence Shield) consists of a mask shield and a detector shield. The mask shield is a layer of plastic scintillator
Scintillator
A scintillator is a special material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light...

 behind the tungsten tiles. It absorbs secondary radiation produced by impacts on the tungsten. The rest of the shield consists of BGO
Bismuth germanate
Bismuth germanium oxide is an inorganic chemical compound with main use as a scintillator. It forms cubic crystals....

 scintillator tiles around the sides and back of the SPI.

The enormous area of the ACS that results makes it an instrument in its own right. Its all-sky coverage and sensitivity make it a natural gamma-ray burst detector, and a valued component of the IPN
InterPlanetary Network
The InterPlanetary Network is a group of spacecraft equipped with gamma ray burst detectors. By timing the arrival of a burst at several spacecraft, its precise location can be found. The farther apart the detectors, or the greater the timing precision of each detector, the more precise the GRB...

 (InterPlanetary Network). Recently, new algorithms allow the ACS to act as a telescope, through double Compton scattering
Compton scattering
In physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect...

. Thus ACS can study objects outside the field of view of the other instruments, with surprising spatial and energy resolution.

Dual JEM-X units provide additional information on targets. They observe in soft and hard X-rays, from 3 to 35 keV. Aside from broadening the spectral coverage, imaging is more precise due to the shorter wavelength. Detectors are gas scintillators (xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...

 plus methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

) in a microstrip layout, below a mask of hexagonal tiles.

INTEGRAL mounts an Optical Monitor (OMC), sensitive from 500 to 580 nm. It acts as both a framing aid, and can note the activity and state of some brighter targets.

The spacecraft also mounts a radiation monitor, INTEGRAL Radiation Environment Monitor (IREM), to note the orbital background for calibration purposes. IREM has an electron and a proton channel, though radiation up to cosmic rays can be sensed. Should the background exceed a preset threshold, IREM can shut down the instruments.

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