INTEGRAL

The European Space Agency's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) is detecting some of the most energetic radiation that comes from space. It is the most sensitive gamma ray observatory ever launched.

INTEGRAL is an ESA mission in cooperation with the Russian Space Agency and NASA. 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 holes.

Mission
Because gamma rays and X-rays cannot penetrate Earth's atmosphere, direct observations must be made from space. INTEGRAL was launched from Baikonur spaceport, in Kazakhstan. The 2002 launch aboard a Proton-DM2 rocket achieved a 700 km perigee. 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 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, Germany, ESA's control centre, through ground stations in Belgium (Redu) and California (Goldstone).

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 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 monopropellant system, containing 544 kg of fuel in four exposed tanks. The titanium tanks were charged with gas to 24 bar 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 (hard X-rays) to 10 MeV (gamma rays). Mechanical resolution is 12 arcmin, but deconvolution can reduce that to as little as 1 arcmin. A 95 x 95 mask of rectangular tungsten 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 Cesium-Iodide tiles (PICsIT- Pixellated Cesium-Iodide Telescope). ISGRI is sensitive up to 500 keV, 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 observes radiation between 20 keV and 8 MeV. SPI consists of a coded mask of hexagonal tungsten tiles, above a detector plane of 19 germanium 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 behind the tungsten tiles. It absorbs secondary radiation produced by impacts on the tungsten. The rest of the shield consists of BGO 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). Recently, new algorithms allow the ACS to act as a telescope, through double Compton scattering. 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 plus methane) in a microstrip layout, below a mask of hexagonal tiles.

INTEGRAL mounts an Optical Monitor (OM), sensitive in the visible and ultraviolet from 500 nm and up. 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.