Integral is the International Gamma Ray Astrophysics Laboratory of the
European Space Agency. It is a cooperative mission with Russia and is
scheduled for launch on 17 October 2002 from the Baikonur Cosmodrome,
Kazakhstan, on a Russian Proton rocket, the Russian contribution to the
programme. It is the world’s most advanced gamma-ray telescope and will
provide first-hand observations of the celestial objects that release some
of the most energetic radiation of the Universe. In particular,
scientists have designed Integral to simultaneously capture gamma rays,
X-rays, and visible light from these objects, allowing astronomers on
Earth to fully analyse them.

Gamma rays are released by the most violent events in the Universe.
Unlike the serene beauty of the stars that we can see with our own eyes,
the gamma-ray Universe is a place of wild explosions, cosmic collisions,
and matter being sucked into black holes or trapped in super-strong
magnetic fields. So far, astronomers have only had glimpses of this
violence; Integral will bring it into sharp focus.

Exploring the turbulent Universe

Gamma rays carry large quantities of energy away from the violent events
where they are created, such as supernova explosions, black holes, and the
mysterious gamma-ray bursts. Integral will find a lot more out about these
powerful gamma-ray sources.

Very massive stars end their lives in big explosions called supernovae.
These outbursts liberate more energy than the combined light of millions
upon millions of stars, much of it in the form of gamma rays. New chemical
elements are created as results of such explosions. In fact, all atoms
heavier than iron are created due to such explosions. For this reason, we
call supernovae the chemical factories” of the Universe. However, we do
not know completely how new atoms are created when a star explodes.
Integral will look into it as one of its first scientific objectives.

After the explosion, each supernova leaves behind a dead ‘heart’. This
heart is incredibly dense and can be either a neutron star or a black
hole. Both can generate gamma rays because they possess incredibly strong
gravitational fields that can capture passing dust, gas and, possibly,
larger celestial objects. When matter falls through a gravitational
field, it heats up and releases energy. In the case of neutron stars and
black holes, the energy released is very intense and is given off in the
form of x-rays and gamma rays.

As well as black holes from supernovae, called stellar black holes, the
Universe contains a variety of far more massive black holes that are found
at the core of some galaxies, the galactic black holes. Galactic black
holes also give off gamma rays, and with such awesome power that you can
detect them almost halfway across the known Universe.

As well as making the most accurate studies of these objects to date,
Integral will also investigate the mysterious blasts of gamma rays that
explode across the Universe about once a day, the gamma-ray bursts. They
can last just a few seconds and can come from any direction in space. The
origin of gamma-ray bursts has remained unexplained for years, from their
first observation in the late 1960s. Today, many scientists think that
gamma ray bursts could be linked to the death throes of the very first
stars. Alternatively, they could be generated by colliding neutron stars,
or could be caused by the explosion of supermassive stars at the end of
their lives, the hypernovae. Integral’s instruments will study gamma-ray
bursts with the highest accuracy ever and may discover something about
their origins.

Integral’s instruments

Integral has four instruments to give the spacecraft maximum versatility
in its task of studying the gamma-ray Universe. Designed to complement
each other, their combined observations will allow scientists to get a
very complete and accurate picture of each celestial target at different
wavelengths.

The first two are dedicated gamma-ray instruments. Imager on Board the
Integral Satellite (IBIS) is the sharpest-resolution gamma-ray camera ever
built. Spectrometer on Integral (SPI) will measure the energy of gamma
rays with exceptional accuracy. In particular, it will be more sensitive
to fainter radiation than any previous gamma-ray spectrometer. The other
two instruments are designed to provide complementary scientific data
about Integral’s targets. The Joint European X-Ray Monitor (JEM-X) will
make observations simultaneously with the main gamma-ray instruments and
will provide images at X-ray wavelengths. The Optical Monitoring Camera
(OMC) will do the same but at visible-light wavelengths. The total weight
of the four instruments is about 2 tonnes, roughly half the launch weight
of Integral.

Integral’s orbit and operations

After launch, Integral will follow an elliptical orbit that is inclined by
51.6? to the Earth’s equator. In this orbit, it will cycle between 9000
kilometres and 153 000 kilometres above Earth, completing one revolution
of the Earth every 72 hours. This eccentric orbit is necessary because
there are ‘radiation belts’ that surround the Earth and these would
interfere with Integral’s ability to see gamma rays. It is important for
Integral to be outside these belts. Its elliptical orbit is designed to
keep it outside the radiation belts for 90% of its trajectory around
Earth.

Once Integral is in orbit, it must communicate with Earth to download its
scientific data and to receive commands. Communicating with and
controlling Integral is a task spread over a number of different sites.
Firstly, astronomers submit proposals for observations to the Integral
Science Operations Centre (ISOC) at Noordwijk, The Netherlands. Experts
at ISOC evaluate the proposals and draw up a list of targets and detailed
observation schedules for Integral. The schedules are sent to the
Mission Operations Centre (MOC) at the European Space Operations Centre
(ESOC) in Darmstadt, Germany. There everything is transformed into
commands that Integral will understand. Signals to and from Integral go
through two tracking stations, one at Redu in Belgium, the second at
Goldstone in California, United States. The MOC also ensures the correct
performance of the spacecraft.

After Integral has collected observations, the raw science data is
forwarded to the Integral Science Data Centre (ISDC) in Versoix near
Geneva, Switzerland. There it is converted into usable data files,
archived, and distributed to the astronomical community. A worldwide
network of space science institutes and observatories will receive the
data very quickly. This is essential especially when sudden and
short-lasting phenomena such as gamma-ray bursts occur. In this case, all
observatories need to receive the information within one minute to be able
to point their telescopes immediately at the area of the sky where the
gamma-ray burst has been detected.

Building Integral

Integral was selected as a mission by ESA in June 1993. The prime
contractor for the spacecraft was Alenia Aerospazio, Turin, Italy. Alenia
involved 26 subcontracting companies from 12 European countries to build
the spacecraft’s service module. This provides the essentials for the
spacecraft such as power (via solar panels), satellite control, and the
communications link to the ground. Alenia was also responsible for
integrating the four science instruments on-board the spacecraft, known
collectively as the payload module. Four consortia of academic and
industrial partners, variously located throughout Europe, built the
instruments.

Integral has faced many technological challenges. However, the greatest
was finding a way to focus gamma rays, which are so powerful they pass
through ordinary mirrors. To overcome this, Integral’s gamma-ray
instruments and its X-ray monitor use a technique called coded-mask
imaging. Instead of focusing, the coded mask blocks some gamma rays,
creating a recognisable shadow on the detector beneath. Ground computer
systems process the data coming from the gamma-ray detector looking for
this shadow. Once it finds the shadow pattern, it groups the gamma rays
together, forming an image. Gamma rays from different astronomical
objects enter the instruments at different angles and so cast different
shadows, allowing gamma rays from multiple sources to be separated.

Integral has been developed and built at a cost of 330 million Euros.
This price does not include the cost of launch, which Russia is providing
free in exchange for observing time on Integral. Neither does the cost
include the price of the science instruments, which have been provided by
academic and industrial consortia. To reduce costs, the design for the
service module was reused from ESA’s XMM-Newton satellite.

Note to editors: historical perspective on gamma-ray astronomy

Scientists have placed small gamma-ray detectors on satellites since the
early 1960s. However, the most extraordinary discovery came in the late
1960s from a series of military satellites designed to monitor the ban on
nuclear bombs being tested on Earth. These satellites detected the
appropriately named gamma-ray bursts, which explode without warning about
once a day, from random directions in the sky.

In 1972, the NASA probe SAS-2 confirmed that the Universe is bathed in a
perpetual shower of gamma rays. In 1975, ESA launched the gamma-ray
satellite COS-B, that worked until being switched off in 1982. COS-B
produced the first map of the gamma-ray sky and identified a number of
bright gamma ray sources. It was followed by the Russian-French mission
GRANAT, in 1989-1998, and NASA’s Compton Gamma-ray Observatory (CGRO), in
1991-2000. The CGRO satellite greatly increased our understanding of
gamma-ray astronomy. Soon we can expect Integral to dazzle the world with
the next leap in technology.

For more information, please contact:

ESA Media Relations Office
Tel: +33 (0) 1 5369 7155
Fax: +33 (0) 1 5369 7960

Kai Clausen, ESA – Integral Project Manager
Tel: +31 (0)71 565 3467
E-mail: Kai.Calusen@esa.int

Christophe Winkler, ESA – Integral Project Scientist
Integral Project Scientist
Tel: +31 (0)71 565 3591
E-mail: Christophe.Winkler@esa.int

Arvind Parmar, ESA – Integral Acting Project Scientist
Tel: +31 (0)71 565 4532
E-mail: Arvind.Parmar@esa.int

More information about Integral and its role are on the following Internet
Web pages:

http://www.sci.esa.int/integral
http://www.esa.int/integrallaunch