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The INTEGRAL spacecraft
Industrial contractors from all over Europe built the
spacecraft, andthe experiments were contributed by academic and
industrial con-sortia, also from various European countries. In
addition, theRussian Space Agency agreed to launch INTEGRAL free,
inexchange for observing time.
The launch is the culmination of almost a decade of work,
startedin 1993. After much preparation, the prime contractor
(AleniaAerospazio) was chosen. The design phase started in 1995
andwas followed by the building of various test models of the
space-craft. This lasted until 1999, by which time the tests were
complete
and the actual spacecraft, known as the flight model, was
con-structed. In 2001, the finished spacecraft began its final
tests,ready for launch at the end of 2002.
The coded mask beingused on SPI. Photo: ESA
Gamma rays from d ifferentobjects pass through differentregions
of the coded maskbefore striking the detector.The resulting pattern
is asequence of overlaid shadowswhich a computer mustdisentangle,
to create animage of the objects.
The INTEGRAL spacecraft is composed of two main sections, the
servicemodule and the payload module. The service module is the
lower part of the
satellite. It provides essentials such as power (via solar
panels), satellitecontrol and the communications link to the
ground. The payload module isconnected to the service module and
consists of the four scientific instru-ments. To reduce the cost as
much as possible, the service module designfrom ESAs XMM-Newton
satellite was reused.
To catch a gamma ray
Because gamma rays and X-rays are so difficultto focus, three of
INTEGRAL's four scienceinstruments rely on a technique called
coded-mask imaging. In this method, traditional mir-
rors and lenses are replaced with a mask, whichcontains a
pattern of holes. Gamma rays canpass through these holes but are
blocked if theyfall onto the mask itself. In effect, the mask
castsa shadow.The gamma rays that make it throughthe holes fall
onto the detector, recording theshadow pattern.
When gamma rays from another direction passthrough the holes,
they arrive from a differentangle and the resulting shadow, cast by
themask, is in a different place on the detector. Inthis way, it
'codes' the radiation so that a com-
puter program, which knows the shape of themask's shadow, can be
used to disentangle theoverlapping patterns and convert them into
animage of the gamma-ray sources in the sky.
Illustration by Medialab, ESA 2002.
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