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Imagine having to watch a wildlife TV programme in black-and-white rather
than colour. Astronomers suffer the same frustration, and lose far more
information, when they view the Universe in only one part of the electromagnetic
spectrum.

NASA’s version of an astronomical colour TV is a suite of four great
observatories, designed to view the Universe across the whole spectrum.
Hubble, tuned to visible light, ultraviolet and near infrared, was the first.
The agency’s long-term programme set next week for the launch of its second
– the Gamma Ray Observatory (GRO). Later this decade, the advanced X-ray
facility and the space-based infrared facility will join Hubble and the
GRO in orbit.

As well as providing a complete picture of the Universe at all wavelengths,
the great observatories are designed to work at far higher sensitivities
and resolutions than their precedessors. That is especially true of the
GRO, because gamma ray astronomy is still in its infancy.

Gamma rays form the most energetic and extensive part of the spectrum,
ranging upwards from energies of some 10 000 electronvolts. By comparison,
the energy associated with visible light is about 1 electronvolt.

Because gamma rays are generated by events within nuclei they are messengers
from the most violent and energetic events in the cosmos, providing details
of supernovas, pulsars and quasars. They travel relatively unmolested through
the Universe. Unlike visible light, clouds of dust do not block their passage.
But the Earth’s atmosphere is impenetrable to all but the most energetic
gamma rays. The rest interact with atomic nuclei in the atmosphere and are
converted to lower-energy radiation.

So, the study of gamma ray astronomy, except in balloons, has had to
wait for the space age. Several satellites, including the European Cos-B
and America’s High Energy Astronomical Observatory, have made enough pioneering
observations to tantalise astronomers with the promise of rich scientific
rewards.

The detail that these early satellites garnered was limited by their
physical size. Astronomical objects, on the whole, do not emit gamma ray
photons in great numbers. To collect enough photons to make a sensitive,
high-resolution picture of the gamma ray sky, large detectors are a must.

Some of the instruments on the GRO are larger than the Cos-B satellite
– about the size of a small car. At 17 tonnes the spacecraft will be the
largest automatic instrument to have been launched and occupies a third
of the shuttle’s payload bay. Despite its size, each of the GRO’s ‘exposures’
will need to last two weeks to record a picture with enough detail to be
useful.

Gamma rays interact with matter in different ways, according to how
energetic they are. It is through the products of these interactions that
astronomers detect them. Four instruments are needed to cover the GRO’s
full range of 20 000 electron volts (20 keV) to 30 billion electron volts
(30 GeV). Each records the arrival of single photons.

The instruments dealing with the lowest-energy gamma rays are the burst
and transient source experiment (BATSE), sensitive to gamma rays from 20
keV to 6 MeV, and the oriented scintillation spectrometer experiment (OSSE),
sensitive to wavelengths from 100 keV to 10 Mev. The two have different
purposes but work on the same principle. They both contain slabs of material
that produce a flash of light when struck by a gamma ray in the right energy
band. These ‘scintillation’ counters are connected to photomultipliers which
amplify the minute flashes to manageable levels.

As the name suggests, the BATSE’s job is to acquire information about
short-lived bursts of gamma rays. From time to time, an enormously intense
burst sears through the Universe. For the few seconds that the bursts last,
they outshine all other gamma ray sources combined. One such, in 1979, appeared
to come from the Large Magellanic Cloud with a luminosity 100 billion times
that of the Sun.

Gamma ray bursts have intrigued astronomers since satellites monitoring
the nuclear test-ban treaty detected them in 1967. Several of the satellites
were deployed to identify the site of any illicit nuclear tests by timing
the arrival of radiation from explosions. ¿ìè¶ÌÊÓƵs from the Los Alamos
laboratory in the US, who were controlling the satellites, realised that
the gamma rays came from outside the Solar System.

The BATSE has eight scintillation counters whose field of view covers
the whole sky not blocked by the Earth. By comparing the number of photons
hitting each detector, astronomers should be able to determine the position
of the source of any bursts to within one degree. That measurement would
not solve the question of how close to Earth a source of gamma rays is,
but it might pinpoint obvious candidates.

The instrument can also record changes in the intensity of gamma rays
over fractions of a millisecond, giving a better understanding of the nature
of gamma ray bursts.

By contrast, the OSSE is designed to point at one source rather than
the whole sky. It contains four sodium iodide scintillation counters with
photomultipliers.

Each of its detectors has its own pointing mechanism. This allows one
part of the instrument to point directly at a source, while another, offset,
records background radiation. The ability to subtract background levels
will allow the instrument to view weak gamma ray sources. If photons arrive
from a burst or from a solar flare – another rich source of gamma rays –
one part of the OSSE can be reoriented without interrupting the rest of
the GRO’s observations.

The satellite’s other instruments exploit two completely different physical
interactions to detect higher-energy gamma rays. The Compton telescope (Comptel)
observes gamma rays from 1 to 30 Mev. It exploits a phenomenon known as
Compton scattering, where high-energy radiation gives up some of its energy
to electrons in a material, before passing through at lower energy.

In Comptel, the Compton scattering takes place in an array of liquid
detectors. The reduced-energy photons, scattered by the liquid, can then
be detected as they impinge on nearby scintillation counters.

The remaining instrument, the energetic gamma ray experiment telescope
(EGRET) detects the highest-energy photons of all, from 10 MeV to 10 GeV.
It relies on the ability of a very high-energy gamma ray to generate an
electron and its antimatter equivalent, a positron, when it encounters metal.

The paths of the particle and antiparticle can be traced as they pass
through a spark chamber underneath the metal detector. The angle between
the two gives the direction of the incoming gamma ray. When the two particles
have passed through the spark chamber, they are absorbed by a crystal and
emit light. Photomultipliers detect the light.

One of the chief problems anticipated for the observatory will be the
task of distinguishing between gamma rays and the high-energy particles
in cosmic rays. These also interact with scintillators to create light flashes,
and are far more abundant than gamma ray photons.

Donald Kniffen, the project scientist for the GRO at NASA’s Goddard
Spaceflight Center at Greenbelt, Maryland, says that trying to see gamma
rays against the cosmic ray background is like trying to view the stars
during the day.

To ease the problem, the GRO’s instruments are protected by shielding
of low-density plastic that allows gamma rays to pass through, but stops
cosmic rays.

The problem is compounded, however, by the fact that when cosmic rays
hit the plastic, gamma rays are produced. Clive Dyer, from Britain’s Royal
Aircraft Establishment, has the unenviable task of advising scientists working
on the GRO how to distinguish between gamma rays from space and those generated
by cosmic rays.

For 15 months after launch, the GRO will conduct the first full survey
of the gamma ray sky. It will do this with sensitivities 10 to 50 times
greater than previous studies, depending on the energy band. The survey
will give astronomers information about gamma rays in a volume of space
some 300 times greater than they have now.

‘As the survey progresses,’ says Kniffen, ‘we’ll get an idea of what
specifically we should do next.’ Also in the first 15 months, NASA is funding
theoreticians to come up with hypotheses for the GRO to test.

But even now, astronomers have a good idea of what they want to study.
Gamma rays are emitted when atomic nuclei are excited. The GRO’s survey
will therefore be able to gather information on the hottest spots in the
Universe. John Bahcall, from the Institute of Advanced Studies at Princeton
University and chairman of the committee that has just set America’s goals
in astronomy for the next decade, says: ‘This is the only part of the spectrum
that allows us to view nuclear processes directly.’

The processes that emit gamma rays at characteristic wavelengths include
interactions of charged particles, such as electrons, protons and heavy
nuclei with magnetic, electric and gravitational fields. The distribution
of gamma rays at different frequencies should give astronomers clues about
the characteristics of the fields.

One nuclear process that astronomers would particularly like to observe
directly is the formation of heavy elements. Theory and evidence suggest
that stars collapse under their own gravitational fields when the nuclear
fuel at their centre is depleted, and that the collapse releases enough
energy to blow the envelope of the star into space in a supernova explosion.

The temperatures of the explosion would be high enough for nuclear reactions
to forge heavy elements. As these radioactive elements decay, they release
gamma rays. The GRO will probe such events.

The observatory will look, too, for evidence of black holes, objects
so massive and with such immense gravitational fields, that not even light
can escape from them. When a black hole inexorably pulls matter into its
maw, that matter heats up to temperatures where nuclear changes occur and
gamma rays may be emitted. Such a black hole may lie at the centre of the
Milky Way, which in astronomical terms, is a minuscule 30 000 light years
away. The GRO will turn its detectors in that direction.

The observatory has also been assigned the job of collecting data to
determine the nature of the background gamma radiation that is apparently
distributed evenly through the Universe. That radiation could simply be
the combination of gamma rays from all galaxies.

NASA is paying $557 million of the GRO’s $617 million cost, and is providing
the launch. Germany, which built the Compton telescope, pays for most of
the remainder, and there are contributions from the European Space Agency,
the Netherlands and Britain.

Kniffen says that NASA plans to run the GRO for four years, but the
spacecraft will carry enough fuel to stay in orbit for 10 years and still
have enough left to position itself for a controlled re-entry to the Earth’s
atmosphere.

Early last year, NASA established the GRO science support centre to
provide backup to scientists wanting to working with data from the observatory.
¿ìè¶ÌÊÓƵs will be able to take raw data and analyse it themselves, or wait
for NASA to process it.

The observatory will send data via the tracking and data relay satellite
to the White Sands ground station in New Mexico. From there the information
goes to Goddard for preliminary processing and is shunted directly on to
the scientists responsible for the different instruments.

Currently, scientists at Goddard are trying to establish ways to make
data from the GRO compatible with a standard for astronomical data called
the flexible image transport system (FITS). The International Astronomical
Union established these standards to enable scientists to take data from
one astronomical instrument, such as an infrared telescope, and combine
it with data from another, perhaps an optical telescope. ‘Unfortunately,’
says Kniffen, ‘standards for data from gamma ray observations are not that
well established, and this is causing us some problems as we will have to
start processing data in a few months.’

Otherwise Kniffen is optimistic. He says the whole system has been checked
extensively from detection of gamma rays through to the electronics that
will pick up signals from the photomultipliers and relay them to Earth.
The failures of Hubble, of course, forced the GRO team to review its preparations
with an even more critical eye. But Kniffen is confident: ‘We believe that
we have a good spacecraft.’