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IN EVEN the clearest night, from the highest mountain and with the largest
of telescopes, the Earth’s atmosphere restricts our view of the Universe.
The atmosphere partially absorbs, refracts and diffracts chunks of the ultraviolet,
visible and infrared regions of the electromagnetic spectrum, limiting the
clarity of astronomical images. These wavelengths carry a wealth of information
about the history and nature of the Universe, data that astronomers cannot
receive in full on Earth. The Hubble Space Telescope, in its orbit 600 kilometres
up, will be the first telescope to lie above these distorting influences.
The $1.2-billion observatory will see so far and so clearly that astronomers
say only that they expect the unexpected from the data that Hubble will
collect.

Riccardo Giacconi directs the Space Telescope Science Institute at Baltimore,
Maryland, which NASA opened in 1983 to run the science programme for Hubble.
In a speech to the annual meeting of the Astronomical League in 1986, he
said: ‘I believe that ultimately the sociological impact of this new knowledge
will be as great as the revolutions started by Copernicus, Galileo, Kepler
and Newton.’ Many of the world’s astronomers appear to agree. For every
application for observing time in the first year that the institute has
accepted, it has turned down five.

Despite this oversubscription, Giacconi has set aside 20 hours on the
telescope in the first year for amateur astronomers. The 20 hours come from
the director’s discretionary time, which is intended for observations of
likely yet unpredictable phenomena such as a new supernova. Giacconi says
of his allocation of time to amateurs: ‘The dazzling discoveries by scientists
become curiously sterile unless they are assimilated in the general culture
and become part of the intellectual heritage of mankind. Amateur astronomers
have always been instrumental in providing that link and their impact on
general education has been enormous.’

Ana Larson, from Seattle in Washington State, is one of the amateurs
selected from several hundred applicants to work with the orbiting observatory.
Like many, her interest in astronomy stems from viewing the Universe through
binoculars as a child. Her enthusiasm waned when she became preoccupied
by college, raising children and running a business. Now, she says, ‘I am
hooked again.’ Larson is currently attending the University of Washington
part-time to take a degree in physics and astronomy. She has already given
talks to her daughter’s schoolmates and is keen to encourage more people
to follow her example. ‘It’s a lot better than doing the laundry,’ she says.

Larson intends to search for gaseous planets, like Jupiter, at early
stages in their lives, as they form alongside new stars. So far, no one
has found any sign of planets outside the Solar System – Hubble could let
us see one. From her observations, Larson hopes to verify theories of how
gassy planets evolve. Her aim is to detect emissions of radiation in the
near-infrared part of the spectrum, as the embryonic planets cool and contract
from very hot to very warm. Like the other amateurs, Larson will have help
in interpreting her results from the institute. She says: ‘The chances of
finding new planets are real slim, and if we are to see them, they will
need to be at least five times the mass of Jupiter.’

The space telescope is named after Edwin Hubble, the American astronomer
who died in 1953. Hubble’s observations did much to support the now generally
accepted theory of the creation of the Universe. According to this big bang
theory, a massive explosion between 12 billion and 20 billion years ago
created the cosmos. Astronomers believe that within the first three minutes
after the big bang, atomic nuclei appeared, and became atoms of hydrogen
and helium about 500 000 years later. Eventually, this gas coalesced into
galaxies. Astronomers do not know when or how this happened. The space telescope’s
ability to capture faint light that has travelled for between 12 billion
and 14 billion light years will give astronomers a much better view of events
early in the life of the galaxies . Today’s ground-based optical telescopes
see only about 2 billion light years back.

Other measurements will refine an understanding of Hubble’s most important
contribution to astronomy. Hubble observed that all other galaxies are moving
away from our galaxy, the Milky Way, and that their distance increases in
a directly proportional relationship with the speed at which the galaxy
is retreating. In other words, the Universe, or space, is expanding at a
steady rate. That expansion rate, a galaxy’s speed divided by its distance
from us, is defined by the Hubble Constant, which was calculated first in
1927. According to the Space Telescope Science Institute, Hubble’s constant
is currently assigned the value of 22 kilometres per second per million
light years. However, this value is uncertain by a factor of two, because
of the difficulty of measuring the distance of faraway galaxies accurately
from telescopes on the ground. Furthermore, it may not apply to the behaviour
of the remotest galaxies, which might be moving away more slowly.

The space telescope, free of interference from the Earth’s atmosphere,
will investigate Hubble’s Constant by observing a class of stars known as
cepheids, in 50 galaxies neighbouring our own. Cepheids brighten and dim
in a regular manner. From watching how they fluctuate, astronomers will
be able to calculate their distance from Earth accurately, and will then
refine their estimates of Hubble’s constant. They can use this improved
value in other astronomical problems, such as finding the age of the Universe
and whether or not the more distant galaxies are slowing down.

The idea for the space telescope can be traced back to Hermann Oberth,
the father of German rocketry. In 1923, he wrote a paper pointing out the
advantages of a telescope orbiting above the Earth’s atmosphere. Nearly
four decades later, a publication sponsored by the US’s National Academy
of Sciences recommended that the country should adopt the development of
a large space-based telescope as a national goal. In 1971, NASA began feasibility
studies. Four years later, the European Space Agency joined the project
, and by 1981 the telescope’s main mirror was ready.

Hubble should have gone into space in the autumn of 1983, but NASA had
to postpone the launch until the beginning of 1986 because of development
problems. Robert Bless, from the University of Wisconsin at Madison, will
work with the space telescope. He says: ‘The project was much more difficult
than anyone anticipated.’ NASA postponed the launch again until August 1986,
but the shuttle Challenger blew up in January of that year, disrupting the
US’s space programme. Now Hubble is scheduled for launch on the space shuttle
Discovery next week.

Once Hubble is in space, mission controllers will send the signals to
unfurl the solar arrays, that will power the telescope, and the antenna
for sending data to Earth via relay satellites in geosynchronous orbits.
Discovery will stay in orbit for two days, with the astronauts on call to
do any repairs necessary. When the shuttle returns to Earth, mission controllers
will run seven months of tests before Hubble begins observing in earnest.

Hubble will detect radiation in the ultraviolet, visible and near infrared
parts of the spectrum, and will record 20 images a day, starting before
the end of the year. It is one of four observatories planned by NASA to
explore the Universe in detail across the entire electromagnetic spectrum.
The gamma-ray observatory is scheduled for launch later this year, while
the agency has plans for an advanced X-ray telescope in the mid-1990s and
an infrared telescope in 1999.

A lifetime in the Sun

The space telescope is designed to last 15 years, but could well operate
for longer. At some stage, NASA will have to send the shuttle up to reboost
Hubble to its original orbit. Exactly when will depend on the length and
magnitude of the current solar maximum. At present, and every 11 years or
so, when the Sun is at its peak of activity, its radiation heats the Earth’s
atmosphere more than at other times in the cycle. The resulting expansion
of the atmosphere increases the drag on satellites such as Hubble, in low-Earth
orbit, and their orbits decay. At the same time as reboosting Hubble, astronauts
will be able to replace some of the scientific instruments it carries with
updated technology. Ray Villard, a spokesman for the Space Telescope Science
Institute, says that such a visit is unlikely before the mid-1990s.

Hubble weighs 11 tonnes, is 13.1 metres long and 4.27 metres wide. It
will take up all of the space for cargo on board the shuttle, and the launch
will be the heaviest for a shuttle to date. Despite its size, the telescope’s
primary mirror, at a diameter of 2.4 metres, is less than half the size
of that in the huge ground-based telescope at Mount Palomar, in California.
Nevertheless, the telescope’s position above the atmosphere will allow Hubble
to observe details separated by as little as 0.1 arc seconds, compared with
the 1 arc second that today’s ground-based optical telescopes can see. Under
the very best of conditions, the resolution of Earth-bound telescopes can
be 0.5 arc seconds, but not for long.

Since the conception of the space telescope, astronomers and engineers
have made great improvements in telescopes on the ground. In particular,
astronomers are working on a new technology, known as active optics, which
corrects light for the distortion it undergoes as it passes through the
atmosphere. But this method is limited to light in the range from visible
to near-infrared, according to Dave Gromowski, a graduate student with a
group working on active optics at the Johns Hopkins University in Baltimore,
Maryland.

Currently, too, active optics cannot help to see stars as faint as those
that Hubble will detect, because the technique needs a bright source close
to the object to allow astronomers to calculate the necessary corrections.
Many of the light sources that Hubble observes will be so far away that
there will be nothing nearby to use to make the corrections. Future developments,
says Gromowski, will improve the technology’s ability to detect faint sources,
but not to the same extent as Hubble. ‘Active optics will revitalise ground-based
telescopes,’ said Gromowski, ‘but there is no way that they can match all
that the space telescope can do. Hubble is going to be very exciting, and
will keep astronomers busy for years.’

NASA chose Lockheed in California as the prime contractor for Hubble,
while Perkin-Elmer, in Connecticut, designed and built the optical system.
Hubble is a reflecting telescope, of the type designed by Isaac Newton and
his contemporaries in the 17th century. A large primary mirror collects
light, and reflects it to a smaller secondary mirror. The secondary beam
concentrates the light further and focuses it on scientific instruments
in the focal plane. The instruments, which collect light over an area about
as large as a dinner plate, record data as photographs or spectra.

The primary mirror is the most important part of the telescope. NASA
decided that the mirror should be optically perfect, meaning that the clarity
of the image in the focal plane should be limited by the nature of light
and not by the technology. In addition, engineers had to produce a mirror
that does not contract and expand when its temperature changes as Hubble
passes between day and night. Since the telescope orbits the Earth once
every 90 minutes, these switches are frequent. On top of all this, the mirror
had to be much lighter than ordinary telescope mirrors, otherwise the shuttle
would never reach orbit. Once Hubble is aloft, scientists want the telescope
to observe both visible and ultraviolet wavelengths, which set another constraint
on mirror design, because different materials are best suited to reflecting
particular wavelengths.

In the end, engineers designed the primary mirror as a sandwich of a
back plate, a honeycomb inner and a mirror. The honeycomb reduced the weight
from 3500 kilograms to 800 kilograms. Corning Glass Works produced the block
for the mirror from a material with very little thermal expansion. Computer-controlled
machines ground the glass so that it was close to its final hyperbolic shape,
but the final polishing was by hand. Next, engineers coated the surface
with aluminium, which reflects about 99.5 per cent of light. Unfortunately,
aluminium is a poor reflector of ultraviolet radiation, and oxidises easily,
blemishing the near-perfect surface. The solution was to add a coating,
much thinner than a piece of paper, of magnesium fluoride, which prevents
oxidation and reflects ultraviolet light. It also allows light to pass through
to the aluminium below. However, it cuts the mirror’s reflectivity for visible
light to 85 per cent. Even so, Hubble’s suite of scientific instruments
will record images 20 times as faint as ground-based telescopes can achieve.

The instruments in the telescope’s focal plane comprise two cameras,
one known as the wide field and planetary camera, and one to look at faint
objects , two spectrometers, a photometer and three fine guidance sensors.
But there are limits to what this battery of equipment can do. Hubble provides
sufficient electricity to operate only two instruments at once. Since the
instruments need 12 hours after being switched on to become stable enough
for observations, the power levels limit the telescope’s operation.

Other constraints arise from outside the telescope. For example, light
from the Sun or reflected light from the Earth and the Moon would blind
the sensitive instruments, so Hubble cannot look at parts of the sky close
to these bodies. Cosmic radiation and solar flares would also damage the
instruments. For most of the time, the Van Allen radiation belts prevent
cosmic rays from the Sun and elsewhere in the Galaxy penetrating to Hubble
as well as to Earth. However, for up to 25 minutes during about eight orbits
per day, Hubble passes through an unprotected region, known as the South
Atlantic Anomaly. Here, the radiation belts dip below Hubble’s orbit, and
the instruments must be protected. Finally, the Earth obscures half of the
sky, so that the telescope can see a particular area for only 45 minutes
out of each 90 minute orbit. The sum of these constraints adds up to a scheduling
nightmare. To get optimum operation from the telescope, hundreds of thousands
of lines of computer code assign the observing time that astronomers request.

Problems in the code

Difficulties in producing this code contributed to the delays in Hubble’s
development. Bless says: ‘Unfortunately, the people who were going to be
actively involved in operating the telescope did not get involved in the
design of software.’ In the four years since Challenger exploded, engineers
have spent a lot of time improving the programs. Asked if he thought the
software would now work, Bless said, ‘We only have to wait a few weeks now
to find out.’

Of the instruments, the most frequently requested, says Villard, is
the wide field and planetary camera. It records all wavelengths from ultraviolet
to near infrared. It is this instrument with which Larson will make her
search for young gaseous planets. The camera can operate either with a wide
field of view or in the narrower planetary mode. By wide field, NASA means
that it can photograph an area 2.6 arc minutes across, about one-twelfth
of the diameter of the Moon as we see it from Earth. In this mode, the camera
will capture tens of distant galaxies on one image. In the planetary mode,
the camera can study all of the planets in our Solar System. Each image
will be 1.1 arc minutes across, wide enough to include the largest planet,
Jupiter, in one frame.

One of the spectrographs will concentrate on faint objects, recording
visible and ultraviolet light, to provide information about the temperature
or chemical composition of a source. The other, the high resolution spectrograph,
will record radiation only in the ultraviolet range. It will allow astronomers
to study the chemical composition of the gas that lies between the stars.

Hubble’s high speed photometer is a very accurate light meter. It can
measure the frequency, intensity and polarity of the light it receives.
It will detect changes in light as rapid as a fifty-thousandth of a second.
Earth’s turbulent atmosphere has so far prevented studies of light variations
that occur much more frequently than once a second. The photometer will
aid research ranging from studies of changes in the rings of Saturn, Jupiter
and Uranus to analysis of the variations in light emitted from stars and
galaxies.

All of the observations require Hubble to point precisely at a given
target, and not to deviate by more than 0.007 arc second in any 24 hours
– about the width of a hair seen from a mile away. For these sensitive instruments,
an exposure might last for several hours; any deviation in the telescope’s
pointing would blur the image. Hubble’s gyroscopes can detect changes of
0.00025 arc seconds. They read out the telescope’s position 40 times a second,
allowing computers to make any corrections necessary to keep Hubble correctly
oriented. A variety of star sensors guide the telescope to its target, but
the most important are the three fine guidance sensors in its focal plane.
These sensors find their target by selecting reference stars from a catalogue
of 15 million stars and 3 million galaxies compiled by the Space Telescope
Science Institute. This star catalogue is 60 times as large as any previous
inventory.

For any observations, the coordinates of two guide stars are fed to
the fine guidance sensors. One sensor begins a search in a spiral pattern
for the first guide star. When the sensor has found a star of roughly the
right brightness in the correct position, the second one searches for another
guide star at the required coordinates. If the relative position of the
two guide stars is correct, their light is relayed to the gyroscopes, providing
a tracking signal. The gyroscopes receive the signal every second, and make
corrections to keep the telescope pointed accurately.

The third fine guidance sensor backs up to the other two. However, if
it is not needed, the sensor records the position of other stars so that
astronomers can calculate accurately their distance from the Earth.

Although Hubble will open a window on the Universe that astronomers
have never had before, some are already thinking of what comes next. Villard
says astronomers would like a larger telescope that can collect more light,
giving images of even greater accuracy and a chance to see objects formed
even closer to the beginning of the Universe. Some astronomers entertain
yet more ambitious plans and would like NASA to build a large telescope
on the far side of the Moon. In the meantime, after nearly 20 years of development,
Hubble waits at Cape Kennedy ready for next Thursday’s launch.

1: ONE ASTRONOMER’S VIEW INTO THE TWILIGHT ZONE

PETER JAKOBSEN, one of the scientists from the European Space Agency,
ESA, working on Hubble, makes the project sound like a bad television show:
‘We are entering the twilight zone of sub-micron resolution.’ But he is
not joking, he is referring to the extreme accuracy with which Hubble should
enable astronomers to see the precise detail of celestial objects. This,
in turn, should tell scientists far more than they can at present guess
about the evolution of the Universe.

Jakobsen is responsible for the European arm of the scientific missions
which rely on ESA’s faint object camera on Hubble. He says that astronomers
really have very little idea what the Universe is like at the level of detail
that the space telescope will make visible. Its optical cameras are far
more sensitive than those used on other recent missions, such as those on
board the Voyager spacecraft. ‘Where Voyager sent us the equivalent of postcards
home from a vacation, as it sped past the planets of the solar system, Hubble
should enable us to produce similar pictures almost routinely,’ Jakobsen
says.

It is unlikely that Hubble will pick out new planets within solar systems
outside our own. This is mainly because of the feeble relative brightness
of such planets against the dazzle of their suns. But Jakobsen believes
Hubble will find brown dwarfs, roughly crosses between stars and planets.
They are much brighter than objects such as the Earth which shines with
reflected light only. The telescope should also find faint stars never seen
from the ground, as a step towards establishing the exact age of stars in
the Universe.

Hubble should also help to unravel one of the abiding mysteries of astronomy,
that of the ‘missing mass’. According to their calculations, cosmologists
have been able to detect only between one and ten per cent of the total
mass of the Universe. Brown dwarfs might account for some of this hidden
mass. According to Jakobsen, brown dwarfs should just ‘jump right out’ at
astronomers using Hubble.

Famous astronomical landmarks, such as our nearest galaxy, in the constellation
Andromeda, will appear to be ten times closer through Hubble, ‘in our front
yard, instead of across the road,’ according to Jakobsen. Bringing galaxies
into the front yard should allow scientists to examine their nuclei and
to see if they harbour black holes, as many suspect. Others will examine
quasars, to try to spot the galaxies which astronomers believe feed the
black holes at the heart of these objects.

The telescope will not just take pretty pictures. It will examine spectra
at wavelengths other than visible light, to detect the interstellar gas
from which all new stars are formed. This should help to check the validity
of the big bang theory of the formation of the Universe. The primordial
clouds of gas fall into two groups, those which sit in haloes around galaxies
and contain heavy elements, and the traces of hydrogen that lie between
stars. Jakobsen says that, according to the big bang theory, within the
first three minutes roughly 10 per cent of the second class of gas should
have been converted to helium. ‘The space telescope can monitor the ultraviolet
region of the spectrum, so we should be able to detect the helium in these
clouds. It damn well ought to be there . . . if it is not, then there’s
something really badly wrong with the big bang theory.’

2: EUROPE’S POWERFUL PART IN THE HUBBLE SPACE TELESCOPE

EUROPE plays a supporting role to America’s lead in the Hubble Space
Telescope project. The European Space Agency, ESA, has provided the faint
object camera, one of the five main observing instruments which make up
Hubble. The agency has also supplied the solar arrays which will power the
observatory. In addition, ESA will provide scientific and technical specialists
for ground stations in the US and West Germany, to help make sense of the
data Hubble sends back. The hardware from ESA has been designed and built
under contract by European industry, and ESA’s participation guarantees
European astronomers at least 15 per cent of the total observing time available.

The faint object camera will be the instrument astronomers choose if
they are looking for distant, faint celestial objects. For this they require
extremely high resolution and maximum contrast against the background sky.
The camera is so powerful that it could distinguish the left headlight of
a car from the right at a distance of 4000 kilometres. It should be able
to pick up objects billions of light years away and some 30 times fainter
than researchers can see from the ground. This will expand the volume of
space accessible to astronomers by a factor of more than a hundred.

ESA was asked to provide this camera because of pioneering work by researchers
at University College,

London, in the development of detectors which can pick up individual
photons of light. Each photon stimulates an image-intensifier tube to produce
a cascade of electrons. These are accelerated and focused so that they fall
on a phosphor screen and create a spot of light. A television camera picks
up each spot and feeds its position to a computer which gradually builds
up a detailed picture.

The camera will operate over a range of wavelengths which includes visible
light, and stretches to the far ultraviolet region of the spectrum. Ordinarily
the Earth’s atmosphere would absorb these shorter wavelengths. A variety
of optical tools such as filters, polarisers, prisms and gratings sit inside
the camera. Astronomers can swing these into the camera’s optical path,
so that, for example, the camera can function as a spectrograph, splitting
the incoming light into its constituent wavelengths.

The images that the camera creates may require exposures lasting up
to ten hours; the whole structure must be highly stable. The instruments’
supports are therefore constructed using composite materials, which resist
thermal expansion. The optical devices are arranged on a bench, the temperature
of which is constantly adjusted so that it does not vary by more than a
quarter of a degree.

Hubble’s solar arrays were built by an industrial consortium led by
British Aerospace. Delays to the mission mean that the telescope will be
in space during a period of maximum solar activity. BAe has therefore had
to coat the entire array of solar cells with a silicone, to protect them
from the high levels of corrosive atomic oxygen that the strong solar wind
will create in Hubble’s orbit. The company has also replaced the silver
connections between the cells with a mixture of silver and molybdenum, that
is less reactive.

The solar panels are designed to produce 5 kilowatts of power, falling
to around 4.3 kilowatts as they degrade over their lifetime of roughly five
years. The panels must survive over 27 000 temperature cycles of more than
200 degrees.