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VK1DSN > SPACE 09.08.97 18:35l 272 Lines 11960 Bytes #999 (0) @ AMSAT
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Relations, Esa Public Information Note 21-97, ISO prolon
Information Note Nr 21-97
Paris 22 July 1997
ISO celebrates its prolonged life with a video of Jupiter
The Infrared Space Observatory ISO ought to be running out of
fuel by now, 20 months after its launch on 17 November 1995,
yet the astronomers and controllers at Villafranca in Spain
are busier than ever. Thanks to meticulous engineering and
some good fortune, the satellite's working life has stretched
from a specified minimum of 18 months to more than 28 months.
ESA's unique space telescope for exploring the cool and cloudy
Universe by infrared rays should, according to present
calculations, remain operational until April 1998.
This is excellent news for astronomers and especially for the
multinational teams, with leaders in France, Germany, the
Netherlands and the United Kingdom, who spent many years
devising the four instruments served by ISO's telescope. The
camera ISOCAM, the photometer ISOPHOT, the Short
Wavelength Spectrometer and the Long Wavelength
Spectrometer span between them an unprecedented range of
infrared wavelengths from 2 to 200 microns.
The atmosphere of Jupiter is one of the cool and cloudy places
attracting ISO's attention, and ESA today releases a video of
unprecedented images of Jupiter. The planet changes its
appearance drastically as the camera ISOCAM scans a range
of 90 different infrared wavelengths. Picture by picture,
ISOCAM picks out different features of the atmosphere's
composition and behaviour. These and other results from ISO
will enable scientists to sharpen their ideas about how Jupiter's
weather works.
"ISO is giving us a new impression of the giant planets of the
Solar System," comments Roger Bonnet, ESA's director of
science. "Not just Jupiter, but Saturn, Uranus and Neptune
too. By observing the planets across its very wide range of
infrared wavelengths, ISO can see features
overlooked even by spacecraft visiting the planets. The
remarkable movie of Jupiter released today represents only a
few per cent of ISO's wavelength range, yet every image tells
its own story."
More information about the Jupiter video appears later in this
Information Note.
How ISO's cold telescope beat the calendar
The need to keep ISO's telescope and instruments chilled to a
very low temperature sets a limit to their useful operating life.
ISO was supplied with more than 2000 litres of superfluid
helium to cool it. Slow evaporation maintains key parts of the
spacecraft at temperatures close to absolute zero, below minus
271 degrees C.
The rate of loss of helium was expected to be about 3 litres a
day, but the cryogenic system could not be tested in exactly
the conditions found in space. Prudence required an allowance
for a possibly faster rate of evaporation, to ensure the
specified 18 months of life. Three of the
additional 10 months that have materialized represent that
safety margin. The rest of the gain in ISO's operating life was
fortuitous, partly because of favourable circumstances during
the launch campaign at Kourou in French Guiana.
ISO was launched in its cold condition, and was expected to
lose helium while waiting on the launch pad. During a
technical check of the Ariane 44P launcher, ISO's engineers
seized the chance to recharge the helium.
The launch then occurred at the very first opportunity, with 99
per cent of ISO's helium aboard instead of an expected 95 per
cent -- a gain of about a month's supply. Another month's
advantage came about because the quick launch meant that
the outer parts of the insulation system of the spacecraft had
little time to warm up in Kourou's tropical climate.
In space, the daily loss of helium turned out to be 17 per cent
less than expected, at the lower end of a range of possibilities
considered by the engineers. That gave ISO an additional five
months of life, bringing the total to about ten months, with the
expected expiry due to occur some time in the month of April
1998.
High hopes for Orion
Some of the prime targets of infrared astronomy lie in the
direction of the Orion constellation, but without the extension
of its life ISO could not have looked safely in that direction in
the sky. The cold telescope must always remain averted from
the intense infrared glow of both the Earth and the Sun. The
orbit is constrained by the need to remain in touch with one or
other of the ground stations, in Spain or California.
As a result ISO's first chance to examine in the Orion sector
comes next month (August 1997).
When astronomers learned, soon after launch, that the coolant
supply would last long enough for Orion to come into ISO's
view, they immediately clamoured for observations. But
operational constraints will cause interruptions in September
and early October. The same geometry that makes Orion safe for
ISO's telescope also puts the satellite into the Earth's
shadow for abnormally long periods on each orbit, cutting off
the power from the spacecraft's solar panels.
Mission controllers are devising strategies to safeguard ISO
during this tricky period, while giving the astronomical teams
as much observing time as possible. Nevertheless, without the
further extension of the spacecraft's life into 1998, the
glimpses in the Orion sector might have
been more tantalizing than satisfying for the astronomers. By
February 1998, ISO will again be able to scan Orion, this time
without any difficulties from the Earth's shadow. Then
sustained observations will build on lessons learned from the
results of 1997. Hopes are high for spectacular discoveries in
ISO's swansong, just before its helium supply finally runs out.
"Astronomers stand to gain hugely from ISO's extended life,"
says Martin Kessler, the project scientist. "We have learned by
now how to get the best results from our completely novel
space observatory, so we can promise to use the extra
observing time very effectively. What we can
look at is important too. The biggest star factories in our
vicinity sprawl across the Orion and Taurus constellations. The
famous Orion Nebula is only their brightest spot. In August -
September 1997 and February-March 1998, ISO will spend
part of its extra life hunting for
newly forming stars hidden in the Orion and Taurus clouds. It's
a big bonus, and the results may well turn out to rank among
ISO's finest achievements."
Jupiter's atmosphere and ISO's video
Impressions by visible light of Jupiter's weather, like the
Earth's weather, are dominated by clouds which are abundant
wherever the gases of the atmosphere are rising. The bright
clouds of Jupiter are concentrated in permanent zones.
Between the cloud zones are bands in various hues created by
the chemistry of the atmosphere. The Great Red Spot is a
long-lived hurricane, wider than the Earth, that has raged for
several centuries in Jupiter's southern hemisphere.
If you had infrared eyes, you would see the Earth's own clouds
radiating in different colours, or wavelengths, according to
their temperatures. You could also recognize various gases in
the air by colours due to their absorptions of particular
wavelengths. Infrared instruments in meteorological and
environmental satellites monitor the Earth's atmosphere
routinely.
Jupiter's atmosphere yields its secrets in the infrared waveband
to ISO, which sees the whole sunward side of the planet.
Three of ISO's four instruments are contributing important
discoveries about the chemical composition and behaviour of
Jupiter's atmosphere. A foretaste, spanning only a few per cent
of ISO's total wavelength range, comes from the ISOCAM
video released by ESA today.
Over half an hour, while Jupiter turned a little and shifted its
Great Red Spot towards the right, ISOCAM obtained 90
images of Jupiter, each of them at a different infrared
wavelength between 2.3 and 11.6 microns. The images change
so dramatically that a viewer could be forgiven for
wondering if it is still the same planet.
Jupiter even disappears from view temporarily, at around 3.3
microns, where methane gas in the Jovian atmosphere mops up
the planet's infrared rays -- in the same way as water vapour,
carbon dioxide and methane in the Earth's air absorb infrared
wavebands. The Jovian blackout gives a
vivid impression of why ISO had to go into space in the first
place, to escape from the Earth's atmospheric barriers to
infrared astronomy.
At the lower end of the wavelength range the Great Red Spot
glows bright in the infrared. The images of Jupiter at around 5
microns show, as hot-spots, regions free of cloud where ISO
can peer deep into the atmosphere. By contrast, the upper
atmosphere is featured at around 7.7 microns, where strong
emissions from the planet's south polar region are
conspicuous. Acting like a swarm of weather balloons, every
one of the images brings new knowledge of the giant planet.
Th)r[se Encrenaz of the Observatoire de Paris-Meudon leads
an ISO team that is examining the weather and chemistry of
Jupiter and the other giant planets Saturn, Uranus and
Neptune. She is pleased with the results so far.
"By observing Jupiter with ISO we can build up a 3-D picture
of the peculiar weather on this giant planet," Encrenaz
comments. "We can also fit into our big picture the local
results coming from NASA's Galileo spacecraft. For example,
it sent a probe into Jupiter and scientists were puzzled by the
results, and now we know that the probe plunged by chance
into one of the dry, cloud-free anticyclones seen clearly by ISO
at 5 microns. ISO's perspective links the winds, clouds,
temperatures and chemistry of Jupiter's atmosphere in
fascinating ways."
Notes about the ISO Jupiter video
The video is available to broadcasters on request, in Betacam
form. Please contact ESTEC
(Tel :+31.71.565.3429) or ESA HQ (Tel: +33
(0)1.53.69.7155).
The video consists of 86 frames shown at a rate of 2.5 frames
per second. Each frame is at a different wavelength between
2.3 and 11.6 microns, as indicated by the moving pointer.
North is at the top. The images were obtained sequentially
over 35 minutes. During that time the Great Red Spot, seen
conspicuously bright below the equatorial at the outset moves
a little to the right.
At the outset and in other early frames ISO sees the cloudy
zones of Jupiter and the Great Red Spot.
Around 3.3 microns the planet goes dramatically dark because
methane gas in the atmosphere
absorbs all the infrared radiation.
At 5 microns, ISO sees deep into the atmosphere, in the belts
between the cloud zones. The bright spots conspicuous north
of the equator are hot dry regions, similar to the one visited by
the Galileo probe.
Around 7.7 microns, ISO is looking at the upper atmosphere
(stratosphere) of Jupiter. The south polar region glows bright.
In the last images, Jupiter is becoming too hot for the camera
The rate of frequency change is not constant. Thus 3 microns
is attained at frames 15-16, 4 microns at 31-32, 5 microns at
44-47, 6 microns at 54-55, 7 microns at 63-64, 8 microns at
69-70, 9 microns at 74-75, 10 microns at 82-83, and 11
microns at 87-88.
ISOCAM operated with an image scale of 1.5 seconds of arc
per pixel in the circular variable filter mode. The equator of
Jupiter spans about 30 pixels.
Note that a gallery of ISO results is located on the WWW at:
isowww.estec.esa.nl/science/
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