The observable Universe may be about 10 percent larger than astronomers have supposed, according to early results from the European Space Agency's Hipparcos mission. Investigators claim that the measuring ruler used since 1912 to gauge distances in the cosmos was wrongly marked.
This ruler relies on the brightness of winking stars called Cepheids, but the distances of the nearest examples, which calibrate the ruler, could only be estimated. Direct measurements by Hipparcos imply that the Cepheids are more luminous and more distant than previously imagined.
The brightnesses of Cepheids seen in other galaxies are used as a guide to their distances. All of these galaxies may now be judged to lie farther away. At the same time the Hipparcos Cepheid scale drastically reduces the ages of the oldest stars, to about 11 billion years. By a tentative interpretation the Universe is perhaps 12 billion years old.
This estimate will provoke much comment and controversy, because the scale and age of the Universe is one of the touchiest issues in cosmology. The best hope for confirming or modifying the result now rests with studies using Hipparcos data on other kinds of stars.
European teams of scientists and engineers conceived and launched the unique Hipparcos satellite, which operated from 1989 to 1993. Hipparcos fixed precise positions in the sky of 120,000 stars (Hipparcos Catalogue) and logged a million more with a little less accuracy (Tycho Catalogue). Since 1993 the largest computations in the history of astronomy have reconciled the observations, to achieve a hundredfold improvement in the accuracy of star positions compared with previous surveys.
Slight seasonal shifts in stellar positions as the Earth orbits the Sun, called parallaxes, give the first direct measurements of the distances of large numbers of stars. With the overall calculations completed, the harvest of scientific discoveries has begun. Among those delighted with the immediate eruption into cosmology, from this spacecraft made in Europe, is ESA's Director of Science, Roger Bonnet.
"When supporters of the Hipparcos project argued their case," Bonnet recalls, "they were competing with astrophysical missions with more obvious glamour. But they promised remarkable consequences for all branches of astronomy. And already we see that even the teams using the Hubble Space Telescope will benefit from a verdict from Hipparcos on the distance scale that underpins all their reckonings of the expansion of the Universe."
Michael Perryman, ESA's Project Scientist for Hipparcos, anticipates a warm debate among astronomers. Should the Hipparcos Cepheid results be taken at face value, with all their implications for the size and age of the Universe? He remains confident that the issue will be settled by other results quarried from the Hipparcos data.
Further Hipparcos studies of variable stars are in progress. Also relevant to the distance scale are differing quantities of heavy elements present in stars of different ages, which can affect their luminosity. Any remaining confusion on this point will be dispelled by mainstream Hipparcos research devoted to the basic astrophysics of stars of different ages of origin, and at different stages of their life cycles.
"Until Hipparcos, the cosmic distance scale rested on well-informed guesses," Michael Perryman says. "The distances we now have, for stars of many kinds, provide for the very first time a firm foundation from which to gauge the distances of galaxies. The work has only just begun. If it should turn out that the Cepheids have given the final answer straight away, that might be surprising. But there will be no reason for astonishment when Hipparcos's direct measurements of stellar distances lead to a revised scale for the Universe."
The Hipparcos Cepheid scale was recently debated at the Royal Astronomical Society in London and at the annual meeting of the American Association for the Advancement of Science in Seattle. It will also be one of the hot topics at ESA's Hipparcos Symposium in Venice,13-16 May. The Venice meeting will celebrate the release of the Hipparcos and Tycho
Catalogues to the world-wide astronomical community. It will also offer the first overview of results obtained by the groups who have had early access to the data, by virtue of their contributions to the Hipparcos mission. The subjects range from the Solar System and the Sun's neighbours among the stars, through special stars and the shape and behaviour of the Milky Way Galaxy, to the link between the starry sky of Hipparcos and the wide Universe of galaxies and quasars. Hipparcos information is accessible on the World Wide Web: http://astro.estec.esa.nl/Hipparcos/hipparcos.html
Hipparcos and Tycho Catalogues
The final products of ESA's Hipparcos mission will be two
major stellar catalogues - the Hipparcos Catalogue and the
Tycho Catalogue - to be published in June 1977 by the ESA
Publications Division, in the form of a 16-volume hard-bound
printed catalogue and a set of 6 CD-ROMs, as ESA Special
Publication SP-1200.
Each catalogue includes a large quantity of very high quality astrometric and photometric data. The astrometric data in the Hipparcos Catalogue is of unprecedented accuracy: positions at the catalogue epoch (J1991.25), annual proper motions, and trigonometric parallaxes, have a median accuracy of approximately 1 milliarcsec. The Hipparcos Catalogue includes annexes featuring variability and double/multiple star data for many thousands of stars discovered or measured by the satellite. The Hipparcos and Tycho Catalogues will remain the definitive astrometric stellar catalogues for many years to come.
One of the four original Cluster spacecraft.
ESA's Science Programme Committee, during its meeting at ESA Headquarters in Paris on 3 April, has agreed on the reflight of a full Cluster mission by mid-2000. After months of intense negotiations and an impressive display of solidarity by all ESA Member States and the scientific community at large in support of the reflight, this mission to investigate the physical interaction between the Sun and our planet is back on track.
The original Cluster mission, lost on 4 June 1996 with the explosion of the first Ariane-5 demonstration flight, is being replaced by Cluster 2, comprising the Phoenix spacecraft (being built with spares from the four original Cluster satellites) and three identical new satellites to be built by a European industrial consortium led by DASA/Daimler Benz Aerospace (Germany).
The satellites will be launched in pairs by two Russian Soyuz launchers in mid-2000 within a short period of time to meet the orbital requirements of the mission. The launchers will be procured through the STARSEM consortium, a French/Russian joint venture.
The choice of the Soyuz launchers, together with major efforts on the part of ESA, the industrial consortium and scientific institutes all over Europe and the US, has enabled the additional Cluster mission cost to be kept down to 214 MECU.
Excellent use of the Hubble Space Telescope continues to provide astronomers in ESA's Member States with a large share of the observing time. ESA has a 15% stake in the Hubble Space Telescope project, earned by providing the Faint Object Camera and the first two sets of solar power arrays, as well as helping to staff the Space Telescope Science Institute in Baltimore. Current European-led programmes account for about 22% of the observing schedule. European astronomers' work spans all aspects of astronomy, from the planets to the most distant galaxies and quasars. The following examples are just a few European highlights from Hubble's second phase, 1994-96.
A scarcity of midget stars
Guido De Marchi and Francesco Paresce of the European
Southern Observatory at Garching, Germany, have taken sample
censuses with the wide-field WFPC2 camera within six globular
clusters- large gatherings of stars orbiting independently in
the galaxy. They found that stars less massive and fainter
than Sun-like stars are ten times more numerous in the Milky
Way galaxy. However, they suggest that very faint stars are
scarce, a finding which is significant for theories as to how
the Universe originated.
Confirmation that very small stars are indeed scarce comes from Gerry Gilmore of the Institute of Astronomy in Cambridge (UK). He leads a European team that analyses long-exposure images in the WFPC2 camera, obtained as a by-product when another instrument is examining a selected object. The result is an almost random sample of well-observed stars and galaxies.
Unchanging habits in starmaking
A remarkable general conclusion is that the make-up of
stellar populations never seems to vary. In dense or diffuse
regions, in very young or very old agglomerations, in the
Milky Way galaxy or elsewhere, the relative numbers of stars
of different masses are always roughly the same. Evidently
Nature mass-produces quotas of large and small stars
irrespective of circumstances. This discovery will assist
astronomers in making sense of very distant and early
galaxies.
Another surprise was spotted by Rebecca Elson of Gilmore's team. Long-exposure images of the giant galaxy M87, in the nearby Virgo cluster have shown that it possesses globular clusters of very different ages. Theory suggests that the clusters were manufactured during collisions of the galaxies that merged into M87, making it the egg-shaped giant seen today. If so, the absence of young globular clusters in the Milky Way may mean that our galaxy has never suffered a major collision.
Accidents in the galactic traffic
Brighter than a million suns, a quasar is the most
powerful lamp in the Universe. Astronomers understand it to be
powered by matter falling into a massive black hole in the
heart of a galaxy. Mike Disney of the University of Wales,
Cardiff, leads a European team that asks why some thousands of
galaxies harbour quasars, in contrast to the billions that do
not. In almost every case that he and his colleagues have
investigated, using Hubble's WFPC2 camera at its highest
resolution, they see the quasar's home galaxy involved in a
collision with another galaxy. Though these findings are not
conclusive, Disney says, "...the important thing is that we
have wonderfully clear pictures to argue about. Quasar
theories were mostly pure speculation before we had Hubble."
The history of the elements
Astronomer Dieter Reimers and his colleagues at the
Hamburger Sternwarte use the Faint Object Spectrograph to
analyse ultraviolet light from distant quasars, which they
also examine by visible light from the ground. Through cosmic
time, they trace the origin of elements like carbon, silicon
and iron, from which planets and living things can be built.
On its way to Hubble, the quasar light passes through various
intervening galaxies and gas clouds, like the skewer of a
kebab. Each object visited absorbs some of the quasar light,
depending on the local abundances of the elements. As more and
more objects are detected, an impression is formed of galaxies
building up their stocks of elements progressively through
time, by the alchemy of successive generations of stars.
Apart from primordial hydrogen, the second lightest element, helium, has also been abundant since the origin of the Universe. The first major discovery after Hubble's last refurbishment came from Peter Jakobsen of ESA's Space Science Department at ESTEC who, in January 1994, detected ionized helium in the remote Universe, by the light of a very distant quasar, 0302-003. Since then Jakobsen has looked for the ionized helium using other quasars. He now suspects that this helium mainly gathered in clumps, rather than scattered freely through intergalactic space. If so, it greatly increases the estimates of the total mass of ordinary matter in the Universe.
Through a lens to the early Universe
Natural lenses scattered through the cosmos reveal distant
galaxies, and make an astronomical tool for Richard Ellis of
the Institute of Astronomy, Cambridge (UK). The strong gravity
of an intervening cluster of galaxies can bend the light from
more distant objects, magnifying and intensifying their
images. In one spectacular case, cluster Abell 2218 creates in
Hubble's WFPC2 camera more than a hundred images of galaxies
lying beyond it. Without the magnifying effect of the cluster,
many of these remote objects would be too faint to study in
detail.
The cosmic scale
The Hubble Space Telescope is being used to measure the
Hubble Constant - both are named after Edwin Hubble who
discovered, almost 70 years ago, that the galaxies are
spreading apart. The Hubble Constant is the rate of expansion
and the most important number in cosmology, because it fixes
the size and the maximum age of the observable Universe.
Since the launch of the Space Telescope in 1990, two
independent teams have tried to fix the constant but their
answers disagree. American astronomers believe there is a
high expansion rate which would make the Universe relatively
young. A lower value for Hubble's Constant, implying an older
Universe, comes from a mainly European team. According to
Gustav Tammann of the latter team, "Time will tell us who is
closer to the right answer."
STS-82 Servicing Mission
Many more important findings may result from the February 1997 Discovery STS-82 Hubble servicing mission, during which astronauts have significantly upgraded the scientific capabilities of the HST.
Work during spacewalks included the installation of two new state-of-the-art instruments. The crew has successfully replaced the Faint Object Spectrograph with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). The Goddard High Resolution Spectrometer was replaced with the Space Telescope Imaging Spectrograph (STIS).
Other tasks included maintenance to the Space Telescope to keep it functioning smoothly until the next scheduled servicing mission in 1999.
Following the announcement by Mr Jean-Marie Luton, at the ESA Council Meeting last December, that he would not be seeking a third term of office as Director General, on 20 March the Council announced the appointment of Mr Antonio Rodotà as his successor for an initial period of four years. Mr Rodotà who is 61 years old and of Italian nationality, was previously Director of the Space Division of Finmeccanica (Italy), Managing Director of Quadrics Supercomputer World Ltd. (Italy/United Kingdom), and a Director of several other companies including Arianespace. A graduate of Rome University and an electronics engineer, Mr Rodotô began his career with Selenia (1966/80), followed by three years as Head of Compagnia Nazionale Satelliti (Italy). He subsequently moved to Alenia-Spazio in 1983, where he held a series of senior positions before becoming Chief Executive in 1995.
At their March meeting, Council, on the recommendation of the Director General, also appointed, for four-year terms of office:
Council also, again on the recommendation of the Director General, extended by two years the term of office of Mr Roger Bonnet, ESA's Director of Scientific Programmes, and asked Mr René Collette, who is currently Director of ESA's Telecommunications Programmes, to assume responsibility as from 1 April 1997 for the new Directorate of Applications Programmes, which will bring together all the approved Earth- observation and telecommunications programmes (other than the Earth-observation scientific programme). Finally, the term of office of Mr Massimo Trella, ESA's Inspector General, has been extended by six months to enable him to complete his task as Co-Chairman of the Ariane-5 Qualification Review Board.
Mr David Dale, to the post of Director of Technical and
Operational Support.
Mr Dale, 54 years old and of British nationality, has a degree in applied physics, together with diplomas in mechanical and electrical engineering. He was previously Head of ESA's Scientific Projects Department.
Mr Hans Kappler, to the post of Director of Industrial
Matters and Technology Programmes.
Mr Kappler, 55 years old and of German nationality, is a physics graduate of the Technical University of Munich. He was previously in charge of technology development at STN Atlas Elektronik in Germany.
Mr Daniel Sacotte, to the post of Director of
Administration.
Mr Sacotte, 51 years old and of French nationality, holds a "Diplôme d'Etudes Appliquées" in astrophysics. He was previously Deputy Director General, Administration, Finance and Human Resources, at Centre National d'Etudes Spatiales (CNES), in France.
An action-packed motion picture from ESA's solar spacecraft SOHO astonishes the experts and will enthral the public. It shows the Sun at Christmas sailing in front of the stars of the Sagittarius constellation and the Milky Way, while blowing its solar wind outwards in all directions around it. Another scene depicts the Sun swallowing a comet. In a subsequent, unconnected event, it emits a plainly visible puff of gas, in a large mass ejection.
The remarkable images come from SOHO's visible-light coronagraph LASCO. It masks the intense rays from the Sun's surface in order to reveal the much fainter glow of the solar atmosphere, or corona. Operated with its widest field of view, in its C3 instrument, LASCO's unprecedented sensitivity enables it to see the thin ionized gas of the solar wind out to the edges of the picture, 22 million kilometres from the Sun's surface. Many stars are brighter than the gas, and they create the background scene.
The results alter human perceptions of the Sun. Nearly 30 years ago, Apollo photographs of the Earth persuaded everyone of what until then they knew only in theory, that we live on a small planet. Similarly the new imagery shows our motion in orbit around the Sun, and depicts it as one star among many - yet close enough to fill the sky with emanations that engulf us.
For many centuries even astrologers knew that the Sun was in Sagittarius in December and drifting towards the next zodiacal constellation, Capricornus. This was a matter of calculation only, because the Sun's own brightness prevented a direct view of the starfield. The SOHO-LASCO movie makes this elementary point of astronomy a matter of direct observation for the first time. The images are achievable only from a vantage point in space, because the blue glow of the Earth's atmosphere hides the stars during the day.
A special allocation of observing time, and of data transmission from the SOHO spacecraft, enabled the LASCO team to obtain large numbers of images over the period 22-28 December 1996. Since then, a sustained effort in image processing, frame by frame, has achieved a result of high technical and aesthetic quality. Only now is the leader of the LASCO team, Guenter Brueckner of the US Naval Research Laboratory, satisfied with the product and ready to authorize its release.
"I spend my life examining the Sun," Brueckner says, "but this movie is a special thrill. For a moment I forget the years of effort that went into creating LASCO and SOHO, and leave aside the many points of scientific importance in the images. I am happy to marvel at a new impression of the busy star that gives us life, and which affects our environment in many ways that we are only now beginning to understand."
A scene from the SOHO Christmas video taken with the C3
coronagraph of the Large Angle Spectroscopic Coronagrahp
(LASCO, Principal Investigator: G.E. Brueckner, Naval Research
Laboratory, Washington D.C.). The angular diameter of the
field of view is 16°; at the location of the Sun this
corresponds to 42 million kilometers (or 28% of the distance
between the Sun and the Earth). The structure in the lower
left-hand quadrant results from the boom that holds the mask
occulting the Sun's disk (whose diameter is indicated by the
circle inside the mask).
Ariane flights V93 and V94 were both successfully launched from the Guiana Space Center in Kourou on Thursday, 30 January, and Saturday, 1 March, respectively.
For V93, an Ariane 44L placed the Argentinian and American telecommunication satellites NAHUEL 1A and GE2 into geostationary transfer orbit.
The International Telecommunication Satellite Organisation's Intelsat 801 was placed into geostationary transfer orbit during V94 by an Ariane 44P version launcher.
ESA has contributed to the development of a faster, more reliable and more accurate satellite distress system for ships at sea, in the framework of a project promoted by Inmarsat, the global mobile satellite operator.
Called Inmarsat-E (for Emergency), this search and rescue system uses dedicated L-band channels (at 1.6 GHz) on the existing Inmarsat telecommunications satellites in geostationary orbit. Inmarsat is committed to offer this service within the Global Maritime Distress and Safety System, currently being implemented by the International Maritime Organization (IMO). Use of the Inmarsat-E system will be free of charge.
The continuing high number of losses of ships at sea and the progress in satellite technology have prompted IMO to adopt regulations requiring most ships to carry Emergency Position Indicating Radio Beacons (EPIRB), small transmitters (about 1W in power) that a ship can trigger in an emergency. The signal is immediately received by the geostationary satellite and relayed down to a coastal Earth station equipped with dedicated and very sensitive receivers. The signal includes the identity of the ship, the type of alert and the ship's position evaluated from a GPS receiver built into the EPIRB (with an accuracy of 200 m). The digital receiver in the Earth station - typically a compact rack - is designed for fully unattended operation. The data is automatically relayed by the station to a search and rescue centre and, consequently, within a few minutes of the alert having been triggered, rescue operations can begin.
For the Inmarsat-E system, ESA has financed the development and the manufacturing of two sets of receivers (contracts were awarded to Nortel-DASA, Germany and Nokia, Finland), while the German Ministry of Transport purchased two other sets. Three receivers have been installed and are now operating in stations at Raisting (Germany), Niles Canyon (California) and Perth (Australia). The Inmarsat-E system was inaugurated worldwide on Thursday, 30 January 1997.
A first series of vibration tests on a structural model of the 8 ton/10m high "Envisat-1/Polar Platform" satellite have recently been performed on the hydraulic shaker "Hydra" in the test centre at ESTEC. It is the first time that a hydraulic shaker of this kind has been used for vibration tests on a satellite system.
Hydra, which has a test platform spanning 5.5 metres, has been developed to perform vibration tests on heavy, as well as geometrically large, elements of spacecraft or launchers. It is capable of testing satellites conventionally, with excitations in the main orthogonal axes, but has additionally been designed for the excitation of transients in 6° of freedom which represents a close simulation of the dynamic inputs to the satellite during its ride on the launcher.
The first tests performed on Hydra were limited to vertical excitations after a preliminary acceptance of the test facility. Full commissioning for all operational modes, including the 6§ of freedom transients, is expected during the course of 1997.
Another step in ESA's preparation for the International Space Station
When the US Space Shuttle Atlantis lifted off on Sunday, 12 January for the STS-81 mission to the Russian space station Mir, it carried the European Space Agency's Biorack facility on its fifth flight in space and its second of three to Mir.
During this nine-day mission, the Biorack was used to study the effects of radiation and the absence of gravity (i.e. microgravity) on plant, fungus, tissue and cell growth. This research will help to determine the effects of long-duration spaceflight on organisms and prepare for the International Space Station (ISS).
The Biorack integrates several scientific facilities in a single rack, a highly desirable attribute given the limited room available onboard a spaceflight. It offers incubators that permit experiments to be performed in different temperature controlled environments; simulated 1-g that allows investigators to distinguish between effects induced by gravity and microgravity; and a "glovebox" or protected workspace for specimen handling.
Many of the 12 experiments carried out during the flight, prepared by scientists from across Europe and one in the USA, are expected to provide information that can be used to prepare for both life and experimental work on board the ISS. Those experiments include the following:
Some of the experiments will be pursued further when the Biorack is flown next, in May of this year, on board STS-84. ESA astronaut Jean-François Clervoy will be the payload commander on the mission and will oversee all of the Biorack experiments.
On 5 March 1997, ESA and NASA signed an agreement in principle under which ESA is to provide additional hardware and services for the International Space Station (ISS) to NASA in exchange for the launch of the European laboratory module on the US Space Shuttle.
The hardware consists of two essential segments of the ISS, the so-called Nodes 2 and 3, and several pieces of advanced technology laboratory equipment (in addition to other elements that it is already contributing). A node is an element used to connect different segments of the ISS. Node 2, for example, will provide the interface between the European and the Japanese laboratory modules.
In return, NASA will launch at no charge, the European laboratory called the Columbus Orbital Facility (COF) on a Space Shuttle flight to the ISS. The COF is presently scheduled to be launched in late 2002/early 2003. Based on an ESA/NASA implementing arrangement which will be established in the near future, ESA will deliver Node 2 to NASA at the end of 1999, for launch in early 2000. Node 3 will follow about two years later.
Under a parallel agreement, ESA will entrust the development of the nodes to the Italian Space Agency (ASI). This will allow Europe to take advantage of the experience gained by Italian industry through the ASI/NASA development of the Mini Pressurised Logistics Module (MPLM) for the ISS.
This measure will also significantly reduce the deficit that Italy has accumulated under ESA's policy of fair industrial return, and was welcomed by the Ministers of ESA's Member States at their meeting held in March.
"With this solution, everyone gains", stated Jörg Feustel-Büechl, ESA's Director of Manned Spaceflight and Microgravity, responsible for Europe's Space Station programme. "NASA will get the important equipment that it was seeking, delivered quickly and on time. ESA will get a Shuttle launch for its COF. Italy will see a great improvement in its industrial return situation. And the Space Station will move one step closer to becoming a reality."
Noordwijk Space Expo offers students a hands-on classroom
experience in the area of remote sensing.
At the end of 1996 the Noordwijk Space Expo (NSE) started a unique educational experience in The Netherlands: a 'Remote Sensing Classroom' was inaugurated to support secondary education (remote sensing has now become part of the official geography curriculum in The Netherlands).
The idea behind a "Remote Sensing Classroom" is to obtain detailed information from original satellite images. Students learn how to manipulate Earth observation data like professional scientists. Various themes such as agriculture, climate and weather are covered.
On advanced PC-stations, students are introduced to a programme called "SaMBA" which allows them to work on various remote sensing tasks. Field research is included whenever possible, led by NSE in cooperation with different institutions.
ESA has a commanding role in space research on comets. Its Giotto spacecraft was the most daring of the international fleet of spacecraft that visited Halley's Comet in March 1986. Giotto obtained exceptional pictures and other data as it passed within 600 kilometres of the nucleus. Dust from the comet badly damaged the spacecraft, but in a navigational tour de force, Giotto made an even closer approach to Comet Grigg- Skjellerup in July 1992. Now ESA is planning the Rosetta mission that will rendezvous with Comet Wirtanen (see figures 1 & 2) and fly in company with it, making observations far more detailed than the fast flybys of Halley's Comet and Comet Grigg-Skjellerup could achieve.
Figure 1. The Rosetta mission is a cometary mission which will be launched in the year 2003 by Ariane-5.
After a long cruise phase, the satellite will rendez-vous with comet P/Wirtanen and orbit it, while taking scientific
measurements. A Surface Science Package (SSP) will be landed on the comet surface to take in-situ measurements.
During the cruise phase, the sayellite will be given gravity-assist manoeuvres once by Mars and twice by the Earth. The satellite
will also take measurements during fly-bys of two asteroids.
Figure 2. An ISOCAM infrared image at a wavelength of 11.5 microns, of Comet P/Wirtanen taken on 7 November 1996.
As for space astronomy, the International Ultraviolet Explorer, in which ESA was a partner, made unrivalled observations of Halley's Comet by ultraviolet light. ESA is also a partner in the Hubble Space Telescope, which saw the historic impacts of Comet Shoemaker-Levy 9 on Jupiter in July 1994, and has recently observed Comet Hyakutake as well as Hale-Bopp (see figure 4). The SOHO spacecraft, built by ESA for a joint ESA-NASA project to examine the Sun, also has a distinctive view of comets. It has observed the hydrogen coronas of comets with its SWAN instrument. SOHO's coronagraph LASCO observed Comet Hyakutake rounding the Sun (when it was invisible to ground-based observers) and has discovered seven new comets very close to the Sun.
ESA's Infrared Space Observatory, ISO provides astronomers with information from comets across a very wide range of infrared wavelengths unobservable from the ground. Besides Comet Hale-Bopp (see figures 3 & 5), ISO has examined Comets Schwassmann-Wachmann 1, Chiron, Kopff, IRAS 1 and Wirtanen. The last of these, Comet Wirtanen, is now making one of its six-yearly visits to the Sun's vicinity.
Figure 3. Image from ISO's camera ISOCAM. It is one of a
series obtained at a wavelength of 15 microns on 1 October
1996. Analysis of the image is still in progress, but the
chief feature is the cloud of dust in the comet's head (coma).
The bright region is about 100,000 kilometres across, or more
than seven times wider than the Earth. At the time, the tail
of Comet Hale-Bopp was largely hidden behind the coma because
of the relative angles of the comet, the Sun and the Earth.
Credit: ISO (ESA), ISOCAM, P. Lamy and the Hale-Bopp team.
Image processed by B. Altieri at ESA Villafranca.
Figure 4. An image of Hale-Bopp by visible light was obtained
by the Hubble Space Telescope, just a week earlier than the
ISO image, is shown for comparison. It covers a region of the
sky (and the comet) one-fifth as wide as the ISO image. At
least five jets of dust are seen emanating from the comet's
nucleus, and lit by sunlight.Credit: HST (NASA & ESA), H.
Weaver and the WFPC2 team.
Figure 5. A spectrum covering a large range of infrared
wavelengths from ISO's photometer ISOPHOT. Here the instrument
operates as a thermometer, taking the temperature of the Hale-
Bopp's dust cloud. The crosses are the measurements by ISOPHOT
and the continuous line is the emission expected from an
object with a temperature of 220 K, or about -50°C. By
this period of observation in October 1996, the dust cloud was
much warmer than in March 1996, when the same instrument
obtained a temperature reading of 120°C. Credit:
ISO(ESA), ISOPHOT, E. Grün and the Hale-Bopp team.
As comets are relics from the construction of the Solar System, and played a major role in the formation of the planets, they are a link between the Earth and the wider Universe of stars. The carbon compounds contained in comets probably contributed raw materials for the origin of life on the Earth, and according to one theory the Earth's oceans were made from cometary ice. Growing knowledge of the composition and behaviour of comets is therefore crucial for a fuller understanding of our cosmic origins.
Emphatic confirmation of this long-standing belief of astronomers comes from the detection of the mineral olivine in Comet Hale-Bopp, by ISO. The 28 March issue of the US journal Science carries a report on this result by a European and American team led by Jacques Crovisier of l'Observatoire de Paris-Meudon.
"ISO sees the same materials in Comet Hale-Bopp as in dust clouds around other stars," Crovisier comments. "A key ingredient of both star dust and comet dust is olivine in crystalline form. This is also one of the main constituents of the Earth's interior. Now we can say with real confidence that we stand on a congealed pile of mineral dust, like that contained in the comets swarming around the Sun 4500 million years ago."
Olivine predominates in the mantle below the Earth's thin crust, and crops up at the surface as the olive-coloured gemstone, peridot. Geologists also value minerals rich in olivine, as important sources of chromium, platinum and diamonds.
Ingredients of Comet Hale-Bopp's vapour and dust emit characteristic infrared wavelengths, many of which are clearly observable only in space. The team took advantage of ISO's unparalleled ability to analyse intensities across a wide band, from 2 to 200 microns wavelength, using three of the spacecraft's instruments: the Short-Wavelength Spectrometer SWS, a short-wave spectrometer within the photometer ISOPHOT, and the Long-Wavelength Spectrometer LWS.
ISO's SWS reveals the olivine dust in the great comet in a distinctive cluster of emission peaks (11.3, 16.5, 19.8, 24.0, 27.6 and 33.9 microns). These are characteristic of crystalline forsterite, a form of olivine rich in magnesium. The infrared fingerprint is wholly different from that of the pyroxenes, commonplace silicates of the Earth's crust.
Last year, astronomers using the same instrument on ISO reported strong hints of olivine, in emissions around 33 microns from dust clouds surrounding half a dozen aged and dying stars. This is where Nature makes the olivine from chemical elements released from the old stars. Other SWS users recorded the full cluster of forsterite emissions in a dust cloud surrounding a young star, where planet formation may be in progress. The similarity between the spectra of this star (called HD 100546) and of Comet Hale-Bopp is astonishing. Although the objects are separated by hundreds of light-years, and huge differences of scale, ISO sees the same dominant mineral in both.
Comets carry ices too in their cargoes - principally frozen water, carbon monoxide and carbon dioxide. ISO has shown them to be the principal ices in interstellar space, where they exist on small grains. In Comet Hale-Bopp, ISO detects and measures these materials as they turn into vapour. Crovisier and his colleagues report in Science the rate at which Comet Hale-Bopp sweats off weight in the heat of the Sun.
On 27 September 1996, when the comet was still 444 million kilometres from the Sun, it was shedding water vapour into space at 10 tonnes per second, carbon monoxide at 11 tonnes per second, and carbon dioxide at 5 tonnes per second. Altogether Comet Hale-Bopp's loss of these materials amounted at that time to 2.2 million tonnes per day. Counting molecules rather than mass, the water vapour, carbon monoxide and carbon dioxide were vaporizing in the ratio 10 to 6 to 2.
The temperature at which the water molecules first formed in space, billions of years ago, was about minus 250°C. Crovisier's team arrive at this answer by distinguishing with SWS the infrared signatures of two kinds of water molecules, at 2.6 to 2.9 microns. When water molecules form at ordinary temperatures, the nuclei of both hydrogen atoms spin the same way, in three cases out of four. At very low temperatures of formation, as in interstellar space, contrary directions of spin become commoner. The best match to the ISO spectrum comes from a ratio of 2.45 to 1 for the two molecular types, corresponding to molecule-making at 25 degrees above the absolute zero of temperature.
Publication in Science of this ISO-based report, together with a number of other scientific papers on Comet Hale-Bopp, coincides with the comet's closest approach to the Sun (at 136 million kilometres) due on 1 April.
Mr Jean-Marie Luton, ESA's Director General (left) and Mr Isao
Uchida, President of NASDA (right) sign a Memorandum of
Understanding representing one of the most important and
advanced space cooperation efforts between the two agencies to
date.
On 18 April, Mr Jean-Marie Luton, ESA's Director General and Mr Isao Uchida, President of the National Space Development Agency of Japan, NASDA, have signed a Memorandum of Understanding concerning the launch and utilisation of ESA's telecommunications satellite Artemis (Advanced Relay and Technology Mission Satellite).
This Memorandum of Understanding, an element of wider collaboration between ESA and NASDA, represents one of the most important and advanced space cooperation efforts between the two agencies to date. It stipulates that NASDA will launch the Artemis satellite on a Japanese H-IIA launcher in the year 2000. In exchange ESA will provide NASDA with data relay capacity through Artemis.
ESA's Artemis which will test and operate new telecommunications techniques, has a two-fold mission. It will provide two-way communications via satellite between fixed Earth stations and mobiles (trucks, trains and cars) all over Europe and adjacent regions. In addition, it will allow the reception of data from spacecraft in low Earth orbit and their retransmission to Earth stations and other users, both by using conventional radio waves and, in a revolutionary way, using laser beams. Artemis will provide communication links with, for instance, ESA's future environmental Envisat satellite, the European elements of the International Space Station, NASDA's Optical Inter-Orbit Communications Engineering Test Satellite OICETS, other Japanese Earth Observation satellites and the French Earth observation satellite SPOT 4.
Artemis is being developed for ESA by a European industrial consortium led by Alenia Spazio (Italy).
ESA Bulletin Nr. 90.