Scientific Research

Five ESA science missions were in orbit in 2000 making scientific observations: Cluster-II, XMM-Newton, SOHO, Ulysses and the Hubble Space Telescope (the last three are joint missions with NASA). The ESA Huygens Titan Probe, carried by NASA's Cassini spacecraft, passed close to Jupiter at the end of 2000 and is currently en route to the Saturnian system. While Cassini is undertaking ‘cruise science’, Huygens is ‘hibernating’, though closely monitored, and will not begin its scientific mission until its separation from Cassini in November 2004.

The last of ISO’s liquid-helium coolant was evaporated in April 1998, signalling the end of its scientific mission. However, with over 26000 observations made, there is a massive data processing, calibration, analysis and archiving task to be completed, which will continue until 2006. The true scientific return from a survey mission of this nature will only start to be realized once the data have been fully reduced, validated and the catalogued results subjected to scientific study (cf. Hipparcos).

Cluster

The Cluster mission is designed to study the three-dimensional, small-scale structure of the Earth's magnetosphere and its environment. The four identical spacecraft are flying in a tetrahedral formation and the separation distances between them can be varied between 600 and 20 000 km, depending on the particular nature of the scientific investigation of the various regions of the magnetosphere to be undertaken.

The Cluster-II spacecraft were launched in pairs on two Soyuz-Fregat rockets on 16 July and 9 August. The Cluster instrument commissioning phase, covering checkout, calibration and instrument operations between the four spacecraft, started in mid-August, once the four spacecraft had successfully reached their nominal operating orbit. Commissioning was completed on schedule in December, so that the true scientific mission phase could begin.

Already during the commissioning period, however, a glimpse of the mission’s scientific potential was provided by observations of the big solar storm on 9 November, which compressed the Earth’s magnetosphere to such an extent that Cluster crossed the magnetopause about one week earlier than predicted. The spacecraft’s passage into and out

Spacecraft potential measured by the EFW electric-field instrument on the ‘Salsa’ and ‘Tango’ Cluster satellites on 10 November (from G. Gustafsson, Uppsala, Sweden)

21 of the magnetosphere is clearly visible in the accompanying figure. The spacecraft were outside the magnetosphere at first, although SC4/Tango made a small incursion at 00:15 UT. After 03:30 UT, both spacecraft drifted in and out of the magnetosphere as it rapidly changed in size. The small differences between the spacecraft provide information about the shape X-ray false-colour image of the and structure of the magnetopause. At the peak of the storm, leading to the strongest low-metallicity, open stellar cluster magnetospheric compression, the spacecraft were exposed to the solar wind for more than NGC 2516: red = 0.3–0.8 keV 2 hours. The compression then decreased and the spacecraft moved back inside the green = 0.8–1.5 keV magnetosphere at around 09:00 UT. SC2/Salsa finished acquiring data at 09:00 UT, while blue = 1.5–4.0 keV SC4/Tango's measurements lasted about one hour longer.

XMM-Newton

XMM-Newton is an X-ray astrophysics observatory launched on 10 December 1999. With a projected lifetime of 10 years, it enables astronomers to conduct sensitive X-ray spectroscopic observations of cosmic sources ranging from nearby Solar System objects to distant quasars. XMM was specifically designed to investigate the spectra of cosmic X-ray sources in detail down to a limiting flux of 10–15 erg/cm2/s. The European Photon Imaging Camera (EPIC) instrument provides an X-ray imaging camera for each of XMM-Newton’s three telescope modules. Two Reflection-Grating Spectrometers (RGS) provide a medium-resolution spectroscopic capability, while the Optical Monitor (OM) enables the satellite to provide simultaneous coverage of the telescope field in the 1700 – 6000 Å waveband.

The year 2000 saw the commissioning of the instruments, their detailed calibration and intercalibration, and ‘performance verification’ observations to demonstrate the depth and extent of XMM's observational capabilities. These observations demonstrated that the instrument sensitivities required to undertake the ‘Halo’ coronal mass ejection as imaged planned mission were being comfortably met. By mid-year, the routine observing phase of by SOHO/LASCO during the close the mission had started, beginning with the guaranteed-time observations for the instrument conjunction of Venus, Jupiter, Saturn and Mercury. The Pleiades is also Principal Investigator teams. The open-time observations for the general scientific visible in the upper left of the image. community were introduced, according to plan, by the end of the year. In parallel with the The white circle in the centre indicates in-orbit activities, the ground segment – in particular operational procedures and data the size of the Sun, blocked out by an reduction and analysis – was brought fully on line. occulting disk to show the surrounding corona. The dark feature pointing towards the lower left corner is the The scientific potential of XMM-Newton was amply demonstrated by the submission and shadow of the support holding the acceptance of 56 papers for an Astronomy and Astrophysics special issue, based largely on occulter observations performed during the calibration and performance-verification phases.

SOHO

SOHO operations ran smoothly in 2000 thanks to the new gyroless control software uploaded to the spacecraft at the end of 1999. Observational highlights during the year included: – The first images of active regions on the far side of the Sun, acquired by applying a new technique known as ‘two-skip far-side seismic holography’ to high-quality MDI instrument data. – The discovery of quasi-periodic changes in tachocline rotational contrast, with a period of 15–16 months; this is in stark contrast to the 11-year sunspot cycle, and is thought to arise from dynamo processes in this region. – The discovery of over 200 Sun-grazing comets – the majority by amateur astronomers using real-time and archival images – bringing the total to over 270. – Unprecedented observations – in terms of completeness, coverage and quality – of coronal mass ejections and other energetic events, elevating space-weather studies to a new level.

22 Ulysses

Ulysses is an exploratory mission to study the properties of the interplanetary medium and Sound waves from the far side of the solar wind in the inner heliosphere as a function of heliographic latitude and solar activity. Sun are reflected internally once before Major mission milestones to date include the first traversal of the south and north polar reaching the front side, where they can be observed with SOHO’s MDI regions of the Sun in September 1994 and July/August 1995, respectively, completion of the instrument first out-of-ecliptic orbit of the Sun in April 1998, and the return to the Sun’s southern polar regions in November 2000. Several weeks earlier, in October 2000, Ulysses completed its 10th The first images of an active region on year in orbit. the far side of the Sun. The two upper images show the magnetic field The first high-latitude passes were characterised by relatively low solar activity as solar cycle strength measured with SOHO’s MDI instrument, whilst the active region was 22 approached its minimum. Ulysses is now engaged in the exploration of the high-latitude facing the Earth – before and after heliosphere near solar maximum, having recorded striking differences at solar minimum and being holographically imaged on the maximum. The accompanying figure compares the solar wind speed during the two periods far side, shown in the lower image in question. Even a cursory glance reveals that the solar wind structure encountered by Ulysses during the approach to solar maximum is fundamentally different from that during the first equator-to-pole transit. The earlier period can be broadly divided into three distinct regimes: the variable, low-speed equatorial wind from the coronal streamer belt through Solar wind speed as measured by which Ulysses moved at the start of the period; the regular excursions from slow to fast wind Ulysses, plotted as a function of solar latitude for the recent descent to the once per solar rotation as Ulysses climbed to mid-latitudes; and the remarkably constant south pole (upper panel), and during high-speed wind (~750 km/s) originating at high latitudes that filled a large volume of the the previous descent in 1992–94 (lower near-minimum heliosphere. In comparison, the solar wind speed measured during Ulysses’ panel) (courtesy of D.J. McComas) return to high southern latitudes in 1998–2000, shown in the upper panel of the figure, is persistently variable. There are no signs of the stable recurrent stream structure seen earlier, and the wind speed rarely exceeds 600 km/s. In fact, over the entire latitude range covered in the plot, the wind most closely resembles that seen near the equator. In general, the magnetic field data follow the same trend as the solar wind speed, showing recurrent features up to latitude ~35º south in 1992–93. Although field compressions related to stream interactions are present in the more recent data, these show no persistent recurrence, and extend to the highest latitudes covered to date.

ISO

During its routine operational phase from November 1995 to April 1998, ISO undertook some 26000

23 individual scientific observations, ranging from objects in our own Solar System out to the most distant extragalactic sources. Since August 1999, all data have been in the public domain. The project is now in a post-operational phase, designed to maximise the scientific exploitation of its vast database and ensure a homogeneous archive as a legacy to future generations of astronomers.

For this post-operational phase, the ISO Data Centre (IDC) has developed the ISO Data Archive, in close conjunction with representative users. This archive has powerful facilities, but is easily usable not only by experts, but also by astronomers without previous knowledge of ISO. It is one of the most technically advanced astronomical archives currently available, is web-based, and is used intensively by the astronomical community. In its first two years of use, the equivalent of 2.8 times the total number of scientific observations in the archive have been downloaded, with the monthly retrieval rate ranging from 10% to over 30%. In total, there are nearly 1100 registered users, still increasing at an average rate of about one per working day. About 100 users actually download data each month. Though the orbital operations were terminated in 1998, the ISO-related publication rate in scientific journals continues to rise.

Hipparcos

The Hipparcos and Tycho Catalogues from the Hipparcos mission, published in 1997, provided a data set that is being used for a very wide range of astronomical research: for investigations into stellar properties, the distance scale, the structure of the Galaxy, studies of extra-solar planetary systems, etc., with Hipparcos-based papers maintaining a roughly constant publication frequency. The Tycho-2 Catalogue, an astrometric and photometric reference catalogue of the 2.5 million brightest stars in the entire sky, was released in February. The catalogue contains positions, motions, brightnesses and colours for more than double the number of stars in the original Tycho Catalogue.

Taken from the wide variety of scientific applications of Hipparcos data over the past few years, the following are just a few examples of results in the area of galactic structure. The Hertzsprung-Russell Diagram for the Hyades open cluster is now beautifully delineated on the basis of the accurate individual trigonometric parallaxes, which resolve the depth of the cluster along the line of sight. Characteristics such as helium abundance and cluster age can be inferred. O and B stars are not randomly distributed among the galactic stellar populations, but generally as unbound associations of young stars. Recent studies have led to significantly improved membership determination, including extension to later spectral types, and improved mean distances out to 600–700 pc. Studying the formation, structure, and evolution of these young stellar groups and star-forming regions is important, since as many as 90% of the stars in the Galaxy may have formed in OB associations. Like bound clusters, these slowly dissolve into the field population as a result of galactic tidal forces. Cepheid variables play a crucial role as distance indicators. The Oort parameters describing galactic rotation imply a Galactic Centre distance of about 8.5 kpc, with an uncertainty of about 5%. By transforming the measured velocities to a Galactic Centre origin, a much more structured extrapolated motion becomes evident, with the phenomenon of differential galactic rotation clearly apparent. Comparison with the sum of all observed local matter confirms that the bulk of the galactic dark matter is distributed in the form of the halo, and not in the form of the disk.

Research in SSD

The research conducted by ESA Space Science Department (SSD) is directed at maintaining scientific expertise and an environment in the different fields necessary to support the Agency’s science missions with a balanced programme, reflecting the breadth of the ESA Horizons 2000 Long-Term Plan. Highlights in 2000 included the following:

In Planetary Science, the main scientific analysis effort concerned planetary surfaces (including ISO spectroscopy of Mars, Neptune and Uranus) and magnetospheres.

24 Observations of four comets (including the Rosetta target 46 P/Wirtanen) shed new light on activity outbursts, C isotopic ratios, and the morphology and rotation of comets. Dust studies were conducted using experiments on GORID/Express-II, Nozomi and Galileo. A dust stream ejected by Io volcanoes was discovered with the Galileo dust experiment.

In Solar-Terrestrial Physics, many results were derived from the analysis of SOHO solar-irradiance and coronal data acquired during the rise from minimum to maximum activity. ‘State-of-the-art’ upper limits were obtained with SOHO for solar gravity oscillations (g-modes). The COSPIN/Ulysses and WIND instruments provided data on the propagation and acceleration processes in the inner heliosphere. In the last quarter of 2000, commissioning of the ASPOC (Active Spacecraft Potential Control) and EFW (Electric Field and Wave) instruments on Cluster-II was completed and the related data analysis was started.

In the field of Stars and Planetary Systems, different phases of stellar birth-to-death evolution were studied using ISO and Hubble Space Telescope (HST) data. Multi-site and space observations were conducted on the non-radial pulsations, activity and environments of stars. Young circumstellar disks have been observed both with HST and ISO, and have been modelled in both the visible and infrared, providing key information on planetary formation.

Astronomical data analysis conducted using HST data included the study of the initial mass The two top panels show mono- function and tidal tails of globular clusters, the dynamics of the Milky Way, the signature of chromatic images (Mg IX 368 Å) merging between galaxies, and the measurement of quasar absorption lines. Using ISO data, recorded by SOHO’s CDS instrument near solar minimum (left) and the Galactic Centre was surveyed in depth, as well as ultra-luminous infrared galaxies. SAX solar maximum (right). The derived observations were analysed to provide detailed timing for high-energy accretion phenomena irradiance from the Mg X (625 Å) line as in X-ray binaries. a function of time is shown in the lower panel (X symbols). For comparison, the Time-resolved, multi-colour observations using the SCAM camera at the William Herschel F(10.7) radio flux is plotted (solid line). Both show the rise in coronal activity Telescope provided insight into the optical emission from AM Her cataclysmic binaries (including a white dwarf), and an X-ray pulsar (neutron star). Using XMM-Newton, sensitive, energy-resolved images were obtained of stellar clusters, nearby galaxies and Active Galactic Nuclei (AGN).

Mosaic of images of planetary nebula NGC 6543 (dying star irradiating previously ejected material) taken with Hubble’s WFPC2 camera

25 Part of the 22 deg2 of the inner galaxy In astrobiology, investigations of complex cosmic organics confirmed the spectroscopic surveyed by ISOCAM at wavelengths of evidence of C60+ fullerenes in space, and the role of the de-hydrogenation of molecules 7 and 15 microns smaller than 40 C atoms in the interstellar medium.

In the Department’s laboratories, qualification tests were performed on the Rosetta orbiter instruments MIDAS, OSIRIS and RPC, and the Rosetta lander’s SESAME probe. They also contributed to the AMIE multi-colour micro-camera and the SPEDE experiment for SMART-1. The on-going research and development activities included the exploitation of the SCAM camera (with a high-efficiency, colour-discriminating STJ superconducting photon counting detector array) and the development of the next-generation devices with larger fields-of-view, higher spectral resolutions and cryogenic-free operation for large telescopes, the development of compound semiconductors, the development of low-mass optics for future- generation X-ray telescopes, and the application of digital signal processors and ASICs in flight instruments.

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