COL 11/8/00 10:18 AM Page 2 r bulletin 104 — november 2000 Solar Orbiter: A Challenging Mission Design for Near-Sun Observations G. Colangelo Mechanical Engineering Department, ESA Directorate of Technical and Operational Support, ESTEC, Noordwijk, The Netherlands O. Pace Science Future Projects and Technology Office, ESA Directorate of Scientific Programmes, ESTEC, Noordwijk, The Netherlands R. Marsden & B. Fleck Space Science Department, ESA Directorate of Scientific Programmes, ESTEC, Noordwijk, The Netherlands Introduction Physics Planning Group, and which was The results from missions such as Helios, proposed as a possible Flexi-mission (F2/F3) Ulysses, Yohkoh, SOHO and TRACE (Transition candidate. Region and Coronal Explorer) have advanced enormously our understanding of the solar The Solar Orbiter Mission as studied embodies corona and the associated solar wind and several totally novel aspects that would allow three-dimensional heliosphere. We have now unique science investigations to be performed. reached the point, however, where in-situ For example: measurements closer to the Sun, together with high-resolution imaging and spectroscopy from – By approaching the Sun to within 45 solar a near-Sun and out-of-ecliptic perspective, radii, or 0.21 Astronomical Units (AU), Solar promise to bring about major breakthroughs in Orbiter would enable close-up remote- solar and heliospheric physics. The acquisition sensing observations of the magnetised solar of these measurements is the primary scientific atmosphere, providing: (a) unprecedented objective of the Solar Orbiter Mission that has high-resolution, and (b) in-situ measurements been studied in the framework of ESA’s Solar of the unexplored innermost region of the heliosphere. – By matching the speed of the Orbiter near The Sun’s atmosphere and heliosphere represent uniquely accessible perihelion to the Sun’s rotation rate, unique domains of space, where fundamental physical processes common to helio-synchronous observations could be solar, astrophysical and laboratory plasmas can be studied under acquired, enabling a better understanding of conditions impossible to duplicate on Earth, and at a level of detail the links between solar and heliospheric that it is not possible to achieve at astronomical distances. A Solar processes. Orbiter Mission designed to take the next step in our exploration of – By increasing the inclination of the orbital these domains was the topic of a technical pre-assessment study plane with respect to the solar equator, the performed in 1999 within the framework of ESA’s Solar Physics first-ever out-of-ecliptic imaging and Planning Group. A Solar Orbiter-type mission was also proposed to spectroscopic observations of the solar ESA by an international team of scientists in response to the recent poles and equatorial corona from latitudes call for mission proposals for two Flexi-missions (F2 and F3). as high as 38º could be acquired. A key feature of the mission is a spacecraft orbit that not only provides Scientific goals multiple, near-Sun passes, but which also carries the Orbiter to Close-up observations of the solar moderately high solar latitudes. This, together with the cost atmosphere: the Sun’s magnetised plasma constraints associated with Flexi-type missions, represented a High-resolution imaging of the solar atmos- significant challenge in terms of mission and system design. phere from Solar Orbiter would represent a major step forward, providing an order-of- 76 COL 11/8/00 10:18 AM Page 3 solar orbiter magnitude improvement in spatial resolution evolution is entangled with other effects such Figure 1. High-resolution over past missions. When operated in concert, as centre-to-limb variations, foreshortening and image of the Sun recorded the scientific instruments envisaged for Solar projection effects. In order to disentangle these by the NASA Transition Region and Coronal Orbiter would enable: effects, it is necessary to have an instrument Explorer (TRACE) mission – a thorough analysis of the time-variability, platform that co-rotates with the Sun. The Solar evolution and fine-scale structure of the Orbiter would, for the first time, provide such an dynamic chromosphere, transition region opportunity and would thus help to resolve old and corona and otherwise intractable problems related to – the study of the Sun’s magnetic activity on the solar dynamo and the diffusion of the multiple scales magnetic field across the solar surface. It would – the investigation of energetic-particle also allow, again for the first time, the evolution acceleration, confinement and release of sunspots and active regions to be followed, – the plasma and radiation processes as well as the influence of the observed underlying the heating of the chromosphere changes on the corona above. and corona to be revealed. In-situ measurements in the inner The Sun is the only star that can be imaged at heliosphere: particles and fields a level of detail such that the physical According to recent SOHO findings, coronal processes responsible for magnetic activity can expansion arises because of the high Figure 2. The Sun as seen by the Extreme-ultraviolet be resolved. temperature of the coronal ions. The Imaging Telescope (EIT) on temperature of the minor species can reach as SOHO (left) and TRACE 8 Helio-synchronous observations: linking the much as 10 K at a few solar radii. In contrast, (centre). Pixel sizes on the photosphere and corona to the heliosphere coronal electrons are comparatively cool. In Sun of EIT and TRACE are Studies of the evolution of solar features such fact, they hardly reach the canonical coronal 1850 and 350 km, as active regions, loops, prominences or temperature of 106 K, and consequently the respectively. The Extreme Ultra-Violet (EUV) imager sunspots are greatly complicated by the fact electric field has a minor role in accelerating the envisaged for the Solar that their evolution time scales are comparable ions. The high pressure of the coronal ions and Orbiter will have a pixel size to the solar rotation period. As a result, the the low pressure of the local interstellar medium of 35 km on the Sun SOHO TRACE SOLAR ORBITER 77 COL 11/8/00 10:18 AM Page 4 r bulletin 104 — november 2000 lead to a supersonic solar wind extending to and the solar wind remain unanswered. A Solar large distances from the Sun (typically 100 AU). Orbiter mission combining high-latitude Yet, even given the insights resulting from vantage points and a suite of remote-sensing SOHO, the detailed physical mechanisms that instruments is the next logical step. For heat the corona and accelerate the plasma to example, progress in understanding the solar supersonic speed remain poorly understood. dynamo will depend on how well we This is largely because the resolution of the understand differential rotation and the SOHO imagers and spectrometers was still not circumpolar and meridional flows near the sufficient. Furthermore, the solar-wind plasma poles of the Sun. The poles appear to rotate has never been sampled directly closer to the comparatively slowly, but the polar vortex is still Sun than 0.3 AU (the perihelion distance of not well characterised due to the serious Helios). limitations of in-ecliptic observations. The Solar Orbiter would provide the first opportunity to The Solar Orbiter would provide the first measure directly the magnetic field at the poles, opportunity of going closer to the Sun than the as well as the surface and subsurface flows Helios space probes. Furthermore, it would there. carry powerful, high-resolution optical instruments in addition to the in-situ experiments. The large-scale unipolar character of the In particular, the envisaged plasma and field magnetic field in coronal holes gives rise to the instruments would have high temporal fast solar wind that expands super-radially into resolution, ranging between 0.01 s and 1 s, the heliosphere. The Solar Orbiter offers the offering unique possibilities to resolve physical opportunity to sample the fast wind at processes at their intrinsic scales. This would in distances where the plasma still retains some turn provide new insights into the plasma memory of the acceleration processes. With kinetic processes that structure the Sun’s regard to the sites of acceleration, SOHO atmosphere, heat the extended corona, and spectroscopic measurements suggest that the accelerate both the solar wind and energetic supergranulation network plays a special role. particles. High-latitude observations would provide a definite answer. The excursion out of the ecliptic: the Sun’s polar regions and equatorial corona Even with new, dedicated missions such as Figure 3. Ecliptic projection Despite the great achievements of Ulysses and STEREO, it is impossible to determine the of the Solar Orbiter’s SOHO, significant scientific questions mass distribution of large-scale structures such trajectory concerning the nature of the Sun, its corona as streamers, and the true longitudinal extent of Coronal Mass Ejections (CMEs). The Solar Orbiter would provide the first observations of the complete equatorial corona and its expansion in the equatorial plane. Furthermore, it would provide crucial information on the three-dimensional form of CMEs. Scientific payload In order to achieve the wide-ranging aims described above, the Solar Orbiter must carry a suite of sophisticated instruments. Owing to the Orbiter’s proximity to the Sun, the instruments could be smaller than comparable Y [AU] instrumentation located at the Earth’s orbit. The payload