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The main scientific goal of Solar Orbiter is to address the central question of heliophysics: ‘how does the Sun create and control the heliosphere?’ To achieve this goal, the spacecraft carries a unique combination of ten scientific instruments (six remote-sensing instruments and four in-situ instruments) towards the innermost regions of the Solar System, to as close as 0.28 AU from the Sun during segments of its orbit. The orbital inclination will be progressively increased so that the spacecraft reaches higher solar latitudes (up to 34° towards the end of the mission), making detailed studies of the polar regions of the Sun possible for the first time. This paper presents the spacecraft and its intended trip around the Sun. We also discuss the main engineering challenges that had to be addressed during the development cycle, instrument integration, and testing of the spacecraft.

After decades of observations of solar energetic particles (SEP) from space-based observatories, relevant questions on particle injection, transport, and acceleration remain open. To address these scientific topics, accurate measurements of the particle properties in the inner heliosphere are needed. In this paper we describe the Energetic Particle Detector (EPD), an instrument suite that is part of the scientific payload aboard the Solar Orbiter mission. Solar Orbiter will approach the Sun as close as 0.28 au and will provide extra-ecliptic measurements beyond ∼ 30 • heliographic latitude during the later stages of the mission. The EPD will measure electrons, protons, and heavy ions with high temporal resolution over a wide energy range, from suprathermal energies up to several hundreds of megaelectronvolts/nucleons. For this purpose, EPD is composed of four units: the SupraThermal Electrons and Protons (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) plus the Instrument Control Unit (ICU) that serves as power and data interface with the spacecraft. The low-energy population of electrons and ions will be covered by STEP and EPT, while the high-energy range will be measured by HET. Elemental and isotopic ion composition measurements will be performed by SIS and HET, allowing full particle identification from a few kiloelectronvolts up to several hundreds of megaelectronvolts/nucleons. Angular information will be provided by the separate look directions from different sensor heads, on the ecliptic plane along the Parker spiral magnetic field both forward and backwards, and out of the ecliptic plane observing both northern and southern hemispheres. The unparalleled observations of EPD will provide key insights into long-open and crucial questions about the processes that govern energetic particles in the inner heliosphere.

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Clusters often grow by adding concentric layers of atoms in such a way that the overall symmetry of the cluster is left unchanged. Icosahedral shell structure was first envisioned by Mackay1 while working on the difficult geometric problem of sphere packing. Shells with icosahedral symmetry have since been identified in clusters composed of inert-gas atoms,2–4 metal atoms,5,6 and even complex molecules. In this report experimental evidence will be presented for shell structures based on geometries other than the Mackay icosahedra. The results which will be discussed include: (a) octahedral shells in Al and In clusters, (b) non-Mackay-like icosahedral shells of metal deposited expitaxially on a C60 molecule and (c) new data on very large cubic shells of alkali halides. In all these cases, the evidence for shells consists of mass spectrometric anomalies that appear periodically when plotted on a (mass)1/3 scale. Each geometry is associated with a unique periodicity.