The Solar Orbiter (SolO) is a Sun-observing probe developed by the European Space Agency (ESA) with a NASA contribution. Solar Orbiter, designed to obtain detailed measurements of the inner heliosphere and the nascent solar wind, also performs close observations of the polar regions of the Sun which is difficult to do from Earth. These observations are important in investigating how the Sun creates and controls its heliosphere. Solar Orbiter makes observations of the Sun from an eccentric orbit moving as close as ≈60 solar radii (RS), or 0.284 astronomical units (au), placing it inside Mercury's perihelion of 0.3075 au. During the mission the orbital inclination will be raised to about 24°. The total mission cost is US$1.5 billion, counting both ESA and NASA contributions. Solar Orbiter was launched on 10 February 2020 from Cape Canaveral, Florida, USA. The nominal mission is planned until the end of 2026, with a potential extension until 2030.
Mission overview During the initial cruise phase, which lasted until November 2021, Solar Orbiter performed two gravity-assist manoeuvres around Venus and one around Earth to alter the spacecraft's trajectory, guiding it towards the innermost regions of the Solar System. At the same time, Solar Orbiter acquired in situ data to characterise and calibrate its remote-sensing instruments. The first close solar pass took place on 26 March 2022 at around a third of Earth's distance from the Sun. The spacecraft's orbit has been chosen to be in resonance with Venus, which means that it will return to the planet's vicinity every few orbits and can again use the planet's gravity to alter or tilt its orbit. Initially, Solar Orbiter was confined to the same orbital plane as the planets, but each encounter of Venus will increase its orbital inclination. For example, following the 2025 Venus encounter it makes solar passes at 17° inclination, increasing to 33° during a proposed mission extension phase, bringing even more of the polar regions into direct view. The spacecraft makes a close approach to the Sun every six months. The closest approaches are positioned to allow a repeated study of the same region of the solar atmosphere. Solar Orbiter is able to observe the magnetic activity building up in the atmosphere that can lead to powerful solar flares or eruptions. Researchers also have the chance to coordinate observations with NASA's Parker Solar Probe mission (2018–present) which is performing measurements of the Sun's extended corona, as well as other ground-based assets such as the Daniel K. Inouye Solar Telescope.
Objectives The objective of the mission is to perform close-up, high-resolution studies of the Sun and its inner heliosphere. The new understanding will help answer these questions:
How and where do the solar wind plasma and magnetic field originate in the corona? How do solar transients drive heliospheric variability? How do solar eruptions produce energetic particle radiation that fills the heliosphere? How does the solar dynamo work and drive connections between the Sun and the heliosphere?
Spacecraft The Solar Orbiter spacecraft is a Sun-pointed, three-axis stabilised platform with a dedicated heat shield to provide protection from the high levels of solar flux near perihelion. The 21 sensors were configured on the spacecraft to allow each to conduct its in-situ or remote-sensing experiments with both access to and protection from the solar environment. Solar Orbiter has inherited technology from previous missions, such as the solar arrays from ESA's BepiColombo Mercury Planetary Orbiter (MPO). The solar arrays can be rotated about their longitudinal axis to avoid overheating when close to the Sun. A battery pack provides supplementary power at other points in the mission such as eclipse periods encountered during planetary flybys.
Communication The Telemetry, Tracking, and Command Subsystem provides the communication link capability with the Earth in X-band. The subsystem supports telemetry, telecommand and ranging. Low-gain antennas are used for Launch and Early Orbit Phase (LEOP) and function as a back-up during the mission phase when steerable medium- and high-gain antennas are in use. The High-Temperature High-Gain Antenna needs to point to a wide range of positions to achieve a link with the ground station and to be able to downlink sufficient volumes of data. Its design was adapted from the BepiColombo mission. The antenna can be folded in to gain protection from Solar Orbiter's heat shield if necessary. Most data will therefore initially be stored in on-board memory and sent back to Earth at the earliest possible opportunity. During nominal science operations, science data is downlinked for eight hours during each communication period with the ground station. Additional eight-hour downlink passes are scheduled as needed to reach the required total science data return of the mission. The Solar Orbiter ground segment makes maximum reuse of ESA's infrastructure for Deep Space missions:
The ground stations, which belong to ESA's space tracking station network (ESTRACK) The Mission Operations Centre (MOC), located at ESOC, Darmstadt, Germany The Science Operations Centre (SOC), located at ESAC, Villanueva de la Cañada, Spain The communications network, linking the various remotely located centres and stations to support the operational data traffic The Science Operations Centre was responsible for mission planning and the generation of payload operations requests to the MOC, as well as science data archiving. The SOC has been operational for the active science phase of the mission, i.e. from the beginning of the Cruise Phase onwards. The handover of payload operations from the MOC to the SOC is performed at the end of the Near-Earth Commissioning Phase (NECP). ESA's Malargüe Station in Argentina will be used for all operations throughout the mission, with the ground stations of New Norcia Station, Australia, and Cebreros Station, Spain, acting as backup when necessary.
Instruments
The science payload is composed of 10 instruments:
Heliospheric in-situ instruments (4)
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