Solar Orbiter reaches first perihelion

Solar Orbiter takes pictures of the Sun from a closer distance than any other spacecraft before.

Today ESA's Solar Orbiter has reached the closest point in its current orbit around the Sun. Only about 77 million kilometres separated the probe from our star. That is just over half the distance between the Sun and Earth. All scientific instruments on board the spacecraft that look at the Sun will be switched on today or in the next few days. Never before have images of the Sun been taken from such a close distance. However, since only four and a half months have passed since the launch and the mission is still in an early phase, the measurement data will be primarily used to calibrate the instruments. Over the next few years, Solar Orbiter will orbit the Sun on increasingly narrow ellipses, gradually coming within 42 million kilometers of the Sun. The Max Planck Institute for Solar System Research (MPS) in Germany is involved in four scientific instruments on board Solar Orbiter.

Six of Solar Orbiter’s ten scientific instruments look at the Sun and its immediate surroundings. These instruments have not yet been fully commissioned. Today's measurements and those in the next few days will primarily serve to calibrate the instruments. "Even though the images from the remote sensing instruments are not yet part of Solar Orbiter's scientific measurement campaign, today is nevertheless a milestone", explains Prof. Dr. Sami K. Solanki, Director at MPS and Principal Investigator of PHI (Polarimetric and Helioseismic Imager) on board Solar Orbiter. "Never before has a telescope imaged the Sun from such a close distance," he adds.

Other space probes have already ventured nearer to our central star. NASA's Parker Solar Probe, for example, has been exploring the Sun from close up for two years. However, these predecessors of Solar Orbiter were and are not equipped with instruments that image the Sun. 

For the scientists a primary focus is the spatial resolution of the measured. Their goal is to reveal structures on the Sun that are small as possible. In the course of the mission, Solar Orbiter’s instruments will provide a sharper view of the Sun than other solar observatories in space. Only a few ground-based solar telescopes, such as the Daniel K. Inouye Solar Telescope on Hawaii, which started its scientific operations at the beginning of the year, can image smaller details with their much larger mirrors. However, unlike Solar Orbiter ground-based telescopes have to make do without the ultraviolet radiation from the Sun. Most of it is absorbed in the Earth's atmosphere.

Ultraviolet radiation in particular is crucial for three of Solar Orbiter’s instruments to which the MPS has contributed. The instruments EUI (Extreme Ultraviolet Imager), SPICE (Spectral Imaging of the Coronal Environment) and the coronagraph Metis look at the hot outer layers of the Sun, which mainly emit light of these wavelengths. During the perihelion passage however, they focus also on another target: the star Regulus in the constellation Leo, which is occulted by the Sun (as seen from Solar Orbiter) during the flyby. Before and after, it will be visible close to the Sun in these instruments’ fields of vision. "Regulus has already been studied in detail by other telescopes such as the Hubble Space Telescope. It is a very hot star, that mainly emits ultraviolet radiation," explains Dr. Luca Teriaca from MPS, who is part of the EUI, SPICE, and Metis teams. Regulus is therefore an optimal calibration star for the researchers.

Despite the rather technical objectives of the next few days, the scientists will of course also look at today's images from a scientific point of view. However, the Sun is currently rather monotonous: for days now, hardly any sunspots or eruptions have been seen. "For calibration, this may even be an advantage”, says MPS scientist Dr. Johann Hirzberger, Operations Scientist PHI.

The images from the perihelion flyby will arrive on Earth in the course of the coming week and will then first be carefully processed. The final images will be published in mid-July.