After its first orbits around the Sun, ESA's Solar Orbiter spacecraft returns to within just 460 kilometers of Earth next Saturday, November 27. The flyby will change the spacecraft's trajectory and velocity allowing Solar Orbiter to fly within a distance of only 50 million kilometers of the Sun during its next orbit. The maneuver also marks an important mission milestone: the commissioning of the scientific instruments is complete; now the regular measurement routine begins. For the instruments PHI, EUI, Metis and SPICE, to which the Max Planck Institute for Solar System Research (MPS) in Germany has contributed, this routine will include simultaneous measurements with research satellites in Earth’s proximity and with ground-based observatories. The current flightpath offers important opportunities to prepare and optimize for this kind of scientific teamwork.
On their way to the outer or inner Solar System, space probes often use flyby maneuvers to gain momentum or decelerate. However, it is a rarity that they venture as close to the respective planet as Solar Orbiter is doing now. On Saturday, November 27, at 5:30 a.m. (CET), the space probe will zoom over North Africa and the Canary Islands at a distance of just 460 kilometers. That is only slightly more than the distance between Earth and the International Space Station. On its route, Solar Orbiter will pass through two areas of space debris that surround our planet: one at a distance of about 36,000 kilometers above the Earth's surface and another at a distance of less than 2,000 kilometers. ESA's control center in Darmstadt will monitor the flyby and can correct the course if necessary.
The flyby is also a special event from a scientific perspective. It offers Solar Orbiter's in situ instruments the opportunity to study how the Earth's magnetic field and the particles trapped within it interact with the solar wind. Combining this data with measurements from the space probes belonging to the Cluster and Swarm mission, which routinely monitor this interaction from an Earth orbit, can lead to a more accurate and complete picture of Earth's environment. "During the flyby, Cluster will be switched to a mode that allows data of the highest quality to be transmitted to Earth," says MPS scientist Dr. Patrick Daly, Principal Investigator of Cluster’s instrument RAPID (Research with Adaptive Particle Imaging Detectors). "These data are an excellent basis for collaborative studies," he adds.
An Opportunity for Cross-Calibrations
Solar Orbiter’s instruments PHI (Polarimetric and Helioseismic Imager), EUI (Extreme-Ultraviolet Imager), SPICE (Spectral Imaging of the Coronal Environment) and the coronagraph Metis, to which MPS has contributed, conduct scientific observations in precisely pre-planned time windows. During the current flight phase, they are in stand-by mode. Simultaneous and coordinated measurements with ground-based solar observatories and solar probes stationed close to Earth (such as NASA's Solar Dynamics Observatory (SDO) and the Japanese probe Hinode) took place earlier this month. At that time, Solar Orbiter was already quite close to Earth. All involved observatories therefore enjoyed a similar view of our star. This is an important prerequisite for future joint campaigns.
"One of the great strengths of Solar Orbiter is that the probe orbits the Sun and can therefore sometimes see its backside," Prof. Dr. Sami K. Solanki, MPS director and PHI Principal Investigator, explains. "At the same time, solar observatories close to Earth such as SDO and Hinode can monitor the Sun’s front. Together, this for the first time creates a view of the whole Sun in its entirety”, he adds.
For this kind of scientific teamwork, the observational data from the different probes must be comparable. "It must be ensured that for the same phenomenon on the Sun the instruments provide the same measurement value," MPS scientist Dr. Johann Hirzberger, PHI Operations Scientist, explains. Therefore, such cross-calibrations are only possible during a phase, such as the current one, when all involved observatories can look at the Sun from a similar vantage point. Another opportunity for joint calibrations will come in March of next year. By then, Solar Orbiter will have already traveled half the distance between Earth and the Sun and will provide data with significantly higher resolution.
Routine Operations Begin
After the Earth flyby, Solar Orbiter's remote sensing instruments begin their normal scientific operations. "We have used the past months since launch to get to know how the instruments behave under space conditions. We've seen what the instruments can do," Solanki says. The scientists are now looking forward to the measurements to come.
Already, the data generated during the commissioning phase is impressive. For example, already in February the PHI team was able to perform coordinated measurements with other probes, providing a first all-around glimpse of the Sun. Earlier this year, the EUI team detected small, bright regions in the solar corona. These so-called campfires occur much more frequently than expected and may help to better understand the processes in the solar corona. Joint measurements by Solar Orbiter's various telescopes make it possible to track these mass ejections from the solar surface all the way into space.
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