As it approaches perihelion, Solar Orbiter’s angular velocity will closely parallel the rotation rate of the Sun itself, offering a unique opportunity to scan areas of the surface for days at a time. This orbit will permit the spacecraft to take measurements of internal magnetism, as well as the triggers and propagation characteristics of emerging solar phenomena. Observing these across multiple latitudes allows scientists to study complex flows deep inside the Sun and better constrain existing and evolving theoretical models for the solar dynamo and coronal magnetic fields.
“We have some generic plans and will tailor them to individual orbits as we get closer,” Horbury says. “Planning for a given orbit starts one year ahead and iterates towards a final detailed plan over six months or so.”
Solar Orbiter’s instruments
Solar Orbiter’s 10 instruments include four in-situ sensors that will run continuously, tracking fields and particles around the spacecraft. Its six remote-sensing detectors will peer directly at the Sun for about 30 days per orbit, using protected apertures cut through its heat shield. Solar Orbiter will work closely with NASA’s Parker Solar Probe. But whereas Parker’s extreme closeness to the Sun — as little as 0.046 AU, squarely inside the corona — promises a fields-and-particles bonanza, its location nixed any chance that it could carry telescopes, thanks to high temperatures. The moderately more benign thermal climes at Solar Orbiter’s distance will allow its imagers to provide additional context for Parker.
Most of the in-situ instruments occupy an extendable boom to minimize interference from spacecraft electronics. The Radio and Plasma Waves instrument from France’s Observatoire de Paris has sensors on the boom and on three monopole antennas, angled 90° apart. UCL’s Solar Wind Plasma Analyser also has detectors on the boom and antennas, plus two more on the main spacecraft body, including a NASA heavy ion sensor. These will study the densities, velocities, temperatures, and compositions of solar wind ions and electrons. “The relative composition of the heavy ions provides a kind of fingerprint, which can be compared with similar information from spectroscopic measurements of the solar surface and thereby confirm the link between the source and the spacecraft,” Owen says. “These measurements should help us reveal how the dynamics of the Sun drive the solar wind and its variability, how that links into interplanetary space and affects Earth’s near-space environment.”
Imperial’s magnetometer will investigate magnetic field evolution, how energetic particles traverse the heliosphere, and why coronal plasmas are so much hotter than the Sun’s surface. The last question has long stumped researchers: “The jury is still out as to whether waves, nanoflares, or something else is doing the extra heating,” Horbury says. “It’s probably a combination of lots of things, but we need to know the contribution of each and how they vary. Helios has given us some strong hints that nanoflares must be playing a role and, with Orbiter, we’ll be able to tie those things down by looking at the solar plasma and fields in unprecedented detail, while looking remotely at the source regions and linking the two.”
Solar Orbiter’s instruments have been a decade in the making. Imperial also contributed magnetometers to Ulysses and Cassini. “There’s a lot of know-how in the design of a sensitive scientific instrument and this flow from one mission to the next, through the expertise of the engineering team,” Horbury says. “It would have been far harder to start from scratch.”