The Solar Orbiter (SolO) spacecraft was launched in February 2020 by a United Launch Alliance (ULA) Atlas V 411 rocket from Cape Canaveral, Florida. The mission is a collaboration between the European Space Agency (ESA) and NASA. The mission is expected to study the poles of the Sun and return unprecedented data and images, as well as some first views of the Sun’s polar regions. The first images were released in July 2020.
NASA and ESA had sent the Ulysses Mission in 1990. That spacecraft too passed over the Sun’s poles but at much farther distances. It did not carry a camera.
Solar Orbiter has been designed to face the Sun at approximately 42 million km from its surface. Before SolO, all solar imaging instruments have been within the ecliptic plane, in which all planets orbit and which is aligned with the Sun’s equator. SolO carries four in situ instruments (which measure the space environment immediately around the spacecraft like the sense of touch) and six remote-sensing imagers (which see the Sun from afar). Scientists will combine observations from both sets of instruments and attempt to answer some important questions, such as what drives the Sun’s 11-year cycle of rising and falling magnetic activity; what heats up the corona to millions of degrees Celsius; what drives the generation of the solar wind and what causes the solar wind to reach speeds of hundreds of kilometres per second, in other words, measuring the composition of the solar wind and linking it to its area of origin on the Sun’s surface; and how all of this affects Earth. The sunspot cycle could also be explained.
The mission is expected to provide information on how the behaviour of the Sun would affect technology such as satellites, navigation systems, power grids, and telecommunication services. More data on the global magnetic field of the Sun would help scientists to forecast space weather events.
The probe is a complement to the Parker Solar Probe launched by NASA in August 2018; however, the Parker probe takes in-situ measures and cannot see the whole picture.
Solar Orbiter has been nicknamed Solar Orbiter ‘Blackbird’ owing to its special thermal protection system that helps it to go into a very hot region. It has an effective 150-kg heat shield made to withstand temperatures up to 520 degrees Celsius. The heat shield must always be pointed straight at the Sun. The heat shield is made up of many layers. The front layer is of very thin sheets of titanium foil to strongly reflect heat. The inner layer nearest to the spacecraft is of honeycomb-patterned aluminium covered in more foil insulation to provide support. The layers are kept in place with thin titanium brackets. Double structural gaps allow most of the absorbed heat to radiate out sideways into space, according to the ESA. Peepholes help the instruments to be used by remote control and they can be shut when not needed.
The titanium foil is coated with a special material, called Solar Black which is made of calcium phosphate, the same material as human bone. (This material has also been used in the medical field to help prosthetics bond with human bone in order to reduce the chance of rejection.) The bone-based coating has stable thermal properties, is electrically conductive, and enabled to resist degradation under the blow of intense solar ultraviolet radiation. Even though some heat would be absorbed by the black powder, it sheds the heat back into space. Black colour was chosen as the shield needed to maintain the same colour during its total mission life; white, after years of exposure to intense solar flux, including high ultraviolet radiation, would change colour.
The orbiter may provide scientists a first good look at the Sun’s poles only after 2025, when it reaches a trajectory of 17 degrees above the ecliptic plane—where the Earth and the rest of the planets orbit. The spacecraft will achieve this vantage point via gravity-assist flybys of Venus, which will boost its inclination.
Solar Orbiter’s steepest viewpoint, 33 degrees above the ecliptic, would come in 2029, when the spacecraft could be well into an expected extended mission (which would start in December 2026). That angle will provide the best images of the Sun’s polar regions, although throughout the mission, the spacecraft will beam back data about this region.
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