Designing a spacecraft that can image the Sun as closely as possible without melting the delicate instruments has been a challenge, but scientists’ luck could be about to change.
Far from being a quiet, stable and benign star, our Sun is constantly changing. It belches out huge energy flares, and – biggest of all – giant storms or coronal mass ejections (CMEs) over a variable 11-year solar cycle. These huge storms on the Sun generate strong accelerated pulses in the solar wind that constantly streams out from it, bathing our whole solar system. During periods of high solar activity these CMEs are more frequent. When they hit the Earth’s magnetosphere they can be visualised as the beautiful aurora light shows around our polar regions.
Extreme events pose a danger to our increasingly technologically dependent society, endangering orbiting satellites, astronauts on board the space station and air passengers flying over the poles. A once-in-a-century event like the solar storm that hit Earth in 1859, known as the Carrington event, could be disastrous for today’s modern technology causing massive power outages affecting millions and lasting for days, wreak havoc with global telecommunication systems, disrupt radio signals, and damage electronics. Researchers hope that the knowledge gained from the Solar Orbiter mission will improve the models used to forecast the worst of the giant storms and space weather in general.
One of the mission’s objectives is to better understand what generates these massive solar storms, and how that ties in with our star’s 11-year cycle of rising and falling magnetic activity. Discovering how that cycle works is another of Solar Orbiter’s mission objectives and is key to a better understanding of solar physics.
The collaborative European Space Agency (ESA) and Nasa project has already yielded new discoveries about the dynamic behaviour of our nearest star. The first images taken by the Extreme Ultraviolet Imager (EUI) reveal miniature solar flares or nano-flares, dubbed ‘campfires’ by the science team. These are millions or even billions of times smaller than the solar flares that can be detected from Earth.
The Solar Orbiter spacecraft in numbers
1,720kg: Launch mass
4: Instruments to sense the solar wind around the spacecraft
6: Telescopes to observe the Sun’s surface
42 million km: Closest approach to the Sun
600°C: Maximum design heat exposure to the spacecraft heatshield
£1.3bn: Cost of mission
10+ years: Length of mission
Solar physicist Daniel Müller is the ESA science mission lead, and has been working on the Solar Orbiter mission since its early planning phase in 2007. He says: “We could see hundreds of these mini flares for the first time in such small sizes. They could help us understand why the corona is so hot relative to the Sun’s surface.” The surface of the Sun is ‘only’ about 5,500°C while the corona, or outer atmosphere, reaches a million degrees. How this is possible is a mystery that the Solar Orbiter science team hopes to solve.
SPICE, short for Spectral Imaging of the Coronal Environment, is a spectrometer built by UK RAL Space that measures the physical state and composition of the Sun’s corona in the ultraviolet wavelengths. It can also measure the speed of the plasma gases in the lower corona that might be responsible for the extreme heating of the upper atmosphere. “Theoretical solar physicists have long thought that these nano-flares might exist, but Solar Orbiter is our first chance to actually see them,” Müller adds.
In late December 2020 the spacecraft passed extremely close to Venus in the first of eight approaches over the next decade designed to use the planet’s gravity to alter its own orbit around the Sun.These flybys will raise Solar Orbiter’s inclination relative to the Sun’s equator, enabling its instruments to image the solar polar regions for the first time ever, perhaps giving us clues as to how the Sun’s magnetic field flips every 11 years.
The six remote sensing instruments (telescopes) that look at the Sun hide behind a specially designed heat shield. Four other instruments perch on a 4.4m boom that extends out to the rear of the spacecraft and monitor the environment around it, constantly sampling the passing solar wind. Combining the data from the two sets of instruments will give solar scientists the information needed to make those vital links between what happens on the Sun’s surface, in its gaseous corona, and how that is manifested in the properties of the solar wind. Each instrument has a lead investigator who is responsible for its development and for the science data.
“One of the cool things about Solar Orbiter is that if one instrument spots something interesting, other instruments can focus on that feature. That interplay is one of the nice features of the Solar Orbiter,” says César García Marirrodriga, ESA project manager and formerly space mission engineer. “Some of the telescopes take 10 high resolution images per second. This would quickly fill up the Solar Orbiter’s memory, so we compress the data on board. Data management is key, and some images are stored on Solar Orbiter for up to six months so that they can be downloaded at speed when the spacecraft is at its closest to Earth during its solar orbit.”
Facts about our Sun
150 million km from Earth.
The vital 3.1m by 2.4m heatshield must protect the six instruments that look directly at the Sun and has an ingenious design. It is constructed of a mille-feuille of multiple layers of titanium foil embossed with tiny dimples. Tiny gaps between the layers help to radiate heat away into the vacuum of space and improve the insulating qualities of the shield. A naked titanium surface would reach an unacceptable 700°C or more, so coating this surface was necessary to withstand the extreme ultraviolet and infrared radiation. The coating also had to be thermally and optically stable under the intense ultraviolet light, and conductive to charged particles to avoid the build-up of static charges that might disturb the precious instrument cargo. It also had to shed no particles under the extreme vibration of lift-off that could contaminate the vital lenses and mirrors.
Testing showed that a white coating would darken over time so the team picked Solar Black, specifically developed by Irish company Enbio for Solar Orbiter. In a single process the metal oxide layer is abraded from the titanium and immediately replaced with the calcium phosphate pigment, based on charred bones, which chemically bonds with the metal surface, becoming part of it. So effective is the shield design that a mere 15cm behind it the temperature is a much more moderate 120°C. Solar Black also coats the high-gain antenna that sticks 4.4m to the rear of the probe where the remaining four instruments are always on during the mission, measuring the solar wind as it passes over the spacecraft.
The six remote sensing instruments need to look directly at the Sun to take their readings but, in order to minimise exposure, engineers cut tiny windows in the heatshield that open at times optimal for observations. But the heatshield alone is not enough to dispel heat accumulation and protect the instruments. SPICE, for example, needs a temperature of -20°C at its detector in order to make accurate measurements. The front of the 80cm-long instrument is at 120°C (just behind the heatshield) and most of the light entering it is in the visible and infrared wavelengths, which have a heating effect.
The design team decided to use pyrolytic graphite thermal straps to run 70-80cm from the heat-sensitive instruments to a set of ten radiators placed on the exterior of the spacecraft which act as heat dumps. This material was invented by Nasa in the 1960s but had never been used in space before. “Pyrolytic graphite consists of layers of carbon, is five times more conductive than copper, and is as flexible as paper. Because of the enormous forces transmitted during launch, this flexibility is vital to protect the delicate instruments to which they are directly attached,” says Walters.
For further thermal optimisation, each of the six remote-sensing instrument teams using complex solutions with mirrors and filters to safeguard the cameras and lenses.
During its years-long journey travelling at 90,000 miles per hour around the inner solar system, the Solar Orbiter will be tracked by and communicate with ESA’s deep-space tracking network Estrack, a global network of ground stations that will relay data to ESA mission control in Darmstadt, Germany. But there will be long periods when the spacecraft cannot communicate with Earth at all. For up to 72 days at a time, Solar Orbiter may be behind the Sun and invisible from Earth, so for those periods the on-board computer and software must run autonomously, maintaining the correct orientation of the heatshield relative to the Sun. The solar arrays must also be reoriented according to the distance from the Sun. Close in they must be angled almost edge-on to it, as they would otherwise overheat with end-of-mission consequences. Glues would cease to act as glues and the solar cells could detach, bend or distort, resulting in loss of power and probable burn-up. “Engineering for the 20 or so extreme heat and cold cycles that the spacecraft must endure is much harder than designing for a more constant environment,” says Marirrodriga in a classic understatement.
Missions like Solar Orbiter are planned years in advance, but its frequent close flybys of Venus may enable some of its instruments to have a closer look at the planet’s outer clouds. A report in September 2020 by Jane Greaves, an astronomer at Cardiff University, recorded the presence of phosphine gas there – a potential marker for biological life – in the cooler upper cloud layer above the scorching planetary surface. The mission scientists are now considering what Solar Orbiter could contribute to the exploration of Venus without jeopardising its original objectives.
“Solar Orbiter is all about establishing a link between what happens on the Sun and how that manifests itself in the solar wind, and ultimately in the space weather that affects our planet. This is just the beginning,” says Müller. “I am sure that there will be lots more exciting science to come.”