Image source: ESA/ATG medialab
Designing a spacecraft that can image the sun as close as possible without melting precision instruments has always been a challenge, but the luck of scientists may be about to change.
The 1.7-ton spacecraft launched from Cape Canaveral in February 2020 is called the Solar Orbiter, which aims to obtain close-up images of the sun from 42 million kilometers away while simultaneously measuring its energy behavior in real time. The 10 airborne instruments will enable it to uniquely combine the study of the solar gaseous coronal event, measure its magnetic field, and sample the solar wind as it flows through the spacecraft as a stream of high-energy particles. In addition to deepening our understanding of how the sun works, there are some practical reasons why it is a good idea to observe and better predict the behavior of the sun.
Our sun is far from being a quiet, stable and benign star, but constantly changing. It ejects huge energy flares, and-the largest-huge storms or coronal mass ejections (CME) in a variable 11-year solar cycle. These huge storms on the sun produce strong acceleration pulses in the solar wind constantly flowing out of the solar wind, bathing our entire solar system. During periods of high solar activity, these CMEs are more frequent. When they hit the Earth’s magnetosphere, they can be imagined as beautiful aurora displays around our poles.
Extreme events pose a threat to our society that increasingly relies on technology, endangering orbiting satellites, astronauts on space stations, and air passengers flying over the poles. A once-in-a-hundred-year event, such as the solar storm that hit the earth in 1859, is called the Carrington event, which can be catastrophic for today’s modern technology, causing a massive blackout that affects millions of people and lasts for several days. , Causing serious damage to the global telecommunications system, interfering with radio signals and damaging electronic equipment. Researchers hope that the knowledge gained from the solar orbiter mission will improve the models used to predict the worst megastorms and general space weather.
One of the goals of the mission is to better understand what generates these huge solar storms and how it relates to our star’s 11-year ascending and declining magnetic field activity cycles. Exploring the working principle of this cycle is another mission goal of the solar orbiter, and it is also the key to a better understanding of solar physics.
A collaborative project between the European Space Agency (ESA) and NASA has made new discoveries about the dynamic behavior of our recent stars. The first images taken by the Extreme Ultraviolet Imager (EUI) revealed miniature solar flares, or nanoflares, called "campfires" by the scientific team. They are millions or even billions of times smaller than solar flares that can be detected from Earth.
4: Instruments to detect the solar wind around the spacecraft
6: The telescope observes the surface of the sun
42 million kilometers: the closest distance to the sun
600°C: Maximum design heat exposure for spacecraft insulation panels
Solar physicist Daniel Müller is the head of ESA's scientific missions and has been working on solar orbiter missions since the early planning stage in 2007. He said: “For the first time we can see hundreds of such small flares. They can help us understand why the corona is so hot relative to the sun’s surface.” The temperature of the sun’s surface is “only” about 5,500°C, while the corona Or the temperature of the outer atmosphere reaches 1 million degrees. How this is achieved is a mystery that the solar orbiter science team hopes to solve.
SPICE is the abbreviation of corona environmental spectral imaging. It is a spectrometer built by the British RAL Space to measure the physical state and composition of the solar corona at ultraviolet wavelengths. It can also measure the velocity of plasma gas in the lower corona, which may be responsible for the extreme heating of the upper atmosphere. "Theoretical solar physicists have long believed that these nanoflares may exist, but the solar orbiter is the first time we have the opportunity to actually see them," Mueller added.
In late December 2020, the spacecraft was the first of eight overflights in the next 10 years, very close to Venus, aiming to use Venus’s gravity to change its orbit around the sun. These overflights will increase the inclination of the solar orbiter relative to the solar equator, allowing its instrument to image the solar polar regions for the first time, and may provide us with clues about how the solar magnetic field flips every 11 years.
Six remote sensing instruments (telescopes) for observing the sun are hidden behind a specially designed heat shield. The other four instruments are perched on a 4.4m boom extending to the rear of the spacecraft, and monitor the surrounding environment, continuously sampling the passing solar wind. Combining the data from the two sets of instruments will provide solar scientists with the information they need to establish important connections between what is happening on the surface of the sun, what is happening in the gaseous corona, and how these things are reflected in the characteristics of the solar wind. Each instrument has a principal researcher who is responsible for its development and scientific data.
"The cool thing about solar orbiters is that if one instrument finds something interesting, other instruments can focus on that function. This interaction is a major feature of solar orbiters," the ESA project Said César García Marirrodriga, manager and former space mission engineer. “Some telescopes can take 10 high-resolution images per second. This will quickly fill up the solar orbiter’s memory, so we compressed the data on board. Data management is the key. Some images will be stored on the solar orbiter for up to Six months so that the spacecraft can quickly download it when it is closest to the earth in its solar orbit."
"The Solar Orbiter is actually a flight laboratory," said Ian Walters, a spacecraft engineer and Solar Orbiter (SO) project manager at Stevenage Airbus Defense and Space, where the probe was built. "In addition to the usual restrictions of packing everything into a 2m-long cube to fit inside the fairing of the Atlas V launch rocket, our biggest challenge was to design a heat shield to protect the instrument from the strong radiation near the sun. At its nearest 42 million kilometers, the sun-facing surface of the solar orbiter will be exposed to temperatures of 500-600°C: this is 13 times the thermal radiation experienced by earth-orbiting satellites, and the probe will face the sun as long as possible The entire journey of up to 10 years."
The vital 3.1mx 2.4m heat shield must protect six smartly designed instruments that look directly at the sun. It is made of multi-layered titanium foil embossed with tiny dimples. The tiny gaps between the layers help dissipate heat into the vacuum of the space and improve the insulation quality of the shield. A bare titanium surface can reach an unacceptable 700°C or higher, so the surface must be coated to withstand extreme ultraviolet and infrared radiation. The coating must also be thermally and optically stable under strong ultraviolet rays, and be able to conduct charged particles to avoid the accumulation of static charges that may interfere with precious instrument cargo. It must also not fall off particles that may contaminate important lenses and mirrors under the extreme vibration of lift-off.
Tests have shown that the white coating will darken over time, so the team chose Solar Black, which was developed by the Irish company Enbio specifically for Solar Orbiter. In a single process, the metal oxide layer is abraded from the titanium and immediately replaced by the calcium phosphate pigment, based on the charred bone, which chemically bonds with the metal surface and becomes part of it. The shielding design is very effective, and only 15 cm behind it, the temperature reaches a much milder 120°C. Solar Black also covers the high-gain antenna, which is attached to the rear of the probe 4.4m, and the other four instruments are always on during the mission to measure the solar wind when passing through the spacecraft.
Six remote sensing instruments need to look directly at the sun to obtain readings, but to minimize exposure, engineers cut small windows in the heat shield, which are opened at the best observation time. But the heat shield alone is not enough to drive the heat accumulation and protect the instrument. For example, SPICE's detector requires a temperature of -20°C to make accurate measurements. The 80 cm long instrument has a front temperature of 120°C (just behind the heat shield), and most of the light entering it is visible and infrared wavelengths, which have a heating effect.
The design team decided to use pyrolytic graphite hot strips, ranging from a thermal instrument to a set of 10 radiators placed on the outside of the spacecraft. These radiators are used as heat sinks at a distance of 70-80 cm. This material was invented by NASA in the 1960s, but it has never been used in space before. "Pyrolytic graphite is composed of a carbon layer, which is five times more conductive than copper, and is as flexible as paper. Because of the huge force transmitted during the launch, this flexibility is essential to protect the precision instruments to which they are directly connected. "Walters said.
To further optimize the heat, each of the six remote sensing instrument teams used sophisticated solutions with mirrors and filters to protect the cameras and lenses.
During a multi-year journey around the inner solar system at a speed of 90,000 miles per hour, the solar orbiter will be tracked and communicated with ESA’s Deep Space Tracking Network Estrak, which is a The global ground station network will transmit data to the ESA Mission Control Center in Darmstadt, Germany. But for a long time, the spacecraft could not communicate with the earth at all. For up to 72 days at a time, the solar orbiter may be behind the sun and invisible from the earth, so during these periods of time, the onboard computer and software must operate autonomously to maintain the correct orientation of the heat shield relative to the sun. Solar panels must also be reoriented according to the distance from the sun. When approaching, they must be almost at an angle to it, otherwise they will overheat due to the end of the mission. Glue will no longer function as glue, and solar cells may separate, bend or deform, resulting in power loss and possible burnout. "Engineering for the 20 or so extreme heat and cold cycles that a spacecraft must endure is much more difficult than designing for a more constant environment," Marirrodriga said in a classic understatement.
Missions such as the Solar Orbiter were planned years in advance, but its frequent close flights of Venus may enable some of its instruments to observe the planet’s outer clouds more closely. Astronomer Jane Greaves of Cardiff University documented the presence of phosphine gas in the cooler upper clouds above the surface of the hot planet in a report in September 2020. Potential sign. Mission scientists are now considering what contribution the solar orbiter can make to the exploration of Venus without affecting its original goal.
"The solar orbiter is to establish a connection between what happens on the sun and its performance in the solar wind and the space weather that ultimately affects our planet. This is just the beginning," Mueller said. "I believe that more exciting science will emerge."
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