Last year, the European Space Agency (ESA) launched the Proba-3 mission to space. Comprised of two spacecraft, the Coronograph and the Occulter, it is a technology demonstrator designed to test techniques for highly-precise satellite formation flying that will pave the way for future multi-satellite missions. This past March, the two spacecraft performed a first-ever feat by flying in tandem for hours without help from mission controllers. They created an artificial solar eclipse in orbit and obtained their first images of the Sun's outer atmosphere (aka corona) in the process.
The two spacecraft accomplish this by flying precisely 150 meters (492 feet) apart, thanks to an innovative suite of navigation and positioning technologies. Once they align with the Sun, the 1.4 m (4.6 ft) diameter disc carried by the Occulter spacecraft covers the Sun's disk so the Coronograph spacecraft can take images using its Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun (ASPIICS) instrument. These observations are crucial to understanding solar wind and Coronal Mass Ejections (CMEs), massive bursts of charged particles that can pose a serious threat to communications, navigation systems, energy grids, and missions in orbit and beyond.
The first rounds of ASPIICS's observations offer a glimpse of the valuable data this mission will provide. "Many of the technologies which allowed Proba-3 to perform precise formation flying have been developed through ESA's General Support Technology Programme, as has the mission itself," said Dietmar Pilz, ESA Director of Technology, Engineering and Quality, in an ESA press release. "It is exciting to see these stunning images validate our technologies in what is now the world's first precision formation flying mission."
Among the many mysteries the mission will investigate is how the Sun's corona reaches temperatures above a million degrees Celsius, far hotter than the surface — something that has long been an open question to the scientific community. Proba-3 will investigate this by studying the corona very close to the Sun's surface and in greater detail than previous missions. Its sophisticated coronagraph lets it detect fainter features by drastically reducing how much 'stray' light reaches the detector.
Meanwhile, its Digital Absolute Radiometer (DARA) will measure exactly how much energy the Sun is emitting (solar irradiance), while its 3D Energetic Electron Spectrometer (3DEES) will detect electrons in Earth's radiation belts, measuring their direction of origin and energy levels. The ASPIICS instrument was developed by an industrial consortium led by Centre Spatial de Liège. The images it takes are processed by the ASPIICS Science Operations Centre (SOC), which was developed by the Solar Influences Data Analysis Center (SIDC) at the Royal Observatory of Belgium (ROB). Said Andrei Zhukov, Principal Investigator for ASPIICS at the ROB:
I was absolutely thrilled to see the images, especially since we got them on the first try. Now we are working on extending the observation time to six hours in every orbit. Each full image – covering the area from the occulted Sun all the way to the edge of the field of view – is actually constructed from three images. The difference between those is only the exposure time, which determines how long the coronagraph's aperture is exposed to light.
Combining the three images gives us the full view of the corona. Our 'artificial eclipse' images are comparable with those taken during a natural eclipse. The difference is that we can create our eclipse once every 19.6-hour orbit, while total solar eclipses only occur naturally around once, very rarely twice a year. On top of that, natural total eclipses only last a few minutes, while Proba-3 can hold its artificial eclipse for up to 6 hours.
In recent years, several European institutes have developed models to simulate observations of the Sun's atmosphere and create "digital eclipses," offering a preview of what scientists would see when examining the corona. This includes the COolfluid COroNal UnsTructured (COCONUT) software developed by researchers at KU Leuven, one of several solar coronal models integrated with ESA's Virtual Space Weather Modelling Centre (VSWMC). These models have helped scientists to research solar phenomena and allowed people, organizations, and industry to prepare for flares. Proba-3's images are already revolutionizing these models by providing real-life examples to compare them to. As Proba-3's mission manager Damien Galano noted:
Having two spacecraft form one giant coronagraph in space allowed us to capture the inner corona with very low levels of stray light in our observations, exactly as we expected. Although we are still in the commissioning phase, we have already achieved precise formation flying with unprecedented accuracy. This is what allowed us to capture the mission's first images, which will no doubt be of high value to the scientific community.
The formation flying we have achieved so far was performed autonomously, but under supervision of the ground control team, who were ready to intervene to correct any potential deviations. Our one remaining task is to achieve full autonomy, when our confidence in the system will be such that we will not even routinely monitor from the ground."
Further Reading: ESA