On 4 December, a PSLV-XL rocket will be launched from the Satish Dhawan Space Centre. It will launch Proba-3, one of the most interesting and technically challenging space missions of recent times, which has been developed by ESA for almost two decades. We are going to tell you about its design and main tasks.
How to see the solar corona
The glare of the Sun hides its corona. It is the outermost, most dilute and hottest layer of the stellar atmosphere, with a temperature of millions of degrees. It is the source of the solar wind and mass ejections. When the latter reach the Earth, they cause geomagnetic storms and auroras.
No wonder the corona is of great interest to scientists. We can see this key element of the solar system thanks to an amazing cosmic coincidence. The fact is that the Sun is not only 400 times bigger than the Moon, but is also located about 400 times further away from it, so their apparent sizes in the sky are almost identical. This allows the Moon to completely cover the solar disc during eclipses, making the corona visible.
The problem is that solar eclipses are rare and their full phase lasts only a few minutes, which is not enough time for full observations of the corona. Therefore, to create artificial eclipses, scientists usually install a shutter called a coronagraph in the telescope’s focus. It acts as an artificial moon, covering the solar disc.
But it’s not that simple. Light scattered on the elements of the optical system creates a bright halo around the coronagraph disc. This creates a blind spot that makes it impossible to see the inside of the corona. Launching the instrument with the coronagraph into space improves the quality of the images, but still, the inside of the corona remains invisible to astronomers.
The best way to overcome diffraction is to move the covering disc as far away from the telescope as possible. However, due to the size limitations of space telescopes, scientists have limited options. For example, the distance between the disc and the telescope inside the SOHO spacecraft is only 70 cm.
The solution to the problem is to use the spacecraft itself as a ‘flapper’. A similar experiment was conducted during the Apollo–Soyuz flight in 1975. Back then, Apollo, which had a cylindrical shape, played the role of a coronagraph. However, due to the bright glare caused by the fuel vapours from the ship’s running engines, it was not the most successful.
But since then, technology has made significant strides forward, allowing us to return to the idea of creating an artificial eclipse using a spacecraft. This is exactly what Proba-3 will do.
The technical device of Proba-3
The research underpinning the Proba-3 project began in 2005. In 2014, the ESA approved the preliminary design of the project. The final green light was given in 2018. The project was carried out by a consortium of 14 states (in addition to ESA member states, it also includes Canada) and more than 40 companies. In total, the design and construction of Proba-3 cost the ESA €200 million.
The design of the project is a compromise between what modern technology can theoretically provide and what can be practically achieved in space. Proba-3 consists of two spacecraft: Coronagraph and Occulter. The former will observe the solar corona, while the latter will act as an artificial moon and cover the Sun.
The Coronagraph weighs 314 kg. In addition to the ASPIICS coronagraph, it is also equipped with a radiometer to measure the total solar flux and an instrument to study electrons from the Earth’s radiation belts.
Occulter weighs 231 kg. It is a cube with a 1.4-metre outer disc that will cover the Sun. It is made of a specially heat-resistant polymer, which should minimise changes in its shape due to temperature fluctuations.
After entering orbit, the vehicles will be deployed in a formation with a distance of 150 metres between them. This distance was considered to be optimal, as it allows for a balance of optical performance, tracking efficiency, and minimises fuel usage.
The key to success is accuracy. In order to successfully eclipse the Sun and see the corona, the maximum error in the distance between the Coronagraph and Occulter must not exceed a few millimetres. This task is assigned to the laser system. It will continuously measure the distance between the spacecraft, which will allow them to maintain formation. To change its position, Occulter is equipped with a set of high-precision engines capable of micro-correcting its course with a thrust comparable to the force of a falling feather.
Date of launch of Proba-3
Proba-3 is scheduled to be launched on 4 December at 12:38 Kyiv time. The Indian PSLV-XL rocket will be used for the launch. It will launch a pair of satellites into a high elliptical orbit around the Earth. At the perigee, they will approach the Earth by 600 km, and at the apogee, they will move away by 60,000 km.
Observations will be made when the satellites are at their apogee. According to experts, the duration of the artificial eclipse during one round will be up to 6 hours. If everything goes as planned, Coronagraph will be able to observe the solar corona at a distance of up to 1.1 solar discs. For comparison, a standard coronagraph has a range of 2.5 solar discs.
Proba-3 has the status of a demonstrator mission. Its success will pave the way for new, more complex and ambitious projects that will allow astronomers to learn even more about the Sun, the processes that take place in its atmosphere, and how they affect the Earth.
