The first such mission. “An extremely difficult task”

The European Space Agency's Proba-3 mission is scheduled to launch on December 4. As assured by Dr. Tomasz Barciński, head of the Satellite Mechatronics and Robotics Laboratory at the Space Research Center of the Polish Academy of Sciences, a truly pioneering system will be launched into space.

– This is the first such precise configuration. It will consist of two satellites. One of them will be used by a coronagraph – a telescope designed to observe the Sun's corona. The second one will act as a so-called oculter – an element covering the solar disk, producing an eclipse just like the Moon. Anyone who has taken photos will notice how difficult it is to photograph a scene with very bright and very dark objects next to each other – for example, taking a selfie with the sun behind you. A disk that is millions of times brighter than the corona would blind the coronagraph detector, so we have to cover it with an occultator – explained the expert.

Coronagraphs have already been sent into space, for example in the SOHO mission. However, previously the oculter was located a few meters away from the telescope mounted on the same satellite. In the case of Proba-3, approximately ten times the distance allows for much greater accuracy in the image of the corona.

– We will be able to descend into very deep areas of the corona, where matter ejected from the Sun into space is “catapulted.” This area was previously only accessible during rare and almost instantaneous total eclipses of the Sun by the Moon. We will observe space almost touching the surface of the Sun – starting at eight percent of its radius – said Dr. Barciński.

The large distance of the occulter from the coronagraph is intended to reduce the problem of diffraction (wave bending) that spoils the image. – The wave nature of light causes photons from the solar disk to “overflow” beyond the edge of the occulter disk and fall into the telescope. By moving the shield away, we reduce the intensity of this phenomenon – described the expert.

As Dr. Barciński pointed out, the main goal of the mission is not to study the solar disk.

– Probe-3 is described as a “technology mission.” In fact, the use of a coronagraph and oculter is an excuse to test the technology of extremely precise keeping two satellites in a constant position. Both elements will be 150 meters apart, with an accuracy of a few millimeters. Similarly, the angular orientation of the satellites will be maintained precisely – deviations will not exceed a few arc seconds (1 arc second is 1/3600 of a degree). It should be remembered that the tandem will be in motion all the time, traveling in an elliptical orbit. In fact, due to the distance between them, both elements will move in two different, although similar orbits. This means that they will tend to move away from each other, so their position and orientation will have to be constantly adjusted. The control systems will have an extremely difficult task – explained the specialist. Scientists and engineers hope that the technology tested in Proba-3 will be able to be used in future space missions – in any mission requiring extremely precise mutual alignment of two satellites. – These will be, for example, programs with the so-called space garbage trucks, in which a robotic manipulator will grab an unusable satellite to remove it from orbit. The captured satellite will probably rotate almost chaotically around its axis. The robot will therefore have to not only approach with great precision, but also start circling the satellite in order to catch it, said Dr. Barciński.

Space tankers will work similarly. – Refueling in orbit will have similar requirements. The fuel tanker will have to connect an appropriate cable to the satellite or other vehicle. Similar precision will be needed here, the expert added. Groundbreaking research will also benefit from this type of technology space conducted with the help of gravitational waves. – In the planned European LISA (Laser Interferometer Space Antenna) mission, three satellites will be placed in space at a distance of as much as 2.5 million kilometers from each other, forming an equilateral triangle. They will send laser pulses between each other, telling the satellites about their relative positions. Gravitational waves will slightly disturb this position. To detect these changes, satellites will have to maintain their position extremely precisely. To sum up, there are many potential applications of the approach tested in the Proba-3 mission – emphasized the CBK PAN expert.

The Center's team was primarily responsible for two parts of the system. – One of the elements we prepared was the computer controlling the coronagraph, the heart of the main instrument of the mission. Space computers are specific – they must meet specific requirements, but they are similar in many aspects. So we used technologies that we had already developed and adapted them to this specific mission – described Dr. Barciński.

The second element was part of the telescope itself operating on the satellite. – We also made a device called a filter change wheel. As its name suggests, it is a wheel with optical filters placed in front of a light-sensitive detector. By turning this wheel, these filters can be selected, depending on the radiation band that will be examined, the expert explained.

– The hardest part was placing the filters in the wheel. It may seem like a simple matter, but it's not. The filters are made of glass, which we had to attach to metal. Vibrations during launch, space vacuum, temperature fluctuations, the need to maintain extreme cleanliness of the optical instrument – these challenges prompted us to develop a special assembly technology. We installed the glass filters in a flexible system so that the force used to press the glass is strictly controlled. Preventing the filter from chipping or even breaking. Another difficulty was that it was a moving mechanism, and the organizers of space missions are always afraid of such elements. If something moves, you need a drive, a control system, a power supply system, a failure detection system and so on – explained the specialist.

The developed technology can probably also be used in other programs. – This mission allowed us to deepen the skills needed to work on European Space Agency projects. Thanks to our participation in Proba-3, we have become an even more recognized group in Europe – as the one that managed to cope with this difficult cosmic mechanism. Our team, including the group of mechanics for which I am responsible, has proven to be very good – he pointed out.

Polish companies also cooperated with CBK PAN, which managed the Polish part of the project – Astri Polska, N7 and Creotech Instruments.

Tomasz Barciński specializes in control theory and mechatronics. He leads teams working on the most important space missions involving Poland, including the Proba-3 mission, the construction of the European ATHENA X-ray space telescope and the Comet Interceptor probe intended to intercept a comet from outside the Solar System. He also worked on the space garbage truck project in the ESA ClearSpace program and the Polish EagleEye observation satellite.