Tiding up the cosmos

It is only recently that the World come to realize just how dangerous space junk can be. Unfortunately, we can’t do anything about the vast majority of these objects. Though after many years the vestigial atmosphere, solar radiation and gravitational force may gradually slow them down, until then they will be a threat to new satellites. We should take care of the future generations and introduce deorbitation systems in every object operating in Earth orbit. PW-Sat2 is going to demonstrate one of the deorbiting techniques – lowering the orbit by using air resistance present at the lower Earth orbits.

The project of Polish students.

“Our second satellite (PW-Sat2), like our previous project (PW-Sat), will be a showcase of a deorbiting technique based on a deorbiting sail. A team comprising students from various faculties of the Warsaw University of Technology began work on the new satellite in 2013. PW-Sat2 will be twice the size of PW-Sat and much more complex. In the PW-Sat2 project education is the first priority. Hands-on experience in a real space project is not the only element. Many diploma theses and semester projects are based on the design, construction and tests of the satellite and payload. In our new project we would like to develop a deorbiting sail structure based on flat springs attached to a cylindrical pin. The satellite will be built according to CubeSat standard as 2 unit CubeSat (20x10x10 cm). The space debris problem is attracting attention all across the world, so in our opinion it is a good time for such an experiment. We hope that PW-Sat2 will be launched into orbit in 2016”, says Dominik Roszkowski, member of Students' Space Association (SKA).

Deorbiting Sail.

The deorbiting structure onboard PW-Sat2 will take the form of a square sail that will open behind the satellite. It will make use of the rarefied atmosphere by greatly increasing the satellite’s drag area and accelerating the orbit degradation. After few weeks in orbit (and plenty of tests), a command will be received (either from Earth or from the on-board computer) and the sail will be deployed. Within a few months the satellite’s velocity and orbit height will decrease significantly. The sail design underwent intensive development in 2014. Several different methods of sail deployment were tested, including use of materials with shape memory (nickel-titanium). Finally a new solution was chosen. A sail made of thin foil will be deployed on four flat springs (similar to those in measuring tapes) wrapped around the central core of the system. The core will be held on an additional spring that will allow the sail to be deployed within a safe distance from the satellite (about 20 cm).

The deorbiting sail is a very competetive system designed to get rid of unnecessary objects from orbit. Our students want to take part in the development of technology which might soon be compulsory for devices operating at the low Earth orbits.

Solar sensor

One of the additional payload experiments is the solar sensor. It will provide orientation and position data which will be compared with commercial ADCS readings. Most satellites have to be pointed in a specific direction in order to work properly, for instance telescopes towards the stars or weather satellites towards Earth. In order to be able to orient the satellite into these desired directions, some manner of engine is needed which will enable such movement, as well as a kind of compass which will determine the direction we need to point our apparatus. The solar sensor device is an example of such a cosmic compass that will provide the information needed for correct positioning of the satellite’s solar panels and cameras. “Our goal is to create a solar sensor, whose size, weight and accuracy will be comparable to commercial solutions. However, the nature of its basic design will make it simpler and thus cheaper to produce. This feature makes it viable to be used on board other CubeSat missions all over the world if proven successful”, says Inna Uwarowa, the coordinator of PW-Sat2 programme.

Cameras

Students decided to put a camera onboard the PW-Sat2 satellite which will allow us to observe some part of the deorbitation sail during its opening. “We wish to verify not only the opening mechanism, but also the effectiveness of the sail drag”, says Mateusz Sobiecki.

“One of the challenges that we will have to face is a transmission of quite a large amount of data (in our case photographs). The satellite’s position and orientation may be a problem after the sail opening, but we will be prepared for such inconveniences”, reckons Kamil Sażyński, the leader of communication department at SKA.  Moreover, even in case of communication failure, the PW-Sat2 will accomplish its mission by itself.

Deployable solar panels

PW-Sat2 will also have two deployable solar arrays in order to increase the energy-gathering area and, thus, the reliability of the satellite. Both panels span an area of approximately 20 cm and are arranged symmetrically. During the launch the arrays will remain closed and will be held in place with a strong Dyneema line. After the detumbling (stabilization) phase, the PW-Sat2 will point at the Sun and the panels will be open. Based on the ADCS and Sun Sensor readings we will try to point the top side of the satellite towards the sun. In order to maintain orientation, the PW-Sat2 will be equipped with a set of magnetotorques. Even in the case of failure or blockage, the panels will deliver enough energy to cover the basic demands of the satellite, such as sail deployment.

From theory to practice

Most elements of the satellite are made by the students themselves. The prototypes undergo many advanced tests. Later on, the systems will be integrated and tested together according to the outlines of the European Space Agency (ESA). All the work on the model which will go into outer space has to be done in a so-called 'clean room' – a place with the highest standards of cleanliness. Then they have to be documented and checked many times by independent specialists. The risk of losing a satellite because of some failure has to be minimlized at each and every step of the production process, because there might not be a chance to launch another. It is a big challenge for our students but they believe they will succeed with help of the experienced scientists and engineers who have been giving lots of support to members of WUT’s Students' Space Association (SKA) since the beginning of this project.

Building a satellite is not an easy task in Poland, and yet it is possible. At the moment, the biggest challenges for our students are, primarily, finding an operator who would launch the satellite, secondly, students need funds for this enterprise. In order to estimate its cost, they have made contacts with space centres all over the world – in China, Japan, USA, Russia and Holland and  found out that they need to get almost 1 million zł.( €250,000). The coming months will be time of testing, purchasing elements and integration of the satellite. It is a big challenge for such a young team, but its members are not giving up and will do anything to bring the mission to a close- in other words, to witness the deorbitation of the PW-Sat2.