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ESTCube-1 ceased working after 2 years in orbit

Estonian first satelliteESTCube-1 ceased to respond on 19-th of May 2015 after two years after launch. The reason was degradation of the photovoltaic elements and consecutive lack of energy for subsystems.

ESTCube-1 was a student project of Tartu University that had been realized under the supervision of the Tartu University and the Tartu Observatory staff in collaboration with the Finnish Meteorological Institute, ESA and other organizations. The satellite was built by following CubeSat standards. It had a shape of 10x10x11.35 cm parallelepiped and a launch mass of 1.048 kg.

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ESTCube-1 on orbit, artistic illustration. Image: Tavi Torim, 2013, by http://www.estcube.eu.

The project was started in summer 2008. Its main goals were obtaining aerospace engineering knowledge by students and promotion of space industry in Estonia. The satellite was aimed to complete a scientific goal as well: it carried an electric sail onboard and should have become first in the world to test this innovative propulsion method.

Electric sail (or e-sail) is an invention of Pekka Janhunen from the Finnish Meteorological Institute. It is a thin positively charged tether which uses the repulsive force of the solar wind protons to propel the spacecraft - so it differs from the conventional solar sail that uses the pressure of solar light photons. The onboard electron gun charges the sail emitting electrons into space and thus creating an excess positive charge. The sail does not require propellant to generate thrust, so lighter and cheaper spacecraft may be created with the use of this propulsion method.

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ESTCube-1 after assembling. Image: Riina Varol, 2013, by http://www.estcube.eu.

ESTCube-1 was provided with solar cells placed on its sides, with a power of up to 3.6 W in the beginning of the mission. Charged particles and micrometeorites hitting the cells provoked their degradation and efficiency dropped over time. Sun sensors, magnetometers and gyros were used for attitude determination and magnetic torquers were applied for attitude control. Communications were performed on amateur radio frequencies. The solar sail experiment hardware consisted of a motor-driven reel with the tether. To aid unreeling, centrifugal force was applied. The body of the satellite was spun up, and a small ball at the end of the tether should have kept it strained. An onboard camera with a 640x480 pix resolution tracked the process, it was also used to photograph the Earth. The onboard computer handled the work of the subsystems. The mission control center was situated in Tartu. ESTCube-1 was controlled using a web-based app elaborated by students.

ESTCube-1 was launched, together with two other satellites, from Guiana Space Center on May 7, 2013 at 5:06 EEST onboard ESA Vega launch vehicle. It was placed to a Sun-synchronous orbit with an altitude of 665 km and the inclination of 980. Upon separation of ESTCube-1 from the launch vehicle, two antennas were deployed, the systems switched on and the satellite started emitting the beacon signal. Soon two-way communication was established, the satellite started executing instructions and making photographs.


The first image of the Earth sent by ESTCube-1. Image: ESTCube, 2013, by https://commons.wikimedia.org.

Soon it was discovered that some construction elements had been magnetized and attitude control was hampered. It took several months to elaborate new control algorithms. By August 2014 the satellite was successfully spun up to 841 deg/sec, in September the reel was unlocked and the unreeling motor was engaged. Unfortunately the motor stalled, most probably due to some mechanical malfunction, all attempts to unreel the tether and to perform the electric sail experiment failed. At the same time all other systems were successfully tested, so the ESTCube-1 mission should be regarded as a big success. The satellite was working in space for more than 2 years, it orbited the globe more than 10,000 times and sent back more than 280 images. It was possible to determine the attitude of the satellite with accuracy up to 20 and to held two-way communications. An important experience of international and national collaboration has been obtained by project participants.

ESTCube-1 will remain in space for about 20 years until it reenters as a result of orbital degradation due to atmospheric drag. More CubeSat projects are planned in Estonia for the intermediate future.

Autor: Vladislav-Veniamin Pustõnski / Green Vironia OÜ