Estonian satellite to enter lunar orbit by 2029

The ESTCube e-sail is able to utilize solar wind as a propulsion source.
The ESTCube e-sail is able to utilize solar wind as a propulsion source. Source: Mario F. Palos / ESTCube

The first Estonian student satellite, ESTCube-1, was launched ten years ago this month. While ESTCube-2 awaits its chance to enter low earth orbit, Tartu Observatory at the University of Tartu is already working on a third and fourth lunar satellites. The first of the satellites in development could reach lunar orbit by 2029.

"The primary objective of the next ESTCubes is to test the e-sail under real-world conditions," Agnes Rohtsalu, a first-year undergraduate student in physics, chemistry and materials science at the University of Tartu, said.

E-satellites, or electric solar sails, are a relatively new technology in which a satellite derives the power that it needs from solar wind, or charged particles from the Sun.

The ESTCube-2, which has yet to be launched into space, has been equipped with a sail, although it serves a slightly different purpose.

The second lunar satellite will be launched into low earth orbit (LEO), as the first one. "On the way back to Earth from low-earth orbit, we will use the e-boat module as a plasma regulator. However, for ESTCube-3 or -4, we want to use it as an e-sail in lunar orbit," Rohtsalu explained.

According to Tartu Observatory Associate Professor Andris Slavinskis, the next lunar satellite will have a secondary objective as well. "We are also trying to return from the Moon's orbit to the Earth's sphere of gravity; this is a good way to show how our propulsion system works," he explained.

Cube satellite becomes a shoe-box

Slavinskis said that when the first satellite was launched, the ESTCube team was already impatient to reach Moon orbit. "However, after finishing the ESTCube-1 mission, it became clear that we needed to redesign the system, so it was simplest to test the satellites in LEO," he said. As a result, the crew upgraded ESTCube-2 to incorporate more deep space technology.

Despite the fact that the launch of ESTCube-2 had been postponed until Christmas, Slavinskis began discussing the next satellite with students in November. "Since then, the students have been working on the engineering side alongside the lectures, and I've mainly been writing proposals to put the design side of the next mission together," he said.

Rohtsalu said that the upcoming third ESTCube will be larger than its predecessors. "While ESTCube-1 consisted of a single cube measuring 10 x 10 x 10 centimeters, the third satellite will have six cubes," she explained. In terms of dimensions, the next satellite will be a cube measuring 20 x 30 x 10 centimeters. "It'll be about the size of a shoe-box," Slavinskis said.

Furthermore, the new satellite will carry a wireless, practically undetectable bellows. "It is about as thick as embroidery thread. We will use it to exit lunar orbit," Rohtsalu said. 

Slavinskis said that the length of the wave is determined by the satellite. "We anticipate a surge of one to five kilometers for the new ESTCube." He said that the final length will be determined by production capacity and how tightly the beam can be packed.

A special issue of the journal Aerospace titled "Advances in CubeSat Sails and Tethers" is being prepared in the background, according to the co-professor. Most of our current research focuses on this particular topic. "It will be a significant advance in the middle of summer," Slavinskis said. Nonetheless, a substantial funding application is due by the end of the year.

The working group, according to Rohtsalu, is in the early phases of developing the next ESTCubes. "It is early to talk about launching," she said, "but perhaps we will launch the next satellite or the one after that in 2029."

New plasma break

As said before, the ultimate goal of the ESTCube team is to send its satellite beyond Earth orbit and into deep space, specifically into the stronger solar winds. The Moon's orbit, according to our estimates, would be the optimum location for this. We have almost direct access to the solar winds there, as Slavinskis pointed out. There, the crew will be able to conduct propulsion tests.

According to Rohtsalu, unlike ELO, the Moon has no magnetic field and no atmosphere. In other terms, the spacecraft is not protected from the effects of solar winds. "We can use this wind as a propulsion force," the student explained.

This propulsion technology is entirely new and has yet been tested. "New propulsion systems are not created every day," she said. "The ESTCube team's already routine work is something new for the global space community."

"Typically, space agencies are very conservative and prefer that technologies develop to a certain level." Only then will they be permitted to go on missions," she added. As a result, the agencies believe that the e-sail technology is not advanced enough.

As a result, according to the co-professor, funding is required for the technology's development. This can be accomplished with the In-Orbit Demonstration and Validation (IOD/IOV)program. "For example, it gives your lunar satellite a free place on a launch vehicle," Slavinskis explained. ESTCube-2 will be launched into orbit with the help of this initiative.

Dedicated support programs are also available from the European Research Council and the Innovation Council. The Finns recently secured a start-up grant for their company Aurora Propulsion Technologies. "They were given a fantastic opportunity to create a plasma brake [a highly compact propellantless solution]. They get a lot of funding from the EU as well as investors' support. "Hopefully, this project will convince the European Space Agency to take plasmas seriously," Slavinskis said.

Work on several fronts

The team is still facing a number of obstacles in developing the next satellite. "Every deep space mission faces the problem of how to communicate and navigate. It works differently than it does in LEO," the associate professor explained. The students have been working on these issues for six months and a basic design is beginning to emerge.

Creating a pair of bellows with twisted wire for safety is challenging as well. "Micrometeorites move through space," he explained, "They will otherwise break the wire quite quickly."

According to the co-professor, as the particle density varies between the ionosphere in LEO and the solar wind in lunar orbit, the new satellite will require a much higher voltage. "The transition from one to ten kilovolts requires additional funding for development," he said.

To that end, the group is working with the Technical University of Dresden in the field of electric propulsion. They are also working with high voltage and electron emissions. "We will use them for the e-sail," Slavinskis said. Similarly, the team works with the Finnish Meteorological Institute, the University of Pisa, Ventspils University of Applied Sciences and Tallinn University of Technology. "We are repurposing many of their technologies for our own needs," he said.

A parallel student endeavor has been launched in addition to the announcement of the third ESTCube initiative. Additionally, the University of Tartu is creating a satellite for evaluating strategic innovations in partnership with Tallinn University of Technology. The latter is expected to be Estonia's fastest satellite in LEO.

"Mihkel Pajusalu's comet interceptor optics system still has issues," Slavinskis said. "I am currently working on establishing new initiatives in Latvia and I am looking for partners." Agnes Rohtsalu plans to focus her studies on building the next ESTCube.


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Editor: Kristina Kersa

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