ESTCube-2 mission for deep space | Space Travel Blog (original) (raw)

Welcome to the second article in the series about the ESTCube-2 mission. Our previous article presented the motivation behind developing and testing the plasma brake (novel deorbiting technology, which can be used for active and proactive debris removal), introduced the project starting from the early days and gave an overview of the satellite’s launch schedule and the team. In this article, we focus on the satellite platform and our ambition to demonstrate technologies required for nanosatellites, which weigh less than 10 kg, to enter the deep-space environment and, in doing so, become nanospacecraft.

Low Earth orbit vs deep space: What’s the difference?

In order to understand the difference between a Low Earth Orbit (LEO) satellite and a deep-space craft, let’s look at some typical technologies that often define an LEO mission. Thanks to the Earth’s magnetic field, angular velocity and Sun direction measurements in combination with orbital models, we can determine the satellite’s attitude (orientation) and orbit. By equipping the satellite with electromagnetic actuators, such as coils and rods, its attitude can be controlled by interacting the actuators with the geomagnetic field. Therefore, in LEO, a satellite can have ≈1° attitude control capability with a set of miniature sensors measuring the magnetic field, Sun’s direction and angular rate and working together with three coils, such as ESTCube-1 had.

In order to perform attitude determination and control in deep space (e.g. outside the Earth’s magnetic field), a star tracker, reaction wheels and a propulsion system are required.

An additional benefit of these technologies is that they provide higher accuracy in attitude knowledge and control. A star tracker camera can be used to track planets and asteroids and estimate the spacecraft’s position. A conventional deep-space mission uses so-called deep-space networks to communicate with and determine the position of a spacecraft. Such communication networks are expensive and, typically, are operated by major space agencies (e.g. NASA and ESA) for their own science missions. So, small and independent interplanetary missions have to develop and implement new communication and navigation solutions. To maximise the utility of small spacecraft while keeping the cost low, advancements in autonomous on-board operations are needed.

Nanosatellites in LEO seldom require propulsive manoeuvers; however, most deep-space missions require for the spacecraft to reach its target. Gas- or water-based propulsion systems used for attitude manoeuvers are not sufficient for ∆v-expensive transfers to interplanetary objects. As explained in our previous articles, the Coulomb drag propulsion concept has two applications – deorbiting with the plasma brake in LEO and propulsion with the electric solar wind sail (E-sail) in deep space. While ESTCube-2 will operate in LEO, its platform includes a star tracker, reaction wheels, a cold-gas propulsion system and an E-sail experiment, all of which will help to design future deep-space missions.

The platform

With the lessons learnt from the ESTCube-1 mission and CubeSats becoming increasingly advanced, the team decided to challenge itself in several ways: first, to develop and host a plasma brake experiment to test the plasma brake’s suitability as effective deorbiting technology: reel out up to 300 metres of a tether (wire), which would require a propulsion system to provide the needed spin for centrifugal tether deployment (see our previous article about the ESTCube-2 plasma brake experiment); second, to consider ESTCube-2 as a precursor of future interplanetary nanospacecraft, which, in addition to the propulsion system, requires the testing of a star tracker and reaction wheels for attitude and orbit control; third, to integrate the satellite platform as tightly as possible, leaving extra room for additional payloads.

A conventional satellite is usually developed for a space agency, weighs hundreds or even thousands of kilograms and costs tens or even hundreds of millions of euros.

However, at the turn of the millennium, CubeSats and private space flight, a.k.a. NewSpace, emerged and disrupted these old ways.

By the middle of the 2010s, more than a hundred CubeSats per year were being launched, Planet Labs were demonstrating 3–5 metre ground resolution images taken by three-unit (about 10×10×30-cm) CubeSats, and SpaceX was launching satellites for commercial customers. In such a world, the ESTCube‑2 team wanted to do more with less – or in other words, increase the productivity of the satellite.

In a typical CubeSat, such as ESTCube-1, the satellite’s systems are stacked on top of each other and connected via a bus connector. Bus, in this case, means the platform – systems performing the satellite’s nominal functions of communications, power production and distribution, computing, attitude control, etc. While the CubeSat stack and the bus connector provide a wonderful way to standardise subsystems and allow for reusability, they take extra space and make less sense when a single team is developing the platform.

The ESTCube-1 design is a typical example of the CubeSat stack. Illustrated by Rute Marta Jansone.

The ESTCube-2 engineering team opted for a custom layout that helps to integrate the bulky components, such as reaction wheels and the star tracker, among the circuit boards. The layout resembles a typical CubeSat stack, but the boards are closer to each other and they include cutouts for the bulky components. Extra space is gained by replacing the standard bus connector with compact connectors, which are positioned at different sides of the circuit board and the functional blocks are placed near them. On a standard CubeSat, side panels cover the satellite, provide mechanical support and radiation shielding, and host solar panels.

On ESTCube-2, the side panels include additional circuit boards that are responsible for interfacing Sun sensors and solar panels and distributing functions of the satellite’s bus. Such ‘smart’ side panels are also used to host a corrosion testing experiment (more details below). In this custom configuration, the ESTCube-2 bus includes all principal components of a deep-space platform, like the GOMSpace NanoProp cold-gas propulsion system, a custom-built star tracker and Hyperion Technologies reaction wheels, into a volume of one CubeSat unit.

The ESTCube-2 bus. Photo by Laila Kaasik.

Secondary payloads

ESTCube-2 features two secondary payloads. The Earth-Observation Payload (EOP) consists of two cameras imaging the planet in two spectral bands at a ground resolution of 20–30 metres, depending on the orbital altitude. The imagers use the ZEISS Sonnar 1.5/50 ZM lens. The spectral bands were chosen to be similar to Sentinel-2’s MultiSpectral Instrument’s bands 4 and 8a; this way, the images captured by ESTCube-2 can be compared to these. The filters are COTS and the central wavelengths of the bands are 660 nm and 857 nm; both of the bands are 30 nm wide (full width at half maximum). The selected bands will be used to estimate the normalised difference vegetation index – a measure that quantifies the amount of chlorophyll in the image frame by comparing the signal difference between the near-infrared band and the red spectral band. This can be calculated because chlorophyll absorbs most of the incident light at 660 nm. However, due to the structure of leaves, vegetation reflects a lot of incident light at near-infrared wavelengths. So, by comparing these two signals we get information on how green the vegetation is that the cameras are imaging.

A render of the ESTCube-2 Earth-observation payload. 3D graphics by Silvar Muru.

The Corrosion Testing in Space (CTS) experiment will study the corrosive behaviour of materials in LEO, where they are exposed to atomic oxygen. For this purpose, a compact satellite module has been developed according to patented corrosion testing systems technology, which allows the testing of up to 15 materials at the same time. It is expected that exposure to atomic oxygen deteriorates these materials in space and triggers a detectable signal once the materials have suffered severe damage. Knowing the thickness of the materials and exposure time in space, it will be possible to calculate the rate of corrosion for the tested materials. Among other materials, the module will also be used to test the performance of a novel nanostructured coating that has been developed at the University of Tartu.

The compact 65 × 41 mm CTS module used for testing up to 15 materials in space. It consists of a circuit board with sensors, on the left, and a cover plate with holes, on the right, to expose tested materials to atomic oxygen in LEO. Photo by Maido Merisalu.

Next steps

The ESTCube-2 satellite’s main mission is to tackle the space debris problem by demonstrating deorbiting with a plasma brake in LEO, as well as to prepare for upcoming deep-space missions. As the satellite is currently in the final design stage and will soon enter the flight-model assembly stage, we have plenty of work ahead and lessons to learn. As tiny as the satellite might be, we have packed it with various experiments – the star tracker and the attitude control system are designed to perform tasks necessary for future interplanetary missions, in addition to the plasma brake, EOP and CTS experiments. It is a great load for a nanosatellite to carry. And that is not all. As the team is constantly expanding the functionality of ESTCube-2 by writing new software, we hope to demonstrate new navigation and control solutions as well. Fingers crossed for our 2023 launch!

References and further reading

Slavinskis et al. ESTCube-2: Launch updates and flight model assembly, integration and test, Space Travel Blog (2022 + on-going updates).

Slavinskis et al. ESTCube attitude. Space Travel Blog (2023).

Slavinskis. The E-sail tale: a historical view and future perspective, Space Travel Blog (2022).

Iakubivskyi et al. Coulomb drag propulsion experiments of ESTCube-2 and FORESAIL-1, Acta Astronautica, 177 (2020) 771–783.

Ofodile et al. ESTCube-2 Attitude Determination and Control: Step towards Interplanetary CubeSats, IEEE Aerospace Conference (2019).

Dalbins et al. ESTCube-2: The Experience of Developing a Highly Integrated CubeSat Platform, IEEE Aerospace Conference (2022).

Ofodile et al. Integrated Anti-Windup Fault-Tolerant Control Architecture for Optimized Satellite Attitude Stabilization, IEEE Journal on Miniaturization for Air and Space Systems, 2(4) (2021) 189–198.

Slavinskis. Deep-space nanospacecraft: the challenges of bringing cubesats to interplanetary space, Space Travel Blog (2022).

Author: Andris Slavinskis
Co-authors: Hans Teras, Janis Dalbins, Kristo Allaje, Silvia Kristiin Kask, Kadri Bussov, Erik Ilbis, Hendrik Ehrpais, Pekka Janhunen, Petri Toivanen, Joosep Kivastik, Maido Merisalu, Mihkel Pajusalu and Antti Tamm
Design: Anna Maskava, Rute Marta Jansone and Guillaume Le Bonhomme
Proofreading: Robert B. Davis

Attribution (text): Space Travel Blog / ESTCube (Slavinskis et al.)
Attribution (images): see captions