A fall of CubeSats

“The ESA in-orbit demonstration phases of these missions had already been completed,” explains Roger Walker, heading ESA’s Technology CubeSats. “The companies overseeing them continued to operate them after the conclusion of our involvement however, gathering additional flight experience to the end, right up until their reentry.”  

CubeSats are small low-cost satellites built up from standardised 10 cm boxes. Originally developed for educational uses these highly capable platforms are increasingly finding operational uses. ESA’s Directorate of Technology, Engineering and Quality employs them to provide the European space sector with early flight demonstrations of promising innovations, typically targeting scientific or Earth-observing goals at the same time. 

The CubeSats had been intentionally deployed in low enough altitudes for their orbits to naturally decay over time, fulfilling ESA space debris regulations. 

SIMBA tracked the Sun 

Developed for ESA by a Belgian-led consortium, the SIMBA (Sun-earth IMBAlance) CubeSat, which was launched in September 2020 and reentered on 14 August, harnessed a highly accurate Attitude Determination and Control System based on an experimental star tracker camera and reaction wheels.  

This enabled SIMBA’s main radiometer instrument to perform measurements of total solar irradiance – how much solar energy Earth is receiving from the Sun – and how much is being radiated back, to determine Earth’s radiation budget – a vital variable for climate studies. 

"Fitting a radiometer aboard such a tiny satellite proved quite challenging," recalls Stijn Nevens of the Royal Meteorological Institute of Belgium, Principal Investigator for the mission. "We had to omit some of the features of conventional radiometers, requiring extra work at the data processing stage, but we learned a lot. For me this has been a long and winding road – it feels weird not having SIMBA in the sky anymore!" 

Tjorven Delabie of the Belgium-based ArcSec Space start-up, spun out of KU Leuven, adds: “Our spin-off company most probably wouldn’t be here if not for the SIMBA mission. It took a startracker algorithm that was previously very much an academic endeavour and demonstrated our approach worked for real in space, giving a lot of traction in the market.  

"So far we’ve sold more than a hundred startrackers around the world, and we’re following up with reaction wheels and ultimately a full ADCS for CubeSats. With SIMBA such an important part of our history, getting the email that it had reenteed was a little bit of a sad moment, almost like someone had died, but we are involved in ESA’s follow-on CubeSpec mission.”  

RadCube monitored space weather impacts to Earth's magnetosphere 

Six days later SIMBA was followed into atmospheric reentry by the RadCube mission from Hungary, Poland and the UK, tasked with surveying space weather.  

Equipped with miniaturised magnetometers to chart disturbances to Earth’s magnetic field as well as a space radiation ‘telescope’ detector, the August 2021-launched CubeSat was in the right place at the right time as the Sun entered its 11-year peak of activity, giving rise to sensing numerous solar storms and active terrestrial auroras. 

And RadCube’s MAGIC magnetometers, based on over the counter ‘magnetoresistive’ sensors typically used in computer hard drives or smartphones, will be reflown on a follow-up ESA CubeSat called HENON, which will venture beyond Earth into deep space in order  to survey space weather. The same MAGIC sensors are also destined to form part of a ‘space weather station’ for the lunar Gateway in orbit around the Moon, the European Radiation Sensors Array. 

“RadCube has been a tremendously valuable opportunity, allowing us to demonstrate the capabilities of MAGIC,” explains Jonathan Eastwood of Imperial College London.  

“To start with we successfully validated the instrument performance and demonstrated that these sensors, an order of magnitude smaller and more power efficient than those used for typical science missions, could still meet the requirements needed for space weather monitoring of magnetic field disturbances in low-Earth orbit.  

"The icing on the cake was RadCube operating during the intense geomagnetic storm of May 2024, the strongest for 20 years. This shows that instruments based on MAGIC’s technology can contribute to ESA’s space safety monitoring goals. We are really excited to now be contributing further MAGIC instruments to ESA’s ERSA and HENON projects.” 

Sunstorm's X-ray analysis of erupting Sun 

On 5 September the reentry of Sunstorm took place, which was made by a Finnish-led consortium and flown on the same launcher as RadCube. It hosted a new type of solar X-ray spectrometer to characterise the strong X-ray pulses that accompany coronal mass ejections – massive eruptions of many millions of tons of material from the Sun’s surface, giving rise in turn to solar storms with the potential to impact satellites in space as well as terrestrial power and communications infrastructure. 

With its basic technology proven in space, a full-size operational version of Sunstorm’s XFM-CS (X-ray Flux Monitor for CubeSats) instrument is now due to be flown on the US NOAA Space Weather Next L1 satellite in 2029. 

Formation flying GOMX-4B traded crypto  

The most recent Technology CubeSat reentry took place on 9 October, marking the end of ESA’s GOMX-4B CubeSat developed by GOMSpace in Denmark. Flown jointly with the Danish Ministry of Defence’s GOMX-4A in February 2018, the pair demonstrated formation flying based on cold gas thrusters and data sharing via inter-satellite links. GomX-4B also hosted the HyperScout hyperspectral imager for Earth observation, provided by cosine Remote Sensing in the Netherlands. 

"The GomX-4B mission played a pivotal role in demonstrating the capabilities of the cosine HyperScout hypersectral imager, showcasing its potential for the first time," comments cosine Managing Director Marco Esposito. 

Follow-up versions of HyperScout have gone on to be flown on ESA’s Earth-observing Federated Satellite Systems (FSSCat) mission and the Hera asteroid mission, while Hera’s Juventas CubeSat has incorporated an evolution of GomX-4B’s cold gas thrusters from GOMSpace Sweden.  

In the extended mission phase GOMX-4B was also able to make several initially unplanned demonstrations by reconfiguring its software and radios on orbit. This included the first demonstration of 5G Internet-of-things communications from a nanosatellite with OQTECH of Luxembourg and the first cryptocurrency transactions made in space with J.P. Morgan. 

“The GOMX-4 mission is now an important part of our legacy, and the products, capabilities and reliability demonstrated through this mission continues to be an important asset for building trust with existing and future customers”, says Lars Krogh Alminde, Co-founder and Vice President Strategy & Business Development. 

A dedicated follow-on mission, GOMX-5, is in development. 

ESA's Technology CubeSats 

ESA supports Technology CubeSats through the ‘Fly’ element of its General Support Technology Programme, with many more in preparation. As well as HENON, CubeSpec, and GOMX-5, all due for launch in 2026, other missions include next year’s GENA-OT which will test a new 16U platform from OroraTech and fly numerous technology experiments. Meanwhile ESA's PRETTY CubeSat for satnav reflectometry testing goes on operating in orbit. 

“These small missions are like stepping stones to the future, demonstrating promising technologies and mission approaches in a cost-effective manner,” adds Roger. “We work with the implementing companies on in-orbit demonstration, but once that work is done, typically within a year, then they are free to carry on using the missions either experimentally or commercially, gathering all-important flight experience well beyond their expected lifetime and maximising the overall return on investment for several years afterwards.”