High-flying Proba-3 will survey Earth’s radiation belts

Beyond the protective bubble of Earth’s atmosphere, particles of diverse energies and charges dart through the vacuum of space in all directions. Whether they were flung off by the Sun during solar flares or originated from powerful explosions in deep space, Earth’s magnetic field can capture and speed them up in a manner resembling a terrestrial particle accelerator – trapping them within the Van Allen radiation belts.

These highly energetic – typically meaning very fast – particles pose various hazards to devices in space. They can disrupt onboard measurements and memories, or even cause permanent damage, while also posing potential danger to astronauts.

These highly energetic – typically meaning very fast – particles pose various hazards to devices in space. They can disrupt onboard measurements and memories, or even cause permanent damage, while also posing potential danger to astronauts.

To give an idea, at one point during its orbit around Earth, the International Space Station (ISS) passes through the tip of the inner Van Allen belt – known as the South Atlantic Anomaly – briefly exposing the astronauts on board to levels of radiation 30 times higher than normal.

Understanding space radiation is therefore crucial for ensuring the safety of people in orbit and for future missions and technologies that will encounter this harsh environment. Mapping particle fluxes advances our understanding of the radiation belts, as well as contribute to space weather forecasting.

What makes Proba-3 such a good fit for this kind of observations is the double satellite’s highly elliptical orbit, which takes the pair more than 60 000 km above Earth and then back down to just 600 km – leading them through both the inner and outer radiation belts surrounding our planet. There are numerous Earth-orbiting missions that traverse both belts, but Proba-3 is unique in crossing an unusually large portion of them.

Starting its space journey aboard Proba-3’s Coronagraph satellite in December 2024, the high-fidelity 3D Energetic Electron Spectrometer (3DEES) will measure ‘angle-resolved electron energy spectra’ in Earth’s radiation belts – meaning that the instrument will be able to capture their direction of origin as well as their energy levels. For the first time, the energy and fluxes of highly energetic electrons will be measured at the same time across six different directions spanning a 180° field-of-view.

In a joint effort of the Centre for Space Radiation at Belgium’s Catholic University of Louvain (UCLouvain), the Royal Belgian Institute for Space Aeronomy and aerospace manufacturer Redwire Space, this instrument will fly as an in-orbit demonstration – to show the technology’s ability to function in the space environment.

Composed of two shoebox-sized parts, the instrument weighs about 6 kg and needs less than eight watts of power (roughly an equivalent of a standard phone charger). The version on board Proba-3 consists of a Panoramic Spectrometer Module (PSM), mounted on the shaded side of the mission’s Coronagraph satellite, and a Docking Module (DM), attached to its inside.

“The PSM consists of three Orthogonal Sensor Modules (OSM) and each OSM can collect particles from two perpendicular directions,” explains Sylvie Benck of Belgium’s Catholic University of Louvain, Principal Investigator for 3DEES. “The most important elements of an OSM are a sensor stack, consisting of four 1.5 mm thick rectangular silicon detectors, plus the two triggering sensors – just 0.14 mm thick silicon detectors of 4 mm in diameter, one for each of the OSM’s two looking directions. When a particle traverses the sensor stack, depending on its energy it will eventually stop in one of the sensors. Based on the energy it deposits in all the sensors it crossed, we can identify and classify the particle. 

“Because the Coronagraph satellite will always be positioned so that its main instrument aperture and solar panel faces the Sun, 3DEES will always look away from the Sun. This means we don’t have to worry about overheating, or any other interference hindering our measurements.”

Sylvie adds: “Formation flying tests and Proba-3’s coronagraph observations will take place around the time the satellites are furthest from Earth, and 3DEES will be switched off as a consequence. This is not a problem for us because we only measure when we enter the radiation belts. This way, useful scientific measurements are being performed across the whole orbit.

“Since we know in advance what the satellite’s orbit will be, we know what its position will be at any given time. Based on this information we can write automatic commands that are uploaded to the satellite, so that the instrument does not need active oversight.”

Although different in layout and operation mode, the instrument’s technology builds on that of its predecessor, the Energetic Particle Telescope (EPT), flying aboard the Proba-V mission since its launch in 2013. More than a decade on, EPT is still collecting data and contributing to ESA’s Space Weather Service Network.

Simon Clucas of ESA’s Space Environment and Effects section explains: “The main difference between EPT and 3DEES is that EPT is a one-directional instrument. It measures the energy and particle species that come into the telescope, but it does not see the full sky. With 3DEES, we can get real time information of the radiation environment from many directions.” 

Proba-3 is scheduled to be launched by Indian PSLV-XL launcher on 4 December.