A team of NASA-sponsored scientists and engineers has developed a novel approach to observing high-energy particles in the near-Earth space environment, incorporating miniaturized sensors into a compact, multi-view particle detection instrument unlike any before it. Built for NASA’s Relativistic Electron Atmospheric Loss (REAL) CubeSat mission, the innovative instrument (also called REAL) enables more complete measurements of how energetic particles are transported and lost in Earth’s radiation environment, opening the door to improved understanding of space weather effects in low Earth orbit (LEO) while also making these unparalleled observations accessible to future small, low-cost spacecraft.
Billions of high-energy charged particles are magnetically trapped around Earth in doughnut-shaped regions called the Van Allen radiation belts. These belts typically form two distinct zones: an inner belt dominated by high-energy protons and an outer belt composed primarily of energetic electrons. Together, they pose a persistent hazard to satellites throughout Earth orbit that modern society depends on, including GPS satellites, and satellites that provide telephone and internet services. The outer belt, in particular, contains so-called killer electrons — particles energetic enough to penetrate satellite shielding and trigger damaging electrical discharges or operational anomalies.
Because of these risks, scientists have spent decades working to better understand and predict how the radiation belts behave. But complicating that effort is how dynamic they can be, particularly the outer belt, where populations of energetic electrons can build up and then rapidly drop off. At times, these electrons are lost from the belts, sometimes plunging into Earth’s atmosphere in microbursts lasting just 100 milliseconds, and other times in longer events that unfold over minutes to hours.
“Radiation particles can be trapped in the Van Allen belts for long periods, going back and forth along magnetic field lines, but if some interaction directs them more closely along Earth’s magnetic field lines, they plunge into the atmosphere,” explained Thomas Sotirelis, a physicist at the Johns Hopkins Applied Physics Laboratory, where the REAL instrument was developed. Sotirelis is the originator of the REAL instrument’s sensor concept and serves as instrument scientist for the REAL mission.
These loss events, known as energetic electron precipitation (EEP), represent one of the primary ways electrons are lost from the radiation belts and play an important role in their dynamics. But while researchers have identified plasma waves as likely drivers of these events, the underlying physics — e.g., whether electron scattering occurs gradually through diffusive processes or rapidly through nonlinear interactions — is still uncertain.
Successfully launched on July 23, 2025, the REAL instrument can distinguish between these potential modes, making it possible to investigate their relative importance and determine which, if any, waves are responsible for electron fallout. Leveraging recent advancements in sensor miniaturization, the instrument includes three sensor heads — a low-, medium-, and high-energy head with two, five, and four simultaneous look directions, respectively — integrated with four electronic boards. Together, they occupy only about half of the REAL CubeSat and use a time resolution sufficient to resolve microbursts of electrons with energies ranging from 40 keV up to 2 MeV. As its parent 3U CubeSat flies in LEO, REAL points along Earth’s magnetic field and can simultaneously measure the quantity, energy, and angle of the particles as they fall into the atmosphere — a first-of-its-kind capability.
“Most CubeSats can observe particles from only a single direction, so they have to spin in order to build up a full picture — and that takes a few seconds, too slow to capture microbursts,” said space physicist Robyn Millan of Dartmouth College, who serves as the REAL mission principal investigator. “With REAL, we’ve managed to squeeze three sensors, each with multiple look directions, into the top of this 100-by-100-millimeter head, allowing us to capture those measurements all at once. We’re really proud of that.”
The high-energy head consists of a 30-millimeter-thick aluminum collimator with four apertures, each spanning 20 degrees of pitch angle. Each aperture connects to an active area on a solid-state detector (SSD) at the base. The medium-energy head similarly uses an SSD base but instead employs five active areas that connect to a 22-millimeter-thick aluminum collimator with five apertures, each spanning 20 degrees of pitch angle. The low-energy head, on the other hand, is a miniature electrostatic analyzer (ESA) consisting of titanium electrodes sandwiched between etched silicon selector slits. These lie on top of a microchannel plate (MCP). The low-energy head uses 36 apertures, two look directions (±40 degrees), and 15 channels to measure electrons with lower energies, from 1 keV to 40 keV.
The pitch-angle-resolved measurements from these different look directions make it possible to distinguish precipitating, quasi-trapped, and trapped electron populations from each other, on timescales as short as 20 milliseconds, thereby more accurately quantifying the rate of electron loss and its impact on Earth’s atmosphere.
Just as importantly, this novel capability demonstrates that measurements once requiring large, resource-intensive missions can now be achieved with compact, cost-effective instruments on small satellites. This shift enables new, more complex mission concepts and technologies and paves the way for CubeSat constellations that could continuously observe Earth’s radiation environment and help us to better protect the space-based systems modern society depends on.
All three sensor heads in REAL are still functioning nominally and continue to collect valuable science data. In fact, the REAL team recently fine-tuned the instrument (changing threshold settings, etc.) to improve its sensitivity.
Project Lead(s): Dr. Tom Sotirelis, Johns Hopkins Applied Physics Laboratory; Dr. Robyn Millan, Dartmouth College
Sponsoring Organization(s): NASA Heliophysics Division’s Heliophysics Flight Opportunities in Research and Technology (H-FORT) program
