Unveiling Energetic Particle Dynamics in the Near‐Earth Environment From CubeSat Missions

The discovery of the Van Allen radiation belts marked a prominent milestone in space physics. Recent advances, through the measurements of two CubeSat missions, have shed new light on the dynamics of energetic particles in the near‐Earth environment. Measurements from CSSWE, a student‐led mission, revealed that the decay of low‐energy neutrons, associated with cosmic rays impacting the atmosphere, is the primary source of relativistic electrons at the inner edge of the inner belt (Li et al., Nature, 2017, https://doi.org/10.1038/nature2464). Recently CIRBE captured striking details of energetic electron dynamics (Li et al., GRL, 2024, https://doi.org/10.1029/2023gl107521), further demonstrating high‐quality science achievable with CubeSat missions.


Introduction
The concept of CubeSats, a class of nanosatellites, originated over two decades ago when Professor Robert Twiggs (then at Stanford University) and Jordi Puig-Suari (CalPoly-SLO) envisioned them as a platform for students to gain practical experience in satellite development (Deepak & Twiggs, 2012).CubeSats are cubeshaped, with their volumes measured in units or "U's."Each unit measures 10 × 10 × 10 cm.Initially, few believed that CubeSats would be capable of conducting meaningful scientific research.
In 2008, the National Science Foundation (NSF)/Atmospheric and Geospace Sciences (AGS) became the first organization to openly solicit proposals for CubeSat missions dedicated to scientific research and student education, with a budget cap of $900K.The Colorado Student Space Weather Experiment (CSSWE) was selected in 2009, receiving $860K to develop a 3U CubeSat mission equipped with a single science payload: the Relativistic Electron and Proton integrated little experiment (REPTile).REPTile was a simplified and miniaturized version of the Relativistic Electron and Proton Telescope (REPT) built at the University of Colorado's Laboratory for Atmospheric and Space Physics (LASP) for NASA's Van Allen Probes mission (Baker et al., 2012(Baker et al., , 2021;;Mauk et al., 2012).
With enhanced maturity of CubeSat technology and perhaps inspired by the demonstrated potentials and scientific achievements of NSF-funded CubeSats, including CSSWE (Blum et al., 2013;Li, Palo, et al., 2013;Li, Schiller, et al., 2013;Li et al., 2011Li et al., , 2012;;Schiller et al., 2014), NASA's Science Mission Directorate (SMD) began funding CubeSat missions for scientific investigations in 2014.In late 2017, the Colorado Inner Radiation Belt Experiment (CIRBE) was selected for a total funding of $4M.CIRBE is a 3U CubeSat mission carrying REPTile-2, a significantly advanced version of REPTile. Figure 1 shows a photo of CSSWE during one of its plugs-out testing (left) and a photo of CIRBE with its solar panels deployed.

Colorado Student Space Weather Experiment (CSSWE)
Due to budgetary constraints and required education component, the CSSWE mission was developed through a series of graduate student project classes in the Department of Aerospace Engineering Sciences (AES) of University of Colorado Boulder, commencing in the spring semester of 2010.The graduate students involved in these classes were primarily enrolled in MS programs and covered their tuition expenses.A few PhD students under the guidance of the Principal Investigator (PI) and Co-Principal Investigator (Co-PI) were supported as Graduate Research Assistants (GRAs) through external research grants and the CSSWE grant.The project classes, held three times a week, spanned four semesters.
Once a week, a class was held at LASP, enabling LASP professionals from various subsystems to provide continuous mentorship throughout the project's duration.This invaluable support helped to streamline the development process and avoid unnecessary detours.All LASP professionals donated some of their time (did not charge fully on the CSSWE grant).A few students were compensated hourly during summer months for part-time work on the project.
Letting students take ownership of the project fostered a strong sense of motivation among the team, resulting in exceptional effort levels.A formal organizational chart outlined the roles and responsibilities of each student, including project manager (PM), system engineer (SE), and subsystem leads.
After each semester, ∼40% of students enrolled were gone, and new students joined the class.To ensure project continuity, detailed documentation, including final reports reviewed by faculty and students, facilitated this handover process.On many occasions, the final reports were returned to the students for completion before a final grade for the class was given.New students were able to quickly familiarize themselves with the project by studying previous reports and engaging with continuing students.
As LASP and AES's first CubeSat mission, CSSWE required the design and construction of a ground station (for Ultra High Frequency uplink and downlink) on the roof of the LASP building.Amateur Radio volunteers Rick Kile and Rob Strieby (from Loveland Repeater Association) provided invaluable assistance by advising on station site selection (with low interference), affordable antenna and radio selection, and system tuning and testing, to ensure a workable command and control link between the ground stations and CSSWE CubeSat.
CSSWE was successfully launched into a low-altitude, high-inclination orbit on 13 September 2012, as a secondary payload under NASA's Educational Launch of Nanosatellites (ELaNa) program.The ground station successfully received beacon packets from CSSWE during its first pass over Boulder.After the completion of the commissioning phase on 4 October 2012, the REPTile was activated.
Ph.D. Graduate students (primarily David Gerhardt, Lauren Blum, and Quintin Schiller) and other students were responsible for mission operations, including uplink and downlink processes.An automated operation scheme was eventually developed and implemented.Lessons Learned: 1. Emphasizing simplicity throughout the project significantly increased the likelihood of success.2. Engaging experienced professionals as mentors from the outset proved invaluable.3. Delegating ownership to students motivated them and facilitated project continuity with detailed documentation.4. Seeking external assistance when facing challenges proved beneficial, as exemplified by CubeSat UHF radio communication expert Jim White's assistance with diagnostics when the communication system was not working.5. Adopting a "robust but not fancy" approach, such as implementing a simple threshold logic for electron and proton measurements and a passive attitude control system (ADCS), was deemed acceptable for the project's scientific goals (Li, Schiller, et al., 2013;Li et al., 2012).

Colorado Inner Radiation Belt Experiment (CIRBE)
With a healthier budget and an overbooked graduate project class (aforementioned), the development of the CIRBE project was led by LASP professionals with students playing supporting roles.With experience and lessons learned from CSSWE, clearly refined and much advanced science requirements were set for CIRBE, which also led to more requirements for the science payload (REPTile-2), other spacecraft subsystems, and the ground stations.For example, the required higher-energy resolution measurements of energetic electrons led to pulse height analysis (PHA) onboard (Khoo et al., 2022), in lieu of the simple threshold method adopted for REPTile on CSSWE.This required more onboard processing and consumed more power, which necessitated deployed rather than spacecraft body-mounted solar panels to meet the power consumption.The higher-energy resolution measurements also meant that more data needed to be downloaded to the ground, which required an Sband receiver for downloading (Li et al., 2022), in comparison to UHF only for CSSWE.
The CIRBE bus (XB1) was procured from a local commercial company, Blue Canyon Technologies (BCT), which has had many years of experience building CubeSat and SmallSat spacecraft, including their advanced active attitude control system (ADCS).The CIRBE team focused on the development of REPTile-2 and its interface with the spacecraft, the integration, and all required tests and other additional tests, including several plugs-out tests when the integrated CIRBE was brought out, standing alone, to receive commands from the ground stations and perform various tests, including sending data back to the ground stations.
Among many unforeseen challenges, the Covid-19 impact was the most significant.LASP, in which the CIRBE mission was housed, had been completely locked down for a few weeks in 2020, and then restricted access for many months, limited occupation for about a year (engineers needed to take turns to work in the lab, no more than 2 were allowed in the same lab).These restrictions reduced the efficiency of the work.CIRBE team meetings were conducted via Zoom.
Another challenge was the delayed delivery of the ordered CIRBE bus (certainly, BCT was also affected by the Covid-19 pandemic).However, the team was able to work on most interface issues using the engineering development unit (EDU), which was ordered at the same time as the flight bus but arrived about a year earlier.
Like many others in the world, the CIRBE team endured the hardships, but persevered and eventually succeeded, even though with some delays.
CIRBE was initially manifested (via NASA/CubeSat Launch Initiative, CSLI) for a launch on US Space Force (USSF) Space Test Program (STP) S28B on Launcher One of Virgin Orbit in early 2022.It was postponed to the second quarter, to the third quarter, and then to the fourth quarter.In September, CIRBE was taken off from the Virgin Orbit Launch, which changed its launch site, to be launched from England in January of 2023.During the last meeting with Virgin Orbit, Rick Kohnert (PM of CIRBE), said clearly that CIRBE was ready to deliver given a 30-day advance notice.A NASA CSLI representative was in the meeting and asked Rick Kohnert to discuss CIRBE offline.It turned out that CSLI had purchased two slots on Transporter-7, however one of the teams, given a slot, was delaying past the Transporter-7 handover readiness date and was unable to deliver.That slot was then given to CIRBE.Our kick-off meeting with Spaceflight, which manages Transporter-7, started on 17 October 2022.Everything went smoothly from then on.We delivered CIRBE in February of 2023 and CIRBE was launched successfully on 15 April 2023.(We also learned from the news media that the Virgin Orbit Launch from England, on 9 January 2023, failed to reach the required altitude due to a failure of the second stage engine.We were disappointed in September of 2022 when we were told that CIRBE would not be on the Virgin Orbit Launch.Now we just feel lucky!) Like CSSWE, the LASP ground station successfully received beacon packets from CIRBE during its first pass over Boulder.After the health of the spacecraft was checked out, REPTile-2 was activated on 19 April 2023, taking science data, and has been functioning flawlessly.
Lessons Learned: 1. Synergistic collaborations among team members of different subsystems are a key to success.We were not afraid of encountering or finding problems.We were trying hard to find problems (and solve them) before the delivery for launch.2. Test early and test often, including performing corner tests (e.g., sending wrong commands on purpose to check robustness of the flight software system), trying to identify any issues.3. Acquiring an engineering development unit or EDU was a good idea, as it remedied some issues associated with the significant delay of the spacecraft.4.Even with launch delay or launch uncertainties, keeping up with the schedule, completing the work, being ready to deliver, and sometimes a stroke of luck (a new launch opportunity) will be more likely to occur.

CSSWE
The CSSWE project has significantly impacted the education and careers of over 65 graduate and undergraduate students, providing them with hands-on experience and team-working experience that have prepared many of them for leadership roles in science and engineering.Five Ph.D. dissertations (Drew Turner, David Gerhardt, Lauren Blum, Quintin Schiller, and Kun Zhang) have been completed based on engineering and science research conducted through CSSWE, and 26 peer-reviewed papers have been published, including two in the prestigious journal Nature (Baker et al., 2013;Li et al., 2017).A major scientific breakthrough achieved through CSSWE measurements was the direct identification and quantification of energetic electrons from cosmic ray albedo neutron decay (CRAND) in the inner electron belt (Li et al., 2017).This discovery resolved a longstanding mystery and had important implications for understanding the dynamics of the Earth's radiation belts.This discovery also led to a series of follow-on studies of CRAND in the inner belt and slot region (Li et al., 2021;Xiang et al., 2019Xiang et al., , 2020aXiang et al., , 2020b;;Zhang et al., 2019Zhang et al., , 2020)).
The project's success led to invitations (2018 and 2019) for the PI to present its findings to policymakers on Capitol Hill, advocating for increased funding for university small satellite programs.

CIRBE
CIRBE's advanced science payload (REPTile-2) and spacecraft (XB1) have enabled unprecedented observations of radiation belt electron dynamics.These observations have revealed novel phenomena, including: • Multi-order-of-magnitude enhancements of outer belt electrons during magnetic storms.
• "Wisps" associated with human made Very Low Frequency waves in the inner belt.
• "Drift echoes" or "zebra stripes" (Ukhorskiy et al., 2014) of electrons across the inner and outer belts, demonstrated in Figure 2. • Bursty precipitation of high-energy (up to 3 MeV) electrons associated with whistler mode chorus waves.
These findings have expanded our understanding of energetic particle behavior in the near-Earth environment and contributed to the development of space weather forecasting and mitigation strategies.

Summary
CSSWE and CIRBE have been highly successful CubeSat missions that have made significant contributions to education, engineering, and science research.CSSWE's measurements led to a major discovery in the inner radiation belt, while CIRBE has unveiled new and unexpected features in the electron population of the near-Earth environment.It should be noted that there have been many more contributions and impacts from other CubeSat missions, as described in Spence et al. ( 2022), a comprehensive review of achievements and lessons learned from successful small satellite missions for space weather-oriented research.Here we focus on CSSWE and CIRBE only, based on the PI's personal perspective.These missions demonstrate the potential of small satellites for advancing our knowledge of space and its effects on Earth's space environment.

Figure 1 .
Figure 1.Left: A photo of CSSWE during one of its plugs-out testing; Right: A photo of CIRBE with its solar panels deployed.

Figure 2 .
Figure 2. Adapted from Figure 4 of Li et al. (2024).(a) Electron fluxes derived from the least-squares method for a pass on 4/ 24/2023; Black curves overplotted on top of the color-coded electron fluxes are selected contours of electron drift period in hr.(b) Electron fluxes using a moving average window of ±19% in energy.(c) Detrended electron fluxes, between (a, b).Lower-right: An image of a zebra and schematic view of Earth's radiation belts and CIRBE's orbit.