Space Radiation Laboratory (Caltech)

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The Space Radiation Laboratory (SRL) is a research group within the California Institute of Technology (Caltech) that develops space instruments and conducts research in cosmic rays, solar energetic particles (SEPs), and heliophysics. SRL has played leading roles in numerous NASA and international missions, often in collaboration with the Jet Propulsion Laboratory (JPL).^[1] SRL-originated instruments and analysis span from early balloon-borne experiments to flagship spacecraft such as IMP-8, the Voyager probes, Galileo, SAMPEX, the ACE observatory, STEREO, and the Parker Solar Probe, providing multi-decade measurements that shaped modern energetic-particle space physics.^[2][3][4]

SRL research focuses on:

• Galactic cosmic rays (GCRs): composition, isotopes, origins, and propagation through the interstellar medium.^[5]
• Solar energetic particles (SEPs): acceleration mechanisms at flares and coronal mass ejection (CME) shocks, including ³He-rich events.^[6]
• Radiation belts: heavy-ion populations in planetary magnetospheres (e.g., Jupiter).^[7]
• Space weather: near-Earth particle fluxes and their implications for astronauts and spacecraft electronics.^[8]
• X-ray astronomy and high-energy astrophysics: instrumentation and observations with missions such as NuSTAR, including imaging of supermassive black holes, mapping of radioactive elements in supernova remnants, and studies of magnetars and active galactic nuclei.^[9][10]

The groundwork for Caltech’s Space Radiation Laboratory (SRL) began in the late 1950s and 1960s with balloon-borne studies of cosmic rays, using technology adapted from nuclear physics and launched from NASA’s Columbia Scientific Balloon Facility in Palestine, Texas. During this era, Ed Stone and Robbie Vogt joined Caltech and helped establish a community centered on particle astrophysics, laying the foundation for SRL.^[11][12] Researchers in Caltech’s physics department began adapting detector technology from nuclear physics, particularly solid-state and scintillation counters, for astrophysical use. These efforts established the measurement techniques that would later be applied to numerous spacecraft instrumentation.^[13]

In the 1970s, SRL transitioned from balloon experiments to space missions. With the Electron/Isotope Spectrometer on IMP-8 (1973), SRL began a legacy of precise spacecraft particle measurements. Just a few years later, SRL played a central role in the Voyager Cosmic Ray Subsystem, enabling decades-long measurements of galactic cosmic rays and solar particles throughout the heliosphere and beyond.^[14][3]

During the 1980s and 1990s, SRL broadened into planetary and heliospheric missions. The Galileo Heavy Ion Counter (1989) measured Jupiter’s intense radiation environment. SRL also contributed to SAMPEX, NASA’s first Small Explorer (1992), and later led key instruments on the ACE (1997), significantly advancing the study of elemental and isotopic composition of cosmic and solar particles.^[15][16][2]

In the 2000s, SRL was pivotal to solar particle missions. The laboratory led the Low Energy Telescope on STEREO (2006), enabling multi-spacecraft investigations of SEPs across the inner heliosphere and training new researchers in heliophysics instrumentation.^[4]

In the 2010s, SRL’s expertise also contributed to high-energy astrophysics instrumentation through Caltech’s leadership of the NuSTAR mission, launched in 2012 under Fiona Harrison. NuSTAR made the first focused hard X-ray images of supermassive black holes, mapped radioactive titanium-44 in the Cassiopeia A supernova remnant, discovered unusual emission from magnetars, and revealed hidden populations of black holes and active galactic nuclei.^[17][18] SRL has co-led the IS⊙IS particle suite on the Parker Solar Probe, contributing to the EPI-Hi instrument. The mission has made the first in-situ energetic-particle observations deep inside the corona, revealing new details about SEP acceleration and propagation, including rare ³He-rich events and intense episodes such as the September 5 2022 event at about 15 solar radii.^{[19][20][21][22]}

Throughout the 2020s, the Voyager CRS continues to monitor galactic cosmic rays beyond the heliosphere, while ACE/CRIS provides ongoing compositional data across solar cycles, ensuring SRL’s measurements remain central to heliophysics today.^[23]

From balloon payloads in the 1960s to near-Sun probes and interstellar spacecraft in the 2020s, SRL, combining Caltech’s academic culture with JPL’s engineering excellence, has shaped the field of energetic-particle heliophysics for over six decades.^[24]

Caltech SRL and the Jet Propulsion Laboratory (JPL) have a long-standing partnership in heliophysics. Many JPL scientists hold joint appointments with Caltech, and SRL graduate students often conduct research alongside JPL staff. The JPL engineering team contributed to instrument integration, thermal design, and testing for ACE, Voyager CRS, and Parker Solar Probe EPI-Hi.^[25]

This collaboration has allowed SRL to combine academic training and fundamental physics research with JPL’s applied expertise in spacecraft systems, environmental qualification, and mission operations. Together, SRL and JPL have co-led NASA instruments for over five decades.^[26]

Over five decades, SRL has provided instruments for many heliophysics missions:

• Edward (Ed) Stone – Longtime SRL leader; PI on ACE/CRIS; Voyager project scientist (1972–2022); led foundational studies of cosmic-ray composition and heliospheric boundary phenomena; served as project scientist for NASA’s Voyager missions, chaired multiple NASA advisory committees on heliophysics, and guided SRL’s role in ACE, STEREO, and Parker Solar Probe instrument development.^[35][36]
• Rochus (Robbie) Vogt – Caltech professor of physics and early SRL leader; guided balloon-borne measurements of cosmic rays in the 1960s and 1970s, oversaw SRL’s transition from high-altitude payloads to space-based instruments; served as PI of the Voyager Cosmic Ray Subsystem, and JPL chief scientist (1977–78).^[37][38][39]
• Alan C. Cummings – Co-I on Voyager CRS and ACE; expert in isotope measurements; advanced understanding of cosmic-ray transport and isotopic abundances.^[40][41]
• Fiona A. Harrison – Caltech professor of physics and current leader of SRL; principal investigator of the NuSTAR mission, which produced major discoveries including focused hard X-ray imaging of supermassive black holes, mapping of supernova remnant ejecta, and studies of magnetars and active galactic nuclei.^[42][43]

• Neil Gehrels – PhD graduate; worked on balloon-borne cosmic-ray experiments and calibration methods for spacecraft detectors in the 1970s; contributed to techniques later applied to the Voyager CRS; later career in high-energy astrophysics.^[44][45]

SRL maintains cleanroom facilities, electronics development labs, and test areas for spaceflight hardware at Caltech. Instrument development includes detector design (solid-state, scintillator, Cherenkov), low-noise electronics, calibration systems, and ground support software. Close collaboration with JPL provides access to environmental test facilities, vibration and thermal-vacuum chambers, and mission integration support.^[46]

SRL scientists and engineers have received numerous awards recognizing their contributions to space science and mission development:

• Edward C. Stone – NASA Distinguished Service Medal (1999), Magellanic Premium (2014), Shaw Prize in Astronomy (2019), and multiple NASA Group Achievement Awards for Voyager and ACE.^{[47][48][49][50]}
• Richard A. Mewaldt – NASA Exceptional Scientific Achievement Medal (2001) for contributions to cosmic-ray composition measurements and leadership of ACE/CRIS and STEREO/LET.^[51]
• Alan C. Cummings – NASA Exceptional Service Medal for decades of data analysis on Voyager CRS and ACE.^[40]
• Christina M. S. Cohen – Recognized by NASA and ESA for leadership in Parker Solar Probe IS⊙IS and STEREO/LET particle science.^[52][53]
• Mark E. Wiedenbeck (JPL) – NASA Exceptional Scientific Achievement Medal (2003) for studies of galactic cosmic-ray composition.^[54]
• SRL and JPL engineering teams – multiple NASA Group Achievement Awards for successful instrument development on missions including Voyager, ACE, STEREO, and Parker Solar Probe.^[53]

The laboratory’s role in training graduate students and postdoctoral researchers has also been recognized by the American Geophysical Union (AGU) and American Astronomical Society (AAS), highlighting SRL as both a research and educational institution in heliophysics.^[55]

• Jet Propulsion Laboratory
• Parker Solar Probe
• Advanced Composition Explorer