Balloon Array for Radiation-belt Relativistic Electron Losses (BARREL) is a balloon-based Mission of Opportunity to augment the measurements of NASA's Van Allen Probes spacecraft. BARREL seeks to measure the precipitation of relativistic electrons from the radiation belts during 2 multi-balloon campaigns, operated in the southern hemispheres (option for 3rd northern hemisphere campaign). During each campaign, 5-8 long-duration balloons would be aloft simultaneously over a one-month period to provide measurements of the spatial extent of the relativistic electron precipitation and to allow an estimate of the total electron loss from the radiation belts. Observations are planned for when the balloon-array will be conjugate with the spacecraft, such that direct comparison is possible between one another.

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BARREL Antarctica 2013 payload paths. Credit: NASA BARREL consists of two Antarctic balloon campaigns conducted in Austral summers of 2013 and 2014. During each campaign, a total of 20 small (~20 kg) balloon payloads will be launched to an altitude of 30-35 km to maintain an array of 5-8 payloads. Each balloon will carry a NaI scintillator to measure the bremsstrahlung X-rays produced by precipitating relativistic electrons as they collide with neutrals in Earth's atmosphere, and a DC magnetometer. The balloons will be launched from the South African Antarctic Station (SANAE IV) and the British station, Halley Bay.

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