This white paper outlines a proposal for a space-based observatory to study the highest-energy particles known: ultra‑high‑energy cosmic rays (E_CR ≳ 1 EeV) and very‑high‑energy neutrinos (E_ν ≳ 1 PeV). These particles are rare and extreme. The authors argue that a dedicated mission in low Earth orbit would use the Earth’s atmosphere and the Earth itself as part of the detector to collect many more of these events than ground observatories can, and with full-sky coverage from a single instrument.

The proposed mission concept follows the POEMMA-style approach with two identical satellites in roughly 525 km altitude orbits, separated by about 300 km and sharing a wide view of the same atmospheric volume. Each satellite would carry a 45° field‑of‑view Schmidt telescope with more than 6 m² of optical collecting area. The focal surface is “hybrid”: a larger near‑ultraviolet camera of fast photomultipliers (1 microsecond sampling) to record fluorescence light from extensive air showers, and a smaller ultrafast silicon photomultiplier camera (10 nanosecond sampling) to record Cherenkov light from upward‑moving showers produced by Earth‑emergent leptons from neutrino interactions. The satellites would be able to slew quickly to follow short transients, and the mission baseline is three years with a goal of five.

Why this matters: at these extreme energies we still do not know where the particles come from, what accelerates them, or how they connect to gamma rays and neutrinos seen at lower energies. A space observatory with very large exposure and full-sky reach could improve the statistics needed to pinpoint sources, measure the mix of particle types (composition), and test particle interactions at energies beyond those accessible on Earth. The paper also highlights applications to fundamental questions such as searches for signatures of super‑heavy dark matter and measurements of hadronic interaction cross sections at the highest center‑of‑mass energies.