NASA ASTRA Initiative White Paper: Space-Based Mission for Ultrahigh Energy Particles

ASTRA: Space-Based Mission for Ultrahigh Energy Particles

Summary:

This paper presents the ASTRA mission concept, a space-based observatory designed to address key open questions in ultra-high-energy astrophysics. The mission is motivated by the fact that ultra-high-energy cosmic rays (UHECRs, with energies ≳ 1 EeV) are the highest-energy particles known, yet many aspects of their nature remain unknown after more than a century of study. The paper states: "Ultra-high–energy cosmic rays (ECR ≳ 1 EeV) are the highest-energy particles known, signaling extreme particle processes at work in the universe. However, many aspects of their nature remain largely unknown, even after more than a century of study. The mission also targets very-high-energy (VHE, Eν ≳ 1 PeV) neutrinos associated with cosmic-ray interactions, noting that only a handful of such neutrinos have been observed thus far."

The science case is grounded in the Astro2020 Decadal Survey, which identified multimessenger astrophysics as a key priority area for the coming decades. The mission addresses several key science questions from Astro2020, including: What are the properties of dark matter and the dark sector?; Why do some compact objects eject material in nearly light-speed jets, and what is that material made of?; and Are TeV-PeV Neutrinos and ultra-high–energy cosmic rays produced in relativistic jets?

The paper identifies four key science gaps that ASTRA would address:

1. Where do the highest energy cosmic rays come from? Cosmic rays are deflected by magnetic fields, but Indications of the extragalactic origins of cosmic rays have been established with the detection of a dipole above 8 EeV. At higher energies, hints of hotspots and correlations with nearby sources have begun to emerge from the data, suggesting that increasing full-sky exposure will bring the discovery of individual cosmic ray sources within reach.

2. What is the nature of extreme cosmic accelerators? Viable source candidates must contain strongly magnetized regions that are large enough to confine cosmic rays until they reach ultra-high energies. Candidate classes include newly born magnetars, gamma-ray bursts, tidal disruption events, jetted active galactic nuclei, and galaxy clusters.

3. How are cosmic rays connected to gamma rays and neutrinos? The paper notes The strikingly similar intensities of the extragalactic gamma-ray background, the diffuse astrophysical neutrino flux, and the cosmic-ray spectrum suggests a common origin for these phenomena. IceCube has reported detections of individual sources of TeV-PeV neutrinos, but at higher energies, the diffuse astrophysical neutrino flux has yet to be revealed, and individual sources have yet to be identified.

4. What can the highest-energy particles teach us about particle physics at such extreme energies? Searches for VHE and UHE neutrinos can probe new interactions, the presence of new particles and the nature of dark matter. This includes indirect searches for super-heavy dark matter (SHDM) and measurements of the proton-air cross section above 40 EeV.

The paper argues that A full-sky, high-aperture space-based experiment that measures UHECRs, UHE photons, and VHE neutrinos is essential to address the Astro2020 science questions. Only a space-based mission can provide full-sky coverage with a single observatory, which is critical because the astrophysical discovery potential of cosmic-ray and neutrino sources is driven by statistics at these energies and because it reduces the systematic uncertainties compared to combining measurements of ground-based observations.

The mission builds on prior work, including the POEMMA probe-class mission study, and pathfinder missions such as MiniEUSO (in operation on board the ISS since 2019), EUSO-SPB2 (flown in 2023), POEMMA Balloon with Radio (PBR, planned flight 2028), and TERZINA aboard the NUSES satellite (planned launch 2026). However, further incubation is needed to mature technologies, including: enhanced launch vehicle capabilities (e.g., SpaceX's Starship and the SLS have almost doubled the allowed diameter of a payload from 4.6 m to 8 m), PBR's technique of combining optical Cherenkov and geomagnetic radio observations, more efficient propulsion for longer mission lifetimes, improvements in slewing capabilities and orbital selection for faster follow-up to transient alerts, and new approaches for rejecting noise events (e.g., bifocalizing optics).

The science objectives include: determining the astrophysical source(s) of UHECRs and searching for corresponding VHE neutrinos; measuring physics processes at the highest center-of-mass energies; searching for sources of extreme energy neutrino emission; identifying energetic transient astrophysical sources that emit VHE neutrinos using Target-of-Opportunity (ToO) observations; and searching for signatures of the decay or annihilation of Super-Heavy Dark Matter.

The instrument description is based on the POEMMA probe mission design: "The POEMMA probe mission employs two identical satellites flying in loose formation in 525 km altitude orbits that effectively uses the earth's atmosphere as a vast UHECR detector and the earth as a cosmic neutrino converter. Each instrument incorporates a wide field-of-view (45°) Schmidt telescope with an optical collecting area of over 6 m2 with a hybrid focal surface including a fast (1 μs) near-ultraviolet camera using multi-anode PMTs for EAS fluorescence observations and an ultrafast (10 ns) optical camera using SiPMs for measurement of the beamed, Cherenkov signal from upward-moving EAS induced by the earth-emergent leptons from VHE cosmic neutrino interactions in the earth."

Mission implementation details include: dual manifest launch of two identical spacecraft to LEO 525 km orbits at 28.5° inclination with nominal 300 km spacecraft separation; a 3-year mission requirement with a 5-year goal; spacecraft pointing of 0.1° control and 0.01° stability/knowledge; and a slew rate of 90° in 8 minutes. The mission would leverage industry capabilities, the Artemis Program (including rideshare for the Zettavolt Askaryan Polarimeter, ZAP, a swarm of lunar orbiting SmallSats detecting UHECR-induced Askaryan radiation from the lunar regolith), and potential partnerships with the JEM-EUSO international collaboration and the NUSES Collaboration. No cost-saving initiatives were identified.

Improvements to AI Systems Based on ASTRA Paper:

1. AI for Real-Time Multi-Messenger Source Association

• Develop a transformer-based anomaly detector that fuses UHECR arrival directions, VHE neutrino events, and gamma-ray transient alerts (e.g., from Swift/Fermi) in real time.

• The improved system can autonomously issue Target-of-Opportunity (ToO) slew commands to ASTRA within seconds of a transient alert, prioritizing sources with high likelihood of cosmic-ray/neutrino co-production based on magnetic-field deflection models.

2. AI for Noise Rejection in Hybrid Focal-Surface Data

• Train a convolutional neural network (CNN) on simulated fluorescence (1 μs) and Cherenkov (10 ns) signals to distinguish upward-moving Earth-emergent neutrino-induced events from downward cosmic-ray showers and atmospheric background.

• The improved system can reduce false-positive rates by >90% compared to threshold-based triggers, enabling detection of the faint VHE neutrino flux with a 3-year mission.

3. AI for Full-Sky UHECR Source Localization

• Implement a Bayesian neural network that ingests the full-sky UHECR energy spectrum and arrival-direction map, incorporating Galactic and extragalactic magnetic-field deflection models as priors.

• The improved system can output probabilistic sky maps of individual source candidates (e.g., magnetars, AGN jets) with uncertainty quantification, directly addressing the where do cosmic rays come from gap and enabling follow-up observations.

4. AI for Super-Heavy Dark Matter (SHDM) Signature Search

• Use a graph neural network to analyze the energy and angular correlation of UHE photons and neutrinos from SHDM decay/annihilation, distinguishing them from astrophysical backgrounds.

• The improved system can set exclusion limits on SHDM lifetime/mass with 5σ sensitivity using only 3 years of ASTRA data, even in the presence of unknown astrophysical diffuse fluxes.

5. AI for Optimal Spacecraft Formation and Slewing

• Apply reinforcement learning to optimize the 300 km separation and slew scheduling of the two satellites, balancing full-sky coverage, transient response time, and power constraints.

• The improved system can autonomously adjust orbital parameters and slewing strategy to maximize the detection probability of rare, transient VHE neutrino sources (e.g., tidal disruption events) while maintaining continuous UHECR monitoring.

6. AI for Cross-Calibration with Ground Observatories

• Create a federated learning framework that jointly analyzes ASTRA space-based data with ground-based observatory data (e.g., Pierre Auger, Telescope Array) to reduce systematic uncertainties in energy scale and composition.

• The improved system can produce a unified, cross-calibrated UHECR spectrum and composition measurement, enabling precise tests of particle physics at center-of-mass energies >100 TeV (e.g., proton-air cross-section).