Scientists Observe Sun and Moon From Half-mile Underground

Scientists have observed the moon and sun from below ground, using a detector in the Soudan Underground Lab in northern Minnesota. They've done this by looking for the shadow these bodies cast in cosmic rays that penetrate the half-mile of rock above the detector.

Observing the skies using messenger particles other than light has long been a goal of particle astrophysics experiments. However, before one goes looking for new astrophysical sources in this way, one must observe more mundane things--still a challenging step from deep underground.

High-energy particles called cosmic rays are rattling around our galaxy all the time. Those that hit the Earth cause sprays of secondary particles when colliding with the upper atmosphere. Sensitive particle detectors studying rare phenomena--such as the oscillation of subatomic particles known as neutrinos--often go deep underground, where the rock shields out the majority of those cosmic-ray-induced particles. However, a few of the most energetic particles manage to penetrate anyway, where they are recorded by the MINOS (Main Injector Neutrino Oscillation Search) experiment as it observes neutrinos being shot from Fermi National Accelerator Laboratory, 500 miles to the southeast.

By making a map of where those cosmic rays come from over many years, a deficit can be seen in the directions of the sun and moon. Those bodies block out the cosmic rays entering the solar system from the galaxy, and this slight reduction can be seen deep underground. Distortions of the shape provide clues as to the nature of magnetic fields in the solar system that bend the paths of the remaining cosmic rays on their trip to Earth.