This paper reviews ways to search for very-high-energy cosmic neutrinos from space. The idea is to use large natural targets — the Earth, the Moon, or the Sun — and to look from orbit or high-altitude balloons for the light and radio signals made when a neutrino interacts and produces a particle shower. The review focuses on using the Earth as the target and using the atmosphere to make optical and radio signals from extensive air showers (EAS).

When a neutrino hits matter it can start a cascade of particles. Those cascades contain many charged particles, mainly electrons and positrons. Moving charged particles can produce optical Cherenkov light — a blue flash made when a particle moves faster than light does in air or ice — and they can also produce radio pulses. In very dense materials like ice, cascades produce Askaryan radiation, a radio signal that can travel through clear ice and sometimes bend (refract) out of the ground so it can be seen from above.

The paper explains the two main detection approaches and their trade-offs. Optical measurements look for Cherenkov light and must be done near astronomical night. They are also affected by moonlight and by clouds in the view direction. Radio measurements look for pulses from the shower and can in principle run almost all the time (nearly 100% duty cycle), but only if human-made and other radio backgrounds are small in the viewing area. Different techniques also respond differently to the three neutrino “flavors” (types), so they are complementary.

Because space and sub-orbital detectors view showers from large distances, the energy needed to make a detectable signal is high. From low Earth orbit or sub-orbital altitudes, the typical energy thresholds are currently above about 1 PeV (petaelectronvolt, 10^15 electronvolts) for optical Cherenkov detection, and about 100 PeV for radio detection. These high thresholds mean the methods target the very-high and ultra-high end of the neutrino energy spectrum.