THOR is a mission proposal, in response to the ESA M4 call, to address one of the most widespread and fundamental, but also one of the least understood, physical processes in the universe: turbulent energy dissipation and particle energization.

Almost all visible matter in space is in a plasma state and most of the energetic processes in the universe occur in plasma environments. While there may be large uniform regions in space showing laminar behavior, most of the plasma environments are in a turbulent state where turbulent fluctuations are continuously generated and dissipated. Energy is cascaded from larger to smaller scales where it may heat plasma and accelerate particles. For example, solar wind turbulence is driven at large scales close to Sun and through cascading to smaller and smaller scales, the turbulence dissipates at kinetic scales throughout the whole heliosphere. In contrast, turbulent fluctuations at shocks may be generated at kinetic scales and they may dissipate very close to their generation region. In general, the dissipation due to the turbulent fluctuations is believed to be one of the main processes of plasma heating and particle acceleration in different astrophysical as well as laboratory plasma environments.

In June 2015, THOR was selected as one of three missions to enter a 2-year study phase.
In November 2017 one of these missions will be selected by ESA for launch in 2026.