Thermal infrared remote sensing has proved its usefulness in various environmental applications, including the evaluation of surface heat fluxes exchanged between the continental biosphere and the lower atmosphere. The concept of a minisatellite, IRSUTE, has been evaluated. It is designed to allow the derivation of accurate (+-50 W m-² or +- 0.8 mm/d water evaporation) estimates of surface heat flux at the field scale (near 40 m). Such estimates will be useful for meteorological, hydrological, and agricultural studies. The instrument will also be useful for environmental monitoring, for example, frost mapping, forest fires, volcano activity, thermal pollution, etc. The IRSUTE concept has been further elaborated by conducting scientific studies (modeling canopy radiative temperature, method for flux estimation, ergodicity problem, emissivity of natural surfaces, correction of atmospheric effect), by analyzing methodological aspects (definition of pixel size, overpass time, time repeat cycle, analysis of angular effects) and system specification (superposition error and MTF effects assessment). These studies have resulted in the definition of a feasible instrument corresponding to the minisatellite category (95 kg), with a high spatial resolution (40 m), four bands in the thermal infrared domain (3.5-4.0 um and three bands between 8.2 um and 11.0 um, with two options), a 1-day revisit, corresponding to an orbite altitude of 540 km. The narrow ground swath (40 km) preludes a global coverage; as a result IRSUTE will focus on a set of test-sites. The phase A study is now complete. The main elements of IRSUTE are defined. This article describes the IRSUTE mission characteristics and the main experimental results which led to the chosen specifications.