Asteroids are remnants of the early Solar System and are possible reservoirs of organic compounds that can be delivered to planets. Collisions with large asteroids can alter the evolution of planets and their moons, and impacts of small asteroids can affect their surface. Measuring the properties and distribution of asteroids is key to understand their role in the formation of the Solar System and the origin of life on Earth, and can serve as a benchmark for exoplanetary systems. Besides, potential collisions with large asteroids pose a threat for life on Earth. We will present our project on detecting and characterising asteroids via their thermal emission using the JWST Mid-Infrared Instrument (MIRI) in the context of Solar System science and planetary defence. JWST MIRI opened up a new window to detect small (10 - 100 m) and cool asteroids by observing at long wavelengths (5 - 28 micron) with unprecedented sensitivity. After four years of operation, a wealth of archival MIRI imaging data is available for mining. To characterise the orbits and the physical properties of these small asteroids, complementary observations with large telescopes in the visible will be crucial. We will present our method and first results in the search for unknown asteroids in MIRI images acquired via MINDS (MIRI Mid-INfrared Disk Survey), a MIRI European Consortium Guaranteed Time Observation program, and illustrate how follow-up observations with the VLT can enhance these discoveries