Mini-EUSO is a telescope observing the Earth in the ultraviolet band (290-430 nm) since 2019, through a nadir-facing UV-transparent window in the Russian Zvezda module of the International Space Station. The main camera has an optical system composed of two 25 cm diameter Fresnel lenses and a focal surface consisting of 36 multi-anode photomultiplier tubes, 64 pixels each, for a total of 2304 channels. The instrument has a square field of view with a side of 44 degrees, a spatial resolution of about 6.3 km on the Earth surface and a sampling time of 2.5 microseconds. Mini-EUSO has also two cameras in the near infrared and visible ranges and silicon photomultiplier
sensors to complement the UV observations. Mini-EUSO has been designed as a small-size version
of the original JEM-EUSO space telescope to demonstrate its observational principle. Mini-EUSO
is in fact potentially capable of observing extensive air showers generated by ultra-high-energy
cosmic rays with an energy above 10 21 eV and of detecting artificial showers generated with lasers
from the ground. Other main scientific objectives of the mission are the study of atmospheric
phenomena (transient luminous events such as ELVES and sprites), the observation of meteors
and among them the search for interstellar meteors and nuclearites such as strange quark matter.
Moreover, Mini-EUSO can map night-time UV Earth emissions, both anthropogenic and natural.
In this work, we will discuss results and performance of the telescope during its first four years of
activity.