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The Bepi-Colombo mission was launched in October 2018, headed for Mercury. This mission is a collaboration between Europe and Japan. It is dedicated to the study of Mercury and its environment. It will be inserted into Mercury orbit in December 2025 after a 7-year long cruise. Probing of Hermean Exosphere By Ultraviolet Spectroscopy (PHEBUS) is an ultraviolet Spectrograph and is one of the 11 instruments on-board the Mercury Planetary Orbiter (MPO). It is dedicated to the study of the exosphere of Mercury, its composition, dynamics and variability and its interface with the surface of the planet and the solar wind. The PHEBUS instrument contains four distinct detectors covering the spectral range from 55 nm up to 315 nm and two additional narrow windows at 404 nm and 422 nm. It also has a one-degree of freedom mechanism that allows observations along a cone with an half angle of 80 ∘ . This paper follows a detailed presentation of the PHEBUS instrument design that was presented by Chassefière et al. (Planet. Space Sci. 58:201–223, 2010). Here we present an update of the science objectives and measurement requirements following the results published by the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission. We also present results of the ground calibration campaigns of the flight unit that is currently on-board MPO. In the last part, we present some details of the observations that will be performed during the cruise to Mercury, such as stellar observation campaigns, interplanetary background observations and planetary flybys. [ABSTRACT FROM AUTHOR]