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Hydrodynamic atmospheric escape is thought to be the main avenue for early evolution in exoplanets ranging from gas giants to terrestrial worlds comma as shown by demographic studies resulting from the Kepler survey. However comma with only three exoplanets to date that display smoking gun evidence of hydrodynamic escape comma most modeling efforts to understand this process have been limited to semi-arbitrary assumptions and artificial test cases. This has a downstream effect that limits our capacity to assess if sub-Jovian planets are able to retain their volatile-rich atmospheres. While hot Jupiters are under no threat of losing significant fractions of their mass comma they are the best targets to study hydrodynamic escape because they have detectable signals of exospheric metals in transmission spectroscopy. These signatures can only be observed by HST in the ultraviolet. In this program comma we propose to observe the transmission spectrum of the ultra-hot Jupiter WASP-76b. We predict that this planet is losing mass at a rate 10 times larger than that of HD 209458b comma and we can detect escape of Mg and Fe at more than 5-sigma confidence in high resolution. We further predict that the escaping material fills the Roche lobe of the planet and is in a state of geometric blow-off. Our results will automatically yield a precise near-UV transmission spectrum of WASP-76 b comma a crucial measurement to probe opacity sources in its stratosphere and rainout of metallic species.