In the last decades it has been realized that Asteroid impact is a natural disaster threat. The goal of planetary defence is to ensure that  humankind is able to prevent the collision of an asteroid with Earth. NASA's DART mission and ESA's Hera mission together will be the first full asteroid deflection test. In 2022, DART will impact Dimorphos, the moon of asteroid Didymos, and change its orbital period. As the Didymos system is an eclipsing binary, groundbased observations will determine the orbit change. Then Hera will come in:  To be launched in 2024, it will reach Didymos end of 2026 and stay at the asteroid pair for 6 months. With a detailed investigation of the Dimorphos system it will allow to understand the DART impact in detail and to scale the outcome of the impact to another asteroid, should the need for asteroid deflection arise. In addition to a planetary defence mission, Hera is the first rendezvous with a binary asteroid, and as such expected to have substantial scientific value by improving our understanding on asteroid formation and evolution. Hera will also demonstrate new technology, the most notable being two cubesats that are going to be released by Hera while at Didymos, and those briefcase-sized spacecraft will perform independent investigations of the Didymos system. We will present the Hera mission and its context in planetary defence, as well as its expected contributions to science and technology development.