Venus: Key to understanding the evolution of terrestrial planets

As we become aware of Earth's changing climate, and as we discover terrestrial planets in other solar systems, we gain ever more reasons to study the Earth's nearest neighbour and closest sibling, the only Earth-sized planet besides our own that can be reached by our spacecraft. For the scientific and programmatic reasons outlined in this document, Venus is a compelling target for exploration. The science themes important for Venus research - comparative planetology and planetary evolution - are common to all of planetary and exoplanetary science. Many of the payloads required - radar and atmospheric remote sensing, in situ mass spectrometers - are common to mission proposals for many other solar system targets, as are mission technologies like high rate deep-space telecommunications technologies. Venus-specific technology developments meriting special attention include high-temperature systems and balloons. Venus is an excellent proving ground for fundamental understanding of geophysical processes of terrestrial planets; an excellent proving ground for techniques of analysis of exoplanets; an indispensable part of our quest to understand the evolution of Earthlike planets. For all these reasons, Venus will be an ever more compelling theme in the coming decades, and we therefore recommend its inclusion in the Voyage 2050 plan. We recommend that ESA aim to have launched at least two M-class Venus missions by 2050, including the EnVision M5 geophysics orbiter, and an in situ element such as a cloud-level balloon; or an L-class mission combining these elements.