Following the findings of the H2020-funded ORISON project, we propose the design and development of a new European astronomical research infrastructure for stratospheric balloon-based telescopes. Our approach is to provide a versatile community-accessible observatory capable of covering unserved scientific needs, including those of star- and planet formation studies and exoplanet research. These are particularly associated with requirements for high spatial and spectral resolution far-infrared (FIR) observations, high sensitivity UV observations, and highly accurate visible light photometry. To a certain extent, these needs could also be addressed by space missions. A balloon infrastructure, however, will offer a more cost-efficient and flexible approach, allowing instruments more adaptable to evolving needs (exchangeable and less risk-adverse), longer overall observation times (possibility to refill cooling fluids), and larger mirror sizes (less severe launcher restrictions). So far, stratospheric telescopes, with the exception of SOFIA, have mostly served a single purpose and did not fly more than a few times. The proposed infrastructure, in contrast, will provide regular flights, allowing it to operate as a true observatory. The infrastructure will be prepared in two parts within the project: 1) Conceptualization of the full distributed infrastructure with several flight platforms, including a suitable governance and operating structure for long-term operations and development. These will build upon a highly sensitive UV platform and lead to a next-generation FIR platform with unprecedented spatial resolution. A development roadmap will summarize the necessary development steps and funding needs. 2) Technical demonstration of the manageability and safe recovery of a versatile high-performance astronomical balloon mission via a prototype carrying a next-generation UV-detector and a visible light camera for photometric tests.

HEMERA will integrate a large Starting Community in the field of tropospheric and stratospheric balloon-borne research, in order to make existing balloon facilities available to all scientific teams in the EU. The project involves major space agencies dealing with balloon infrastructures, companies operating the balloons, companies providing the necessary technology and scientific experts. Balloon borne instruments are suitable for a wide range of science fields, such as atmospheric measurements, climate and environment related investigations, astronomy and astrophysics, space instruments validation and testing of new technologies. The objectives of HEMERA are to: - Provide better and coordinated balloon access to the troposphere and stratosphere for scientific and technological research - Attract new users to enlarge the community accessing the balloon infrastructure - Enlarge the fields of the science and technology research conducted with balloons - Improve the balloon service offered to scientific and technical users - Favour standardisation, synergy, complementarities and industrialisation through joint The infrastructure deals with a wide range of mission characteristics including altitude, flight duration, instrument mass and volume. Flights can be performed during all seasons at various latitudes, satisfying various scientific needs. Currently, the HEMERA programme will use the Zero Pressure Balloons for stratospheric flights (relevant for measuring Essential Climate Variables and for astrophysics), additional types of balloons can be considered. HEMERA will give access to launch bases and is thus capable of conducting scientific balloon campaigns, in Europe and abroad, necessary in particular for scientific issues in the context of climate change. HEMERA includes 3 major components : - Operational activities to organize and conduct the flights - Development of innovative technologies and infrastructure to optimize the balloon offer - Networking