Astronomical observations are inherently limited by the surrounding conditions in which they are carried out. The boundaries of ground-based observations continue to be pushed further by more sophisticated observation and data processing techniques, larger telescopes and arrays, technological advances such as adaptive optics, and increasingly precise measurements that allow to filter out more parts of atmospheric influence. However, fundamental limitations imposed by the Earth’s atmosphere and the ground-based location remain. Among those are the inability to access certain spectral regions, including the ultraviolet (UV) and large parts of the infrared (IR), but also limitations to photometric accuracy due to atmospheric variations. The answers to many fundamental, yet still unresolved astrophysical questions, such as those about the detailed mechanisms of astronomical engines, the secrets of exoplanet atmospheres, or the distribution of water in our own solar system, closely linked with questions about its own formation and evolution, thus lay obscured behind this atmospheric curtain. Europe houses first-class astronomical, astrophysical, and astrochemical science communities that strive to further answer these questions and to gain a better understanding of the Universe and mankind’s own place within it. European astronomical observatories and instruments in many domains belong to the world class and serve a multitude of researchers internationally. To add further puzzle pieces to the answers, more capable and efficient observatories will be needed particularly for the UV and near- to mid-infrared in the near term and for the far infrared (FIR) in the long term. The most common way to do so is to move observatories to space, outside of Earth’s atmosphere, as the James Webb Space Telescope will do for the near- and mid-infrared ranges below 28 µm. Even in the age of micro- and nanosatellites, new launch providers and related developments, space observatories are, however, intrinsically expensive and bear operational limitations themselves: development times are long, updates or corrections of the instrumentation are usually not possible after launch, operating material such as cryogenic coolant fluids (see the Herschel Space Observatory) cannot be refilled or replaced. Furthermore, comparably conservative approaches towards new technologies are used to minimise risks of failure. For many aspects of observational quality, such as access to spectral regions, photometric stability, or lack of turbulence, and corresponding applications, though, it is sufficient to move into the high stratosphere at 30 to 40 km altitude, above 99% of Earth’s atmospheric mass. Balloon-based observatories can cater this region while maintaining accessibility and flexibility of instruments similar to ground-based observatories.
While balloon telescopes have been used for several decades, they are not free of challenges and limitations: current balloon designs are limited to about 3.7 t of suspended payload mass; unguided landings pose a certain risk of damage to the payload gondola; and accurate telescope pointing with remaining perturbations from the balloon is challenging. In addition, the design and operation of large stratospheric balloon missions requires specialized expertise that most research groups interested in astronomical observations do not have.
Technological advances in material science, miniaturisation of electronics, and renewed interest in high-altitude platforms for various applications are leading to important steps to overcome some of the technical limitations. The challenge of mission design and operation is an infrastructural one, however. While operating institutions that provide observing time and instrument space on observatories exist in the ground- and space-based domains (such as ESO or ESA, respectively), such an institution is missing for balloon-based astronomical observations.
The Long-Term Plan for ESBO
The European Stratospheric Balloon Observatory (ESBO) will fill this gap by providing an operating organisation for balloon telescopes. Besides providing an operational and governance structure, ESBO will provide several telescope platforms that can be flown on stratospheric balloons from different locations worldwide. It will thereby constitute a distributed research infrastructure with fixed administrative and service sites, but flexible deployment locations allowing observations of astronomical targets both in the Northern and the Southern hemisphere. The implementation of ESBO will follow a step-by-step approach to ensure sustainable long-term operation and earliest possible exploitation of scientific services. As a first step that can be implemented within the next three to five years, we foresee a flight platform with a UV telescope (see also “Prototype and UV Science Cases”). Further in the future, we foresee additional and larger flight systems, with a particular focus on the far infrared astronomy community.
In order to ensure continuous access to far infrared observations after the end of the Herschel Space Observatory in 2013, and in the future to succeed the airborne Stratospheric Observatory for Infrared Astronomy (SOFIA), we foresee a FIR telescope exceeding the angular resolution of both Herschel and SOFIA by a factor of 1.5 to 2 as a second step for ESBO. The balloon-based location would make such a system highly efficient, allowing it to provide the same amount of observation hours during a six-weeks flight as SOFIA currently provides during a year. This system can be operational within a timeframe of approximately 10 to 15 years.