By Hot Air Balloon to the Stratosphere

The European Stratospheric Balloon Observatory Design Study (ESBO DS) – to date, only a handful of scientists in Stuttgart, Tübingen, Munich, Spain and Sweden have set their sights on establishing a European balloon-based observatory as a long-term objective. But, the international team drawn from the fields of Spaceflight, Astronomy, Astrophysics and Extra-terrestrial Physics is set to grow to around 20 experts in the near future.

By Hot Air Balloon to the Stratosphere Cosmologists are working on more flexible and cost-effective telescope platforms FUTUR 22 The European Stratospheric Balloon Observatory Design Study (ESBO DS) – to date, only a handful of scientists in Stuttgart, Tübingen, Munich, Spain and Sweden have set their sights on establishing a European balloon-based observatory as a long-term objective. But, the international team drawn from the fields of Spaceflight, Astronomy, Astrophysics and Extra-terrestrial Physics is set to grow to around 20 experts in the near future.

“Untethered from the Earth, the great balloon is floating completely weightlessly in the sky” – For researcher Philipp Maier, dreams based loosely on David Bowie's “Major Tom” are successively coming true. More flexible, lighter, more cost-effective, with room for larger telescopes: that's the motto of the engineer from the University of Stuttgart's Institute of Space Systems (IRS). Maier is the ESBO DS project coordinator. The three-year interdisciplinary project, which is funded under the EU's Horizon 2020 Agenda, is aimed at augmenting the field of Astronomy research environment with images of stars observed from an altitude of 30 to 40 kilometers above the Earth, taken from a helium-filled lighter-than-air balloon with a diameter of between 50 and 70 meters.

The University of Stuttgart, which is acting as project consortium coordinator, will be playing a leading role in the effort. The Stuttgart- based researchers are responsible for building the telescope for the visible light spectrum, whilst the University of Tübingen will supply the main scientific instrument for observations of primarily variable stars in the ultraviolet range. The Swedish Space Corporation (SSC), according to Project Coordinator Maier, “one of the most experienced organizations in the world for sounding rocket and stratospheric balloons”, will build the balloon gondola, which will serve as an instrument platform.

The higher into the atmosphere the flight goes, the fewer atmospheric, and therefore, obstructive particles will obscure the telescope’s view. Beyond a majority of the Earth’s obstructive atmosphere, at altitudes of between 12 and 14 kilometers, at which the airborne telescope SOFIA orbits, for example, there is a largely unobstructed view of planets in other solar systems. “This enables research into whether or not our solar system is anything special”, says Maier: “We can determine whether planets and also atmospheres in other solar systems may support life”.

However, as NASA’s Hubble mission shows, the cost of such high-altitude flights also increase astronomically. It was almost a billion euro over budget because another space shuttle had to be launched to rectify a polishing flaw in the telescope's main mirror. In terms of interstellar research, the ESBO DS project is far more modest, both in terms of cost and the entry-level size of the telescope. The latter will have a diameter of 50 centimeters in the prototype. As Maier explains: “the cost of a major balloon mission with a balloon diameter of 70 meters are typically between 15 to 20 million euro".

The project he is coordinating will be addressing two problems simultaneously: Earthbound observatories are limited by the atmosphere, whilst satellite- borne missions at altitudes up to the second Lagrangian Point (L2 Point), such as the Herschel observatory, are limited by cost. Balloons, on the other hand, can be launched multiple times: the liquid helium required for cooling can be replenished and repairs can be made between flights. For each mission carried out under the ESBO DS project and its precursor project, ORISON, in which the IRS was also involved, different instruments are carried aloft depending on the respective research objectives, which makes the balloon flights affordable for a larger research community. The only limitation is that visibility is a bit restricted, even at an altitude of 30 kilometers, Which is why Maier sees balloon-based research as supplementary to, rather than a replacement for, satellite-based space research.

The objectives of the EU project are ambitious: In the longer-term, the ESBO team plan to launch a long-wavelength-infrared telescope with a diameter of five meters. “The size just needs to be increased for a better spatial resolution” as Maier explains. The ESBO DS (prototype) will eventually result in the ESBO, at which time the findings from research carried out in the ultraviolet to near-infrared ranges during both the ORISON and ESBO DS projects will the longer-term, the ESBO team plan to launch a long-wavelength-infrared telescope with a diameter of five meters. “The size just needs to be increased for a better spatial resolution” as Maier explains. The ESBO DS (prototype) will eventually result in the ESBO, at which time the findings from research carried out in the ultraviolet to near-infrared ranges during both the ORISON and ESBO DS projects will inform the construction of a long-wavelength-infrared telescope.

As such, the ESBO would become a long-term research facility for many institutes and research facilities. As Maier summarizes: “We want to create an organization that operates various balloon-based telescopes and provides observation timeslots and transportation opportunities for scientific research”. The project encompasses a range of topics from innovative optics to lightweight construction, which could be of interdisciplinary interest to many different institutes. That gives the project participants cause for optimism regarding the future of the ESBO. The next generation of long-wavelength-infrared observatories should become available within a period of around 15 years, based on the findings from the prototype project.

Susanne Roeder