Hang Test for Sunrise III

The balloon-borne solar observatory prepares for its next research flight

The solar observatory Sunrise III, which will observe the Sun from a helium balloon at an altitude of 35 kilometers, has reached an important milestone: today scientists and engineers at the Max Planck Institute for Solar System Research (MPS) in Germany performed measurements with sunlight for the first time. For the so-called hang test, the huge gate of the MPS balloon hall opened to allow the observatory to look directly into the Sun. The test is an important part of the current preparations for the multi-day stratospheric flight of Sunrise III in early summer next year. During this flight, the observatory will collect unprecedented observational data from the Sun's lower atmosphere, the chromosphere.

At around 10.45 a.m., the nine-meter-high door to the balloon hall of the MPS at Justus-von-Liebig-Weg in Göttingen opened. With the help of the hall crane, the Sunrise III staff had alreadytransported the seven-meter-high gondola, which in its flight configuration contains a solar telescope, scientific instruments, a system for image stabilization, and the on-board electronics, inside the hall to just before the huge opening. The Sun is shining; everything is ready to go.

Approximately seven months before the planned launch of the Sunrise III mission, preparations at MPS are in full swing. Almost all subsystems are operational: the telescope, which with its 1-meter primary mirror is the heart of Sunrise III; the SUSI (Sunrise UV Spectropolarimeter and Imager) and SCIP (Sunrise Chromospheric Infrared Spectro-Polarimeter) instruments, developed and built by MPS and by a Japanese consortium led by the National Astronomical Observatory of Japan; the image stabilization system from the Leibniz Institute for Solar Physics in Freiburg (Germany); and the on-board electronics. The seven-meter-high gondola was delivered in recent weeks from the Applied Physics Laboratory of Johns Hopkins University (USA), assembled on site in Göttingen and "married" to the telescope and instruments.

"With the current tests, we are checking whether all of Sunrise III's systems are working smoothly together, and can make further improvements and refinements if necessary," explains MPS scientist Dr. Andreas Lagg, Sunrise III project manager. "In this way, we can already now optimize the processes for the observations during the flight," he adds. While space probes typically use the long journey through space to their destination to commission science instruments, Sunrise III must operate on a much tighter schedule. "The flight lasts only a few days. After launch, we therefore want to start scientific observations as soon as possible," said Dr. Achim Gandorfer, Sunrise III project scientist. 

Sunlight for SUSI and SCIP

In the balloon hall, the Sunrise III team now begins with the first part of the hang test, the so called pointing: the observatory is to align itself automatically with the Sun - as required during the flight. This process is monitored and controlled from 6000 kilometers away by engineers from Johns Hopkins University in the USA. "Because of the Corona pandemic, many of our international partners were unable to be here on site and lend a hand themselves during the installation of their hardware contributions," says Dr. Achim Gandorfer, describing the work of the past weeks and months. "Instead, we had to rely on their advice and expertise via video conference. We are now taking a similar approach to the hang test," he adds.

Around 11.05 a.m., the continent-spanning task is successfully completed; Sunrise III is peering perfectly into the Sun. For the first time, the Sunrise III team now opens the cover that has protected the 1-meter main mirror inside the telescope until now. Sunlight falls onto the sensitive mirror and is transmitted from there to the scientific instruments mounted above. Now the commissioning and calibration of the instruments can begin. "No artificial light source can replace sunlight for these measurements," says Dr. Andreas Lagg.

The scientific and technical team has until spring of next year to complete the commissioning. When the snow masses beyond the Arctic Circle slowly melt, Sunrise III will travel by truck, disassembled into individual parts and securely packed in crates, to the launch site, Esrange Space Center, in Kiruna (Sweden).

On June 1st, the solar observatory will be ready for launch. As soon as the wind and weather conditions in Kiruna are favorable, the balloon filled with 850 kilograms of helium will lift the six-ton observatory to an altitude of 35 kilometers. From there, the wind will carry Sunrise III westward. After several days of flight, it will land by parachute in northern Canada.

Midnight Sun and UV radiation

"Because we are flying in summertime and beyond the Arctic Circle, Sunrise III will be able to look non-stop at the Sun during the entire flight," says Sunrise III Principal Investigator Prof. Dr. Sami K. Solanki of MPS, describing one of the mission's advantages. At an altitude of 35 kilometers, the observatory has left most of Earth's atmosphere behind; air turbulences barely obstruct the view. "Unlike ground based telescopes, Sunrise III has access to ultraviolet radiation from the Sun at this altitude," Solanki continues. This portion of the Sun's radiation originates primarily in its hot atmosphere and contains valuable information from this region. Because the ozone layer of Earth's atmosphere largely absorbs the Sun’s ultraviolet light, it is not available to ground based telescopes.

Sunrise III's instruments are specialized to process light from different wavelength ranges. In this way, Sunrise III simultaneously looks at a region more than 2000 kilometers high, extending from just below the visible surface of the Sun into the upper chromosphere. Better than any other observatory - whether in space or on Earth - Sunrise III's observational data from this region can be assigned to an exact altitude. This makes it possible to precisely track, for example, how small bursts of radiation, known as microflares, occur or how wave-like phenomena propagate in the solar atmosphere.

"Sunrise III can make an important contribution to understanding how the Sun manages to heat its outer envelope to unfathomable temperatures of more than one million degrees Celsius," Solanki says. "To answer this question, it takes a very special view of the Sun."

About the mission

The balloon-borne solar observatory Sunrise III is a mission of the Max Planck Institute for Solar System Research (MPS, Germany). Sunrise III looks at the Sun from the stratosphere using a 1-meter telescope, three scientific instruments, and an image stabilization system. Significant contributors to the mission are a Spanish consortium led by the Instituto de Astrofísica de Andalucía (IAA, Spain), the National Astronomical Observatory of Japan (NAOJ, Japan), the Johns Hopkins Applied Physics Laboratory (APL, USA) and the Leibniz Institute for Solar Physics (KIS, Germany). Other partners include NASA's Columbia Scientific Ballooning Facility (CSBF) and the Swedish Space Corporation (SSC). Sunrise III is supported by funding from the Max Planck Foundation.

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