The Gondola for the Sunrise iii Balloon-Borne Solar Observatory

Sunrise iii is a balloon-borne solar observatory dedicated to investigating the physics governing the magnetism and dynamics in the lower solar atmosphere. The observatory is designed to operate in the stratosphere, at heights around 36 km (above 99% of Earth’s atmosphere), to avoid image degradation due to turbulence in the Earth’s lower atmosphere, to gain access to the NUV wavelengths down to 309 nm, and to enable (when flown during summer solstice) observing the Sun uninterruptedly 24 hours/day. It is composed of a balloon gondola (equivalent to a spacecraft bus) carrying a 1-m aperture telescope (the largest solar telescope to-date to fly in the stratosphere on a balloon) feeding an imaging vector magnetograph and two spectropolarimeters aiming at acquiring high spatial resolution high cadence time series maps of the solar vector magnetic fields, plasma flows, and temperature in the photosphere and chromosphere.

In July 2024 Sunrise iii successfully completed a six and a half days long stratospheric flight from Kiruna (Sweden) to Northern Canada at an average altitude of 36 km. This was the third successful flight of the Sunrise observatory, which had previously flown in 2009 and 2013. For this flight it was upgraded substantially with a new and improved suite of three instruments carried by a completely new gondola with upgraded pointing control system.

This article focuses on describing the design and flight performance of the Sunrise iii gondola and all its subsystems. It describes the gondola mechanical structure, its power system, its command and control system, and in particular its pointing control system which was key for achieving high spatial and spectral resolution images of the solar photosphere and chromosphere by the three instruments.