Stratospheric Balloon Missions for High Resolution Submillimeter-FIR Astronomical Spectroscopy

Many spectral lines that are key tracers of star formation lie in the submillimeter and far-infrared. This wavelength range is almost completely blocked for astronomical observations by terrestrial water vapor, and studies must be carried out from stratospheric or space platforms. The termination of the NASA/DLR airborne observatory SOFIA and the lack of any space mission for at least the remainder of the decade leaves stratospheric balloons as the only platforms for carrying out observations throughout the 1000 – 5000 GHz frequency range. I will present the science goals and technology overview of two NASA Antarctic balloon missions that are being constructed to observe some of the most important fine structure lines of abundant elements. The Galactic/Extragalactic Ultra/Long Stratospheric Terahertz Observatory (GUSTO; C. Walker, [University of Arizona], PI) will observe lines of N+, C+, and O0. Employing a 90cm diameter telescope, GUSTO will map the central portion of the Milky Way and the Large Magellanic Cloud. The Astrophysics Stratospheric Telescope for High Spectral Resolution Observations at Submillimeter wavelengths (ASTHROS; J. Pineda, [JPL], PI), will observe both fine structure lines of ionized nitrogen. With a 2.5m diameter telescope, ASTHROS will have high angular resolution and be able to make detailed measurements of the electron density in ionized regions. Both GUSTO and ASTHROS employ heterodyne front ends using Hot Electron Bolometer (HEB) mixers and frequency-multiplied local oscillator sources to achieve high spectral resolution (R > 106). Both missions employ digital ASIC spectrometer systems to carry out the spectral analysis over a ~4 GHz-wide band. GUSTO is expected to be launched in December 2023, and ASTHROS a year or two later, ushering in a new era of studies of the role of feedback in controlling the rate of star formation.