GUSTO (Galactic / Extragalactic ULDB Spectroscopic Terahertz Observatory)

GUSTO (Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory) is a NASA led balloon-borne mission aimed at investigating interstellar processes within our galaxy and the Large Magellanic Cloud (LMC) using a telescope equipped with high-resolution terahertz receivers to map these regions in key interstellar lines, specifically [CII] at 158 µm, [OI] at 63 µm, and [NII] at 205 µm. The complex experiment was born on the heritage of previous balloon-borne instruments, mainly the STO or Stratospheric Terahertz Observatory. The collaboration behind the initiative includes Johns Hopkins University providing the gondola, the University of Arizona which is responsible for the telescope and instrument which incorporated detection technologies developed by NASA's Jet Propulsion Laboratory, the Massachusetts Institute of Technology, Arizona State University, and SRON Institute for Space Research from the Netherlands.

At left we can see an artistic rendering of the GUSTO payload in flight (click to enlarge). GUSTO's instrument system features a 0.9-meter Cassegrain telescope on a high-altitude balloon platform, optimized for detecting terahertz spectral lines of [CII], [OI], and [NII]. This telescope includes a primary mirror made from low-expansion borosilicate glass, shaped to handle temperature fluctuations during flight, and a secondary mirror constructed from diamond-turned aluminum, mounted within a carbon fiber reinforced polymer support structure to reduce distortion. Thin titanium flexures further stabilize the primary mirror's alignment, limiting flexure errors to within 10 arcseconds.

The optical system incorporates calibration and flip mirrors, with frequency-selective surfaces that direct specific terahertz frequencies to corresponding mixer arrays. Light from the telescope is combined with local oscillator (LO) beams using dielectric beam splitters, guiding the signals to cryogenically-cooled mixer arrays housed in a cryostat. Three 8-pixel heterodyne mixer arrays, each dedicated to a targeted spectral line, down-convert terahertz signals to intermediate frequencies (IF) between 1 and 3 GHz. Low-noise amplifiers (LNAs) enhance these signals, employing niobium nitride bridges to improve sensitivity and provide the wide bandwidth needed for velocity measurements.

Each mixer array has a dedicated LO, including a quantum cascade laser (QCL) for the 4.7 THz line. The QCL is cooled to maintain narrow linewidths essential for accurate [OI] emission detection. This LO setup includes a Fourier phase grating that divides the QCL output into eight paths, aligning with each pixel in the mixer array. For the [CII] and [NII] lines, the LO employs Schottky diode sources operating at room temperature, with frequency locking for precise measurements.

Signal processing is managed by digital autocorrelator spectrometers connected to each mixer pixel. These spectrometers, organized into modules, translate signals into spectral data across thousands of channels, then transmit the data to a central computer via Ethernet. The spectrometers are constructed with HIFAS ASICs, which offer resilience to noise and high dynamic range, while the modular design supports scalable integration across all three frequency bands.

The cryogenic system, essential for reducing thermal noise, includes a 150-liter helium tank and vapor-cooled shielding, with temperatures maintained by a Stirling cryocooler. This cooling setup stabilizes the system for up to 75 days, while an active vibration-damping mechanism minimizes disturbances, ensuring stable alignment and reducing observational noise.

Balloon launched on: 12/31/2023 at 6:35 utc
Launch site: Williams Field, McMurdo Station, Antarctica  
Balloon launched by: Columbia Scientific Balloon Facility (CSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon Aerostar - SF 39.57 (39.500.000 cuft)
Flight identification number: 736N
End of flight (L for landing time, W for last contact, otherwise termination time): 2/26/2024 at 23:24 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 57 d 7 h 38 m
Landing site: 380 km S of Mawson Station, Antarctica

The balloon was launched from McMurdo station on December 31 2023 at 6:35 UTC as mission 736N. In the following weeks the balloon performed two complete turns to the south pole without altering the typical route followed by the NASA balloons launched in the austral summer. However, during the third turn, the balloon started to follow a more erratic flight pattern consistent with the breaking of the circular pattern of the polar vortex and also with the increasingly pronounced changes in altitude that the balloon experienced when the sun began to gradually settle lower and lower on the horizon.

Finally, the mission was terminated on February 26, and the payload landed 300 miles South of Mawson Research Station. GUSTO established a new record for a NASA heavy-lift, long-duration balloon mission remaining aloft for 57 days, 7 hours, and 38 minutes.

• GUSTO website at NASA
• GUSTO website at SRON
• 4x2 Hot electron bolometer mixer arrays for detection at 1.46, 1.9 and 4.7 THz for a balloon borne terahertz observatory ArXiv Physics - Instrumentation and Detectors
• Balloon Telescope GUSTO lands on Antarctica after record-breaking flight Delft University of Technology website
• GUSTO Balloon Observatory Mission Ships Out in Preparation for Antarctic Launch Johns Hopkins University APL Press Release
• NASA Balloon Mission Designed to See the Space Between Stars JPL press release
• NASA's Balloon Mission GUSTO Mapping the Space Between the Stars Johns Hopkins University APL Press Release
• NASA's GUSTO balloon telescope will map part of the Milky Way The Verge Magazine
• Thermal Design, Analysis, and Testing of GUSTO 2020 International Conference on Environmental Systems