HASI (Huygens Atmospheric Structure Instrument)

HASI which stands for Huygens Atmospheric Structure Instrument was a multisensor package designed to measure the physical quantities characterizing Titan's atmosphere onboard the HUYGENS lander. The probe was part of the NASA/ESA/ASI mission CASSINI devoted to study Saturn and his moons that was designed to enter and brake in Titan's atmosphere and parachute a fully instrumented robotic laboratory down to it's surface. The mission, launched in October of 1997, reached Saturn's orbit on July 2004. Orbital entry of Huygens occured on January 15, 2005. The probe studied the clouds, atmosphere, and surface of Titan in its descent.

Between 1995 and 2004, as part of the research program in preparation for the mission, different agencies participating of the program performed a series of balloon-borne missions using different mockups of the probe, and specially built gondolas for testing different sub-systems and sensors. Also several stratospheric drop tests were made to simulate orbital entry into Titan's atmosphere.

The objective of this first flight of a series of three, was to perform a drop test from a balloon over Sicily, Italy. For this purpose scientists from the University of Padova, designed and built a specially engineered probe, similar to the one launched to Titan suitable for a balloon launch and capable of collecting all HASI scientific measurements during the parachute drop in the Earth atmosphere. The main goal was to check HASI operations at instrument level and to verify sensors performance during a realistic functional test in dynamic and environmental conditions similar to those foreseen for the Titan's descent. The test would also provide real data for the exercise descent trajectory reconstruction software and to investigate possible aerodynamic disturbances on pressure and temperature measurements induced by the Huygens probe configuration. Furthermore, data from balloon flight campaigns had a secondary scientific aspect, since they provided a local detail description of physical properties of the Earth's atmosphere: density, pressure and temperature vertical profiles along with magnetic field data.

At left above we can see an scheme of the balloon probe. The 1:1 scale mock-up was developed trying to keep both weight and costs at an acceptable level. The main structure was a platform composed of two circular plates of high resistant aluminium alloy, which represented a good trade off between structural properties and mass reduction. Both plates were kept at 50 mm distance gap by nine poles. This main structure was connected with the metallic plate on top of the probe by three poles, that ensured thermal and electric junction among all parts of the system. On the bottom of the main platform was located the battery plate, hosting the main power supply, that in the flight configuration was sheltered in a safe position by surrounding polyurethane foam.

On the border of the main platform an external aluminium ring was fixed, where, external instruments and three box-like extensions were attached in a 120º configuration. These extensions were parts of the Separation Subsytem which in the real mission provided mechanical attachment and separation from the Orbiter. The lower part of the probe showed a set of vans, whose function was to induce a rotation rate similar to the one envisioned for the Titan descent.

The overall external dimensions of the mock-up were 1304 mm of diameter and 631 mm of height while the internal allocated volume for the payload was approximately 0.3 cubic meters. Weight of the mock-up was close to 100 kg and in order to have maximum parachute efficiency a ballast of six lead bricks, having a total 66 kg mass, was added before launch.

A telemetry box linked the probe to the parachute and balloon. It consisted of an aluminum tubular structure measuring 0.8 m x 0.8 m x 1.0 meters covered with expanded polyestyrene insulating panels and housed the radar transponder, the telecommand antenna, the GPS system and a VAISALA meteorological sonde. Most of the box volume was occupied by the ballast canister containing 100 kg of small lead spheres.

The instruments on board this first mission were: accelerometers, pressure and temperature sensors belonging to the HASI probe, one tilt sensor, a three axis magnetometer, the beagle 2 UV sensor and additional housekeeping sensors.

Balloon launched on: 6/15/2001 at 6:19 utc
Launch site: Base di Lancio Luigi Broglio, Trapani, Sicily, Italy  
Balloon launched by: Agenzia Spaziale Italiana (ASI)
Balloon manufacturer/size/composition: Zero Pressure Balloon Raven - 100.000 m3
End of flight (L for landing time, W for last contact, otherwise termination time): 6/15/2001 at 9:02 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): ~ 3 h
Landing site: In southern Sicily, Italy
Payload weight: 300 kgs
Overall weight: 1400 kg

The balloon was launched by dynamic method at 6:19 utc on June 15th, 2001 from the balloon launch base of Trapani in Sicily. After a nominal ascent phase the balloon reached an altitude of 26.7 km from where the drop test was performed. The payload touched ground in southern Sicily at 9:02 utc.

Althought the flight proceeded flawlessly and the payload was recovered almost intact on ground, the acquisition system failed. However, temperature and pressure profiles during ascent and descent phases were reconstructed from the telemetry GPS positioning and velocity of the system. That allowed the scientific team to perform and improve models for the reconstruction of the Huygens probe's descent in Titan's atmosphere and for atmosphere and wind analysis.

• HASI Stratospheric Balloon Experiments CISAS - Centro Interdipartimentale di Studi e Attività Spaziali (via Archive.Org)
• The Huygens Atmospheric Structure Instrument (HASI): expected results at Titan and performance verification in terrestrial atmosphere 2nd International Planetary Probe Workshop, April 1, 2005