Description of the payload
In 1983 a group of scientists proposed to the Swedish National Space Board the construction of a general purpose stabilized balloon platform capable of carrying a 200 kg payload for stratospheric flights lasting up to 12 hours. It was built by the Swedish Space Corporation and was used to perform astronomical observations of the interstellar medium in the far infrared spectral range in eight missions spanning the 15 years of duration of the project.
The platform base -see image at left- on which instruments and telescope were mounted was octagonal shaped. The entire gondola had a width of 3 meters and a height of 2.9 meters, weighting less than 500 kg. It's mechanical design was planned keeping in mind that it would be used repeated times in several flights so was developed to avoid damage to vital parts of the instruments at landing. Thus, the central telescope mount was located inside a lifting yoke which was in turn surrounded by an impact protection cage. Crush pads mounted below the gondola were also included in each flight to absorb the landing shock.
The azimuthal stabilisation was achieved by means of a cold gas system that used compressed nitrogen carried in pressurised tanks located below the gondola. During the flight, the gas passed through a buffer tank at an intermediate pressure whose flow was controlled very precisely by four proportional valves and let out through pairs of nozzles. There were two separate systems, the first one with high power thrusters for coarse control and the second one with low power thrusters for fine pointing.
During the observational phase of the flight, the proportional valves received signals from an analogue control system which used the sun as a reference. This system was composed by position sensitive photodetectors and a projected light spot. There was a coarse sensor, which used a slit for projection, and a fine sensor, which used a lens. In addition there was an independent, redundant sensor. Since sun sensors of this type worked best as nulling instruments, the angle to the sun was established by measuring the rotation of the platform on which they were mounted.
To facilitate the orientation of the telescope two TV cameras were used, along with two inclinometers that provided the information on the vertical attitude of the gondola. At any time of the flight two balance weights could be ordered to move along a pair of perpendicular rails in order to restore balance.
The PIROG gondola got their power from lithium battery packs with a total capacity of 300 Ah. The gondola itself consumed approximatelly 150 watts leaving more than 300 watts to the experiments for a 10 hour flight. The electronics inside the gondola were contained in four racks with their own power supply.
Scientific and housekeeping data was collected by a Pulse Code Modulation (PCM) system in both analogue and digital form and converted to a bi-phase format before being sent to the ground by a P-band transmitter. Telemetry data was sent with a speed of 256 Kbits/s throught near 800 channels with a sampling rate of 20 Hz, and with a resolution 16 bits. The telecommand system received commands sent from the ground station and interfaced them to the control system. Finally the signals from the two TV cameras were sent one at a time to the ground by a S-band transmitter.
For all these communication systems there were an onboard backup unit or some kind of redundancy.
During his useful life, the platform was progresivelly improved. The initial model (MK I) carried a 30 cm telescope coupled with a cooled Fabry-Perot spectrometer, a second generation of the instrument (MK II) incorporated a 60 cm telescope with a heterodyne receiver, and the final incarnation of the platform used the same telescope along with a SIS receiver. Several other improvements were made specially on the onboard electronics.
Nowadays, the gondola is diplayed at a museum located in the European Space Range, near Kiruna.
Details of the balloon flight and scientific outcome
Launch site: Centre de Lancement de Ballons CLBA, Aire Sur L'Adour, Landes, France
Balloon launched by: Centre National d'Etudes Spatiales (CNES)
Balloon manufacturer/size/composition: Zero Pressure Balloon model 402z Zodiac - 402.000 m3
Balloon serial number: 402z Nº 71
Payload weight: 797 kgs
Gondola weight: 525 kgs
PIROG 8 -the ninth and last mission of the program- was the second one to be performed outside Sweden. The balloon was succesfully launched on September 25, 1997 from the CNES balloon base in Aire sur l'Adour, southern France.
The duration of the flight at float altitude (39.5 km) was 8 hours.
The PIROG 8 experiment consisted of a 60 cm diameter Cassegrain telescope, with a new SIS receiver developed for observations of the 425 GHz O2 line. The receiver was combined with a 400 channel auto-correlator with selectable resolution. In this flight the main observational target was the giant molecular cloud associated with NGC7538 and W51.
External references and bibliographical sources
- A Sensitive SIS Receiver at 424/440 GHz for the PIROG-8 Experiment: Performance and Considerations on Stability Submillimetre and Far-Infrared Space Instrumentation, Proceedings of the 30th ESLAB Symposium
- Chemistry and rotational excitation of O_2_ in interstellar clouds. I. Predicted emissivities of lines for the ODIN, SWAS, PRONAOS-SMH and PIROG 8 submillimeter receivers stronomy and Astrophysics, v.324, p.221-236
- Low interstellar abundance of O2 confirmed by the PIROG 8 balloon experiment Astronomy and Astrophysics, v.339, p.L81-L84 (1998)
- O_2 Search in the Milky Way : new upper limits from ground (POM 2) and balloon-borne (PIROG 8) experiments The Physics and Chemistry of the Interstellar Medium, Proceedings of the 3rd Cologne-Zermatt Symposium
- Results of the PIROG 8 Balloon Flight with an Embarked Experiment Based on a 425/441 GHz SIS Receiver for 02 Search 9th International Symposium on Space Terahertz Technology, 1998, p.253
- The PIROG Program 13th ESA Symposium on European Rocket and Balloon Programmes. p.57