Venera 11 & Venera 12

Venera spacecraft at the Memorial Museum of Cosmonautics, Moscow

* Mission Overview

Venera 11 was launched on 9 September 1978. It, and its sister, Venera 12, which was launched on 14 September 1978, traveled to Venus with a separation of ~0.02 AU. Later, the separation increased to 0.5 AU. The Venera spacecraft could either be spin stabilized with a spin period of several hours, or 3 axis stabilized. In general, the satellite was spin\ stabilized before encountering Venus, and 3 axis stabilized thereafter. The Venera 11 mission was terminated in February 1980. The Venera 12 mission ended in April 1980.

* Instrumentation

There were 2 separate instruments aboard the Venera 11 & 12 satellites: the SIGNE 2 experiment and the KONUS experiment.

Veneras 11 and 12 were part of the French-Soviet collaboration SIGNE 2. The SIGNE 2 project consisted of French-built gamma ray detectors on the Soviet satellites Venera 11, Venera 12, and Prognoz 7. These 3 satellites then formed an interplanetary gamma-ray burst triangulation network. Two features made this program unique in its time: it was the first time that identical detectors had been used simultaneously for gamma-ray burst studies on separate spacecraft over interplanetary distances, and the experiments used the largest dedicated gamma-ray burst detectors put into orbit.

Both Veneras had two dedicated gamma-ray burst detectors, arranged in such a way that they viewed 4-pi steradians. Each detector consisted of a 4.5 X 3.7 cm NaI(Tl) crystal surrounded by an 8mm thick plastic anticoincidence jacket. The crystal and plastic were viewed from the side by photomultiplier tubes.

The gamma-ray burst detector operated in low time resolution "waiting" modes in the absence of a burst; low energy resolution spectra and higher energy resolution calibration spectra were transmitted in this mode. Typically, the data were accumulated into 120.0 s time bins, 1-31 energy channels. Detection of a burst triggered the storage of high time resolution count rates and spectral data. The trigger criterion was an excess count rate 8 sigma above normal in a 250 ms interval. The maximum time resolution was 1/512 s. The energy ranges examined for burst detection ranged from 50-270 keV to 600-3000 keV, depending on the detector mode. During a burst, there were 1-6 energy channels used, and, for a "typical" burst seen with Venera 11 & 12, the nominal energy range was 80-800 keV. The readout time was ~4 s. However, two successive bursts could only be detected if the interval between them was 3-20 minutes. In addition, the storage of information on-board the spacecraft limited the number of distinguishable bursts to 1 every 2 hours.

Also aboard the 2 spacecraft was the KONUS experiment. It consisted of 6 scintillation detectors, a set of 6 devices for gamma-ray burst detection against the current background, 6 counters to measure the count rate in each sensor, a 320-channel time analyzer, and a 128 channel PHA. Each scintillator was a NaI(Tl) crystal with 80 mm diameter and 30 mm thickness. The background was measured over the energy range 50-150 keV for count rate, and over the range 30 keV - 2 MeV for spectra. Eight seconds of preburst data was recorded when a trigger was issued, with a resolution of 0.25 s. The temporal profile of the burst was recorded in 3 ways: 2 s with 15.625 ms resolution, 32 s with 0.25 s resolution, and an additional 32 s with 1 s resolution. The PHA trigger mode was a 16 channel, quasi-logarithmic energy scale mode which measured 8 energy spectra in 8 successive intervals of 4 s each. The threshold sensitivity at the 6-sigma trigger level was ~4 X 10-7 erg/cm2.

* Science

A catalog of the Konus experiment results was published in a series of papers by Mazets et al. in 1981. Other results can be found in Mazets and Golenetskii (1981), where 85 bursts were discussed. Based on their logN-logS plots, they believed that bursts must be of galactic origin. The data showed that the total energy released by a typical gamma burst event was 1040-1041 ergs.

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