Mission Overview

Venera 11 was part of a two-spacecraft mission to study Venus and the interplanetary medium. Each of the two spacecraft, Venera 11 and Venera 12, consisted of a flight platform and a lander probe. Identical instruments were carried on both spacecraft. The flight platform had instruments to study solar-wind composition, gamma-ray bursts, ultraviolet radiation, and the electron density of the ionosphere of Venus. The lander probe carried instruments to study the characteristics and composition of the atmosphere of Venus.

Venera 11 was launched into a 177 x 205 km, 51.5 degree inclination Earth orbit from which it was propelled into a 3.5 month Venus transfer orbit. After ejection of the lander probe, the flight platform continued on past Venus in a heliocentric orbit. Near encounter with Venus occurred on December 25, 1978, at approximately 34,000 km altitude. The flight platform acted as a data relay for the descent craft for 95 minutes until it flew out of range and returned its own measurements on interplanetary space. The platform was equipped with a gamma-ray spectrometer, retarding potential traps, UV grating monochromator, electron and proton spectrometers, gamma-ray burst detectors, solar wind plasma detectors, and two-frequency transmitters.

Launch Information
Launch Date: 1978-09-09 at 03:25:39 UTC
Launch Vehicle: Proton Booster Plus Upper Stage and Escape Stages
Launch Site: Tyuratam (Baikonur Cosmodrome), U.S.S.R

Trajectory Details
Type: Flyby
Central Body: Venus
Closest approach time: 1978-12-25 00:00:00 UTC

Orbital Parameters
Periapsis 6.619999885559082 RV
Apoapsis
Period
Inclination 0.0°
Eccentricity 0.0

Instrumentation

Gamma-Ray Spectrometer

The objectives of this investigation were to measure solar and cosmic gamma-ray bursts, to accurately measure their position in conjunction with measurements from other spacecraft, and to determine the energy spectra and temporal charcteristics of the bursts. The instrumentation consisted of two scintillation detectors. One pointed towards the sun; the other pointed at 180 deg from the first. The detectors measured 0.08 to 2.5. MeV energy loss in 7 channels. The detectors had a sensitivity of 5.0E-6 ergs/sq cm for each gamma-ray burst detected.

Retarding Potential Traps

The objective of this investigation was to study the energy spectra of the ion and electron components of the solar wind at varying distances from the sun. The instrument was a retarding potential analyzer which measured ions from 0 to 4.5 keV and electrons from 0 to 300 eV. The detector had a flux sensitivity of 3.0E5 to 3.0E9 particles/sq cm-s. It was operated at intervals during the mission.

UV Grating Monochromator

The objectives of this investigation were to measure scattered UV radiation from interplanetary space and Venus by analyzing spectra lines at 304, 584, 736, 869, 1048, 1216, 1300, 1356, and 1500 A. Determinations of line spectra for H, He I, He II, O I, Ne I, Ar I, and CO were made when the spacecraft was close to Venus. Line intensities for H, He I, and He II were determined while the spacecraft was in interplanetary space. The detector consisted of a multichannel grating monochromator with the optical axis oriented in the anti-solar direction. This investigation was operated at selected intervals during the mission including a scan of the solar-illuminated disk of Venus.

Electron and Proton Spectrometer

The objectives of this investigation were to measure the spectra and angular distribution of electrons and protons in the solar wind. It used proportional counters, Geiger counters, and semiconductor and scintillation detectors. Electrons from 5 to 500 keV and protons in two ranges, 0.05 to 1 MeV and 30 to 200 MeV, were measured. The instrumentation had a dynamic range up to 5.0E5 particles/sq cm-s-sr.

Gamma-Ray Burst Detectors

The objective of this investigation was to determine the coordinates of gamma-ray bursts to within 2-3 deg. The instrumentation consisted of six identical scintillation detectors with their orientation along the geometric axis of the spacecraft. They had a measurement range of 20 to 300 keV with a sensitivity of 1.0E-6 ergs/sq cm-s.

Proton Spectrometer

The objectives of this investigation were to study proton acceleration in the interplanetary medium and the solar-activity processes involved in the origin of charged particles. The instrumentation consisted of a semiconductor spectrometer with a Si n-p detector. It had 10 energy channels covering from 0.1 to 100 MeV, and was sensitive to a flux › 1.0E4 protons/sq cm-s at 10 MeV.

Two-Frequency Transmitters

The objectives of this investigation were to study the electron concentration distribution in the ionosphere of Venus and to study fluctuation of electron concentration in interplanetary and near-sun plasmas. This investigation used radio transmissions in the centimeter and decimeter range.

Solar Wind Plasma Detectors

The objective of this investigation was to measure the energy spectra of the solar wind ion and electron components. It also measured separately protons and alpha particles at varying distances from the sun. The investigation used electrostatic analyzers and a cylindrical Faraday cup. Electrons were measured from 10 to 200 eV in 24 steps, total ion concentrations from 0.25 to 5 keV in 24 steps, protons from 0.25 to 5 keV in 24 steps, and alpha particles from 0.5 to 10 keV in 24 steps. Spectral measurements took 192 s. The flux sensitivity was 5.0E7 to 1.0E10 particles/sq cm-s. The instrument was operated at intervals during the mission.

Descent Craft

The Venera 11 descent craft carried instruments designed to study the detailed chemical composition of the atmosphere, the nature of the clouds, and the thermal balance of the atmosphere. Separating from its flight platform on December 23, 1978 it entered the Venus atmosphere two days later at 11.2 km/sec. During the descent, it employed aerodynamic braking followed by parachute braking and ending with atmospheric braking. It made a soft landing on the surface at 06:24 Moscow time on 25 December after a descent time of approximately 1 hour. The touchdown speed was 7-8 m/s. Information was transmitted to the flight platform for retransmittal to earth until it moved out of range 95 minutes after touchdown.

It is unknown whether the Lander Probe carried an imaging system. No mention of it occurs in the Soviet literature examined by the author. Two other experiments on the Lander did fail, and their failure was acknowledged by the Soviets. Some U.S. literature on the subject notes that the imaging system “failed” but did return some data. Among the instruments on board was a gas chromatograph to measure the composition of the Venus atmosphere, instruments to study scattered solar radiation and soil composition, and a device named Groza which was designed to measure amospheric electrical discharges. Results reported included evidence of lightning and thunder, a high Ar36/Ar40 ratio, and the discovery of carbon monoxide at low altitudes.

Descent Craft Instrumentation

Mass Spectrometer

The object of this experiment was (1) the detection and determination of the relative concentrations of Noble gases in the Venusian atmosphere, (2) determination of the isotopic composition of the Noble gases, and (3) detection of small amounts of such materials as ammonia, methane, and sulfur-containing compounds. The mass spectrometer was of the radio-frequency type with a range of 11-105 atomic mass units. The resolution (M/delta M) at 10% of peak amplitude was about 35. The volume sensitivity limit for molecular nitrogen was 0.2%, for molecular oxygen 0.05%, for inert gases 1-5 ppm, and for methane 5 ppm. The mass spectrometer cycled approximately every 8 s and acquired a total of 176 spectra (Venera 11 + 12). The instrument was operated in five gain modes: 0.1, 1, 10, 100, and 1000. The experiment was active during the Venusian descent, from altitudes of 23 km to 1 km above the surface. A source of systematic errors in the determination of minor atmospheric components such as nitrogen and argon was the error in the composition of the preflight model gas mixtures. The systematic error is estimated at about 20%.

Gas Chromatograph

The object of this experiment was the quantitative determination of the chemical structure of the Venusian atmosphere using a gas chromatograph. The gas sample was analyzed by three columns and detectors in series. The first column, filled with Polysorb, served to separate sulfur-containing compounds and the moisture in the carbon dioxide. The second column, filled with molecular sieves, served to separate the low-boiling point gases such as helium, hydrogen, argon, oxygen, nitrogen, krypton, methane, and carbon monoxide. The third column, filled with reduced manganese, served to separate argon. Neon was used as the carrier gas and hence can not be determined. The sensitivity threshold of the instrument was between 1E-5 and 1E-3 volume percent. The gas chromatograph cycled every 6 minutes. The experiment was active during the descent from 42 km to the Venusian surface. Eight analyses of the atmosphere were completed.

Spectrophotometer and Scanning Photometer

This experiment was designed to gather information on the spectral composition of scattered solar radiation in the Venusian atmosphere. The upward-looking spectrophotometer continuously scanned the spectrum in the visible and near-infrared regions (0.43-1.17 micrometers) with a wavelength resolution of 0.022 micrometers. The scanning photometer made a circular scan in the vertical plane, recording the radiation in four broad wavelength ranges. The experiment was active during the descent at altitudes below 64 km and at the surface, making more than 500 measurements. The instrument was calibrated during descent by means of miniature incandescent lights mounted in both instruments.

Electrical Activity

This experiment was designed to search for natural electrical discharges (i.e., lightning) in the atmosphere of Venus. A super long wavelength (8-95 kHz) radio receiver with an external loop antenna of 25 cm diameter served as the sensing instrument. Four frequency channels, at 10, 18, 36, and 80 kHz with bandwiths of 1.6, 2.6, 4.6 and 15 kHz, respectively, were monitored. The data were processed with an onboard acoustical receiver and a spectroanalyzer. The threshold sensitivity at 10 kHz was 0.3 microvolts, at 80 kHz it was 1 microvolt. The spectroanalyzer had a dynamic range of 90 dB. The dynamic range of the acoustical receiver was 74 dB. The instrument operated during the descent below 60 km and on the surface of Venus. A possible source of error was charging of the spacecraft during descent.

Chemical Composition of Aerosols

This experiment was designed to determine the chemical composition of aerosols in the atmosphere of Venus via x-ray fluorescence. Aerosols were collected on fibrous cellulose acetate filters exposed to the atmosphere during descent. The filters collected particles with a radius greater than 0.4 micrometers with 98% efficiency. The instrumentation consisted of an x-ray detector and a multichannel amplitude analyzer. Fluorescence emission was excited by Fe-55 and Cd-109 sources. Xenon, Ti, and Mn were used for calibration. The experiment was conducted in a carbon dioxide atmosphere at 20 degrees centigrade under Venusian ambient pressures. Two spectra were obtained by the instrument, one in the energy range of 1-7.5 keV and a second from 1.5-15 keV. The experiment functioned from an altitude of 64 to 49 km above the surface, where the filter melted. Elements with Z less than 13 were not detected with this experiment.

Temperature and Pressure Profiles

This experiment was designed to determine the temperature and pressure of the atmosphere of Venus from an altitude of 105 km to the surface. Temperature and pressure were measured indirectly from 105-70 km by means of inertial load measurements during aerodynamic braking. Temperature and pressure were measured directly at altitudes below 50 km. Four manometers of the aneroid barometer type, four platinum resistance thermometers, and an accelerometer were used to conduct this experiment.

Science

The data produced a spectrum of the diffuse background over the energy range 14-200 keV.

Summary