SCIENCE ON THE SURFACE OF VENUS: 1972 Campaign objectives

The Venera 7 landing on the surface of Venus was a jubilant success for the Soviets. Once the pressure and temperature at the surface were finally confirmed, the NPO – Lavochkin engineers scaled back the pressure design limit from 180 bar to 105 bar and used the saved mass for a stronger parachute and more scientific instruments. In
anticipation of their next generation of larger more complex Venus landers, a photometer was added to determine the illumination at the surface. All of the previous entry probes had been targeted at the night-time hemisphere, mainly to ensure direct-to-Earth comm uni cations. but day-side light-level measurements were required in order to design imagers for future landers. So the 1972 missions were to land in early morning daylight at sites near the terminator from which it would still be possible to transmit to Earth. A redundant deployable antenna was ineluded as a precaution against poor primary antenna pointing or obscuration of the line of sight by rough terrain.

Spacecraft launched
First spacecraft:	Venera 8 (3V No.670)
Mission Type:	Venus Atmosphere; Surface Probe
Country і Builder:	USSR/NPO-Lavoehkin
Launch Vehicle:	Molniya-M
Launch Date! Time:	March 27, 1972 at 04:15:01 UT (Baikonur)
Em ounter Da tej Time:	July 22, 1972
Outcome:	Successful, transmitted from surface.
Second spacecraft:	Cosmos 482 (3V No.671)
Mission Type:	Venus Atmosphere/Surface Probe
Country і Builder:	USSR/NPO-Lavoehkin
Launch Vehicle:	Molniya-M
Launch Date ‘: і ime:	March 31, 1972 at 04:02:33 UT (Baikonur)
Outcome:	Failed to depart Earth orbit.

Two launches were attempted, the first successfully dispatching Venera 8 and the second stranding its spacecraft in parking orbit. Venera 8 was the ultimate success of the 3MV series and. as events transpired, the last of its type. It achieved all that the Soviets had worked so hard for over so many years and so many attempts. It was the final reward for dogged persistence. During its construction, NPO-Lavochkin was working on the new generation of Luna spacecraft to undertake sample return, rover and orbiter missions, and the or biters and landers for Mars, both of which would use the Proton launcher, so this was the final planetary campaign to employ the 8K78M Molniya.

Venera 8 supplied the data needed to design the much more sophisticated landers that would be delivered by the next generation of advanced Venera spacecraft to be launched by the Proton rocket beginning in 1975.

Spacecraft:

The carrier spacecraft for Venera 8 was essentially the same as for all missions since Venera 4, but the entry probe was modified. The pressure design limit was reduced to accommodate additional science instruments and the parachute was strengthened, although the size of the canopy was the same as for Venera 7 in order to make the same rapid descent through the atmosphere. Since the probe was to land further from the center of the planet as viewed from Earth, the antenna transmission pattern was changed from the egg-shape that was appropriate when Earth was at the zenith to a funnel-shape for when Earth was low on the horizon. In case the capsule were to come to rest on its side, a second antenna was provided that was to be ejected onto the surface and this was a flat disk with a spiral antenna on each side to enable it to work irrespective of how it settled.

A new honeycomb composite material was used as the primary insulation of the lander. Further thermal protection was provided by using lithium nitrate trihydrate, a phase-change material that absorbs heat by melting at 30’C. In addition to forming ‘thermal accumulators’ inside the pressure vessel, this jacketed the instruments that projected outside.

Launch mass: 1,184 kg

Entry capsule mass: 495 kg

Payload:

Carrier spacecraft:

1. Solar wind charged particle detector

2. Cosmic ray gas discharge and solid state detectors

3. Ultraviolet spectrometer for Lyman-alpha measurements

Figure 13.1 Depiction of Venera 8 deployed on the surface with ejected parachute and deployed second antenna (courtesy NPO-Lavochkin).

Figure 13.2 Venera 8 spacecraft in test at Lavochkin.

Descent I landing capsule:

1. Temperature, pressure and density sensors

2. Atmospheric chemical gas analyzer

3. Broad-band visible photometers (2)

4. Gamma-ray spectrometer

5. Radio altimeter

6. Doppler experiment

An accelerometer was to measure atmospheric density during the descent prior to parachute deployment. The altimeter had been redesigned to provide an accuracy of several hundred, meters for the instruments that would operate during the parachute descent. The atmospheric composition experiment now included an ammonia litmus

Figure 13.3 Venera 8 probe. Radio altimeter deployed at left, primary antenna in the center, secondary antenna and deployment mechanism at the right. Small cylinders on the rim are the two photometers, one on each side, and the gas analyzer.

test, and the atmospheric structure experiment carried four resistance thermometers, three aneroid barometers, and a capacitance barometer. A pair of single-channel broadband cadmium sulfide photometers were carried to measure the integrated downward flux with a 60 degree field of view in the wavelength range 0.52 to 0.72 microns. The optical unit was outside the capsule, mounted on top inside a separate unit scaled against high pressure and insulated against high temperature. The light reached the electronics by a 1 meter long light guide of fiber optic. The photometers were sensitive over the range 1 to 10,000 lux and encoded logarithmically.

The gamma-ray spectrometer was mounted inside the hermetically sealed probe. It was sensitive to emissions from potassium, thorium and uranium, and had been calibrated for these elements against a suite of Earth rocks.

Mission description:

Venera 8 was launched on March 27, 1972, made its midcourse correction maneuver on April 6, and arrived at Venus on July 22. The solar panels charged the batteries of the capsule and a system in the cruise module pre-cooled the capsule by circulating air through it at -15C’C. After being released 53 minutes prior to entry, the capsule hit the atmosphere at 11.6 km/s at 08:37 UT at an angle of 77 degrees on the sunlit side.

Figure 13.4 Venera 8 probe diagram: 1. Parachute housing cover; 2. Drogue parachute; 3. Main parachute; 4. Deployable radio altimeter antenna; 5. Heat exchanger; 6. Heat accumulator; 7. Internal thermal insulation; 8. Program timing unit; 9. Heat accumulator; 10. Shock absorber; 11. External thermal insulation; 12. Transmitter; 13. Pressurized sphere; 14. Commutation unit; 15. Fan: 16. Cooling conduit from carrier; 17. Deployable secondary antenna; 18. Parachute housing; 19. Primary antenna; 20. Electrical umbilical; 21. Antenna feed system; 22. Cover explosive bolts; 23. Telemetry unit; 24. Stable quartz oscillator; 25. Commutation unit.

approximately 500 km from the morning terminator. Eighteen seconds later it had slowed to 250 m/s and deployed its pilot parachute. The reefed main parachute opened at 60 km altitude and the canopy was fully opened at 30 km. The instruments were activated at 50 km and transmitted data during the 55 minute descent. There was a clear line of sight to Yevpatoria. The capsule thumped onto the surface at 10.70°S 335.25°E. It was 6:24 Venus solar time and the solar zenith angle was 84.5 degrees. The parachute was jettisoned on impact and the secondary antenna was

deployed onto the surface. The capsule transmitted for another 63 minutes reporting measurements on the surface, starting with a 13 minute stream from the primary antenna, then a 20 minute stream from the secondary antenna, and finally a 30 minute stream from the primary.

The Venera 8 carrier spacecraft returned measurements on the upper atmosphere and ionosphere prior to breaking up in the atmosphere.

The second spacecraft to be launched failed to depart from low Harth orbit due to a fourth-stage misfire when the failure of a timer caused the engine to stop after only 125 seconds. It was stranded in a highly elliptical orbit and designated Cosmos 482. At the end of June a fragment separated. This was probably the entry capsule, and it remained in orbit when the main spacecraft re-entered on May 5, 1981.

Results:

The Venera 8 capsule returned a wealth of data about the atmosphere and surface. It determined atmospheric density from accelerometer data in the altitude range 100 to 65 km and directly measured atmospheric temperature, pressure, composition and down-welling light flux from 55 km down to the surface. Although imprecise, these first profiles of the solar flux versus altitude were sufficient to confirm that the high temperatures were caused by the greenhouse effect. The illumination at the surface was measured and the pattern of change in attenuation attributed to clouds. Profiles of the speed and direction of horizontal winds from 55 km down to the surface were obtained from Doppler data. The wind speed was 100 m/s above 50 km, 40 to 70 m/s in the haze layer near 45 km, surprisingly rapid at 20 to 40 m/s below this down to 20 km, and only about 1 m s from 10 km to the surface. The wind was super-rotating coincident with the motion of the high ultraviolet clouds.

The first report from the radio altimeter was at an altitude of 45.5 km and it gave a total of 35 readings, the last at 900 meters. The capsule drifted 60 km horizontally as it descended. The altimeter produced a ground profile with two mountains 1,000 and 2,000 meters tall, a hollow 2.000 meters deep, and a gentle upward slope toward the landing site. Two echo intensity profiles were obtained from which it w as possible to compute the dielectric constant and a surface density of 1.4 g/ec. The photometers made 27 measurements, and the light level declined steadily from 50 to 35 km as the probe descended through the clouds. Venera 8 was the first to distinguish three main optical regions in the atmosphere: two cloud layers w’ith a thicker upper layer of fog from 65 to 49 km and a lower haze layer from 49 to 32 km. Then the light level was essentially constant to the surface, indicating a relatively clear atmosphere below the clouds. The illumination in this part of the atmosphere was comparable to a cloudy day on Harth at tw ilight. The w eak surface brightness indicated that only 1 % of the incident sunlight reached the surface. On the other hand, the Sun was only 5 degrees above the horizon. The important finding was that the illumination wras sufficient for the next lander to operate a camera.

The gas analyzer returned a composition of 97% carbon dioxide, 2% nitrogen.

0. 9% water vapor, and 0.15% oxygen. Although the ammonia test gave a positive detection at altitudes between 44 and 32 km with readings of 0.1 to 0.01 %. this was compromised by sulfuric acid which also reacts positively. A significant point is that the gas analyzer eon firmed the presence of sulfuric acid in the clouds. This had been offered as an explanation of why the clouds were so arid and yet were able to form cloud droplets. And the fact that such droplets would reflect sunlight so efficiently explained why the planet had such a high albedo.

On the surface, Venera 8 reported a pressure of 93 + 1.5 bar and a temperature of 470 ± 8’C. confirming the measurements by Venera 7 and in good agreement with an extrapolation of the data from the Venera 4, 5 and 6 probes down to the surface using models of the adiabatic temperature lapse rate.

The gamma-ray spectrometer made measurements in the descent, and tw o on the surface. It reported 4% potassium. 6.5 ppm thorium, and 2.2 ppm uranium indicative of a more granitic than basaltic composition. However. this result was contested and all later Venera landers found more common basaltic compositions. Radar mapping many years later showed that Venera 8 landed in an upland volcanic region that was probably older than the lava plains that constitute most of the planet. Alternatively, a potassium-rich basalt that is relatively rare on Earth could account for this particular data.