ПІЕ FIRST SUCCESS AT VENUS: 1967

Campaign objectives:

By the end of 1965 the Soviets had failed in a total of sixteen launches to Venus and Mars. Ten of these failed attempts were aimed at Venus, including the most recent Venera 2 and 3 missions that had come so close to achieving their goals. Adding to the frustration was the fact that by this time the US had succeeded with close flybys at Venus in 1962 and at Mars in 1964. Nevertheless, the Soviets were encouraged by their near successes, and were determined to push on. Realizing that the US was to attempt another Venus flyby mission in 1967. the Soviets wanted to outdo them with two missions to pierce the cloudy veil of the planet and obtain new information on its mysterious atmosphere and surface.

Spacecraft launched
First spacecraft:	Venera 4 (IV No.310)
Mission Type:	Venus Atmosphere/Surface Probe
Country/ Builder:	1 JSSR/NPO-Lavoch ki n
Launch Vehicle:	Molniva-M
Launch Date: Time:	June 12, 1967 at 02:39:45 UT (Baikonur)
Encounter Date/ Time:	October 18, 1967
Outcome:	Successful.
Second spacecraft:	Cosmos 167 (IV No.311)
Mission Type:	Venus Atmosphere/Surfaее Probe
Country і Builder:	1 JSSR/NPO-Lavoch ki n
Launch Vehicle:	Molniya-M
Launch Date: Time:	June 17, 1967 at 02:36:38 UT (Baikonur)
Outcome:	Failed to depart Earth orbit.

After Venera 2 and 3 the robotic planetary program was transferred from OKB-1 to NPO-Lavochkin. Beginning in April 1965 Babakin decided not to send any more llyby missions to Venus after the 1965 campaign and Lavochkin began to revise the 3MV spacecraft for the 1967 window for this planet, concentrating heavily on entry and landing. Working from 3MV blueprints supplied by OKB-1 and insight drawn from the Venera 2 and 3 experience. Babakin’s engineers devised improvements to the thermal control and other systems. Lavochkin did more ground testing and built two new test facilities, one a thermal vacuum chamber completed in January 1967 to test the spacecraft under simulated flight conditions and the other a centrifuge rated at 500 G to test the entry and descent system. The first test of an entry probe in this chamber at the 350-450 G load expected for high angle Venus entries near 11 krn/s destroyed its internal components. As the earlier descent capsules would certainly not have worked, the design had to be modified. This revitalized effort was rewarded immediately with the USSR’s first truly successful planetary mission in 8 years of trying, with Venera 4 yielding in-situ data on the atmosphere of Venus. It began a new and much more fruitful era in the Soviet investigation of this planet.

Spacecraft:

Carney spacecraft:

These spacecraft were the first 3MV for Venera missions built by NPO-Lavochkin which, in particular, greatly improved the thermal control system that had caused so much trouble with Venera 2 and 3. The hemispherical fluid radiators on the ends of the solar panels were deleted and a new system of heat transfer pipes located behind the parabolic antenna, which itself served as a radiator since it faced in the opposite direction to the solar panels. Liquid coolant was abandoned in favor of gas coolant. The communication system was also improved and the omnidirectional antenna was replaced by low gain spiral cone antennas mounted on booms connected to the solar panels and angled in flight to keep Earth in the radiation pattern. As previously, the spacecraft had to be turned to aim its high gain antenna at Earth, but this was only during scheduled communications sessions and operations at Venus.

Like its predecessors. Venera 4 w? as 3.5 meters tall, the solar panels had a span of 4 meters and the parabolic high gain antenna w as 2.3 meters in diameter. The panels measured 2.5 square meters but. as previously, were sparsely populated with cells. The noticeable difference between Venera 4 and its predecessors in the 3MV series were the change in the solar panels to a more rectangular shape and the absence of the hemispherical radiators.

Entry vehicle:

For the 1967 mission the entry capsule was strengthened to resist stresses as high as 350 G and given an internal damper to reduce shock effects during entry and landing. At 1 meter in diameter it was 10 cm larger than the previous probes and nearly spherical with an ablative surface and a covered opening in the rear hemisphere for deployment of the parachute and antennas. It was the first of a series of entry probes which would be progressively better suited to survive the descent down to the surface. The internal mass distribution was bottom heavy to ensure the proper pointing on entry and aerodynamic stability during the descent. It was pre­cooled to -10 C by a system in the main module prior to separation, and operated a re-circulating fan thereafter. The capsule was intended to transmit atmospheric data

Figure 10.13 Venera 4 spacecraft, front and back views. These publicity photos do not show the thick ablative material on the entry system or the thermal blankets.

Figure 10.14 Venera 4 spacecraft diagram (from Space Travel Encyclopedia): 1. Carrier vehicle; 2. Star sensor; 3. Sun sensor; 4. Attitude control gas tanks; S. Earth sensor; fi. Magnetometer; 7. Parabolic antenna; 8. Omnidirectional spiral antennas; 9. Thermal radiator; 10. Solar panels; 11. Propulsion system; 12. Attitude control microengines; 13. Cosmic ray detector; 14. Entry vehicle.

and radar data on descent, survive the impact and make measurements on the surface. The 28 amp-hour battery, which was rechargeable by the spacecraft during the cruise, could sustain 100 minutes of independent operation. The capsule design pressure was 10 bar with a margin up to about 18 bar, and the maximum survivable temperature for the parachute was 400°C.

The Venera probes were targeted to the center of the planetary disk as seen from Earth for optimum communications directly back to home. A helical antenna on top of the descending capsule was used to direct a radiation pattern to the zenith, and the telemetry was sent at 1 bit/s on 922,8 MHz using a pair of redundant transmitters. Measurements were sent back every 48 seconds. If the capsule w’ere to splash down in an ocean, which few people believed was likely, it would float and a ‘sugar seal’ would release a semaphore signal to signify this fact.

Figure 10.15 (left) show’s the entry vehicle without its upper insulation layers. The two ports arc for testing the insulation system on this engineering model. Inside the thick, porous and lightweight ablative material is ihe descent capsule itself shown in Figure 10.15 (right). Hanging out over the side are the radio altimeter antennas that spring out when the parachute deploys. In accordance with international regulations, the capsule was sterilized prior to launch.

Five levels of redundancy w’ere provided to ensure separation from the spacecraft. First by direct command from Earth, second by the on board sequencer, third by the triggering of a G switch on atmospheric entry, fourth by a sensor activated if Earth communications were interrupted by reorientation on entry and, as a last resort, the bands attaching the capsule to the spacecraft would burn through during initial entry.

Launch mass: 1,106 kg

Entry vehicle mass: 383 kg

Figure 10.15 Venera 4 entry system and enclosed descent capsule.

Figure 10.16 Venera 4 descent capsule diagram (from Space Travel Encyclopedia): 1. Outer heat shield; 2. Structural frame; 3. Probe walls; 4. Altimeter deployment system; 5. Heat exchanger; 6. Communication antenna; 7. Altimeter antenna; 8. Avionics unit; 9. Battery; 10. Insulation; 11. Shock absorber.

Payload:

Carrier spacecraft:

1. Triaxial fluxgatc magnetometer

2. Solar wind charged particle detector

3. Lyman-alpha and atomic oxygen photometers

4. Cosmic ray gas discharge and solid state detectors

It had the same instruments as the Venera 2 and 3 cruise modules, except that the cosmic ray instrument included a second gas discharge counter of a different type.

Descent I landing capsule:

1. Temperature, pressure and density sensors

2. Atmospheric chemical gas analyzers

3. Radio altimeter

4. Doppler experiment

The temperature, pressure and density sensors were the same as on Venera 3. The gas analyzers used eleven cells to measure carbon dioxide, molecular nitrogen.

molecular oxygen and water vapor. The composition was identified by how the atmosphere reacted with the material in each cell, such as by the electrical conductivity of chemically absorbing surfaces; or by reactive heated filaments; or by how the internal pressure varied with specific absorptive materials. The experiment was to take a set of readings at parachute deployment and then again 347 seconds later. The instrument wras the same as flown on Venera 3 but included a hydrometer Гог water vapor measurement. A radio altimeter was carried for the first time to obtain absolute altitudes and confirm landing on the surface. The system w:as built by the Research Institute for Space Device Engineering and adapted from one used in aircraft. To conserve bandwidth, it did not issue continuous data, but only a semaphore to indicate falling through the altitude of 26 km. The Doppler experiment required no hardware on the capsule, utilizing the frequency shift of the carrier wave of the transmitter to determine the line of sight velocity of the probe as it descended through the atmosphere.

Some of the instruments carried by previous probes had to be deleted in order to release mass for the radio altimeter and the structural strengthening. The gamma-ray instrument, wave motion sensor, and photometer were sacrificed. But, as always, it carried a medallion with the coat of arms of the USSR and a bas relief of Lenin.

Mission description:

The first spacecraft was launched successfully towards Venus on June 12. 1967, and became Venera 4. The second was stranded in parking orbit on June 17, when the fourth stage did not ignite because the turbopump had not been pre-cooled. It was named Cosmos 167 by the Soviets and re-entered 8 days later. Venera 4 performed well during cruise, reorienting itself every few days to point its high gain antenna at Earth for a communication session. A midcourse correction was made on July 29 at a range of 12 million km from Earth. It arrived at Venus on October 18 and released the entry capsule at 04:34 IJT, at which time it was 44,800 km over the night side. The carrier spacecraft sent measurements on the upper atmosphere and ionosphere until it broke up in the atmosphere. The capsule entered the atmosphere at 10.7 kiu/s and slowed through a peak deceleration of 350 G. At a pressure level of 0.6 bar and a speed of 300 m s it shed the rear cover and deployed the 2.5 square meter drogue parachute. Several seconds later it deployed the 55 square meter main parachute and radio altimeter antennas. The instruments were turned on at 55 km altitude, at w hich time the rate of descent was 10 m/s. The mechanical commutator interrogated each instrument in turn and fed the data to the transmitter. It transmitted for 93 minutes on its parachute descent before falling silent. It reached the surface at 19 N 38 E, in darkness near the morning tenninator. It was 4:40 Venus solar time and the solar zenith angle was 110 degrees. Including three intended test flights, this was the first successful Soviet planetary mission after twenty attempts, and the first successful entry probe by either spacefaring nation.

Jodrell Bank reported receiving signals from the surface, not realizing that these had been sent during the descent. Thinking the capsule had reached the surface in an

Figure 10.17 Venera 4 descent sequence (from Space Travel Encyclopedia)-. 1. Separation; 2. Unstabilized free flight; 3. Entry and stabilization; 4. Braking parachute deployed; 5. On main parachute, transmitter and altimeter on, acquiring and transmitting data on descent; 6. Surface impact, main chute release.

operational slate, the Soviets reported that it had landed. But it slowly became clear that this could not be the case. The data from Mariner 5, which flew by Venus a day after Venera 4 arrived, indicated that the surface temperature was much higher than the final measurement reported by the entry probe. A series of meetings by Soviet and American scientists conducted over the next 2 years decided that the probe had succumbed to the increasingly hostile environment and had been disabled while still far above the ground. Nevertheless, as the first mission to transmit data from within a planetary atmosphere it achieved a major scientific milestone. The data return was significant and demonstrated just how hostile was the environment of Venus. It was evident that future probes would have to be further strengthened.

Results:

During its descent the Venera 4 entry probe returned more than 23 sets of readings by the atmospheric structure experiment. They began at an altitude of 55 km, and the atmospheric temperature was measured over the entire 93 minute descent. The initial temperature was 33nC and it increased to 262nC. The initial pressure reading was 0.75 bar, and the instrument reached its limit of 7.3 bar long before the probe ceased to transmit. Using atmospheric models constructed from the data at the time, it was concluded that the signal was lost at an altitude of 24 km. Atmospheric density
was obtained by plugging the temperature and pressure data into the hydrostatic equation and the result tested against the parachute descent characteristics. Doppler data (i. e. changes in received master oscillator frequency) provided altitude profiles of wind speed and direction, both horizontal and vertical, but the measurement errors were large.

The atmospheric composition experiment showed the atmosphere to be composed mainly of carbon dioxide:

carbon dioxide molecular nitrogen molecular oxygen water vapor

The percentage of carbon dioxide was initially disputed because the expectation was that at least 50% of the atmosphere would be molecular nitrogen, and American scientists were skeptical. But later missions would prove Venera 4 correct. The arid nature of the atmosphere was also unexpected. The model of Venus as a watery world had to be completely scrapped.

The aircraft-derived radio altimeter was designed to send a signal semaphore at an altitude of 26 km, but it had not been adequately adapted for the Venus mission and actually sent its signal at twice that altitude, 52 km. This was a principal cause of the confusion over whether Venera 4 had reached the surface or not. Atmospheric data and Doppler measurements showed that the probe had descended through about 28 km during transmission and the altimeter semaphore indicated that the top level was 26 km. The last measured temperature of 262 C and the derived pressure of 18 bar were about what was expected at the surface at the time. However, measurements of the planet’s microwave brightness made by terrestrial radio telescopes had indicated values of about 325 C. The chemical analysis by Venera 4 showing the dominance of carbon dioxide required a reanalysis of the radio-telescope microwave brightness based on atmospheric models with less carbon dioxide. A new analysis in 1967 explained some of the unusual features of the microwave spectrum of Venus as due to carbon dioxide, and resulted in surface conditions of about 427 C and 75 bar that were inconsistent with the Venera 4 probe having reached the surface. Atmospheric models based on Mariner 5 data also showed far higher temperatures and pressures at the surface. One suggestion was that Venera 4 had landed on a large mountain, but Carl Sagan pointed out that radar studies of the planet had found no such large edifice. Extrapolation of the Venera 4 atmospheric profile indicated conditions at the surface at the impact site to be 500°C and 75 bar. Eventually the data wns reconciled by Avduevskv, Marov, and Rozhdestvensky (1969) using an adiabatic model of the Venusian atmosphere which confirmed loss of signal at 18 + 2.5 bar at an altitude of about 24 km and extrapolated conditions at the surface as 442 C and 90 bar.

The signal ceased near the pressure limit of the capsule, but it is possible that the probe exhausted its battery near the 18 bar level after 93 minutes of operation. In any case the capsule would have been crushed and thereafter the parachute would have burned, leaving the capsule to free fall to the surface.

Prior to breaking up, the main spacecraft provided the first in-situ measure­
ments of the close-in magnetic field, thermosphere, ionosphere, and solar wind interaction. In 1962 Mariner 2 had flown past Venus at 34,773 km. which was too great a range to detect a magnetic field or magnetospheric signature. Venera 4 found no intrinsic planetary magnetic Held. The low fields detected were due to interaction of the solar wind with the ionosphere. No radiation belts were found, and an extended corona of atomic hydrogen was discovered reaching 10,000 km into space from the planet.