REACHING THE SURFACE OF VENUS: 1970 Campaign objectives

The Venera 4, 5 and 6 missions were major successes for the Soviets, sending back detailed information while descending on their parachutes, as they were designed to, but they did not survive all the way to the surface. The controversy about the surface conditions in the wake of the Venera 4 and Mariner 5 missions was not resolved in time to permit modification of the Venera 5 and 6 probes to survive the temperature and pressure at the surface. It was decided to launch them anyway in order to obtain more information on atmospheric conditions, which they did well.

Spacecraft launched
First spacecraft:	Venera 7 (3V No.630)
Mission Type:	Venus Atmosphere/Surface Probe
Country і Builder:	USSR NPO-Lavochkin
Launch Vehicle:	Molniya-M
Launch Date ‘: 7 ‘ime:	August 17, 1970 at 05:38:22 UT (Baikonur)
Em ounier Da te l Time:	December 15, 1970
Outcome:	Successful, transmitted from surface.
Second spacecraft:	Cosmos 359 (3V No.631)
Mission Type:	Venus Atmosphere/Surface Probe
Country і Builder:	USSR/NPO-Lavochkin
Launch Vehicle:	Molniya-M
Launch Date: Time:	August 22, 1970 at 05:06:09 UT (Baikonur)
Outcome:	Failed to depart Karth orbit.

For the 1970 Venus opportunity the Soviets were determined to reach the surface. They now had unequivocal scientific proof that the pressure at the surface was about 100 bar and the temperature exceeded 450 C. But after years of controversy among scientists, the engineers were wary and they designed the new capsule to withstand 180 bar and a tempera lure of 540 C for 90 minutes. The additional mass required a significant reduction in the number of science instruments.

One mission was dispatched successfully but the other was a launcher failure. The entry capsule of Venera 7 was the first to reach the surface in an operational state and became the first successful planetary lander. Over the decade 1960 70 the Soviets had seventeen unsuccessful attempts at Venus missions, seven of which were intended to reach the surface. Now’ persistence had paid of!’. At the same time, their first Lunokhod rover was traversing the surface of the Moon.

Spacecraft:

Carrier spacecraft:

The carrier vehicle was unchanged from Venera 4, 5, and 6, but fewer instruments were carried to accommodate the larger mass of the entry system.

Entry vehicle:

The descent capsule was significantly modified as described above. In particular the entry system was made more egg-shaped to accommodate extra thermal insulation and a new’ shock absorber. A new’ spherical pressure vessel was used instead of the previous flat-capped hemisphere. The pressure vessel w as made of titanium, and to minimize weak points it had the fewest possible number of feed-throughs and welds. The temperature and pressure sensors w ere on the exterior of the shell, under the top hatch. The design was verified in a new test chamber at 150 bar and 540"C.

The goal was to reach the surface as fast as possible without losing the capsule in order to maximize its lifetime on the surface. A single parachute was used. On first opening, it was reefed by a cord wrapped around the shroud lines to limit its area to 1.8 square meters. Being smaller than the main parachute of previous missions, this would produce a faster descent. But the reefing cord was designed to melt at 200°C, deep in the atmosphere, opening the parachute to its full 2,5 square meters in order to achieve a soft landing. The parachute was built to survive the high temperatures at the surface. The capsule was pre-coolcd to -8°C prior to being released by the carrier spacecraft to maximize its survival time. The total design lifetime of the capsule was 90 minutes.

Launch mass: 1,180 kg (entry capsule 490 kg)

Figure 12.1 Venera 7 spacecraft.

Payload:

Carrier spacecraft:

1. Solar wind charged particle detector

Descentjlanding capsule:

1. Temperature, pressure and density sensors

2. Radio altimeter

3. Gamma-ray spectrometer

4. Doppler wind experiment

Due to the additional mass required to accommodate the very high pressure limit, the spacecraft had only the solar wind charged-particle detector and it was necessary to delete the atmospheric composition experiment and airglow photometer front the descent capsule. The aneroid barometer could measure pressures of 0.5 to 150 bar, and the resistance thermometer had a range of 25 to 540°C. Density was measured by an accelerometer during the entry phase. A gamma-ray instrument was added for measurement of surface rock type.

Mission description:

Venera 7 was dispatched successfully on August 17, 1970, and conducted midcourse maneuvers on October 2 and November 17. The second launch on August 22 failed

Figure 12.2 Venera 7 descent capsule.

to depart low Earth orbit when the fourth stage misfired. As a result of a sequencer problem and a power system failure the engine ignited late and shut down after only 25 seconds. This vehicle was designated Cosmos 359 by the Soviets and re-entered on November 6. Venera 7 initiated its planetary encounter activities on December 12 when the capsule batteries were charged by the solar panels. The internal equipment compartment was activated and chilled down to -8°C. The capsule was released at 04:58:44 UT on December 15. It struck the atmosphere at an altitude of 135 km at

11.5 km/s. By the time it was down to 54 km it had been slowed to 200 m/s, and a pressure reading of 0.7 bar triggered the deployment of the parachute just above the cloud layer. The capsule transmitted for 35 minutes during its descent in darkness. It survived the impact at 05:34:10 UT and a weak signal was received for another 23 minutes. The landing site was at 5°S 351 °E, where ir was 4:42 Venus solar time and the solar zenith angle was 117 degrees.

■ Parachute Deployed Figure 12.4 Doppler frequency plot for the Venera 7 descent capsule (from Don Mitchell).

This success was not immediately obvious, and it was originally thought that the capsule had failed to reach the surface. The 35 minute descent to the surface turned out to be a wild ride. After the first 13 minutes the reefing cord melted away and the parachute opened fully, just as it was meant to do. Six minutes later the parachute ripped, and over the next several minutes the descent rate increased and the capsule oscillated wildly as the rip extended. A few minutes before reaching the surface the parachute failed and the capsule fell freely. All of this was evident from the Doppler shift in the transmitter carrier frequency. The capsule hit the surface at 16.5 m/s, the signal disappeared into noise, and it was concluded that the capsule must have been destroyed. There was no immediate announcement of the success of Venera 7. After the New Year holiday, an expert in signal processing re-ran the data tapes and in all the noise found the barely perceptible signal from the capsule on surface. The signal strength had reduced to 3% at impact, returned to full strength for one second, then dropped back to 3% for the next 23 minutes before terminating. The capsule would seem to have bounced on impact and come to rest tilted at about 50 degrees to the vertical, aiming the radiation pattern of its antenna well off Earth and resulting in a very low received power. The team members were elated by this discovery. It may not have been a graceful touchdown, but it was another first for the Soviet planetary program.

The Venera 7 carrier spacecraft returned measurements on the upper atmosphere and ionosphere prior to breaking up in the atmosphere.

Results:

The Venera 7 probe measured the temperature of the atmosphere from an altitude of 55 km to the surface, but a commutator failure resulted in no pressure or altimetry information being transmitted. Initially il was thought the temperature measure­ments returned were internal but fortunately they did turn out to be atmospheric, and when combined with Doppler data and with thermodynamic and aerodynamic modeling it was possible to construct a profile of temperature and pressure down to

the surface. Altitude profiles of; horizontal wind speed and direction were also obtained from the Doppler data and aerodynamic modeling. There was a fast wind at high altitudes in the same retrograde direction as the axial rotation. This con finned astronomical evidence from ultraviolet cloud flow that the upper atmosphere was ‘super-rotating*. Wind speeds of less than 2.5 m s were measured at the surface. The probe temperature sensor oscillated between binary readings of 457 C and 474 C on the surface. The computed surface pressure was 92 bar. Doppler data at the moment of touchdown, plus the fact that the capsule survived the high speed impact, implied that the surface was harder than sand but no harder than pumice. No surface composition measurements were returned because of the stuck commutator.