Over the last half a century or so, NASA has managed to land operating payloads on a number of bodies in our solar system. Naturally the first of these was our closest celestial neighbor, the Moon, where NASA’s Surveyor 1 landed on June 2, 1966 (see “Surveyor 1: NASA First Lunar Landing”). A decade later was NASA’s Viking 1 which landed on Mars on July 20, 1976. Someone well versed in space history trivia could also cite NASA’s NEAR (Near Earth Asteroid Rendezvous) mission. After spending a year investigating the asteroid 433 Eros from orbit, NEAR was carefully brought down onto this asteroid’s surface on February 12, 2002 where it continued to transmit data for another week even though it was not designed for surface operations.

After NEAR, most people’s recollections begin to falter: one could cite Huygens which landed on Titan in 2004 but this was actually an ESA spacecraft (carried by NASA’s Cassini spacecraft) as was Philae which landed on the comet 67P/Churyumov–Gerasimenko in 2014. Then there was the tiny near Earth asteroid 25143 Itokawa which was visited in 2005 by Japan’s Hayabusa sample return mission. The only other world in the solar system which has been landed on is Venus whose surface exploration history is dominated by a series of well known Venera missions launched by the now defunct Soviet Union during the 1960s to 1980s (see “Venera 9 and 10 to Venus”). What some may find surprising is the NASA has also landed a spacecraft on the hostile surface of Venus which survived for over an hour – although it was never intended to do so.

Pioneer-Venus Spacecraft

NASA’s first (and so far only) Venus “lander” was actually one of a quartet of atmospheric probes which were part of the Pioneer Venus program. Run by NASA’s Ames Research Center, the Pioneer Venus program sent out a pair of spacecraft during the 1978 Venus launch window to study our nearest planetary neighbor with emphasis on its dense, hot atmosphere and its interaction with the Sun. These spacecraft were to continue the exploration of our hellish twin started by NASA’s three earlier Marnier flyby missions in 1962, 1967 and 1974.

Both Pioneer Venus spacecraft used a spin-stabilized bus built by Hughes Aircraft Company based on its HS-507 comsat. Each spacecraft was launched separately by an Atlas SLV-3D rocket employing a Centaur D-1AR upper stage. The common spacecraft design was squat cylinder 2.53 meters in diameter and 1.22 meter tall with the outer face covered with solar cells to provide power. The 533-kilogram Orbiter was fitted with a Thiokol Star-24 solid retrorocket that produced 17.8 kilonewtons of thrust to brake the spacecraft’s approach speed by 1,050 meter per second to enter an elliptical 24-hour orbit around Venus. The Orbiter carried 45 kilograms of instruments to study the atmosphere of Venus as well as a primitive radar system to make the first global maps of its hidden surface.

The second Pioneer Venus spacecraft was the 875-kilogram Multiprobe. This spacecraft carried a total of four atmospheric probes each protected by conical Deceleration Modules fitted with carbon phenolic heat shields: a single Large Probe with a mass of 316 kilograms and a trio of 93-kilogram Small Probes called Day, Night and North based on their targeted areas on Venus. The intent was for the Multiprobe carrier to release its probes some weeks before its encounter so that they could enter the Venusian atmosphere at scattered locations. They would then perform in situ measurements of the atmosphere and its clouds during a descent that would last almost an hour transmitting their data directly back to Earth. The probes would impact the surface at a speed of about 9 meters per second and were not expected to survive. The unprotected carrier was also equipped with instruments to study the composition of the upper atmosphere of Venus before it burned up 110 kilometers above the surface.

Each probe consisted of a pair of titanium alloy hemispheres bolted together to form a pressure vessel. The pressure vessels of the of the Small Probes were filled with xenon at Earth atmospheric pressure instead of dry nitrogen to help limit heat flow. They were each attached to a conical Deceleration Module with a diameter of 76 centimeters whose carbon phenolic ablative heat shield protected the probe during the worst of the entry heating. The pressure vessels of the Small Probes remained attached to their Deceleration Modules as they descended towards the surface in a free fall without a parachute. Each Small Probe carried 3.4 kilograms of instruments to study atmospheric structure, characterize the clouds and measure the net flux of energy coming down towards or up from Venus. Power was supplied by an 11 amp-hour silver-zinc battery and data were transmitted directly back to Earth some three-light minutes away using a ten-watt S-band transmitter.

The single Large Probe, which is sometimes referred to as the Sounder Probe, had a pressure vessel with a diameter of 73 centimeters protected inside a Deceleration Module with a diameter of 1.5 meters. Unlike the Small Probes, the Large Probe discarded its Deceleration Module after entry and deployed a parachute at an altitude of 64 kilometers to slow its descent through the planet’s cloud layers. The inside of the pressure vessel was lined with 2.5 centimeters of Kapton insulation to help limit heat flow. All the experiments and electronics were mounted on beryllium shelves and the vessel was pressurized with dry nitrogen.

The large probe carried 28.2 kilograms of instruments which performed measurements similar to those of the Small Probes plus additional equipment to study atmospheric composition and cloud properties. The Probe’s neutral mass spectrometer used an Intel 4004 chip making it the first NASA spacecraft to employ a microprocessor. After descending 19 kilometers over 16½ minutes, the Large Probe would jettison its parachute and freefall for another 39 minutes before impacting the surface. Precise radio tracking of the ten-watt S-band transmitter of this and the three Small Probes would allow scientists to precisely track the positions of the descending probes allowing winds and atmospheric density to be inferred.

The Mission

The first spacecraft of this series to be launched was the Pioneer Venus Orbiter on May 20, 1978 during the first day of its three-week launch window. Called Pioneer Venus 1 (and sometimes Pioneer 12) after being successfully sent on its way by Atlas-Centaur 50, the Orbiter followed a long Type II trajectory that took it out to an aphelion of 1.3 AU outside of Earth’s orbit before it started arcing towards Venus. This trajectory was chosen to minimize the approach velocity at orbit insertion 6½ months later.

Next was the Pioneer Venus Multiprobe which lifted off from LC-36A at 3:33 EDT on August 8, 1978. The Atlas-Centaur 51 worked nearly flawlessly to place what was now called Pioneer Venus 2 on a faster Type I trajectory that would reach Venus in four months. An initial trajectory error of 14,000 kilometers was corrected by a series of thruster burns on August 16 with a delta-v of 2.25 meters per second.

As the Multiprobe spacecraft approached Venus, preparations were begun to deploy its probes. At 2:37 GMT on November 16, the Large Probe was deployed while still 11.1 million kilometers and 23 days out from Venus. The three small probes were released at 13:06 GMT on November 20 some 9.3 million kilometers out from Venus. This release was precisely timed so that the 48 RPM spin of the Multiprobe carrier spacecraft spread out the Small Probes in a shotgun fashion to reach their entry targets. All four probes were powered down during their long coast towards Venus to conserve limited battery power. Timers would fully awaken the probes about 21 or so minutes before entry. After releasing the last of its probes, the Multiprobe bus performed a small maneuver to target its entry point over Venus.

The first to reach Venus was the Pioneer Venus Orbiter which initially entered an elliptical 23.4 hour orbit with a periapsis of 379 kilometers on December 4, 1978. The orbit was trimmed two days later to establish a 24-hour period. The first of the Multiprobe segment of the mission to reach Venus was the Large Probe which started entry into the atmosphere of Venus at 18:45:32 GMT on December 9, 1978. In just 38 seconds, the velocity of the Large Probe plummeted from 11,700 to just 200 meters per second with a peak braking load of 280-g. Shortly thereafter, the Large Probe deployed its parachute and pulled itself clear of its aeroshell to begin its descent science. Four minutes later, the Small Probes started hitting the Venusian atmosphere one after another for the next 6½ minutes experiencing peak braking loads of 200 to 565-g depending on the exact entry trajectory followed.

With all four probes now descending through the increasingly hot and dense atmosphere of Venus, the Large Probe cut lose from its parachute at 19:03:28 GMT to begin its 45-kilometer freefall to the surface below. All four probes transmitted data but some of the instruments experienced malfunctions because of electrical and/or chemical activity in the atmosphere. The first probe to hit the surface was the Large Probe at 19:39:53 GMT followed by the North Probe two minutes and 47 seconds later. Both probes went silent on impact as expected. Next came the Day Probe at 19:47:59 GMT then Night Probe four minutes and six seconds later. Unlike the first two probes, they did not go silent immediately upon impact. The Night Probe lasted for two seconds but the Day Probe kept on transmitting for seconds then minutes and longer.

The Day Probe had come down at 31.3°S, 317.0°E in the western part of a volcanic region now known as Dione Regio. The Sun was just 9.5° above the horizon at landing with the local time being the equivalent of 6:42 AM here on Earth. The surface pressure was measured to be 91.5 bars. Although the temperature sensor ceased working about 12 kilometers above the surface, the surface temperature was estimated to be 456° C. Data returned by the probe’s nephelometer indicated that dust had been raised by the impact which cleared after a few minutes. This hinted that the Day Probe had come down in some soft, dusty deposits which cushioned the impact of landing allowing it to survive to become NASA’s first Venus lander. While the pressure increased after landing, the increase was consistent with the rise in the interior temperature indicating that no leak had developed in the pressure vessel at impact.

Meanwhile, as the Day Probe continued to transmit data back to Earth at 16 bits per second, the Multiprobe carrier with its instruments hit the atmosphere of Venus and burned up as expected at 20:22:55 GMT some 110 kilometers above the surface. With the interior temperature slowly rising to 126° C, the Day Probe’s battery was finally exhausted at 12:55:34 GMT ceasing all transmissions after 67 minutes and 37 seconds – the longest any spacecraft had transmitted data from the surface of Venus up until that time. NASA’s first “mission” on the surface of Venus had come to an end.

With the end of the Multiprobe flight, the data analysis began as did the process of trying to make sense of what had been observed. These results were supplemented by data returned by the separate Soviet Venera 11 and 12 missions which deployed landers that came down on December 25 and 21, respectively (breaking the Day Probe’s fresh longevity record by transmitting from Venus’ surface for 95 and 110 minutes, respectively). The longest surviving element of the Pioneer Venus mission was the orbiter which continued to transmit data from Venus until October 8, 1992 with its orbit finally decaying just two weeks later. To this day, the Pioneer Venus program’s Day Probe remains the only NASA spacecraft to return data from the surface of Venus.

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Related Video

Here is an excellent NASA documentary on the Pioneer Venus mission entitled “The Venus Pioneers”.

Related Reading

“Venera 9 and 10 to Venus”, Drew Ex Machina, October 22, 2015 [Post]

“The Future That Never Came: Pioneer Saturn/Uranus Probes”, Drew Ex Machina, August 26, 2015 [Post]

General References

Richard O. Fimmel, Lawrence Colin and Eric Burgess, Pioneer Venus, NASA SP-461, 1983

Jim Loudon, “Pioneer Venus: A First Report”, Sky & Telescope, Vol. 57, No. 2, pp. 119-123, February 1979

Alvin Seiff et al., “Structure of the Atmosphere of Venus up to 110 Kilometers: Preliminary Results from the Four Pioneer Venus Entry Probes”, Science, Vol. 203, pp 787-790, February 23, 1979