Pioneer Venus probes
Pioneer Venus probesTwo US probes, an orbiter and a multiprobe, launched in May and Aug. 1978, respectively, towards Venus. The orbiter, Pioneer Venus 1, entered a highly inclined elliptical orbit around Venus on Dec. 4 1978, approaching to within 150 km of the surface. It was designed to make observations over 243 days (Venus' rotational period) and do radar mapping of the surface, cloud studies at ultraviolet and infrared frequencies, and magnetometer studies.
The multiprobe, Pioneer Venus 2, released one large probe on Nov. 15 of 316 kg and three small probes on Nov. 19, each of 93 kg. These probes penetrated the Venusian atmosphere on Dec. 9 at different locations and continuously gathered and relayed data as they approached and hit the surface. One small probe survived the hard landing and continued transmitting for a further hour. Pioneer Venus 2 – the interplanetary ‘bus' – entered the atmosphere on Dec. 9, shortly after its payload, as an upper-atmosphere probe.
The probe measurements revealed a surprisingly large amount of primordial argon, krypton, and neon in Venus' atmosphere, which relates to the early history of the planetary atmosphere. They also provided evidence of the layered structure of the clouds and confirmed the role of the greenhouse effect in maintaining the high surface temperature. Radar maps of the surface showed evidence of massive plateaus, volcanic areas, and large rift valleys greater than those seen on Earth. A major surprise was the evidence of lightning since it is not compatible with the meteorological observations. It is possible that volcanic activity at the time of the Pioneer mission may be related to this surprising observation.