Which was the first spacecraft to soft land on another planet and transmit data back to Earth?

Venera 7 was a Soviet spacecraft, part of the Venera series of probes to Venus. When it landed on the Venusian surface on December 15, 1970, it became the first spacecraft to soft land on another planet and first to transmit data from there back to Earth.

The lander was designed to be able to survive pressure of up to 180 bars (18,000 kPa) and temperatures of 580 °C (1,076 °F). This was significantly greater than what was expected to be encountered but significant uncertainties as to the surface temperatures and pressure of Venus resulted in the designers opting for a large margin of error. The degree of hardening added mass to the probe which limited the amount of mass available for scientific instruments both on the probe itself and the interplanetary bus. The interplanetary bus carried a solar wind charged particle detector and a cosmic ray detector. On the lander there were temperate and pressure sensors as well as an accelerometer to measure atmospheric density. The probe also carried a radar altimeter.