December 4th, 1978: Pioneer Venus 1 goes into orbit around Venus.
Formally approved by NASA in August 1974, the Pioneer Venus project comprised two spacecraft to explore the atmosphere and surface of Venus. Both spacecraft used a basic cylindrical bus. Pioneer Venus 1, the orbiter, was designed to spend an extended period in orbit around Venus mapping the surface using a radar package. After a six-and-a-half-month journey, the spacecraft entered an elliptical orbit around Venus at 15:58 UT Dec. 4, 1978.
It was the first American spacecraft to enter orbit around Venus, about three years after the Soviets accomplished the same feat. The initial orbital period was 23 hours, 11 minutes, which was altered within two orbits to the desired 24 hours—a maneuver that would allow the orbit’s high and low points (about 99 miles or 160 kilometers) to occur at the same time each Earth day. Data from the radar mapper allowed scientists to produce a topographical map of most of the Venusian surface between 73 degrees north latitude and 63 degrees south latitude at a resolution of about 47 miles (75 kilometers).
The data indicated that Venus was much more smooth and spherical than Earth. The orbiter identified the highest point on Venus as Maxwell Montes, which rises about 6.5 miles (10.8 kilometers) above the mean surface. Infrared observations implied a clearing in the planet’s atmosphere over its north pole. In addition, ultraviolet light photos showed dark markings that covered the clouds in the visible hemisphere.
Cameras also detected almost continuous lightning in the atmosphere. The spacecraft confirmed that Venus has little if any magnetic field. Because of the nature of its orbit, Pioneer Venus 1 passed through the planet’s bow shock twice per revolution, and using its magnetometer, scientists were able to observe how the planet’s ionosphere interacted with the solar wind.
The mapping radar was switched off March 19, 1981, after having mapped 93 percent of the band between 74 degrees north latitude and 63 degrees south latitude. It was reactivated in 1991, 13 years after launch, to explore the previously inaccessible southern portions of the planet.
In May 1992, Pioneer Venus 1 began the final phase of its mission, maintaining its periapsis between 93 and 155 miles (150 and 250 kilometers) until propellant depletion. The last transmission was received at 19:22 UT Oct. 8, 1992, as its decaying orbit no longer permitted communications. The spacecraft burned up the atmosphere soon after, ending a successful 14-year mission that was planned to last only eight months.