The Mariner Venus-Mercury spacecraft, also known as Mariner 10, is seen through the open doors of JPL's 25-foot space simulator in July 1973.

The 85-foot-high stainless steel cylinder was used to simulate conditions in space: very low air pressure, extreme cold and intense solar radiation. After the spacecraft was suspended in the chamber, a series of vacuum pumps exhausted the air from the chamber. Liquid nitrogen was pumped through the piping of the inner chamber walls to reduce the temperature to about minus 300 degrees Fahrenheit. The light from thirty-seven high intensity xenon arc lamps was projected onto a mirror and reflected down into the test area. This artificial solar beam was up to 10 times the intensity of sunlight reaching Earth's atmosphere.

The Mariner 10 was the first spacecraft with a dual-planet mission: scientific observation of Venus and Mercury. It was also the first to use gravity assist, with the gravitational field of Venus being used to propel it on to Mercury.