The microwave radiometer was automatically calibrated twenty-three times during the mission by a sequence originating in the data conditioning system, so that the correct functioning of the instrument could be determined before the encounter with Venus. INFRARED RADIOMETER The infrared radiometer was a companion experiment to the microwave instrument and was rigidly mounted to the microwave antenna so that both radiometers would look at the same area of Venus with the same scanning rate. The instrument detected radiation in the 8 to 9 and 10 to 10.8 micron regions of the infrared spectrum. The infrared radiometer had two optical sensors. As the energy entered the system, it was “chopped” by a rotating disk, alternately passing or comparing emissions from Venus and from empty space. The beam was then split by a filter into the two wavelength regions. The output was then detected, processed, and transmitted to the Earth. The infrared radiometer measured 6 inches by 2 inches, weighed 2.7 pounds, and consumed 2 watts of power. The instrument was equipped with a calibration plate which was mounted on a superstructure truss adjacent to the radiometer. MARINER’S SCIENTIFIC OBJECTIVES Equipped with these instruments and with the mechanism for getting the measurements back to Earth, Mariner II was prepared to look for the answers to some of the questions inherent in its over-all mission objectives: 1. The investigation of interplanetary space between the Earth and Venus, measuring such phenomena as the cosmic dust, the mysterious plasma or solar winds, high-energy cosmic rays from space outside our solar system, charged particles from the Sun, and the magnetic fields of space. 2. The experiments to be performed near Venus (at about 21,150 miles out from the surface) in an effort to understand its magnetic fields, radiation belts, the temperature and composition of its clouds, and the temperature and conditions on the surface of the planet. CHAPTER 9 THE LEGACY OF MARINER If intelligent life had existed on Venus on the afternoon of the Earth’s December 14, 1962, and if it could have seen through the clouds, it might have observed Mariner II approach from the night side, drift down closer, cross over to the daylight face, and move away toward the Sun to the right. The time was the equivalent of 12:34 p.m. along the Pacific Coast of the United States, where the spacecraft was being tracked. Mariner II had reached the climax of its 180-million-mile, 109-day trip through space. The 35-minute encounter with Venus would tell Earth scientists more about our sister planet than they had been able to learn during all the preceding centuries. SPACE WITHOUT DUST?