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Vanguard 2, also known as Vanguard II, was launched on February 17, 1959 atop a Vanguard SLV 4 rocket. The satellite was designed to measure cloud-cover distribution over the daylight portion of its orbit. That primary mission was meant to last for 19 days. Its secondary mission was to provide information on the density of the atmosphere. That mission was designed to last for the lifetime of its orbit, estimated to be 300 years. The satellite was last successfully tracked on June 25, 2009.
The spacecraft is a sphere 50.8 cm in diameter and made of magnesium. It is gold-plated internally and covered with aluminum and silicon oxide externally to provide thermal control for the instrumentation. Communication, now defunct, was provided by a 1 W, 108.03 MHz telemetry transmitter and a 10 mW, 108 MHz beacon transmitter. A command receiver was used to activate a tape recorder that relayed telescope experiment data to the telemetry transmitter.
Vanguard 2 was selected by the experimenters for use in determining upper atmospheric densities as a function of altitude, latitude, season, and solar activity based on its symmetrical shape. This is possible because the spacecraft would lag its predicted positions slightly, accumulating greater and greater delay due to drag of the residual atmosphere. Scientists can measure the rate and timing of orbital shifts and the relevant atmospheric parameters could be back-calculated, since the body’s drag properties are known. Using this simple computational method, it was determined that atmospheric pressures, and drag and orbital decay, were higher than anticipated, when the Earth’s upper atmosphere tapered into space gradually.
The optical scanner experiment onboard was designed to obtain cloud-cover data between the equator and 35° to 45° N latitude. This experiment is fairly simple. As the satellite orbits, two photocells, located at the focus of two optical telescopes, which are aimed in diametrically opposite directions, measure the intensity of sunlight reflected from clouds, land masses, and areas of the sea. The satellite motion and rotation caused the photocells to scan the earth in successive “lines”. Solar batteries turned on a recorder when the Earth beneath the satellite was in sunlight only. About 50 minutes of data were obtained during each orbit. The measurements were stored on tape. Ground stations received the data when the command receiver was activated, causing the entire tape to be played back in 60 seconds, erased, and rewound for the next orbit. The weather experiment went flawlessly, but the optical instrument’s data was poor because of poor orientation of the spin axis.
Here are some articles about Vanguard 2 for Universe Today.
Can We Put Weather On A Budget?
We’ve also recorded an entire episode of Astronomy Cast all about the Earth’s natural satellite, Moon. Listen here, Episode 113: The Moon, Part 1.