|Mission type||Weather satellite
Air density experiment
|Operator||Department of the Navy|
|Harvard designation||1959 Alpha 1|
|Mission duration||Weather observation: 19 days
Atmospheric density studies ongoing
|Manufacturer||Naval Research Laboratory|
|Launch mass||10.8 kilograms (24 lb)|
|Start of mission|
|Launch date||17 February 1959, 15:55UTC|
|Launch site||Cape Canaveral LC-18A|
|Semi-major axis||8,127.72 kilometers (5,050.33 mi)|
|Perigee||560 kilometers (350 mi)|
|Apogee||2,953 kilometers (1,835 mi)|
|Mean anomaly||11.85 degrees|
|Epoch||24 January 2015, 14:43:44 UTC|
Vanguard 2 or Vanguard II is an Earth-orbiting satellite launched February 17, 1959, aboard a Vanguard SLV 4 rocket as part of the United States Navy's Project Vanguard. Vanguard 2 success was an important part of the space race between the United States and the Soviet Union. . Vanguard 2 took the first photo of earth from a satellite and was also the first weather satellite.  The satellite was designed to measure cloud-cover distribution over the daylight portion of its orbit, for a period of 19 days, and to provide information on the density of the atmosphere for the lifetime of its orbit (about 300 years). 
As of July 21, 2015, Vanguard 2 was still in orbit.
The spacecraft is a magnesium sphere 20 in (50.8 cm) in diameter. It contained two optical telescopes with two photocells. The sphere was internally gold-plated, and externally covered with an aluminum deposit coated with silicon oxide of sufficient thickness to provide thermal control for the instrumentation.
Radio communication was provided by a 1 W, 108.03 MHz telemetry transmitter and a 10 mW, 108 MHz beacon transmitter that sent a continuous signal for tracking purposes. A command receiver was used to activate a tape recorder that relayed telescope experiment data to the telemetry transmitter.
Satellite drag atmospheric density
Because of its symmetrical shape, Vanguard 2 was selected by the experimenters for use in determining upper atmospheric densities as a function of altitude, latitude, season, and solar activity. As the spacecraft continuously orbited, it would lag its predicted positions slightly, accumulating greater and greater delay due to drag of the residual atmosphere. By measuring the rate and timing of orbital shifts, the relevant atmosphere's parameters could be back-calculated knowing the body's drag properties. It was determined that atmospheric pressures, and thus drag and orbital decay, were higher than anticipated, as Earth's upper atmosphere tapered into space gradually.
This experiment was very much planned prior to launch. Initial Naval Research Laboratory proposals for Project Vanguard included conical satellite bodies; this eliminated the need for a separate fairing and ejection mechanisms, and their associated weight and failure modes. Radio tracking would gather data and establish a position. Early in the program, optical tracking (with a Baker-Nunn camera network and human spotters) was added. A panel of scientists proposed changing the design to spheres, at least twenty inches in diameter and hopefully thirty. A sphere would have a constant optical reflection, and constant coefficient of drag, based on size alone, while a cone would vary with orientation. James Van Allen proposed a cylinder, which eventually flew. The Naval Research Lab finally accepted 6.4-inch spheres as a "test vehicle," with twenty inches for follow-on satellites. The payload weight savings, from reduced size as well as decreased instrumentation in the early satellites, was considered acceptable for the initial launches. Afterwards, the later Vanguard rockets had some test instrumentation removed, lightening them enough for the 20" bodies. 
As the three Vanguard satellites are still orbiting, with their drag properties essentially unchanged, they form a baseline atmospheric dataset fifty years old and counting. Vanguard 2 has an expected orbital lifetime of 300 years. 
The optical scanner experiment was designed to obtain cloud-cover data between the equator and 35° to 45° N latitude. As the satellite circled Earth, two photocells, located at the focus of two optical telescopes aimed in diametrically opposite directions, measured the intensity of sunlight reflected from clouds (about 80%), from land masses (15 to 20%), and from sea areas (5%). The satellite motion and rotation caused the photocells to scan the Earth in successive "lines". Separate solar batteries turned on a recorder only when the Earth beneath the satellite was in sunlight and about 50 min of data per orbit were obtained. The measured reflection intensities were stored on tape. Ground stations interrogated the satellite by signaling its command receiver, which caused the entire tape to be played back in 60 s. The tape was then erased and rewound.
For the planned 19 days of the weather experiment, the equipment functioned normally. The satellite was spin stabilized at 50 rpm, but the optical instrument's data was poor because of an unsatisfactory orientation of the spin axis.
After the scientific mission ended, both Vanguard 2 and the upper stage of the rocket used to launch the satellite became derelict objects that would continue to orbit Earth for many years. As of September 2013[update], both objects remain in orbit.
- Vanguard 1
- Sputnik 1
- Sputnik (rocket)
- Weather satellite
- Timeline of artificial satellites and space probes
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