Vanguard 2 (or Vanguard 2E before launch) is an Earth-orbiting satellite launched 17 February 1959 at 15:55:02 GMT, aboard a Vanguard SLV-4 rocket as part of the United States Navy's Project Vanguard. 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 the first weather satellite and one of the first orbital space missions, the launch of Vanguard 2 was an important milestone in the Space Race between the United States and the Soviet Union. Vanguard 2 remains in orbit. Previous satellites Before the successful 1959 launch of the satellite that became known as Vanguard 2, multiple attempted launches of satellites named "Vanguard 2" were made in 1958. All of these launches failed to reach orbit. The satellites that failed to reach orbit were: Vanguard 2A: launched 29 April 1958, by the Vanguard TV-5 rocket Vanguard 2B: launched 28 May 1958, by the Vanguard SLV-1 rocket Vanguard 2C: launched 26 June 1958, by the Vanguard SLV-2 rocket Vanguard 2D: launched 26 September 1958, by the Vanguard SLV-3 rocket The satellite whose launch was successful and that became known as the Vanguard 2 was the Vanguard 2E. Spacecraft The spacecraft is a magnesium sphere in diameter. It contains 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 watt, 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. The power supply for the instrumentation was provided by mercury batteries. Instruments Optical scanner 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" (akin to