|Names||Vanguard Space Launch Vehicle-4|
|Mission type||Weather satellite|
Air Density Experiment
|Operator||Naval Research Laboratory|
|Harvard designation||1959 Alpha 1|
|Spacecraft type||Vanguard 2E|
|Manufacturer||Naval Research Laboratory|
|Launch mass||10.75 kg (23.7 lb)|
|Dimensions||50.8 cm (20.0 in) of diameter|
|Start of mission|
|Launch date||17 February 1959,|
15:55:02 GMT 
|Launch site||Cape Canaveral, LC-18A|
|Contractor||Glenn L. Martin Company|
|End of mission|
|Last contact||15 March 1959|
|Decay date||2259 (estimated)|
~ 300 years orbital lifetime 
|Reference system||Geocentric orbit |
|Regime||Medium Earth orbit|
|Perigee altitude||559 km (347 mi)|
|Apogee altitude||3,320 km (2,060 mi)|
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. As of March 2021[update], Vanguard 2 was still in orbit.
Before the successful 1959 launch of the satellite that became known as Vanguard 2, multiple attempted launches of "Vanguard 2" named satellite were made in 1958. All of these launches failed to reach orbit. The satellites that failed to reach orbit were:
The satellite whose launch was successful and that became known as the Vanguard 2 was the Vanguard 2E.
The spacecraft is a magnesium sphere 50.8 cm (20.0 in) 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 watt, 108.03 MHz telemetry transmitter and a 10 mW, the 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 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 a whisk broom scanner). Separate solar batteries turned on a recorder only when the Earth beneath the satellite was in sunlight and about 50 minutes 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 seconds. 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.
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 lead its predicted positions slightly, accumulating greater and greater advance as it spiraled lower and faster due to the 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 gradually tapered into space.
This experiment was very much planned prior to launch. Initial Naval Research Laboratory (NRL) 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 50.8 cm (20.0 in) in diameter and hopefully 76 cm (30 in). 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 16 cm (6.3 in) spheres as a "test vehicle", with 50.8 cm (20.0 in) 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 50.8 cm bodies.
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 February 2021[update], both objects remain in orbit.
As the three Vanguard satellites are still orbiting, with their drag properties essentially unchanged, they form a baseline atmospheric dataset sixty years old and counting. Vanguard 2 has an expected orbital lifetime of 300 years.