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Vanguard 1 (ID: 1958-Beta 2 ) was the fourth artificial Earth orbital satellite launched (after Sputnik 1, Sputnik 2, and Explorer 1), but the first satellite to be solar powered. Although communication with it was lost in 1964, it remains the oldest manmade satellite still in orbit. It was designed to test the launch capabilities of a three-stage launch vehicle as a part of Project Vanguard, and the effects of the environment on a satellite and its systems in Earth orbit. It also was used to obtain geodetic measurements through orbit analysis.

Spacecraft design
The spacecraft is a 1.47 kg (3.2 lb) aluminum sphere 165 mm (6.4 inches) in diameter. It contains a 10 mW, 108 MHz transmitter powered by a mercury battery and a 5 mW, 108.03 MHz transmitter that was powered by six solar cells mounted on the body of the satellite. Six short antennas protrude from the sphere. The transmitters were used primarily for engineering and tracking data, but were also used to determine the total electron content between the satellite and ground stations. Vanguard also carries two thermistors which measured the interior temperature over sixteen days in order to track the effectiveness of the thermal protection. A backup version of Vanguard 1 is on display at the Kansas Cosmosphere and Space Center in Hutchinson, Kansas.

Mission
The three stage launch vehicle placed Vanguard into a 654 x, 134.2 minute elliptical orbit inclined at 34.25 degrees on March 17, 1958. Original estimates had the orbit lasting for 2,000 years, but it was discovered that solar radiation pressure and atmospheric drag during high levels of solar activity produced significant perturbations in the perigee height of the satellite, which caused a significant decrease in its expected lifetime to only about 240 years. The Vanguard 1 transmitted its signals for nearly 7 years as it orbited the earth.

Radio beacon
A 10 mW transmitter, powered by a mercury battery, on the 108 MHz band used for International Geophysical Year (IGY) scientific satellites, and a 5 mW, 108.03 MHz transmitter powered by six solar cells were used as part of a radio phase-comparison angle-tracking system. The tracking data were used to show that the shape of the Earth has a very slight north-south asymmetry, occasionally described as pear-shaped with the stem at the North Pole. These radio signals were also used to determine the total electron content between the satellite and selected ground-receiving stations. The battery-powered transmitter provided internal package temperature for about sixteen days and sent tracking signals for twenty days. The solar cell powered transmitter operated for more than six years. Signals gradually weakened and were last received at Quito, Ecuador in May 1964 after which the spacecraft was optically tracked from Earth.

Satellite drag atmospheric density
Because of its symmetrical shape, Vanguard 1 was used by experimenters for use in determining upper atmospheric densities as a function of altitude, latitude, season, and solar activity. As the satellite 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, together with the body's drag properties, the relevant atmosphere's parameters could be back-calculated. It was determined that atmospheric pressures, and thus drag and orbital decay, were higher than anticipated, since Earth's upper atmosphere does taper off into space gradually.

This experiment was extensively planned prior to launch. Initial Naval Research Laboratory (NRL) proposals for the project 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 have properties that varied with its orientation. James Van Allen of the University of Iowa proposed a cylindrical satellite based on his work with Rockoons, which became Explorer 1, the first American satellite. The Naval Research Laboratory finally accepted a sphere with a 6.4-inch diameter as a "test vehicle", with a diameter of twenty inches set for the follow-on satellites. The weight savings, from reduced size as well as decreased instrumentation in the early satellites, was considered to be acceptable.

Since three of the Vanguard satellites are still orbiting in the 2010s, with their drag properties essentially unchanged, they form a baseline data set on the atmosphere of Earth that is 50 years old and counting.

Post mission
After the scientific mission ended in 1964, Vanguard 1 became a derelict object—just like the upper stage of the rocket used to launch the satellite had after it finished the delta-v maneuver to place Vanguard 1 in orbit in 1958—and both would continue to orbit Earth for many years. , both objects remain in orbit.

50th anniversary
The Vanguard 1 satellite holds the record for being in space longer than any other man-made object. On March 17, 2008 it logged its 50th year in Earth orbit.

A small group of former NRL and NASA workers had been in communication, and a number of government agencies were asked to commemorate the event. The Naval Research Laboratory commemorated the event with a day-long meeting at NRL on March 17, 2008. The meeting concluded with a simulation of the satellite's track as it passed into the orbital area visible from Washington, D.C. (where it is visible from the Earth's surface). The National Academy of Sciences scheduled some seminars to mark the 50th anniversary of the International Geophysical Year, which were the only official observances known.