Vanguard 1 is the oldest still orbiting artificial satellite, though there is no longer communication with it. It is a small earth-orbiting satellite that was designed to test the launch capabilities of a three-stage launch vehicle 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 was a 1.47 kg aluminum sphere 152 mm in diameter. It contained a 10 mW, 108 MHz mercury-battery powered transmitter and a 5 mW, 108.03 MHz transmitter powered by six solar cells mounted on the body of the satellite. Six short aerials protruded 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 carried two thermistors which measured the interior temperature over 16 days in order to track the effectiveness of the thermal protection.

Mission

The three stage launch vehicle placed Vanguard into a 654 x 3969 km 134.2 minute orbit inclined at 34.25 degrees on March 17, 1958. Original estimates had the orbit lasting for 2000 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 battery powered transmitter stopped operating in June 1958 when the batteries ran down. The solar powered transmitter operated until May 1964 (when the last signals were received in Quito, Ecuador) after which the spacecraft was optically tracked from Earth.

Mission Results

Radio Beacon

A 10 mW mercury battery powered transmitter on the 108 MHz band used for 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 earth was 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 16 days and sent tracking signals for 20 days. The solar cell powered transmitter operated for more than 6 years. Signals gradually weakened and were last received at Quito, Ecuador, in May 1964.

Satellite Drag Atmospheric Density

Because of its symmetrical shape, Vanguard 1 was selected by the experimenters for use in determining upper atmospheric densities as a function of altitude, latitude, season, and solar activity. This experiment was not planned prior to launch. Density values near perigee were deduced from sequential observations of the spacecraft position, using optical (Baker-Nunn camera network) and radio and/or radar tracking techniques. This experiment obtained reasonable density values. Vanguard 1 has an expected orbital lifetime of 240 years.