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Vanguard rocket explodes shortly after launch at Cape Canaveral (December 6, 1957).

Vanguard TV3 was the first U.S. attempt to launch a satellite into orbit around the Earth. It was a small satellite designed to test the launch capabilities of the three-stage Vanguard rocket and study the effects of the environment on a satellite and its systems in Earth orbit. It also was to be used to obtain geodetic measurements through orbit analysis.

At its launch attempt on December 6, 1957, the booster ignited and began to rise but about 2 seconds after liftoff, after rising about four feet (1.2 m), the rocket lost thrust and began to settle back down to the launch pad. As it settled against the launch pad the fuel tanks ruptured and exploded, destroying the rocket and severely damaging the launch pad. The Vanguard satellite was thrown clear and landed on the ground a short distance away with its transmitters still sending out a beacon signal. The satellite was damaged, however, and could not be reused. It is now on display at the Smithsonian Air and Space Museum.

The exact cause of the accident was never determined, presumably it was due to a fuel leak between the fuel tank and the rocket engine, possibly due to a loose connection in a fuel line or low fuel tank pressure allowing some of the burning fuel in the thrust chamber to leak back into the fuel tank.

The payload was a 1.36 kg aluminum sphere 152 mm in diameter, nearly identical to the later Vanguard 1. 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 to measure the interior temperatures in order to track the effectiveness of the thermal protection.