Vanguard I, the oldest satellite still orbiting the Earth, will complete 50 years in space on Monday.

Vanguard-I - the first ever solar-powered space vehicle - was launched on March 17, 1958 and is only six inches across and weighs about 1.5 kg.

Its small size, compared to the Soviet Union's 200-pound Sputnik-I, caused then-Soviet Premier Nikita Khrushchev to dub it "the grapefruit satellite."

Since its launch 50 years ago, the tiny satellite has gone 196,990 times around the earth and travelled 5.7 billion nautical miles - the distance from Earth to beyond the planet Pluto and halfway back.

During that time, it has provided a wealth of information on the size and shape of the Earth, and set a number of space records as well.

Vanguard I also introduced much of the technology that has been applied in other US satellite programmes, for example, it proved that solar cells could be used for several years to power radio transmitters.

Vanguard's solar cells operated for about seven years, while conventional batteries used to power another onboard transmitter lasted only 20 days.

Most importantly its successes had set the pattern for a multitude of other space ventures in the US and abroad and also served as a springboard for the Naval Research Laboratory’s (NRL) scientists to launch several series of space probes to study various aspects of radiation phenomena.

Now the oldest man-made satellite in orbit, Vanguard-I has been 100 percent successful in meeting its scientific objectives.