An experimental satellite launched in the first months of the space race is about to celebrate 50 years in orbit.

Vanguard I is the oldest surviving man-made satellite in space and was the first to provide measurements showing the Earth is slightly pear-shaped instead of perfectly round.

When the 50th anniversary of the US satellite’s launch on March 17, 1958, arrives on Monday it will have made more than 196,990 Earth orbits.

At 6in in diameter and 3lb in weight, it was dubbed “the grapefruit satellite” by Nikita Khrushchev, then the leader of the Soviet Union.

But while tiny by modern standards, and small compared to the 23in-long Sputnik 1 launched the previous October, its achievements were enormous.

It was the first to use solar power and it sent back a wealth of information on the size of the Earth, its air density and temperatures.After the humiliation of watching the Soviet Union get satellites into space first, its successful launch and deployment came as a welcome fillip to the United States and the West.

President Dwight Eisenhower publicly announced its deployment a little more than two hours after the launch and it was reported in The Times on March 18, 1958, under a headline: “Up ‘for a very long time’.”

In The Times it was recorded that scientists knew within ten minutes that it was fully operational but decided not to tell the public until the first orbit was completed.

The satellite has travelled 5.7 billion miles, the equivalent of flying from here to Neptune and back, plus a round trip to Mars. It was the second successful satellite launched by the United States but it has remained in orbit longer than any of its predecessors, all of which burnt up when they reentered the Earth’s atmosphere.

The United States suffered an embarrassing setback in December 1957 when it attempted to launch its first satellite, only to see the rocket explode on the launch pad. Only in February the following year did it succeed in getting its first satellite, Explorer I, into orbit. At the time of its launch, Vanguard I was reported to be capable of staying in orbit for five or ten years, though it later became clear that it was designed to last for 200 years.

More recent analysis suggests it will continue circling the Earth for 2,000 years unless it is knocked off course. It was launched by the US Naval Research Laboratory (NRL) from Cape Canaveral in Florida as part of Project Vanguard, headed by John Hagan.

Much of the technology for the project was based on Germany’s war-time V-2 rockets. One of the satellite’s most important achievements was to test solar cells in space, allowing space-craft using them to continue much longer in orbit than those with conventional batteries.

While other satellites ran out of power after about three weeks, Vanguard I’s solar power enabled it to continue transmitting information back to Earth for seven years. It finally stopped broadcasting in 1964.

Its orbit, a little over 2 hours 10 minutes, was so stable that it sent back observational data of the Earth’s surface that allowed cartographers to improve maps of a range of Pacific islands.

Placed in orbit

— About 3,000 satellites orbit the Earth

— Orbits range from 155 miles above the Earth’s surface to 22,300 miles

— There are six main types: scientific research, weather data, communications, navigation, earth observations and military

— The largest satellite is the International Space Station

Sources: Nasa & Times Database

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