What was Sputnik?
Sputnik was the world's first artificial satellite, launched by the USSR on 4 October 1957.
It was a major technical achievement and propaganda coup for the Soviets.
The event marked the beginning of the space age and also the space race between the US and the USSR.
After blasting off on an R-7 rocket from the Tyuratam missile range in Kazakhstan, Sputnik took 98 minutes to orbit Earth on an elliptical path.
In the early 1950s, Soviet rocket engineer Sergei Korolev had campaigned for an artificial satellite to be launched before the start of International Geophysical Year (IGY) - guaranteeing a first place finish ahead of America.
Designated to begin in July 1957, the IGY was timed to coincide with a period when the Sun would be emitting a large amount of radiation.
Western scientists were planning a variety of experiments to study Earth's upper atmosphere during this period. A satellite equipped with an array of scientific instruments was intended to form the centrepiece of proceedings.
In January 1956, Korolev got his wish. The Soviet government approved his plan after hearing the US was preparing for its own satellite launch.
Did Sputnik carry experiments into space?
Sputnik was able to transmit some temperature and density measurements to the ground, demonstrating that spacecraft could supply valuable information from space.
But it was a very rudimentary satellite and did not contain any specific experiments. It consisted of a polished sphere measuring 58.5cm (22.8in) in diameter and weighing 84kg (184lb) with trailing, whip-like antennas.
Inside were two one-watt radio transmitters and three silver-zinc batteries - two for powering the radio beacons and one for a ventilation fan. The sphere was filled with nitrogen gas pressurised to 130 kilopascals (1.3 atmospheres).
1. Polished thermal shield (1mm thick)
2. Long, trailing antenna (one of four) to transmit signal
3. Aluminium hemisphere with two radio transmitters
4. Three silver-zinc batteries to power transmitters
Korolev's team of engineers had originally planned to launch a more complex satellite weighing over 1,000kg and carrying an array of scientific instruments. This was known as Object D.
By mid-1956, this project was lagging behind schedule and Korolev was beginning to worry that the Americans would beat him to space.
The Soviet team decided to scrub the original plan and opt for a lighter, simpler design. Korolev ordered his team to work around the clock on the new project, called PS-1.
At 1928 GMT, on 4 October 1957, an R-7 rocket blasted off from the remote Kazakh steppe carrying PS-1 on board.
Hours later, the satellite engineers heard the "beep, beep, beep" of Sputnik's signal coming in over the radio, confirming the satellite had entered orbit around the Earth.
What were the immediate effects?
Soviet state media were slow to recognise the significance of the launch. Pravda devoted only a few paragraphs on its front page to "Sputnik" (the Russian word for satellite).
The rest of the world was stunned by the launch; before Sputnik, the US had been widely expected to win the satellite race.
The news provoked mass hysteria in the United States. Many Americans linked the Sputnik launch with a Soviet capability to deliver ballistic missiles to targets in the US.
The New York Times declared that America was in a race for survival. Senator Lyndon Johnson warned: "Whoever controls space controls the world".
According to Sergei Khrushchev, son of Sputnik-era Soviet premier Nikita, the relatively nonchalant reaction in the USSR was due to a general belief that, step-by-step, their country would edge ahead of America as a world power.
Boris Chertok, 95, who was Korolev's deputy, said: "At that moment we couldn't fully understand what we had done.
"We felt ecstatic about it only later, when the entire world ran amok. Only four or five days later did we realize that it was a turning point in the history of civilization."
How were US satellite projects doing?
In 1955, the White House had solicited proposals from government agencies to undertake development of an artificial satellite.
A US Army proposal, spearheaded by German-born rocket scientist Wernher von Braun, was eventually rejected in favour of the US Navy's Project Vanguard.
Wernher von Braun's satellite proposal was originally rejected
Von Braun had designed the V-2 missiles for Nazi Germany during WWII, but surrendered to American forces in 1945. He was transported to the US - along with many other V-2 engineers - to develop missiles for America.
The German rocket designer, who had long harboured dreams of working on space vehicles, refused to give up after the initial rejection of his proposal.
He continued to seek approval for launching his own satellite; but his repeated pleas fell on deaf ears, even as Project Vanguard slipped behind schedule.
Then, in November 1957, the Soviet Union rubbed salt in America's wounds by launching Sputnik 2, carrying a female dog called Laika on board.
The Pentagon gave into von Braun and allowed him to join the satellite race.
Vanguard made it to the launch pad first. On 6 December 1957, the Navy rocket climbed into the air from Cape Canaveral, Florida, in a televised launch. Unfortunately, it only climbed a few feet before losing thrust and exploding on the launch pad.
The US was spared further humiliation when von Braun successfully launched his Explorer I satellite on 29 January 1958.
The space race had begun in earnest.