Let's go!" With these words Yuri Gagarin on April 12, 1961 blasted-off on the Vostok rocket that took him into orbit as the first man in space.

There was no countdown, American-style, at the Baikonur cosmodrome, just tense exchanges between Gagarin and the father of the Soviet space programme, Sergei Korolev, that occasionally lapsed into banter to break the tension.

Here is a chronology of the extraordinary days in April 1961 and moments on April 12 that saw the Soviet Union write a new chapter in human history.

April 9 -- Yuri Gagarin is officially chosen for the mission scheduled for April 12. His reserve is named as Gherman Titov who would later on August 6 became the second man in space with a day-long flight.

April 10 -- Staying at the Baikonur cosmodrome ahead of his launch, Gagarin writes to his wife to bring up their two daughters as "worthy members of the new Communist society" should anything happen to him.

April 11 -- Gagarin and Titov spend the night in a small wooden hut in Baikonur. "I'm leaving tomorrow and I don't even believe it myself," he writes.

April 12

5:30 am Moscow time: The two cosmonauts are awoken. After a medical examination and a light meal they climb into their orange space suits and their helmet. Gagarin does not shave, respecting the superstition of Soviet pilots before an important flight.

5:45 am: Gagarin and Titov are taken towards the launch site in a blue bus. It stops on the way for Gagarin to take a bathroom break, a halt that has now become a ritual tradition for other launches.

6:50 am: Gagarin gets out of the bus close to the launch pad. He is embraced by Korolev. A worker writes the letters "USSR" in Russian on his helmet.

7:07 am: Gagarin boards his Vostok-1 capsule. Gagarin awaits the end of preparations singing popular songs and joking with his colleagues over the radio. Famously, Korolev assures him he is well supplied with food and warns him he may get fat.

At the last moment, the technicians carry out a repair to seal number one which had turned out not to be completely hermetically sealed.

9:07 am: The rocket fires. Gagarin utters the famous words "Let's go" as his pulse reaches 150 and Vostok blasts off.

9:09 am: Gagarin is silent for several minutes, as his body is subjected to unprecedented G-forces.

9:12 am: Gagarin speaks again over the radio "I see the earth, it's magnificent".

10:02 am: Soviet official radio solemnly announces the news to the people.

10:09 am: Gagarin says over the radio he has returned from under the Earth's shadow.

10:25 am: The spaceship returns into the earth's atmosphere, its retrorockets carry out braking but the craft starts spinning.

10:35 am: The craft stabilises.

10:49 am: Gagarin ejects at an altitude of 7,000 metres, as planned.

10:55 am: The craft itself lands with the help of a parachute into a field close to Saratov in the Volga region.

10:57 am: Gagarin lands by parachute some three kilometres (two miles) distant. The first people he sees are a peasant woman and her granddaughter whom he reassures that he is not an alien.

A helicopter then arrives to take the triumphant cosmonaut to the nearby city of Engels, where he receives a telegram of congratulation from Soviet leader Nikita Khrushchev. Gagarin also speaks to his wife, Valentina.

April 13 -- Gagarin recuperates at an elite dacha in the Volga city of Kuibyshev, today known as Samara.

April 14

-- Gagarin lands at Moscow's Vnukovo airport. Thousands of Russians take to the streets decked with flowers and posters to give him a triumphant welcome which culminates in a drive in an open top car on Red Square.

-- Standing on Lenin's Mausoleum, Gagarin reports back to Khrushchev who then breaks with formalities by embracing and kissing the cosmonaut. He gives a speech which is broadcast by all Soviet radio and television.