Ahead of International Day of Human Space Flight, let's take a look at some key moments in the history of space exploration.  The Sputnik 1 satellite became the first man-made object to leave the earth's atmosphere after the USSR launched the device on October 4, 1957. Two monkeys became the first creatures to be returned from space alive after the United States launched the animals on a Jupiter 2 missile on May 28, 1959.  Yuri Gagarin became the first person to reach space after he was launched from Baikonur Cosmodrome in Kazakhstan in a Vostok spacecraft by the USSR on April 12, 1961. On June 16, 1963, Valentina Tereshkova became the first woman to enter space when she orbited the Earth 49 times across three days on a USSR spacecraft. Russian cosmonaut Alexei Leonov conducted the first-ever spacewalk after being launched in a two-man Voskhod spacecraft on March 18, 1965. Despite the Soviets' success in space, the US was the first country to have astronauts on the moon after Neil Armstrong made “one giant leap for mankind” on July 20, 1969.  Russia launched Salyut, the first space station, on April 19, 1971, with the first crew to dock on the station dying while re-entering the atmosphere. On February 7, 1984, Bruce McCandless became the first astronaut to carry out a spacewalk without being attached to a spacecraft.  The assembly of the International Space Station (ISS) began on November 20, 1998, with the launch of the first module, named Zarya. Since then, 200 people from 15 countries have visited the station. Californian billionaire Dennis Tito became the first tourist to visit space when he paid $20 million for an eight-day trip on the ISS.