There are a lot of things that pose a threat to our planet—climate change, natural disasters, and solar flares, for example. But one threat in particular often captures public imagination, finding itself popularised in books and films and regularly generating alarming headlines: asteroids.

In our solar system there are millions of space rocks known as asteroids. Ranging in size from a few metres to hundreds of kilometres, these objects are mostly left over from the formation of our planets 4.6 billion years ago. They are building blocks that didn't quite make it into fully fledged worlds.

Asteroids and other objects that make a closest approach to our sun of less than 1.3 astronomical units (1 astronomical unit, AU, is the Earth-Sun distance) are known as

Sixty years ago on Monday cosmonaut Yuri Gagarin became the first person in space, securing victory for Moscow in its race with Washington and marking a new chapter in the history of space exploration.

Decades later, his journey has become shrouded in myth after many details about the historic mission were for years kept secret by the Soviets.

Here are five things to know about Gagarin's legendary flight:

'Let's go!'

A trained steel worker turned military pilot, Gagarin was selected from thousands of candidates to undergo the rigorous training required for a space flight.

Apart from showing excellent results in his tests, Gagarin, then aged 27, also reportedly stood out by removing his shoes before entering the Vostok spacecraft designated for the mission, a custom in Russia when entering a home.

On April 12, 1961, as Gagarin's flight took off from the Baikonur spaceport in Kazakhstan, he exclaimed his iconic catchphrase "Poekhali!", or "Let's go!" in Russian.

Risky business

The flight lasted just 108 minutes as the Vostok completed one loop around the Earth.

Once Gagarin safely returned home, the success of his mission outshone the fact that not everything went according to plan.