For six months the International Space Station (ISS) will be his home.
The ISS was launched back in 1998 and astronauts have been living on board ever since.
It's an important place to carry out tests and research that will help future space missions to places like the Moon and Mars.
Take a look at the ISS in numbers:

This includes a "development bank" to support small businesses and a promise to bring fast broadband to everyone.
Leader Carwyn Jones said it was "an ambitious plan ... focused on the economy, growth and fairness".
He said the aim was "to build economic growth and security for everyone in Wales".
"Over the last few years the Welsh economy has punched above its weight with high profile inward investments coming through Aston Martin and TVR," Mr Jones said on Tuesday, launching the manifesto at a college in the south Wales valleys.
"Our manifesto today seeks to build on those firm foundations with big and bold new ideas such as a new development bank for Wales and tax cuts for all small businesses to help boost economic growth even further in the years to come."
Details: What is in the Welsh Labour manifesto?
Mr Jones added: "Today is about taking our country forward. We ask the people of Wales to join us once again on that journey."
The focus on the economy comes after he wrote in the Sunday Telegraph there was more he could do to "whip government into shape" in how it helps businesses.
Welsh Labour was left feeling shocked and bruised by 2015's general election result and the party says it's been in listening mode ever since.
It's been all over the country asking people what they want from Labour.
The product of that consultation is this manifesto.
There are plenty of attractive promises, such as increased free childcare, more apprenticeships and an extension of its school-building programme.
But perhaps the more interesting aspect is what's not in the manifesto.
While it commits the party to an M4 relief road, it doesn't give a specific route.
And there's a promise to cut the number of councils, but we're not told how many there will be.
So Labour is keeping its options open on some controversial topics, perhaps with an eye on post-election discussions with other parties.
Major Tim Peake is getting ready to blast into space to complete his first mission.
[NEXT_CONCEPT]
Welsh Labour has launched its assembly election manifesto with a "plan for prosperity" the party says will "get the country moving".