Astronaut Capt Scott Kelly's year in space

The longest-serving American astronaut in space is returning to Earth after living on board the International Space Station for 340 days.

While there, Capt Scott Kelly shared fantastic images of our planet via Twitter and Instagram, as well as videos documenting everyday life in space.

"22 September 2015 - Day 179. The Nile at night is a beautiful sight for these sore eyes."

Capt Kelly's mission will help Nasa understand the long-term effects of living in space. His identical twin, Mark, stayed grounded during the year, and Nasa is conducting research on both men to see how a year of weightlessness affects the human body.

"Christmas in July! Great gift for my 100th day in space! Only 250 more to go - not that I'm counting."

The research is critical, as it will take a year of space travel to reach Mars.

"More #Africa #EarthArt hues"

In October, Capt Kelly was one of two astronauts on board the International Space Station to make their first spacewalk outside - to do some repairs and maintenance.

Capt Kelly and Dr Kjell Lindgren had to grease the station's big robot arm, reroute some cables and remove some insulation.

The spacewalk had to be delayed after Dr Lindgren switched on the water flow in his suit too soon.

"#SpaceWalkSelfie Back on the grid! Great first spacewalk yesterday. Now on to the next one next week."

Travelling more than 220 miles above the Earth, at 17,500mph, Capt Kelly orbited the globe more than a dozen times a day.

This image shows the view over Africa.

During his stay on station, he had the opportunity to photograph various geographical locations, including the Bahamas, pictured here.

"Pictures like this make me really regret that my watercolours didn't make it up here," Capt Kelly wrote on 22 November 2015.

On 29 September, Capt Kelly had an Antipodean view.

He wrote: "My favourite colour is blue. But it's green I miss most. #New Zealand, you were everything I expected."

Some images proved puzzling to some of Capt Kelly's followers.

On 11 February, this shot raised some questions.

He wrote: "Posted this pic last week. Many of you asked, 'What is it?'"

Capt Kelly revealed all on Tumbler.

He wrote: "Photo located on the western edge of the Sahara desert at centre Mauritania in north-west Africa. There is a giant quartzite circle called Richat Structure. It is approximately 24 miles across. This volcanic bulge that never erupted and was levelled by erosion makes for interesting Earth art."

On 27 August, Capt Kelly captured one of many amazing shots of the aurora borealis.

He also marked Halloween with another shot of the spectacular light show.

On 27 February and near the end of his time on the space station, Capt Kelly wrote; "Of all the sunrises I've seen on my year in space, this was one of the best! One of the last too. Headed home soon."

Unsurprisingly, Capt Kelly's final post before returning was a picture of Earth; "#Countdown We're down to a wake-up. #Earth. I'm coming for you tomorrow! #GoodNight from @space_station! #YearInSpace."

All photos: @stationcdrkelly / Nasa