The US president has congratulated two astronauts undertaking the first ever female spacewalk at the International Space Station (ISS), telling them “you’re doing an incredible job”.

US astronauts Christina Koch and Jessica Meir made history as they floated out of the orbiting space laboratory at 12.38pm BST on October 18 to replace a faulty power unit.

Part way through their spacewalk they received a call from Donald Trump, who told them a mission to Mars was on the horizon.

Speaking to the astronauts from the White House alongside vice-president Mike Pence and his daughter Ivanka Trump, Mr Trump said: “Well Christina and Jessica I would just like to, if I could, just thank you.

“Jessica I know you’ve being doing this and you’ve been working hard and Christina I’ve studied your resume it’s really incredible what you’ve done, what you’ve both done.

“Your lives have been incredible and now you’re in a place that as I said very few people will ever get that experience.”

He added: “You’re doing an incredible job. This is a first step because we’re going to the moon and then we’re going to Mars.”

This is Ms Koch’s fourth spacewalk and a first for Ms Meir, who has become the 15th woman to venture out of the ISS and into the vacuum.

The spacewalk was originally planned to last five-and-a-half hours but the two astronauts extended it in order to complete some extra, unplanned tasks, Nasa said.

The duo were assisted by Luca Parmitano of the European Space Agency, who is also the mission commander, and Nasa Flight Engineer Andrew Morgan.

The pair are working to replace a battery unit on the Port 6 truss structure – the attachment points for external payloads received by the ISS.

It stopped working after new lithium-ion batteries were installed on the space station last week.

There have been 227 spacewalkers since the first spacewalk in 1965 and all of them have included a male astronaut or cosmonaut.

The US space agency had originally planned an all-female spacewalk in March – which included Ms Koch and fellow astronaut Anne McClain – but had to cancel it because of a shortage of medium-size suits.

Ms McClain originally thought a large-size spacesuit would be fit for purpose but later concluded that a medium would be safer.

She returned to Earth in June and Nasa sent a second medium-size spacesuit up to the station earlier this month.

The route astronauts Christina Koch and Jessica Meir took for their spacewalk
The route astronauts Christina Koch and Jessica Meir took for their spacewalk (PA Graphics)

Spacesuits are made of many interchangeable parts – designed to accommodate astronauts with widely varying body sizes.

Body measurements of each astronaut are taken and plotted against the size ranges available for each spacesuit component.

The suits are then assembled – usually four months prior to flight.

While there are no differences between a male and female spacesuit, female astronauts usually wear a smaller size.

Both Ms Koch and Ms Meir are wearing medium-size suits, with Ms Koch’s spacesuit featuring red stripes.

In a recent interview, Ms Koch spoke about the significance of this spacewalk, saying: “I think it’s important because of the historical nature of what we’re doing.

“In the past women haven’t always been at the table.

“It’s wonderful to be contributing to the space programme at a time when all contributions are being accepted, when everyone has a role.

“That can lead in turn to increased chance for success. There are a lot of people who derive motivation from inspiring stories of people who look like them, and I think it’s an important story to tell.”

The spacesuits Ms Koch and Ms Meir are wearing
The spacesuits Ms Koch and Ms Meir are wearing (PA Graphics)

Ms Koch, an electrical engineer, has been living in space since March while Ms Meir, a marine biologist, joined the ISS crew in September.

Both are members of Nasa’s Astronaut Class of 2013.

Ms Koch is more than 200 days into her mission and is slated to set a record for the longest single spaceflight by a woman – with an expected total of 328 days in space.