Two NASA astronauts are due to make history this week by taking part in the first ever all-female spacewalk.

Astronauts Christina Koch and Jessica Meir will work to fix the power system of the International Space Station (ISS), the habitable satellite which is in orbit 220 miles above the Earth's surface.

The momentous spacewalk was due to take place on October 21. But NASA announced on Tuesday it would be pushed forward to Thursday or Friday.

In a change to their brief, the pair will replace a faulty battery charge-discharge unit, which is currently preventing a new lithium-ion battery installed earlier this month from powering the ISS.

The ISS is fuelled by a collection of thousands of solar cells known as arrays. Battery charge-discharge units control how much charge batteries which collect energy from these arrays receive.

NASA officials explained during a press conference attended by Space.com that the issue stems from a battery pack which was swapped in April.

Despite the broken unit, NASA gave assurances in a statement that the crew is safe and their laboratory experiments onboard the ISS have not been disrupted.

The postponed spacewalk due to take place on 21 October would have been the fourth of 10 planned to take place between October and December, as part of what is known as Expedition 61.

Commanded by the European Space Agency's Luca Parmitano, the mission includes NASA astronauts Koch, Meir, and Andrew Morgan, as well as Russia's Aleksandr Skvortsov and Oleg Skripochka.

Meir and Koch had originally planned to help replace nickel-hydrogen batteries with "newer, more powerful" lithium-ion batteries on the far side of the ISS's port truss, according to NASA. The work is a continuation of an upgrade of the stations power system which started in January 2017.

The second half of the sequence, expected to start in November, will see crew members fix the space station's alpha magnetic spectrometer.

Meir joined Kochwho holds the record for the longest single spaceflight by a womanat the satellite in late September, and will spend over six months on the ISS.

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The first all-woman spacewalk was controversially postponed in March as there were not enough medium-sized space suits on the ISS to fit both women. NASA astronaut Anne McClain was scheduled to join Koch in updating the ISS's power sources. But McClain arrived back on Earth in June, completing a 204-day mission.

In March, McClain became the 13th woman to complete a spacewalk, followed by Koch a few days later.

McClain tweeted back in March: "This decision was based on my recommendation. Leaders must make tough calls, and I am fortunate to work with a team who trusts my judgement. We must never accept a risk that can instead be mitigated. Safety of the crew and execution of the mission come first."

Dr. Scott G. Gregory, lecturer in astrophysics at the University of Dundee, told Newsweek the all-female spacewalk is "both historic and inspirational."

"Despite many talented female scientists and engineers, spacewalks have been a male-dominated activity," he said.

"Although there have been female spacewalkers before, it has always been with a male counterpart. This first all-female spacewalk is long overdue. Jessica Meir and Christina Koch are following their childhood dreams of being astronauts and 'walking' in space. It represents years of dedication, study, hard work, and pushing limits and that is truly inspirational."

Gregory said he'll be watching the live stream of the spacewalk with his 5-year-old daughter.

"We'll be following the news as Jessica Meir and Christina Koch write their own history."

"All over the world lots of little girlsand little boyswill be watching and they'll be dreaming that they'll be walking in space when they're grown up," he added. "If that inspires them to dedicate their lives to scientific or technological endeavours, this can only be a positive for all of society."

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What the Women of the First All Female Spacewalk Will Do at the ISS - Newsweek