Why are these astronauts 'stuck' in space?

Butch Wilmore and Suni Williams left Earth on Boeing's Starliner spacecraft on 5 June

Nasa says they are still unsure about how to bring home two of its astronauts that are 'stuck in space'.

Butch Wilmore and Suni Williams set off from Florida on Boeing's Starliner spacecraft on 5 June after multiple delays.

They were only meant to stay in space for around a week, however their craft developed a number of issues - meaning they were unable to head back.

The US space agency say that the two astronauts were “doing great”, and that they will make a decision soon on how to get them down to Earth.

What's the latest?

The pair were expecting to return from space after eight days

Suni Williams and Butch Wilmore travelled to the International Space Station (ISS) at the beginning of June, expecting to return back to earth after eight days.

But Nasa discovered a number of issues with the Starliner spacecraft they were due to travel on, including several helium leaks in its capsule.

The problems mean the two astronauts have to remain on the ISS while engineers carry out a technical investigation.

One alternative option to bring them back, is by using a craft from another company, SpaceX, however it isn't easy to change how the astronauts travel back to Earth.

That's because astronauts’ space suits are designed specifically for the spacecraft they are flying in.

Butch Wilmore and Suni Williams are the first crew to fly Starliner, which Boeing and Nasa hope will make regular trips to the ISS in the future.

Despite the issues, Nasa says it has “confidence” in Starliner and that the machine could be used to return the astronauts home in an emergency.

The US space agency says it will continue looking at the best way of bringing the astronauts back, before making a decision by the end of the month.

What is Boeing's Starliner?

The Boeing Starliner is a capsule designed to transport crew and equipment into space.

Nasa asked Boeing - which is a private company that makes planes - to develop the Starliner so they could have a US-made spacecraft to transport crew to and from the ISS.

It's made up of two main sections - a reusable crew module, which can fit up to seven astronauts inside, and a service module with thrusters and engines to power the craft.

The capsule, which is where the astronauts sit, has been designed to be as safe as possible and has an escape system that propels the crew away from the rocket, if there's a problem during launch.