Elon Musk's Starship rocket set for fifth test flight

SpaceX's Starship is set to take off later on its fifth test flight as Elon Musk pushes ahead in his quest to build the most powerful operational rocket system in history.

For the first time SpaceX will attempt to catch the Super Heavy booster, which sits at the bottom of the two-stage vehicle, as it returns to the launchpad in Texas.

Being able to land the booster safely increases its chances of being rapidly reusable, which would reduce the costs of spacefaring.

It was unclear until Saturday whether the launch would even take place after SpaceX awaited approval from US authorities.

A fifth flight in just 18 months is an extraordinary feat for the team at SpaceX. The first two flights saw the vehicle blown apart not long before its missions.

However, SpaceX argue this is all part of its development plan - to launch early in the expectation of failure so that it can collect as much data as possible and develop its systems quicker than its rivals.

Since the last flight in June, SpaceX has said that its engineers have collectively worked for 12,000 hours to replace Starship's entire heat shield with additional protections. During its last flight, as it re-entered the Earth's atmosphere, some of the protective tiles on the surface were destroyed as the Ship was enveloped by super-heated, ionised gas.

This time SpaceX is hoping to launch the 121m-tall (397ft) system but rather than having the whole system splashdown in the Indian Ocean like the last flight, they want the Super Heavy to fly back to the launchpad where it can be caught between a pair of giant mechanical arms called the "chopsticks".

The initial stages of the ascent will be the same as the previous four outings, with the Ship and booster separating two and three-quarter minutes after leaving the ground.

But then the booster will head back towards the launch site at Boca Chica in Texas and it will reignite its engines to slow the vehicle down from supersonic speeds. Sonic booms are expected to be heard in the surrounding area.

At the launch pad is the world's tallest rocket tower standing 146m-high (480ft) with two giant robotic arms. These will catch the rocket booster before re-stacking it on the orbital launch mount for its next flight.

Catching the booster rather than getting it to land on the launch pad reduces the need for complex hardware on the ground and will enable rapid redeployment of the vehicle in the future.

If the Flight Director does not think it is possible then a call will be made before the boostback burn - when the rocket turns around - for the booster to land in the Indian Ocean. This will be less than three minutes into the flight.

Elon Musk and SpaceX have grand designs that the rocket system will one day take humanity to Mars, making our species "multi-planetary".

The US space agency, Nasa, will be watching the flight attempt closely. It has paid the company $2.8bn (£2.14bn) to develop Starship into a lander capable of returning astronauts to the Moon's surface by 2026.

In space terms that is not that far away so Elon Musk's team have been eager to get the rocket re-launched as soon as possible.

But the Federal Aviation Administration (FAA) , the US government body that must approve the flight, had previously said there would be no launch before November as it reviewed the company's permits.

Since last month the agency and Elon Musk have been in a public spat after the FAA said it was seeking to fine his company, SpaceX, $633,000 for allegedly failing to follow its license conditions and not getting permits for previous flights.

Before issuing a license the FAA reviews the impact of the flight, in particular the effect on the environment.

In response to the fine Musk threatened to sue the agency and SpaceX put out a public blog post hitting back against "false reporting" that part of the rocket was polluting the environment.

Currently the FAA only considers the impact on the immediate environment from rocket launches rather than the wider impacts of the emissions.

Dr Eloise Marais, professor of atmospheric chemistry and air quality at University College London, said the carbon emissions from rockets pale in comparison to other forms of transport but there are other planet-warming pollutants which are not being considered.

"The black carbon is one of the biggest concerns. The Starship rockets are using liquid methane. It's a relatively new propellant, and we don't have very good data of the amount of emissions that are coming from liquid methane," she said.

Dr Marais said what makes black carbon from rockets so concerning is that they release it hundreds of miles higher into the atmosphere than planes.

"They're putting pollutants in layers of the atmosphere where they stay for two and a half, or three years, compared to pollutants close to the surface of the earth that go away after about a few days to a week. So the longer that they stay in the atmosphere, the bigger the impact they have," she said.

In April, Nasa released its first space sustainability strategy in which it said "the chemicals used during launch raise concerns about atmospheric impacts". It did not lay out specific solutions but committed to work with its climate team on this issue.