American aerospace giant Boeing is very soon going to make another attempt to fly its first crewed mission to the International Space Station aboard a Starliner spacecraft. We can show you these live pictures. That is the scene on the launch pad right now at Cape Canaveral. It is going to launch in less than five minutes now. There will be two veteran astronauts on board. They're already on the craft. They are ready to go. A successful launch would make Boeing the second commercial operator to ferry crews to the International Space Station after Elon Musk's SpaceX X. Boeing did try to do this with the Starliner at the start of May, on the sixth of May, but that previous attempt was aborted just hours before liftoff. Well, I've got two eminent minds to join me to talk us through this moment. Our science correspondent, Jonathan Amos, is with us, as is Dr. Simeon Barber, who's a space scientist at the Open University. Thank you, both for joining us. I've got to say I'm particularly excited about this because about six years ago, I made a program for the BBC, and I got to see the Starliner being built.

I got to go inside the simulator where they trained the crew. And this is, Jonathan, as we were saying, it's a big moment because commercial crew, that's what they're hoping is the future of space travel, isn't it?

It's a big deal for the US Space Program. It is a particularly big deal for Boeing. You can't have escaped the news in recent years that the company has had a number of difficulties with its airplanes. And this vehicle, too, the capsule on the top of the rocket has had problems. You talk about six years ago. Back in 2010, I discussed with Boeing executives their plans to introduce a commercial capsule. They thought it would be launching with people on board in 2015. That's almost a decade ago. That is how hard it has been for Boeing to develop this capsule. They've had any number of technical issues. And even today, this capsule is launching with a helium link. It's a very small helium leak in the propulsion system. They think it's perfectly safe, but it speaks to the problems that Boeing have had in getting this system ready to fly. So they need a flawless ascent today. They need everything to go perfectly well. And then the capsule, which is Starliner, can join the fleet of other vehicles that the space agencies use to take astronauts up to and down from the international space station. So you mentioned the SpaceX and their Dragon capsule.

There is also still the Russian Soyuz capsule. This would make a third, and you need redundancy, because if you have a problem with one of the other vehicles, one steps forward that's in reserve. So it's very important for everybody concerned.

We had a countdown time on the screen until just a couple of moments ago, those live pictures from NASA. Let's see if we can listen in and actually hear what's going on there right now.

Okay, console. Step 60, report launch vehicle status. Atlas lead. Atlas tank pressures and helium bottles stable. Atlas propulsion system stable. Roger. Centra lead. Centra tank pressures and Helium bottles stable. Centra Our propulsion system stable. Has gas readings nominal. Roger. Vehicle electrical.

And Dr. Simeon Barber is still listening to this as well. So these are the final pre-flight checks. And words like stable, that is exactly what their mission controller wanting to hear right now.

Absolutely. Words like go, words like stable. Nominal has been my favorite word over the last two hours listening in to this.

And just describe to us as well, Dr. Barber. I've got to say, Anna. Oh, yeah. Yeah, Jonathan, go for it.

Yeah, I'm just going to say we have a scrub for the day. So just a couple of minutes before launch, they have decided to call it off. It's not absolutely clear why that is. But when you-Is that why the Timer disappeared?

Basically, they can't hang on, right?

You have to go on the button. You have to go on the button. In order to be able to catch the space station as it comes overhead, you essentially launch to the second. And to resolve whatever issue they have today, they don't think that they can get through it and then get back on the clock. So they're going to have to come back. Probably tomorrow is the first opportunity that they'll get.

That'll be disappointing for us. So, Jonathan, is that why? Because We noticed that Timer was there. It got to about three and a half minutes, and then it disappeared from the live pictures. So do we think that was the indication that things weren't going as planned?

Yeah. I mean, these vehicles are festooned in sensors. They may well have seen something that they didn't like. If they'd had time, they might have been able to work through it. When you have a launch like this, they build in what they call planned holds. That's to say they stop the clock for a period of time, and that allows them then to troubleshoot. But when you get up to that moment of launch, there is no time to troubleshoot. So they make a decision to scrub for the day, to make all the systems safe. They'll get all the systems safe, they'll detank the rocket, and they'll get the two astronauts off the top, and they'll go back to their quarters, and they'll have another day on Earth.

Stay with us, Jonathan. Dr. Simeon Barber, the excitement in your voice just a couple of minutes ago when this still looked like it was going ahead, and you were saying you've been watching these pictures for the last couple of hours. As Jonathan was touching on, this is going to be another disappointment for Boeing.

It's a huge disappointment. The Even in the last 12 minutes before launch, a problem occurred, which was to do with a fan on one of the astronaut's suits, and that fan needs to be working in order to take off. It seemed that that had been fixed, and we're hoping that that was the last of the last minute problems. I'm juggling between a mobile phone feed and listening to yourself. They're solving problems real-time here, but ran out of time on this one. And of course, this is human space flight, so safety is the number one concern for the stop.

And Simeon, just talk us through what we're looking at here, because obviously, you've got that huge Atlas rocket with the Starliner itself just right on the very top. Jonathan was talking about the complexity of what we're seeing Can you just talk us through what these pictures actually show.

What we're seeing is a five-metre capsule, which is where the astronauts are sitting, strapped in, so two astronauts. They're basically sitting on top of 300,000 liters of fuel. This is basically a massive system of electronics and computer control, firing massive engines of different types, which allows it to break free of Earth's gravity. In the first two minutes of that flight, when it finally goes, it'll be burning 2 tons of fuel every second. That tells you why everything has to be right before you hit that final go button. You want to make sure that everything is performing normally. I don't know what that issue was, but someone's made that call, and there'll now be a controlled depressurization of the fuel system on the rocket, and they'll get the crew off, and then they'll try and go again tomorrow.

And they are extraordinary live pictures that we're just seeing now. And this, I presume, Jonathan, I know that you're seeing a similar picture. It looks like they're actually moving around to take those astronauts off, as you would expect. Jonathan, they are veteran experienced astronauts. They will be used to delays like this, but disappointed all the same.

Yeah, you're just seeing there, Anna, the crew access arm coming into position. So that will allow the ground crew to approach the capsule, to unseal that door, and then get Butch Wilmore and Sonny Williams out of the capsule and then take them down into a waiting vehicle, which will take them back to their quarters. But yeah, I mean, Wilmore and Williams are two of the most experienced NASA astronauts in the astronaut corps. So yeah, they'll be disappointed. But they know what the game is, right? They've flown Shuttle, they've flown Soyuz. This was supposed to be the first American vehicle since Dragon, the sixth astronaut vehicle in total, and it would have been nice to see it fly. But as Simeon was saying, they act out of an abundance of caution here. They're not going to take any risks. They don't need to, especially when they can come back. I think they've earmarked another two or three days shortly this week. Then they may have to stand down for a while if they can't fix the problem that they have. Hopefully, it'll be something when they take the vehicle back to its shed that But they'll be able to tinker with something and then get it right.

And they won't have to take the capsule on the top and really get under the bonnet to try and change some component out. And we will be back tomorrow or maybe the day after.

And Simeon, we have to say the frustration will be added to. We mentioned SpaceX, who are already doing Commercial Crew and have been managing it for a little while. So that will no doubt add to Boeing's frustration.

I'm sure it will. It just was so close today. I feel for the whole crew and the whole team, but just going back to the astronauts, they've been living in quarantine for many weeks now. So to make sure that they're fit and healthy, they're not taking any bugs up to the international space station. So can you imagine what it's like to be having... It's like waiting to go on the most amazing holiday of your life, even though they're working. And these flight delays. They must be so painful to take in their stride. But they're professionals, as John said, and I'm sure they'll be ready when the spacecraft's ready to go.

And in fact, we can see the faces of both of them now. Just some of those little gestures are certainly giving away a hint of that frustration. But as Jonathan, as you were saying as well, safety is all important, and Sadly, we remember those moments when space travel goes wrong. And of course, there are many moments where it's incredibly successful, but there's a history to this, and there's a reason why they make every single effort, don't they, to keep those astronauts and also that very expensive spacecraft as safe as possible.

Yeah, I mean, the origin of this vehicle, really, you can take it right back to 2003 and the loss of Columbia, the shuttle. And I think everybody then decided that the basic design of shuttle was not safe. There was this period in time from the launch pad to quite high up where the astronauts were very vulnerable. They couldn't escape their vehicle if they ran into trouble. And so NASA decided they'd retire the shuttle, and then they take a different course with new vehicles that were inherently safer. This capsule, if they have a problem on the launch pad right from the moment of go, they can get that capsule off with a series of thrusters. They can push it away from the rocket and keep those astronauts alive. And not only did NASA decide that they wanted to go with safer vehicles, they wanted to go with a new approach as well. They wanted to go with a commercial approach. So rather than owning and operating their own vehicles like they did with the space shuttle, they would essentially contract out. They would buy in the service, a taxi service, to use or push the analogy further.

It's rather like the company that contracts out its IT or its payroll. Nasa would do the same. It would contract out transportation. It would buy a service off SpaceX, and it gave a contract to to Boeing to do exactly the same. They would hail the taxi whenever they needed an astronaut to go to the space station. They would put their hand up and Boeing or SpaceX would come and say, Yeah, there's the seats. Get in and we'll take you. And It's gone very well for SpaceX. They've done this particular flight, the first crew test flight. They did that back in 2020. But this is the stage that Boeing are at now. Meanwhile, SpaceX have gone on and they've done six operations international flights for NASA. And really, Boeing need to get a move on because the International Space Station is going to be retired in 2030. And if they launch this vehicle once a year, those six flights which they are contracted to, that will take them right up to the end of the space station. Now, there probably will be commercial space stations after that, which they can sell a transport service to. But for NASA, this is a key moment.

Jonathan Amos, our science correspondent, Dr. Simeon Barber, space scientist with the Open University. Thank you both for your expertise. I'm hoping you'll both join us again tomorrow when they go for launch again. And we'll just leave you with those pictures for now. Now, those live pictures that we were seeing from Kate Canaveral of, as we were saying, two veteran, highly experienced, but ultimately today disappointed astronauts. As you were watching the live pictures with us, you might have noticed, as we were saying, that countdown in the top left-hand corner of the screen got to about three and a half minutes before launch when it vanished, and that was the first sign that all was not going as planned, but they are expecting to go again tomorrow. You can still hear. I'm just going to let you listen just one last time.

Cat, TVM system enabled. Readings nominal. Roger.

They're just doing their final... They were doing their checks at the beginning, and now they're doing all of their various checks to make things safe before they're actually taken off the flight. But back tomorrow, they say that they will again try and launch in the suitable window in 24 hours time. And of course, when it it does happen, you'll be able to follow it along with us here on BBC News.