Revealed: How Boeing's faulty Starliner could crash into the ISS in worst case scenario

Boeing's faulty Starliner spacecraft will finally begin its journey home today after three months spent docked to the International Space Station (ISS).

But experts have raised concerns over a small chance that Starliner could malfunction again, and potentially send it careening into the ISS. 

'There is an extremely small probability that if all of the thrusters fail, the Starliner could impact the ISS,' former US military Space System Commander Rudy Ridolfi told DailyMail.com.

'This will be like NASCAR for NASA, some watch just for the crash,' Ridolfi added. 

Starliner's technical issues - including thruster failures - have lefts its crew stranded on the ISS for the last three months. 

NASA officials decided it was too risky to bring astronauts Sunita Williams and Barry Wilmore home on the faulty spacecraft. They will remain on the ISS until next year, which is why Starliner will return to Earth unmanned today. 

Shortly after 6pm ET today, Starliner will autonomously undock from the ISS and then begin a six-hour flight back to Earth.

The spacecraft launched June 5, carrying Williams and Wilmore to the ISS.

By the time the spacecraft reached the ISS, five of its 28 reaction control system thrusters had failed. 

As a result, Starliner's first attempt to dock to the space station was waived off. But ultimately, the spacecraft successfully docked and Williams and Wilmore safely boarded the ISS.

As for whether these propulsion issues have been resolved, 'I would say no,' Harvard University astronomer and astrophysicist Jonathan McDowell told DailyMail.com. 

Although NASA and Boeing have identified overheating as a likely source of the issues, 'they still don't really fully understand why the thrusters are behaving the way they are, and so that means that they can't say for sure that they're not going to act up again,' he said.

'Having said that, I fully expect tonight to go fine,' he continued. But in a highly unlikely worst-case-scenario, thruster issues during the undocking could cause Starliner to collide with the ISS. 

If one of the thrusters fails to stop firing when directed, for example, 'it could push the Starliner into a collision with some other part of the space station,' he said. 

Crashes like this have happened before. In 1997, a Russian space station was damaged when a cargo spacecraft missed its docking port and smashed into one of the station's modules.

The collision punched a hole in the space station 'and the astronauts had to rush to  close the hatches to that module before all the air ran out of the whole space station,' McDowell said. 

'That was a very, very scary thing. So that's the thing you want to avoid,' he added.

Additionally, if more than a certain number of the thrusters fail, there's a chance that Starliner might be stuck drifting toward the ISS and ultimately crash, McDowell previously told Business Insider. 

'That could happen, but it would have to be very specific ones that fail,' Ridolfi wrote in a statement sent to DailyMail.com. 

'There are five 'sets' of thrusters and if they can still rotate the spacecraft, sets of thrusters can take over for ones that are out,' he continued.

If thruster failures do send Starliner on a collision-course with the ISS, a crash won't happen right away, Ridolfi said.

That's because although the two objects will be close to each other, they will be in slightly different orbits, he explained.

'US Space Forces (USSF) will conduct what is called 'conjunction analysis' to show when the two objects could collide,' he said, adding that it could become an issue about every 90 minutes as each object completes an orbit.

But the ISS also has its own thrusters that it can use to avoid a collision, which it does several times a year, Ridolfi said. 

Although, 'it's a pain to move the ISS mainly because it’s a pig and moves very slowly,' he added. 

Plus, mission control will be 'watching like a hawk, so the minute that any one of the thrusters starts to misbehave, they're going to shut it down and take over with a backup thruster,' McDowell said. 

But even if Starliner manages to successfully undock from the ISS, it won't be out of the woods quite yet. After the undocking, the spacecraft will have to re-enter Earth's atmosphere. 

'At that point, you've really got to have the thrusters working right, because they're steering the ship as it's firing its main engines, and it's important that the main engines fire in the right direction,' McDowell said.

'The worst case scenario there is that you lose the thruster system completely, for example, and you can't steer the spacecraft while it's making the de-orbit burn,' he added.

In that case, the Starliner capsule would re-enter at the wrong angle and speed, either causing it to burn up or remain temporarily stranded in orbit before re-entering uncontrolledly some time later, he explained. 

The issue with an uncontrolled de-orbit burn is that Starliner has heat shielding to help it survive re-entry. Therefore, 'you could imagine chunks of this thing surviving to the surface,' McDowell said.

'And if you were very unlucky, that could happen over somewhere populated. And you could have a repeat of the incident we had earlier this year when some poor guy had pieces of spacecraft go through the roof of his house,' he continued. 

But this scenario is highly unlikely. McDowell thinks it's more probable that thruster issues could cause Starliner to burn up over the Pacific Ocean instead of landing in White Sands Space Harbor, New Mexico as intended.

In the event that Starliner's thrusters misfire during re-entry, there's little that mission control can do to resolve the issue, McDowell said.

If problems arise right from the start, mission control could shut everything down, keep Starliner in orbit for another day and try again tomorrow, he explained.

'But once you're well into the burn, if things go start going wrong there, you're already on a trajectory where you're going to hit the atmosphere in some form anyway. So you've just got to hope that everything stays right,' he said. 

The risks associated with possible thruster mishaps both during Starliner's undocking and its re-entry likely played into NASA's decision not to bring Williams and Wilmore home on the spacecraft.

The space agency announced their decision at an August 24 press conference. 

Instead, Williams and Wilmore will remain on the ISS until at least February 2025, when SpaceX's Crew Dragon spacecraft will shepherd them back to Earth. 

At another press conference on Wednesday, NASA officials stated that they have been working to mitigate any risk to the ISS during Starliner's undocking, and that tests of the spacecraft's thrusters showed that they were working well.