Image copyright NASA Image caption The Soyuz system is the only means to get crew to and from the ISS

There was always the risk that when the US shuttles were retired, continued operations on the International Space Station (ISS) could be left more vulnerable should there be a failure on the Russian Soyuz rocket system.

Soyuz has become the sole means of getting people to the 400km-high outpost. If it can't fly, no-one can. It's the classic single-point failure with no back-up.

Everyone knew this was a possibility; few I suspect thought we'd be in such a position so soon after the last American orbiter was withdrawn from service.

The failure last week of an unmanned freighter to the ISS has left the Russians questioning the reliability of their usually ultra-reliable Soyuz fleet, and the planned manned flight to the station on 22 September has now been postponed as a consequence.

The commission set up to investigate Wednesday's failure believes already it has a good idea of what went wrong, and so it may not be too long before normal service is resumed.

But the ISS partners have to make contingencies, and if Soyuz rockets are out of service for a while, the station will see its crew complement reduced - perhaps all the way to zero.

The driving factors are the 200-day "use-by" rules on the Russian-built capsules that carry astronauts home from the ISS, and the safety protocols that demand all landings in the Kazakh Steppe must take place in daylight.

The second regulation needs no explanation; the first might. Russian engineers will certify, or "guarantee", the performance of the return capsules for a fixed period.

Beyond this 200-day limit, the confidence in the performance of the vehicle drops off, and in particular in the status of the propellant in their manoeuvring thrusters.

So although there are plenty of supplies on the orbiting platform right now to sustain a crew all the way through to June next year, the regulations mean the astronauts have narrow windows in which to make their return to Planet Earth.

And if they have to leave and no-one goes up to replace them, the ISS begins to empty.

Image copyright NASA

The crew will fall from six to three in late September. That's a given. And if Soyuz rockets cannot be returned to service by mid-November to take up relief astronauts in a fresh capsule, then the station will be vacated by its remaining residents.

The ISS has been continuously manned since November 2000; and for the Americans especially, an empty station would be a significant blow to morale.

Having lost the use of the shuttle, the orbiting outpost is now the core of their human spaceflight programme. It wouldn't sit well to see it flying overhead with no-one inside.

It would be a blow also to the image of the station and its scientific programme.

Of late, the number of astronaut hours dedicated to research has been running close to 50 a week - a significant increase on the assembly phase of the ISS when crewmembers had much less time for science.

Many experiments could run autonomously or by remote operation from the ground. The hugely expensive and much-talked-about Alpha Magnetic Spectrometer mounted on the top of the station is a case in point.

It needs next-to-no-intervention from crew members to gather its data.

The science that would come to a shuddering halt are the experiments that require a human subject (the astronauts are the guinea pigs in many physiological studies), or the ones that need occasional human intervention, either to change test parameters or to fix components that unexpectedly fail.

Image copyright AP Image caption Safety demands that all capsule landings in the Kazakh Steppe must take place in daylight

You also lose the serendipitous observation of a passing human eye that notices something unusual or interesting in an experiment that has been missed by a camera.

The ISS partners considered a temporary mothballing of the station after the Columbia shuttle accident in 2003. It's a bigger object now, but the procedure would be the same.

The hatches between the modules would be closed, and all onboard systems would be configured for control from the ground.

Nasa says it could run the station in this state "indefinitely", excepting multiple failures on critical components.

The imperative would be to maintain a stable orientation, enabling manned vehicles to dock again at some later point.

Everyone will be hoping such a contingency is not needed come November.

"If you look at probability risk assessments, some of the numbers are not insignificant," said Mike Suffredini, the US space agency's (Nasa) ISS manager. "There is a greater risk of losing the ISS when it's unmanned than if it were manned, and that's why when we made our decision after the Columbia accident to keep the station manned - that's exactly why. The risk increase is not insignificant."

It is worth remembering that even if the shuttles were still flying, the situation now facing the ISS partners would not be massively different. Shuttles could only ever stay at the station for a few days.

If they were magically available today to take astronauts up to the platform, those crew members would still need "lifeboats" to bring them home in an emergency or at the end of their tour - and those capsules are launched on Soyuz rockets.

The situation, however, will throw some light back on to Nasa's readiness - or lack of it - to bring in a replacement for the shuttle. These capsules, provided by the private sector, are still three or four years away from flying. Their entry into service should make ISS operations significantly easier.