Space junk tracker LeoLabs has been monitoring the paths of a defunct Russian satellite and a discarded Chinese rocket segment.

It sees them converging over Antarctica at 00:56 GMT (01:56 BST) on Friday.

Other experts who've looked at the available data think Kosmos-2004 and the ChangZheng rocket stage will pass with a far greater separation.

With a combined mass at over 2.5 tonnes and relative velocity of 14.66km/s (32,800mph), any collision would be catastrophic and produce a shower of debris.

And given the altitude of almost 1,000km, the resulting fragments would stay around for an extremely long time, posing a threat to operational satellites.

Neither Kosmos-2004, which was launched in 1989, nor the ChangZheng rocket stage, launched in 2009, can be moved. So, there is no possibility to influence the event.

LeoLabs offers orbital mapping services using its own radar network.

Dr Moriba Jah, an astrodynamicist at the University of Texas at Austin, has worked out the miss distance to be about 70m.

And the Aerospace Corporation, a highly respected consultancy, comes to a similar conclusion.

With more and more satellites being launched, there's certainly growing concern about the potential for collisions.

The big worry is the burgeoning population of redundant hardware in orbit - some 900,000 objects larger than 1cm by some counts - and all of it capable of doing immense damage to, or even destroying, an operational spacecraft in a high-velocity encounter.

This week, the European Space Agency released its annual State of Space Environment report.

It highlighted the ongoing problem of fragmentation events.

These include explosions in orbit caused by left-over energy - in fuel and batteries - aboard old spacecraft and rockets.

On average over the last two decades, 12 accidental fragmentations have occurred in space every year - "and this trend is unfortunately increasing", the agency said.

Also this week, at the online International Astronautical Congress, a group of experts listed what they regarded as the 50 most concerning derelict objects in orbit.

A large proportion of them were old Russian, or Soviet-era, Zenit rocket stages.