Part of the Chinese Long March 5B rocket went into an "uncontrolled reentry phase" - in plain English: "falling to Earth and out of control".

We are suddenly aware of danger from the skies but also that space is congested.

We may have imagined that space was vast and empty but it turns out that we are cluttering it up.

So what's going on?

"Space junk, or space debris, is any piece of machinery or debris left by humans in space," according to the Natural History Museum in London.

"It can refer to big objects such as dead satellites that have failed or been left in orbit at the end of their mission. It can also refer to smaller things, like bits of debris or paint flecks that have fallen off a rocket."

On top of that, some bits of debris are solid lumps of metal and others are paper thin. Each shape might cause different sorts of damage in a collision with a satellite. Some are no bigger than specks, but travelling at more than 20,000 kilometres per hour.

Big and tiny, flat or heavy and solid, these bits of junk are flying at great speed. Even the smallest piece would destroy what it hit - like a satellite.

It is big - but the debris tends to stay in the same orbit. It doesn't spread throughout the immensity of space. The higher bits of junk (further than about 500 kilometres from the Earth's surface) will stay there but lower bits might not.

Humans have been sending up satellites since 1957 and by now, nobody quite knows how many objects are out there, circling the Earth.

NASA reckons that there are more than 500,000 human-made bits of orbiting debris the size of a marble or larger.

Imagine the earth as a ball surrounded by layers of circling objects, some of them satellites. There are about 2000 operating satellites orbiting Earth at the moment plus another 3000 dead ones.

And then there are clouds of much smaller bits of debris "that could nonetheless prove disastrous if they hit something else," according to the National History Museum.

Professor Steven Freeland of Western Sydney University and Bond University said the world economy depends on satellites. "If we didn't have space, our economy would collapse. If we make space unsustainable that will have a devastating effect," he told this paper.

There have already been serious near misses. A year ago, two "dead" satellites passed within metres of each other. "NASA often moves the International Space Station when it calculates a higher-than-normal risk of collision with debris," according to Professor Freeland.

The risk of collision increases markedly. In the late 1970s, a retired NASA scientist, Donald Kessler, warned of collisions generating more debris which, in turn, would generate more collisions and so on in a spiral.

The scenario was called the Kessler Syndrome: "a cascading, self-sustaining runaway cycle of debris-generating collisions can arise that might ultimately make low-Earth orbit too hazardous to support most space activities".

Junk has fallen to Earth. In 1977, Kosmos 954 came to earth, with its nuclear reactor. In the end, it fell in uninhabited wilderness in north-east Canada but it could easily have been a city.

"It was just a quirk of fate that Kosmos 954 did not land on Toronto or Quebec City, where the radioactive fallout would have necessitated a large-scale evacuation," Professor Freeland said.

"In 2007, pieces of debris from a Russian satellite narrowly missed a Chilean passenger plane flying between Santiago and Auckland. As we send more objects into space, the chances of a calamitous crash-landing will only increase."

"In 1979, the 77-tonne US space station SkyLab disintegrated over Western Australia, peppering the area around the southern coastal town of Esperance with fragments."

Esperance's council issued NASA with a fine for littering.

In the case of Kosmos 954, Russia paid Canada about $3 million (about $9 million in today's money). Canada thought the compensation was way too low compared with the cost of the clean up and anti-nuclear precautions across a swathe of its territory.

There are treaties and agreements about responsibility for space debris. Broadly, they say that the damage caused by the debris generated by a mission should be paid for by country or agency which launched the mission.

The United Nations Outer Space Treaty suggests that a launching state "shall be absolutely liable to pay compensation for damage caused by its space objects on the surface of the Earth or to aircraft, and liable for damage due to its faults in space."

Professor Freeland says that the difficulty with the law is it addresses a potential disaster after it's happened but the urgent task is to prevent it happening in the first place.

All kinds of clever technological solutions have been proposed, nets, magnets, harpoons, lasers and robots among them. One aim would be to remove dead satellites from orbit by dragging them down into the atmosphere, where they would burn up.

A new laser at Mount Stromlo outside Canberra can knock larger debris from orbit.

But there is no one-size-fits-all solution for removing large rockets at the heavy end to tiny particles at the smallest level.

And having the capability of removing satellites sets up another complexity, according to Alice Gorman of Flinders University - she blogs under the name Dr Space Junk.

"The big issue is that any successful technology that can remove an existing piece of debris can also be used as an antisatellite weapon," she told Scientific American. "This is a whole other can of worms that requires diplomacy and negotiation and, most importantly, trust at the international level."

Scientists agree that the problem is serious and will get worse - but also that the solutions are complex, and we are nowhere near finding them.