After a week of speculation, the core stage of a Chinese Long March 5 rocket, dubbed CZ-5B, landed in an uncontrolled reentry in the Indian Ocean, near the Maldives. The rocket had transported part of China's new space station into orbit.

But its core stage — about 30 meters long and 5 meters in diameter — could have landed anywhere, even on land.

A year ago, a pipe from a previous Long March 5 rocket reportedly landed on someone's house in Cote D'Ivoire. Hence, all the fear and criticism this year.

NASA's administrator, Bill Nelson, said it was "clear" that China was "failing to meet responsible standards regarding their space debris."

Nelson is not alone with his opinion. But the picture is more complicated than that. And the US is by no means innocent.

More grey than black and white

"There is no doubt," says Alice Gorman, an associate professor at Flinders University in Adelaide, Australia, and author of Dr Space Junk vs the Universe, that "China has been a bit naughty."

"They are relying on the fact that most stuff either burns up [on reentry through the Earth's atmosphere] or falls over ocean or sparsely populated land. But given their experience last year, when the retaining rods [from another rocket] hit a village in Cote D'Ivoire, that optimism is a bit misplaced," Gorman told DW.

Chinese aerospace experts rejected any international concern before the rocket core came down on May 9. One expert, Song Zhongping, was quoted in the Global Times as saying that it was "completely normal" for rocket debris to return to Earth.

And Zhongping is right — it is quite normal for bits of rocket, satellites and even space stations to splash back down. And China's not even the worst offender. There are other nations and commercial companies doing it, too.

"As I've said, the main polluters of space are the US and USSR/Russia," says Gorman.

South Pacific spacecraft cemetery

The majority of space junk lands somewhere in the ocean. That's simply because there's more ocean than land.

Mission designers will target specific regions, such as the South Pacific Ocean Uninhabited Area (SPOUA), near Point Nemo.

Point Nemo is one of the Earth's "poles of inaccessibility." It is the farthest point from land in any direction on the planet.

In a blog post from 2018, the European Space Agency writes that more than 260 spacecraft have fallen in that zone since 1971. The number increases annually.

Perhaps unsurprisingly, Point Nemo is known as the "spacecraft cemetery." But it's not the only ocean region where spacecraft fall.  

"Point Nemo? It's sort of there, but it's like everywhere in the South Pacific between New Zealand and Chile," says Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, USA.

"The punchline is that it's not very concentrated," McDowell told DW. "And, more and more now, people are using [other spots].

When SpaceX launches its Falcon 9 rocket — a heavy-lift vehicle that towers over the Long March 5 by a further 13 meters — to send up small Starlink satellites, it lands its rocket stages to the South West of Australia, on a boundary between the Indian Ocean and the Southern Ocean, says McDowell. "Some of the Russian ones go down in the Indian Ocean, and some go down in the North Atlantic" — for instance, in Baffin Bay.

It depends on launch and mission goals

Where and how you come down depends on where and how you go up.

So, for instance, if you launch from the Russian spaceport at Plesetsk, says McDowell, you go up over the Arctic, but by the time you come back again, on the second northward trajectory, the Earth has turned about 20 degrees. "So, you're now coming up over the Atlantic, and you dump in there." 

On some missions, the main stage of a rocket will remain "suborbital" — in space at over 100 kilometers (62 miles) above sea level but below Low-Earth Orbit at about 160 km, and that makes it easier to drop rocket stages in a controlled way.

Even then, however, things can get precarious, especially when rockets are launched in-land, rather than from a coast.

A number of boosters have fallen near populated areas in China, once near an elementary school, and at a test site in Kazakhstan. Both cases released toxic orange clouds of what's called "BFRC."

Once a rocket enters orbit, things get more complicated. And the deeper a rocket goes, the harder it gets to deorbit.

It's more expensive, because you have to keep the rocket alive, as it were, with extended battery life and/or a restartable engine that gets fired after the rocket has delivered its "payload" — a satellite or supplies for the International Space Station.

But only then can you control its reentry. Many rocket stages just get left up in orbit.

It all depends. And it bears repeating that China's not the only "naughty" nation. McDowell estimates there are about 20 Falcon 9 upper stages "in orbit as junk that will eventually reenter" in some form or other.

A threat to marine life?

There is a trend in the industry to change, says McDowell. It wants to leave less debris in space for fear of a growing congestion that could either interfere with earthly communications systems or impede further space exploration.

But that means more stuff will have to come down. There is even talk of deorbiting the International Space Station in 2028, and dropping it at a final resting place in the South Pacific.

The impact on the ocean is — despite assertions that space junk becomes nice, natural habitats for marine life — largely unknown.

When junk falls at Baffin Bay, an icy point off Greenland, the threat to local seal, whales, bears and walrus, is under-researched.

In the South Pacific, scientists have discovered and revived 100-million-year-old microbial life at the South Pacific Gyre — essentially the same region as Point Nemo.

That microbial life may mean little to our daily lives, but microbes at extreme environments, such as hydrothermal vents, do sustain other life, such as the yeti crab, and may have even played their part in the origins of our own, human life.

"Some spacecraft fuels are toxic — hydrazine, for example. But cryogenic fuels are not toxic," says Gorman. "There are metals like beryllium and magnesium, they are usually in alloy form, but beryllium is pretty nasty no matter what." 

So, there are potential environmental impacts, says Gorman, "but I don't think people have thoroughly assessed that yet."

"Salt water can corrode things easily, but we have a million shipwrecks across the world and shipwrecks generally become habitats," she says. "And the priority really should be what's in orbit. That's by far the bigger risk."