Sometime over the weekend, the 30-meter long 20+ ton first stage core booster of a Chinese Long March 5B rocket will re-enter Earth's atmosphere in an uncontrolled manner, with pieces of it crashing somewhere on the planet.
The thing is, we don't know where.
Now, don't panic. It's a big planet, and the odds of it hitting anyone are low… but not zero. To be more clear, this is a worrisome event but not one to be freaked out about.
[UPDATE (Sunday May 9, 20201): The rocket fell in the Indian Ocean at 02:24 UTC Sunday morning, with pieces reportedly coming down roughly 50 kilometers (!!) west of Maldives. That's very close, and as I say in this article below, terribly irresponsible of China to allow this to happen, especially since they have not commented even now on why and how this huge rocket was allowed to re-enter in an uncontrolled way. I have not heard any updates on any actions that might be taken about it, although the new NASA Administrator, Bill Nelson, had this to say:
“Spacefaring nations must minimize the risks to people and property on Earth of re-entries of space objects and maximize transparency regarding those operations.
“It is clear that China is failing to meet responsible standards regarding their space debris.
“It is critical that China and all spacefaring nations and commercial entities act responsibly and transparently in space to ensure the safety, stability, security, and long-term sustainability of outer space activities.”
If there is more news I will update this post.]
Why don't we know where it will hit? There are several factors here. For one, it's slowed by the very thin upper reaches of the Earth's atmosphere, and the density of the air is so low there that even a small change affects orbiting objects strongly (the density also changes with solar activity; space weather can make the upper atmosphere puff up, and it reaches higher).
For another, video from an observer on the ground shows the booster rapidly changing brightness over time in a cyclical way, indicating it's tumbling as it orbits —when we see it broadside it reflects more sunlight toward us and it appears brighter than it does when we see it end-on. That means the cross-section it presents to the atmosphere changes all the time, making it even more difficult to predict its orbit and fall.
Third, it's moving fast. Orbital speed in low Earth orbit is roughly 8 kilometers per second, so every second you're off in your prediction means you're 8 km off in your prediction of where it is.
The small amount of drag it feels right now is stealing orbital energy from it, so it drops lower. As it drops lower the air gets thicker, so it drops faster. Eventually it hits a part of the atmosphere thick enough that as it rams through the air the gas ahead of it compresses violently and heats up, creating an artificial meteor. It “burns up”.
The problem here is that not all of it will burn up. Fuel tanks, for example, are relatively lightweight for their size, and they're made of heat-resistant material. They can survive the hottest part of re-entry and get to the ground. This, obviously presents a danger.
The orbit of the booster is slightly elliptical, and orbital dynamics means things get a little weird. Once per orbit, as it's nearest the Earth (what we call perigee), the increased drag tends to lower its apogee, the point of its orbit where it's farthest from Earth. This has the effect of making the orbit more circular, but dropping it down lower. My friend and astronomer Jonathan McDowell is an expert in tracking such things, and has been posting updates on Twitter.
The rocket launched on April 29, carrying Tianhe, the core module of a new Chinese space station called Tiangong. Tianhe appears to be in orbit and doing fine, but the core stage booster's apogee is dropping rapidly. The orbit is decaying as expected, and re-entry is predicted between May 8 and 9. It probably won't be possible to narrow it down more until Friday May 7 at least. Follow McDowell on Twitter to get updates, and I'll be tweeting too.
What are the odds it'll hit over land? Relatively low. The Earth is mostly water, but you also have to account for the tilt of the rocket's orbit, too. At a roughly 42° tilt, the path it takes over the Earth is over land about 25% of the time. A lot of that land is uninhabited, dropping the odds of injury further. The orbit, however, takes it as far north as New York City and as far south just past the southernmost part of Australia. Anyplace from 42° N and 42° S latitude is under the path of the rocket.
If there is damage or injury from this, the consequences for China are difficult to ascertain. An interesting article at The Planetary Society has the details.
The Long March 5B rocket launched on April 29 and is a two-stage rocket with four strap-on boosters for added thrust. It's a beast, one of the most powerful rockets in use, which is what's needed to launch the heavy components the China National Space Administration (the official name of the space agency) wants to meet their goals. That's why the first stage core booster is so big: 33 meters long, 5 wide, and a mass of 21 metric tons without propellant.
In general, the first stage of a rocket doesn't get to orbit. It gets the second stage as high and fast as it can, then is dropped so that the second stage can take over and boost the payload to orbit. This multi-stage method has been used for decades to launch payloads; by dropping the first stage the rocket isn't carrying dead weight with it, which saves a lot of fuel. With current fuels, it would be impossible to launch heavy payloads without using this system.
In most modern launches the first stage is suborbital; the US launches from Florida to the east so that a first stage drops into the Atlantic. The second stage has enough fuel left over to de-orbit itself safely, targeting someplace uninhabited (usually the vast expanse of the Pacific Ocean). That doesn't always happen, though. SpaceX had a malfunction on a recent launch that left a Falcon 9 second stage to de-orbit uncontrolled, and it burned up over the Pacific Northwest of the US, causing quite a bit of consternation.
But the first stage of this Long March did get to orbit. It's unclear why it didn't de-orbit itself, but this happened before with the previous (and first) Chinese Long March 5B rocket core, which was sent to orbit in May 2020 and also had an uncontrolled re-entry, with some pieces crashing down in Africa after it re-entered over the Atlantic Ocean.
Let's hope they're not making a habit of this. Their history of raining launch debris down over villages in China itself doesn't inspire confidence in their de-orbit mitigation. Their media seem like they're actively trying to downplay the event, too.
For a positive note on this, if it does burn up harmlessly within eyesight of land, the view will be spectacular. It will look like a very bright slow-moving meteor, with pieces breaking off and burning up on their own.
Still, let's hope this does take itself down over the Pacific or some other uninhabited territory. Stay tuned.