Mini-Throughput: Tumbling Chinese Rocket Edition

Space probe of Tianwen-1 Mission was launching by Long March 5 Y4 carrier rocket, 24 July 2020. Photo by China News Service, CC BY 3.0 , via Wikimedia Commons

So, the headlines probably sounds a bit alarming this week:

The Pentagon has said it is tracking a large Chinese rocket that is out of control and set to reenter Earth’s atmosphere this weekend, raising concerns about where its debris may make impact.

The Chinese Long March 5B rocket is expected to enter Earth’s atmosphere “around May 8,” according to a statement from Defense Department spokesperson Mike Howard, who said the US Space Command is tracking the rocket’s trajectory.

The rocket’s “exact entry point into the Earth’s atmosphere” can’t be pinpointed until within hours of reentry, Howard said, but the 18th Space Control Squadron will provide daily updates on the rocket’s location through the Space Track website. The Chinese rocket was used to launch part of their space station last week. While most space debris objects burn up in the atmosphere, the rocket’s size — 22 tons — has prompted concern that large parts could reenter and cause damage if they hit inhabited areas.

Let’s get this out of the way first: the danger posed by this event is small and the danger to you personally is basically zero. Most of the rocket body is going to burn up in the atmosphere. Some debris might reach Earth but the most likely place it will hit will be the ocean, simply because most of the Earth’s surface is ocean. Even if it doesn’t hit the ocean, there are large uninhabited areas where it could come down. When Skylab, which was much larger than the rocket stage, came down in a semi-controlled fashion, it scattered debris over Western Australia. No one was hurt although some of the pieces were quite large.

An oxygen tank from Skylab which crashed to Earth in Western Australia in 1979.

Last year, a similar Long March stage fell into the ocean, shortly after passing over New York and may have had some debris hit the Ivory Coast, damaging some buildings. So, it’s certainly possible that there could be some property damage done. And if we’re very unlucky, someone could get injured. Even a small piece moving at high speed could ruin someone’s day. But the odds are against it. And the idea that this could cause some kind of mass devastation is hysteria.

So why don’t we know when and where it will hit? Because projecting this kind of thing is tricky. While we can track the stage pretty accurately, we have no control over it. It is moving extremely fast and its interaction with the atmosphere depends on the rather unpredictable behavior of the atmosphere in fine grain and rather unpredictable orientation of the spacecraft when it hits. Re-entry patterns tend to be very dramatically shaped. Things fall to Earth very slowly until suddenly they fall very rapidly. The atmospheric drag grows exponentially, bringing them down suddenly and spectacularly. When projections are made for spacecraft re-entries in advance, the error bars tend to be years long. But even when you’re close to re-entry, it’s difficult to pin down the exact moment and thus the exact location. The spacecraft is moving five miles every second. Even if you could narrow the re-entry time down to ten minutes — and know how it’s going to plunge through the atmosphere as it disintegrates — that would only tell you what continent it might hit.

I have talked about the problem of space debris before. And rocket bodies are the object of biggest concern when it comes to space debris. There are, in fact, over 2000 spent rocket bodies in orbit around the Earth, about half from Russia and quarter from the US. But that isn’t quite what we’re talking about here. The Long March stage of the Chinese rocket isn’t going to be up there long, so it’s not really a space debris concern. And, in fact, most countries are not trying to mitigate the amount of space debris they put up on rocket launches.

No, what is getting the attention here is size. The Long March stage will be one of the largest objects to ever make an uncontrolled re-entry. Most heavy lifters drop their stages into the ocean soon after launch. The Long March leaves its core stage in low orbit, rather than a sub orbit, which means it will drop…somewhere. Hopefully not on someone but, you know, somewhere. And while we’re in a bit of glass house on this sort of thing, it’s still not acceptable. It would not be hard to modify the Long March series to not drop these things on us every year or so and hope everything works out OK.

So, while this is not something to panic about, it is something that we should be talking about. Because if you allow enough 20-ton Chinese rocket stages fall to Earth in an uncontrolled fashion, it is inevitable that something bad will happen. Not “destroy a city” bad but maybe “kill somebody” bad. And it seems very avoidable.

In the meantime, I would suggest you follow astronomer Jonathan McDowell who’s been doing some great commentary on the subject with graphs and videos.