A Space Debris Expert Weighs in on the Massive Chinese Rocket Body Falling Uncontrollably to Earth

The 21-metric-ton rocket body, Long March 5B, is expected to reenter Earth’s atmosphere this weekend. How did this happen, and where will it hit?

The Chinese Long March 5B successfully launched the core module of China’s first space station. Now the empty rocket body is plummeting towards Earth. Credit: CASC
The reentry prediction for the Long March 5B rocket body from the Center for Orbital and Reentry Debris Studies. Note: The ground traces shown in the above image extend the full uncertainty window. Credit: The Aerospace Corporation

How often does an uncontrolled reentry of this size occur? Have there been other instances that compare?

This stage and its predecessor last May are the sixth and seventh largest objects to ever reenter. The mass of this core stage is 21 metric tons. The list of other comparably-sized objects that have reentered includes early space stations — like Mir, Skylab, Salyut 6 and 7 — and the Saturn V second stage that launched Skylab. These represent a mix of controlled and uncontrolled reentries, however.

What normally happens with a rocket stage like this and what was different this time?

Normally the first stage of a rocket and its strap-on boosters are not designed to reach orbit. Their trajectories are planned so that the stage and any strap-on boosters fall into a safe area, usually in the ocean. In this case the first stage core of the rocket reached orbit. That means that it was no longer able to control where it would reenter without a deorbit maneuver.

What is a “deorbit maneuver”? Could that have been attempted?

A deorbit maneuver uses a satellite or rocket stage’s engines to drop the low point of its orbit to choose where it hits the earth. This is called a controlled reentry. By doing this a large object can be targeted for an unpopulated region of the ocean where its debris will not injure anyone. The ability to conduct a deorbit maneuver is dependent on the design of the vehicle and the mission. It is not uncommon for rocket operators to plan for deorbit maneuvers and controlled reentries as large rocket stages tend to pose larger risks to people on the ground.

How much of the Long March 5B rocket stage is expected to survive reentry and reach the Earth’s surface?

The general rule of thumb is that 20–40% of the mass of a large object will reach the ground, but the exact number depends on the design of the object. In this case, we would expect about five to ten metric tons. Generally, for an upper stage, we see small and medium tanks survive more or less intact, and large engine components. The large tanks and the skin of this core stage are likely to come apart. We will also see lightweight material such as insulation fall out. The melting point of the materials used will make a difference in what remains. You can learn more here.

How likely is it that I will get struck by space debris from this reentry? As of right now, do we know where the debris is likely to land?

The probability that any random reentry will land in the ocean is about 3:4, since the Earth is about 75% covered by oceans. Also, the majority of the land mass is either uninhabited or lightly inhabited: deserts, mountains, forests, and open prairie, farms, or grasslands. The probability that a piece of space debris will land on a city or a densely populated area is usually relatively small. What makes this reentry particularly noteworthy is that it will occur between 41.5 deg N and 41.5 deg S latitudes, where the vast bulk of the world’s population lives.

What can the space industry be doing to avoid these types of uncontrolled reentries from happening in the future?

There are established methods for analyzing the reentry breakup of any object we launch into space and determining the casualty risk to people on the ground. If the risk exceeds a threshold, one should mitigate the risk by either altering the design or changing the reentry. For rocket stages, this usually means planning a controlled reentry into a broad, open area such as an ocean — the southern Pacific is a common target. Controlled reentries, particularly for a large object, require considerable planning and will have a significant impact on the design and payload capacity of the stage. Nevertheless, this is a preferred approach in international standards and is rapidly becoming a global norm.

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