Another Uncontrolled Chinese Rocket Body is Plummeting to Earth. Any Questions?
Our space debris expert is back with answers about the giant rocket stage left behind after the July 24 Long March 5B Launch.
On April 29, 2021, China launched the first module, Tianhe, of its Tiangong space station on their heaviest rocket to date, the Long March 5B. The rocket body traveled with the module to orbit, resulting in an uncontrolled reentry of the rocket’s core stage. The rocket’s path crossed several populated areas and captured the world’s attention before eventually landing in the Indian Ocean near the Maldives on May 8.
This past Sunday, July 24, 2022, the second module of the space station, Wentian, launched on an identical Long March 5B rocket. At this point, it appears that the launch is similar to previous flights with an uncontrolled reentry of the rocket body expected.
As we wait to learn more, we are sharing some of the best questions we’ve received from debris watchers around the world, answered by our resident space debris expert, Marlon Sorge.
Follow us on twitter @AerospaceCorp for reentry updates as they happen and to submit your questions.
Q: How often does an uncontrolled reentry of this size occur? Have there been other instances that compare?
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.
This stage and its two predecessors, launched in May 2020 and May 2021, are the sixth-, seventh-, and eighth-largest objects to ever reenter. The mass of this core stage is about 22.5 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.
Q: What is a “deorbit maneuver”?
A deorbit maneuver uses a satellite or rocket stage’s engines to drop the low point of its orbit and 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.
Q: 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 nine 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 items such as insulation fall out. The melting point of the materials used will make a difference in what remains.
Q: How do we know when and where debris will land?
The data sets our team uses to make predictions are generated when an object being tracked passes over one of a collection of sensors across the planet.
The Space Surveillance Network (SSN) is operated by the U. S. Space Force and tracks objects in space. The SSN has radar and optical sensors at various sites around the world as shown in the figure above. These sensors observe and track objects that are larger than a softball in low Earth orbits and basketball-sized objects, or larger, in higher, geosynchronous orbits. The sensors can determine which orbit the objects are in and that information is used to predict close approaches, reentries, and the probability of a collision. Other nations also run space object tracking systems.
Q: How do I read the debris predictions?
For the current Long March 5B reentry, experts at Aerospace’s Center for Orbital Reentry and Debris Studies (CORDS) are monitoring its journey using sophisticated modeling to predict when and where the 22-ton piece of space debris will crash to earth.
The predicted reentry path shown in the image above indicates a window of uncertainty around the prediction of ± 20% of the remaining prediction time. As an example, if the predicted time of reentry were in five days, the window of uncertainty would be ± 1 day.
As the time to reentry shrinks so does the uncertainty. The yellow satellite icon indicates the center of the reentry window, the blue track is the first half of the window, and the yellow track is the second half.
The final reentry location for the current Long March 5B will be displayed once it has reentered.
Looking for more information about this graphic? Check out A Quick Guide to Understanding Orbital Debris Predictions.
Q: Why are updates limited to two to four per day with such a fast-moving object?
Updates are made each time a new orbit measurement is made by the Space Surveillance Network. Those updates come each time an object passes within sight of one of the SSN’s radars. As debris orbits get lower, they become more difficult to view from the sensor sites. Fortunately, the SSN has sensors around the world making it possible to get repeated updates every day even when orbits are very low.
Q: Why do Aerospace’s orbital predictions differ slightly from the Space Force or other agencies?
There are a variety of ways to predict the reentry, and models differ. The predictions are highly sensitive to the modeling assumptions including how we think the sun will affect the earth’s atmosphere which affects how quickly an object falls out of orbit. We and the U.S. Space Force use slightly different models so we get different answers. These different answers tend to fall within each other’s uncertainties, so just because they are not identical does not mean they don’t agree. We are constantly refining our models and are satisfied with our approach in the face of the unknowns.
Q: Are reentries visible from the ground?
Reentries can be visible from the ground if they hit in the right place and at the right time. If the reentry happens over the ocean or an unpopulated region of land it is unlikely people can see it. If it happens over a populated region, it helps if it’s dark. The darkness makes it easier to see the pieces of debris because they tend to be glowing from their hot descent from space.
Q: Do weather patterns affect the path of reentry? Is this why it’s difficult to calculate exactly where it will land? What about solar flares?
The sun’s activity, like solar flares, is one of the main uncertainties affecting our ability to accurately predict reentries. The sun is pouring out a lot of energy which heats the earth’s atmosphere. If the amount of energy changes even a little, as with a solar flare, the atmosphere will expand or shrink — changing how much it pulls on a reentering object and affecting the timing. Predicting exactly what the sun will do is notoriously difficult and a major uncertainty in reentry predictions.
Q: Can people report sighting a reentry?
Yes! We greatly appreciate sighting reports. Sighting information can help us pin down the location of the reentry and improve our models for the next reentry.
Enter your sighting via our submission form.
Q: How much debris came down from the May 2021 Long March 5B reentry? Was there a debris field?
There was certainly a debris field. Typically, 20%-40% of the mass of an object survives reentry. That would put the amount of debris from the CZ-5B at four to nine metric tons. This CZ-5B was very large, but much of the debris field was over the ocean so anything that hit would be undersea. Not easy to find!
Q: If space debris were to land in your yard, do you get to keep it?
There is a United Nations treaty that governs found debris—the 1967 Outer Space Treaty. It states that countries keep ownership of objects they launch into space, even after those objects reenter and return to earth. The country that launched the object, in this case, China, could request the return of any parts that survived reentry. It is also worth noting that the treaty says that the launching country is also internationally liable for damages.
Q: This launch was part of the ongoing assembly of a space station needing more launches to complete. As the pace of launch accelerates globally, will these sorts of debris reentry events become more common?
As the number of satellites grows rapidly, reentries overall will also likely increase in frequency. One of the best ways to control the growth of orbital debris is to get the satellites out of orbit at end-of-life. This is often done through reentry.
In most of these cases, the satellites are small. Some are even designed to have very little mass survive to the ground. Larger objects like the CZ-5B rocket body don’t easily burn up. It is a common practice for larger objects such as rockets to perform a controlled reentry — using their engines to target an isolated region of the ocean to reenter, making the risk to people extremely small.
Q: Can we shoot down the rocket body?
The United Nations Outer Space Treaty says that objects launched into space remain the property of the launching country. The rocket body remains the property of China even as it is reentering so shooting it down would not be appropriate.
Q: What are some of the potential geopolitical ramifications of an uncontrolled reentry?
There can be international ramifications for uncontrolled reentries. In 1978, the Soviet satellite Cosmos 954 reentered over Canada and deposited radioactive material along its reentry footprint. Fortunately, the region was not highly populated. The Canadian government billed the Soviet Union for expenses. This is why it is important that all space operators behave responsibly as their actions can affect other nations.
More information on Space Debris from The Aerospace Corporation Center for Space Policy and Strategy:
Marlon Sorge is a technical fellow for the Space Innovation Directorate of The Aerospace Corporation. For more than 30 years, he has conducted space debris research and analysis in a broad range of fields including debris risk assessment, fragmentation analysis, operations support, debris mitigation technique implementation, debris event reconstruction, satellite design for debris survivability, orbital and suborbital range and space safety, ballistic debris management, debris environment projection, collision avoidance, orbital reentry prediction, and national and international mitigation guideline and standards development.