May 13, 2021
Your Chinese Rocket Reentry Questions Answered
The responses to our updates about Long March 5B have been overwhelming. Our Center for Orbital Reentry and Debris Studies expert, Marlon Sorge, answers some of the most-asked questions about the reentry that captured the world’s attention.
The remnants of the large Chinese rocket body known as Long March 5B plummeted into the Indian Ocean on May 8 after an uncontrolled and dangerous reentry. Experts at Aerospace’s Center for Orbital Reentry and Debris Studies (CORDS) monitored its journey using sophisticated modeling to predict when and where the 22-ton piece of space debris would crash to earth.
While at first glance, it may seem the CORDS reentry prediction seems half a world away from the reentry point, upon closer inspection, it’s clear that the predicted orbital path and timeframe for reentry were very close to where the CZ-5B rocket body went down in the Indian Ocean.
Traveling at roughly 18,000 mph and orbiting the planet every 90 minutes, CZ-5B reentered the atmosphere approximately 40 minutes before the last estimate on the same revolution around the earth as the CORDS prediction and within the ± two hours margin of error. The full uncertainty for time of reentry is represented on the plot below by the yellow and blue lines. Each tick mark along the line represents just a five-minute interval on the orbital path. The blue portion of the line represents time before predicted reentry, and the yellow portion is time after reentry.
The graphic below shows the narrowing of the reentry windows from the start of CORDS’ predictions:
CORDS’ predictions were steady throughout the week. This final predicted reentry window was made from the last available TLE around five hours and 30 minutes prior to reentry and included the reported reentry time, well within the 20% prediction uncertainty.
During the event, Aerospace received an overwhelming response with many queries about how and why Long March 5B ended up on this uncontrolled reentry path. Marlon Sorge, CORDS Principal Engineer, answers some of the most asked questions from our followers on Twitter.
Q: How do we know when and where the rocket body landed?
A: 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: Why do Aerospace’s orbital predictions differ from the Space Force or other agencies?
A: 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 then affects how quickly an object falls out of orbit. We and 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 the same does not mean they don’t agree. We are constantly refining our models but are satisfied with our approach in the face of the unknowns.
Q: Why are updates limited to two to four per day with such a fast-moving object?
A: Updates are made each time a new orbit measurement is made by the Space Surveillance Network. Those updates come each time the CZ-5B passes within sight of one of the SSN’s radars. As the CZ-5B’s orbit got lower, it became 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 the CZ-5B’s orbit was very low.
Q: Do weather patterns affect the path of the rocket’s reentry? Is this why it’s difficult to calculate exactly where it will land? What about solar flares?
A: 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 rocket and affecting when it will reenter. Predicting exactly what the sun will do is notoriously difficult and a major uncertainty in the reentry predictions.
Q: Was the reentry visible from the ground?
A: It is likely that the reentry would have been visible from the ground if it 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 that 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: Did anyone capture the reentry as it passed over?
Q: How much debris came down? Is there a debris field?
A: There was certainly a debris field. Typically, 20%-40% of the mass 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, and the previous CZ-5B reentry resulted in debris falling to Earth. In this case, much of the debris field would be over ocean so anything that hit would be undersea. Not easy to find!
Q: Can people report sighting a reentry?
A: Yes! We at CORDS greatly appreciate sighting reports via our sighting submission web page. Sighting information can help us pin down the location of the reentry and improve our models for the next reentry.
Visit us online at aerospace.org/CORDS.
Q: How close did the rocket come to landing in a populated area?
A: The plots at the top of this article show our last prediction window. The rocket body could have fallen anywhere along the yellow and blue lines. Most of the path was over ocean or unpopulated regions but it did pass over the central and Eastern United States, the Eastern Mediterranean coast and Northern Spain. It could have landed in any of those areas had it come in earlier or later.
However, even if it had fallen in those areas, the chance of you getting being struck by debris is very low. The chance of someone being hit is much higher if the reentry occurs in a populated area rather than the ocean, of course.
Q: If space debris were to land in your yard, do you get to keep it?
A: There is a United Nations treaty that governs this, 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 the 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 damage.
Q: This launch was part of the ongoing assembly of a space station estimated to need 10 more launches to complete. As the pace of launch accelerates globally, will these sorts of debris reentry events become more common?
A: As the number of satellites rapidly increase, 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 once they are 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 will not 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 for reentry. This makes the risk to people extremely small.
The hope is that events like the CZ-5B reentry will become less common in the future.
Q: Why didn’t we shoot down the rocket body?
A: The United Nation Outer Space Treaty says that objects launched into space remain the property of the launching country. The CZ-5B remained the property of China even as it was reentering so shooting it down would not be appropriate.
Q: What are some of the potential geopolitical ramifications of an uncontrolled reentry?
A: There can be international ramifications for uncontrolled reentries. In 1978 the Soviet satellite Cosmos 954 reentered over Canada and deposited radioactive material along the 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 from The Aerospace Corporation:
Marlon Sorge is a principal engineer 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.
