Chinese rocket stage reentered highlighting need for space surveillance

Do you remember our story about the Chinese reusable rocket test flight on December 3?

The first stage of the Zhuque-3 rocket, built by the Chinese company LandSpace, was designed to be reusable, but on this first test flight it did not manage to land softly. Otherwise, the launch from the Jiuquan Space Center went well, as the rocket’s second stage worked as planned and carried a mass simulator into space. This payload did not separate from the second stage, but instead remained orbiting the Earth together with it.

Until today, when they will fall back down uncontrollably.

The second stage is about 12 meters long and 4.5 meters in diameter. Its estimated mass is between 7.5 and 11 tons. An object this massive and large will not completely burn up upon reentry into the atmosphere; some fragments will survive all the way to the Earth’s surface. For example, the rocket engine, built to withstand extreme heat, and possibly titanium propellant tanks are unlikely to be fully destroyed.

Finland’s Space Situational Awareness Center and the EU Space Surveillance and Tracking Center have been tracking the falling rocket stage and predicted that it would have reentered on Friday between approximately 1–2 PM. The exact time cannot be stated precisely because factors such as atmospheric density along the flight path, as well as the object’s orientation and rotation, affect how much the thin upper atmosphere slows its orbital speed.

The lower the rocket stage falls, the more the atmosphere slows it down, and the more accurately the reentry time can be estimated. Eventually the object falls faster and faster, until at around 80 kilometers altitude it begins to break apart as it plunges through the atmosphere at high speed and heats up intensely.

Most satellites and other objects falling into the atmosphere are completely destroyed due to air resistance and extreme heat. For example, today, January 30, seven Starlink satellites and a U.S. Department of Defense experimental satellite, Blackjack Aces-4, are also reentering.

Although the rocket stage has been closely monitored recently, including with telescopes used by the EU Space Situational Awareness Center (the headline image shows a photo taken on January 28 by a Polish tracking telescope located in Chile), it cannot be observed continuously. Therefore, the actual reentry cannot be usually directly witnessed — unless it happens over a populated area.

However, the possible impact area can be estimated. The rocket stage orbits the Earth at an inclination of 56.94° relative to the equator, meaning it can only fall within a region extending from the equator to about 57° north and south latitude. The flight path can also be calculated before and after the predicted reentry time. The reentry can only occur along this ground track, so for example the Chinese rocket cannot fall onto Finland.

On its final orbits it passes over areas such as Scotland, Denmark, Poland, Belarus, and Ukraine. At the time of writing, the rocket stage has most likely already fallen, but the location is not yet known.

Because most of the Earth is covered by oceans, the most likely outcome is that the remnants of the rocket stage will splash down into the sea.

Responsible launch providers, such as the European Arianespace and the American SpaceX, guide rocket stages to fall back in a controlled manner after the payload has been delivered and the rocket has completed its mission.

Sometimes, however, this does not succeed. For example, on the maiden flight of Ariane 6, the rocket’s upper stage failed during an engine burn, and the stage is still in space. Similarly, last year SpaceX’s Falcon 9 upper stage engine did not perform as planned, and the stage fell uncontrollably over Poland. Rocket debris was found on the ground, but fortunately no one and nothing was hit.