China should give advance warning that such a large element may fall, rather than always having to detect it remotely, with space detection and observation radars. China should be more cooperative and when it has such a large piece of hardware falling to Earth, give plenty of warning. Even if it knows what the trajectory is, say so. 

It is common for elements of this kind to fall, but not this big. A few months ago there was another alarm about another piece of a Chinese rocket, but not as large.  

We are not 100% sure because they are elements that are uncontrolled. Once they do their job, they go into orbit, but as they have no propulsion system, they fall due to the effect of the atmosphere and their fall is not controllable. At some point they may fall in an inhabited area or at least on land, not in the sea, which is where they normally fall, over the ocean, and this does not cause any problems.

In a re-entry, we cannot predict precisely where the debris of the object will fall (if it reaches the surface of the Earth and does not burn up in the atmosphere, as in this case, because it is a large object). Because of the high speed at which it moves, it travels a long way in a short time and an error of a few seconds in the prediction of the time of entry means kilometres in the journey.  

Normally, we paint the areas where it may fall, such as all those through which the object flies within 20% of the time remaining until it enters the surface, around the predicted date of its entry. If there is one day left, we paint for about four hours, which is about two full circles of the Earth. That's why we see these traces in many places where there is a certain risk of it falling. 

It has finally fallen in the Pacific. We are lucky that most of the Earth's surface is water and not land, so the probability of it falling into an ocean is very high.

Every day, space debris falls to Earth, but the small fragments disintegrate in the atmosphere and the large ones usually drift in a controlled manner over the Pacific Ocean, where the risk is very low. It is irresponsible of the Chinese space programme to let the main stage of its 20-tonne CZ-5B Long March rocket fall in an uncontrolled manner. This is the fourth launch of this rocket, which is being used to launch the modules of the new Chinese Space Station. The first launch, in 2020, ended with some pieces of the rocket impacting in the Ivory Coast.  

This morning we woke up to the prediction that, after this fourth launch, the main stage could fall on some Mediterranean countries, including northern Spain. Although the probability of the debris hitting an aircraft is low, it seems reasonable to restrict air traffic as has been done, since such an impact on an aircraft would probably be fatal for all its passengers, and would foreseeably open up an unprecedented international conflict. According to the US military space force, re-entry has finally occurred over the Pacific Ocean.  

Space activities have a lot to do with what happens on Earth, which is why collective reflection and the development of a Space Ethics leading to a well-founded Space Law is necessary. In this reflection, we should not only see the "mote in the eye of the beholder", nor let ourselves be guided by the propaganda of great powers and companies. China is doing some things irresponsibly, the United States, and companies like SpaceX, too.  

Space activities belong to all of humanity and, if we want to avoid disaster, we need a multilateralism that has been conspicuous by its absence lately. The severe climate, energy and ecological crisis is the ultimate example of how this lack of multilateralism is turning into "collective suicide". In a few days a new climate summit will start in Egypt. Let's be clear: while world leaders do nothing, time is running out.

Re-entry of launch rockets is quite common. Whenever an artificial satellite is launched into space, at the same time a part of the rocket remains in orbit and will eventually fall to earth. This has been the case since Sputnik 1 and will continue to happen. 
 
These pieces of the rocket re-enter the atmosphere after some time (days or weeks) and the collision with the air destroys them. Only in the case of very large rockets can fragments survive to reach ground level. 
 
However, there are two circumstances that make this particular case special. The first is that it is a large rocket, because it has launched no less than a complete module for the Chinese space station, and that requires a considerable launcher. Secondly, satellite launches, both large and small, are becoming more and more frequent. 
 
The risk is very small. It is reasonable to restrict air traffic in potentially affected regions because we should not risk losing an aircraft in flight, no matter how small the danger. But, at ground level, the likelihood of large debris arriving, falling in areas where it can cause damage and, in addition, personal injury, is astronomically small. The population can therefore rest assured. 
 
No matter how small the risk, accumulated over time and with an increasing rate of releases, sooner or later damage to property or people will occur. This is why organisations (public and private) launch satellites but take measures to ensure that their rockets re-enter in regions where they cannot cause any damage. Systems to force a safe re-entry are standard technology and are certainly within the reach of a power like China, which is capable of building its own space station in little more than a year. 
 
It would therefore be desirable to promote the establishment of international regulations requiring rocket launchers to ensure safe re-entry.

Uncontrolled rocket bodies and satellites reenter every one or two days. What is special about this Chinese stage is its large mass, about 20 tons. It was the 4th of this type placed in orbit: The second reentered over the Indian Ocean, but the first and the third ones scattered debris on the ground as well, in Ivory Coast (2020) and in Borneo, at the end of July this year. Due to the fact that 88% of the world population is overflown by this stage trajectory and considering its large mass, the probability of someone in the world being hit by a falling debris, between 42° north and south, is about 1 in 500, i.e. more than the 1 in 10,000 threshold for which, at international level, a controlled re-entry is recommended.

Concerning the risks from uncontrolled reentries in general, the last estimates from Carmen Pardini and myself are that currently the global casualty probability is around 2% - 3%  per year. This is the probability that one generic person around the world will be hit in a year by the fragment of a space object that reentered uncontrolled. Approximately 1/3 of the risk comes from satellites, 2/3 of the risk comes from rocket bodies. For any specific individual this risk is still very low, much lower than any other risk we take in everyday life. However, the situation is evolving, due to the changes in space activity, and if appropriate measures are not taken, this risk is bound to grow in the coming years.