What is a runaway star

The red point CZ-5B moves like a sine curve across the world map, and pretty quickly. He needs a few minutes to fly over North America, he's already over the Atlantic, then Africa, Asia, occasionally he wanders across Europe. What the Orbiting Now website shows is the trajectory of a remnant of a Chinese missile-type Long march 5B. It is currently orbiting the earth at more than 27,000 kilometers per hour. The altitude, however, is less than 200 kilometers and is steadily decreasing. Presumably at the weekend, the 18-ton rocket stage will enter the earth's atmosphere and crash in an uncontrolled manner.

Tiangong"Heavenly Palace" is the name of the space station that China's space agency is currently assembling in orbit. The core module called "Heavenly Harmony" brought the now out of control Longer-marchRocket into orbit on April 29th, everything initially seemed to be going according to plan. A great success for the Chinese space program, which is appropriately celebrated in the state media. On the other hand, the Chinese authorities have so far been silent about the failed re-entry. In any case, the maneuver does not create harmony on Earth. Esa speaks of the "worst uncontrolled re-entry" for more than 20 years.

Impact in Germany is out of the question, while southern Europe lies within the risk zone

Originally, the rocket upper stage was supposed to be maneuvered in such a way that it returned to Earth in a controlled manner. However, something must have gone wrong. From the object's trajectory, it is possible at least roughly to estimate when it will re-enter the atmosphere. Esa estimates that it could be 11 a.m. German time on Sunday morning, American sources give the time a few hours earlier. "However, this is fraught with great uncertainties," says Manuel Metz from the German Aerospace Center. At very high altitudes, the density of the atmosphere is not exactly known, says Metz. The outermost air envelope is also constantly influenced, for example by solar radiation. This makes it difficult to predict how objects flying through will behave. Re-entry is also possible a day earlier or later.

In addition, the air resistance depends heavily on construction details of the rocket stage, but these are largely unknown. Its length is estimated at 33 meters, with a diameter of five meters. "That's pretty big," says Metz. Will individual rubble also reach the earth? "You have to assume that there is a high probability." As a rule of thumb, a satellite burns up about 80 percent when it re-enters. In the case of a rocket stage, however, it can behave differently. It usually has heat-resistant engines and tanks that "can be relatively resistant to reentry," says Metz.

Already last year parts of a rocket were of the type Long march fell uncontrollably to the earth. Debris fell in the Ivory Coast at that time, including a twelve-meter-long pipe.

However, at least individual regions can now be excluded as possible impact points. Since the orbit axis of CZ-5B is slightly inclined to the equator, the burnt-out rocket stage never flies more northerly than 41.5 degrees in the northern hemisphere, or more south than 41.5 degrees in the southern hemisphere. In Europe, the border line runs across southern Europe, so an impact in Germany is out of the question. According to Esa, parts of Portugal, Spain, Italy and Greece could theoretically be hit. Large parts of North and South America and Asia are also within the risk zone, as well as all of Africa and Australia. The zone will only be able to be narrowed down a few hours before the impact.

However, you don't need to worry if you are in this zone. Three quarters of the earth's surface are covered by water and large parts of the land masses are uninhabited, explains Esa in a press release. Everyday risks, such as when driving a car, are far greater than the risk of being killed by space junk.

Note: An earlier version of the article stated that the Chinese space station was called Tianhe, "Heavenly Harmony". However, this is only the name of the core module. The entire space station is called Tiangong, "Heavenly Palace".