ISS battery pack: “Will not fall into territory as a compact individual part”

It is the largest object to have been ejected from the ISS to date – a battery pack weighing 2,600 kilograms. The head of ESA space safety, Krag, believes it is unlikely that debris will fall on Germany.

tagesschau.de: A battery block is expected to fall to earth tomorrow. Why did the ISS send the object towards Earth in the first place?

Holger Krag: When objects on the ISS are no longer needed, they are disposed of. There are two options: Either you load them into one of the existing spaceships, which then dispose of them on their return through a controlled re-entry. Or, as in this case, they are expelled, and then it is up to the forces of nature to determine exactly when and where the objects re-enter.

Re-entry time and location are difficult to predict

tagesschau.de: How accurately can one predict when NASA’s battery pack will enter Earth’s atmosphere?

Krag: When objects re-enter in an uncontrolled manner, it is in the nature of things that it is very difficult to predict the time and place of re-entry. The object is gradually slowed down by the atmosphere and thus loses its orbit. The big uncertainties come from the fact that we cannot predict exactly how dense the atmosphere will be.

It depends on many factors and is largely left to chance. In this respect, even a few hours in advance, you can’t exactly name the location. You might be able to exclude some continents, but you definitely can’t narrow the forecast down to one country or one city.

Debris is expected to reach the atmosphere Friday

tagesschau.de: When do you expect to re-enter?

Krag: We expect re-entry sometime tomorrow. So a period of many hours comes into question. One must not forget: During this period, the object orbits the Earth several more times. One orbit around the Earth takes an hour and a half. If the uncertainties today are almost half a day, then one can imagine that the area cannot yet be narrowed down at all.

tagesschau.de: Is the entire Earth a possible re-entry area?

Krag: Currently the area in question is anything between 51.6 degrees south latitude and 51.6 degrees north latitude. This is the area that the object geographically flies over. Now we will see over the course of the hours whether this can be narrowed down even better. But as I said, a continent can be flown over within a few minutes.

Germany is probably not affected

tagesschau.de: Which path does the object follow?

Krag: The object follows a similar path to the ISS from which it came. One can at least say which areas will not be affected, namely those above 51.6 degrees north latitude. Because this limit of 51.6 degrees now runs through Germany, which is the northernmost point that can be reached for the object, one should not think that Germany is now particularly affected.

tagesschau.de: Can you quantify how high the danger is for Germany?

Krag: If you look at the entire landmass between 51.6 degrees south and 51.6 degrees north – that’s a large part of the entire earth’s surface, of which Germany only takes up a very small part. This ratio also roughly corresponds to the probability that Germany, of all places, will be affected by this event.

There could be a small piece every ten or 20 kilometers

tagesschau.de: How often do pieces of debris from space hit the Earth?

Krag: This is not an isolated case. Every year we have around 100 tonnes that re-enter in an uncontrolled manner. This is spread over several satellite and rocket stages and around 50 individual objects. So sizes like the two and a half tons we have now are not that rare. This happens several times a year. Why this is causing greater panic in this case may be because objects coming from human spaceflight are always under greater scrutiny and this object is the largest object ejected from the ISS to date.

tagesschau.de: How long does it take after entering the Earth’s atmosphere for debris that has not burned up to hit the Earth?

Krag: This happens very quickly. From an altitude of 100 kilometers, where re-entry occurs when the object begins to disintegrate, to the ground takes only ten minutes. However, the battery pack will not fall as a compact individual part in a very narrow area, but will rather be distributed in a longer trail of debris. One would expect a smaller part every ten or 20 kilometers in the affected area.

Trail of light in the sky

tagesschau.de: How do you notice the re-entry?

Krag: You can only say that the re-entry has occurred when someone has observed the light trail in the sky or when we expect the overflight of a station and then it no longer occurs. We use radars that are suitable for observing objects at such deep orbital levels. There are several on Earth, geographically distributed.

tagesschau.de: If a piece of debris actually hits an inhabited area, how much danger does the battery piece pose?

Krag: It’s best never to touch a piece of debris, but these batteries used in space are no different than batteries we use on Earth. So this doesn’t pose any major danger. When we talk about danger, it is more about the mechanical danger that comes from just the object falling.

The interview was conducted by Ute Spangenberger, SWR