The German space center DLR is reporting that ROSAT — an astronomical satellite launched in 1990 — re-entered the Earth’s atmosphere just after 01:50 UTC last Sunday, October 23, burning up over the waters of the Bay of Bengal, to the southwest of Thailand. This was during the day, which is why there were no reports of it coming in, and no reports of debris hitting the ground. There are no reports of anyone getting hit, either, of course.
I was thinking about why it’s so hard to know exactly where and when a satellite comes down. New Scientist has a short, to-the-point article about it. In a nutshell, low-Earth orbit satellites like ROSAT are a few hundred kilometers above the Earth’s surface, circling every 1.5 hours or so. The Earth’s atmosphere doesn’t just stop if you go high enough; it thins out slowly, and even at 300 km there is still some air, though incredibly tenuous. However, over days, months, and years, a satellite moving through it feels a drag, feels air resistance. This drops its orbit, putting it into thicker air, which slows it faster, dropping it more… this process is very slow at first, but at some point increases rapidly, and the satellites drops into air thick enough that, at its high speed, causes it to burn up. I’ll note it’s mostly due to gas compression, not friction, that heats the satellite up.
And that’s why it’s so hard to know in advance where these things will come down. Read More