It should give you a feeling of déjà vu: a defunct satellite's orbit is decaying, and because that orbit is circular it's going to be impossible to predict where and when along its ground track it's going to happen. A few large pieces will make it to the ground, and there's a one-in-many-trillions chance that you will be hit if you live between 53 degrees north and south latitude. It will come down some time this week, between October 20 and 25.
This time it's not UARS (which was in a similar situation and which wound up falling into the Pacific Ocean, by far the likeliest outcome for events of this type), it's ROSAT. RoSat (Röntgen Satellite), an x-ray observatory, was launched in 1990. It spent the first six months of its mission performing an x-ray all-sky survey, and the subsequent 8 years performing targeted observations of x-ray sources. According to the German Aerospace Center (DLR), its mission was productive:
Almost all known kinds of astrophysical objects could be observed with the help of ROSAT, mainly those experiencing high-energy processes or with temperatures of several million degrees Celsius. Examples of this include active galactic nuclei that seemingly contain black holes, as well as galaxies and galaxy clusters. In our own Milky Way, the objects observed included normal stars, X-ray binaries, neutron stars and supernovae remnants. Not only that, but objects in our own Solar System such as the Moon, planets and comets, surprised observers by proving to be X-ray sources as well. The results have been documented in more than 8000 publications.
ROSAT was decommissioned on February 12, 1999. At that time, its orbit was at an altitude of about 575 kilometers. Since then, DLR states on a page where it is currently updating ROSAT's orbital status, atmospheric drag has caused the orbit to decay. According to the ROSAT_Reentry Twitter feed, the orbit is currently at about 229 kilometers with an 89-minute period. (I don't actually know who is maintaining @ROSAT_Reentry but the neat graphics on the associated website appear to be made with current space-track data, for what that's worth.)
Most of what I wrote about UARS' reentry last month is true as well for ROSAT's: once it gets to an altitude of around 120 kilometers, the atmosphere will be thick enough to be exerting very strong drag forces on the spacecraft. After that point, ROSAT will fall very fast. According to the NASA Orbital Debris Program Office, solar arrays usually break off the spacecraft at an altitude of about 95 to 90 kilometers; the rest of the spacecraft breaks up at between 84 and 72 kilometers, when aerodynamic forces exceed the spacecraft's structural strength. Components made of low-melting-point materials (like aluminum) burn up at altitude, while components made of higher-melting-point materials (like titanium, steel, and carbon-fiber) survive longer. If an object is contained inside a housing, the housing must burn up before the interior receives significant heating.
The big difference between ROSAT and UARS is that although it was less than half the mass of UARS to begin with (2400 versus 6000 kilograms), it has many more large, heat-resistant components than UARS did. According to a February article in Der Spiegel, as much as half the mass of ROSAT could make it all the way through the atmosphere to the ground. Among the parts most likely to survive, according to Spaceflight Now, is the 400-kilogram primary mirror. Since it's massive and relatively compact, atmospheric drag will have less of an effect on the descent of this piece, so it will likely "go long" in the debris field.
As with UARS I'm not likely to blog about this again until after it's crashed; watch my Twitter feed for real-time updates that I find worth repeating, or ROSAT_Reentry for much more detail.