NASA’s UARS (Upper Atmosphere Research Satellite) re-entered the Earth’s atmosphere last night between 8:20pm and 10:10pm Pacific Time. At the time of this writing (2am? Oops), the exact time and location of re-entry are unknown, though NASA says it was over the Pacific Ocean. There were some unconfirmed reports that it fell over northern Canada.
What I found most fascinating is why they were unable to predict the exact decay of this orbit: solar radiation. Solar radiation (think heat from the Sun, though it would be more precise to say photons from the Sun) heats up the Earth’s atmosphere, specifically the upper atmosphere where satellites in Low-Earth Orbit can be as their orbits decay. This heating makes that part of the Earth’s atmosphere both larger and less dense. NASA likens this to a marshmallow being toasted over a campfire. So now your satellite is travelling through a different density of atmosphere, where it will experience more or less drag depending on what exactly is happening between the Sun and the Earth’s upper atmosphere. Complicated, especially if you can’t see that density changing.
~ A l i c e !