03-25-2017, 07:20 PM
Paul - for your info, the 250-mile altitude is an average. Because the ISS is in low earth orbit (LEO) it interacts with an extremely rarified atmosphere, i.e., there are still molecules from the Earth's atmosphere with which it interacts - and that can change daily depending on solar activity. The effect is minute but over time adds up. The orbit is also affected simply by passing over more dense parts of the Earth's crust and the orbit of the Moon. It's a very complicated interaction that is almost impossible to model, but things are improving as we learn more. It also shifts orbit slightly whenever there is a threat of a collision with orbital debris. Even magma shifts below Kilauea affect orbits of LEO satellites.
This is why it's not a good idea to predict sightings for the ISS, Iridium flares or other LEO satellites too far in advance as the orbits are likely to change. The adjustments tend to be very small but over a few days make a big change to predictions.
This is why it's not a good idea to predict sightings for the ISS, Iridium flares or other LEO satellites too far in advance as the orbits are likely to change. The adjustments tend to be very small but over a few days make a big change to predictions.