When predicting the next acquisition of a satellite with your location, NASA SkyWatch will simulate the movement of the satellite as it orbits the Earth. This is called state vector propagation. When propagating a state vector, NASA SkyWatch will account for many "real world" effects that the Earth has on the satellite. One of these effects is aerodynamic drag. Yes, we still do have some amount of drag even in Earth orbit! In order to account for drag, you need to provide some idea of how big the satellite is and what it weighs. This will enable NASA SkyWatch to predict the satellite's position as closely as possible when computing the next acquisition.

As with your location, precision is not required in the satellite's weight and area. Typical shuttle values are 225,000 pounds for weight and an area of 2,751 square feet. For the International Space Station, good values to use are 410,000 pounds with an area of 4,000 square feet. If you select your input vector from an ephemeris in the vector source pull down menu, the weight and area will be supplied for you.

If you don't know anything about the satellite, a good generic input might be 5,000 pounds with an area of 100 square feet. This will allow the satellite to propagate fairly well for about 72-96 hours with minimal loss in accuracy. Data after 96 hours could have significant error and should be treated as suspect. Acquisition times could be off on the order of minutes and predicted sightings may not be possible contrary to what NASA SkyWatch predicts. Or even worse, a sighting may be possible when NASA SkyWatch indicates otherwise!