Can we predict where debris will land?

It is very difficult to predict where debris will actually land. A one-minute error in predicting the reentry time will change the debris’ location by nearly 300 miles. The few surviving pieces of the object will be spread over a long footprint, so that two nearly identical fragments may impact many miles apart. For example, there were a total of four spheres on the Delta stages that left debris in Texas and South Africa, but only one sphere was found in both cases. The others most likely survived, but impacted some distance away and were not recovered.

Since predicting where specific pieces will land is very difficult, analysts generally predict the location of the debris footprint to indicate the general area where debris will land. Unfortunately, predicting where the debris footprint will be on Earth’s surface for a specific object’s reentry is also difficult.

The primary difficulty with predicting the footprint location relates to the uncertainties in predicting the object’s lifetime in orbit. Given sufficient tracking of the object during the orbit decay combined with measurements of the sun’s activity over extended time (solar activity can have dramatic effects on the upper atmosphere), it is possible to obtain a fairly good approximation of the date and time (but not location) of an object’s final reentry.

In general, there is about a ±10 percent uncertainty of the time of the final reentry. Considering that orbiting objects are travelling at more than 7 km/sec, a prediction made at the beginning of the object’s last orbit, which could take about 90 minutes to complete, could be off by as much as ±9 minutes, equivalent to more than 7,000 km on the ground. This means that the debris footprint could be located anywhere along this 7,000-km-long path.

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