To get rid of large objects in orbit, such as with the Mir station, the European ATV cargo vessel or its Russian Progress counterpart, they are, whenever possible, manoeuvred to place them in a trajectory intercepting the dense layers of the atmosphere over risk-free areas such as the South Pacific, far from inhabited islands and air or maritime traffic.
For wreckage, such as the last stages of launchers or large, out-of-control satellites, this is not possible. They do however represent a hazard, because not all of their structures will be destroyed during re-entry. This is in particular the case with certain tanks or engine combustion chambers, which are designed to withstand high heat and pressure.
It is therefore difficult to model the date and consequently the geographical location of re-entry. This is due to the fact that the density of the atmosphere varies with solar activity and we cannot know the exact attitude of the object, nor its movement, which leads to uncertainty in the ballistic coefficient, which determines the extent of atmospheric friction. By assessing this coefficient with an uncertainty of 10%, we obtain a re-entry window and ground footprint.Ten days before re-entry, the uncertainty is one day, while ten hours before, it is one orbit, giving us a ground footprint 40,000 km long.
Figure 1 - Uncertainty concerning the ground footprint, for an LEO object, for a date of re-entry estimated a few hours before the actual date of re-entry
Prior to the “short-term” estimation of the re-entry phase of a space object (i.e. a few days to a few hours in advance), an analysis is performed of the space objects liable to re-enter the Earth’s atmosphere within a few weeks to a few months. This “medium – long-term” estimate enables the event to be anticipated and the characteristics of the objects concerned to be analysed (e.g. nationality of the space object, mass, etc.).
“Medium – Long-term” re-entry prediction for a space object in low orbit
Figure 2 gives the calculated re-entry date on the X-axis and the predicted re-entry date on the Y-axis. The blue circles represent the calculated re-entry dates and the grey lozenges the uncertainty around this date. The red line represents the date on which this object actually re-entered the Earth’s atmosphere