On Ground Risks

To get rid of large objects in orbit, such as 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 debris, such as the last stages of launchers or large, out-of-control satellites, this is not possible.

Figure 1
Controlled re-entry of the ATV over the Pacific (Artist’s impression)

In general, we estimate the relative error for the uncontrolled re-entry of a space vehicle at 10%.

This uncertainty is mainly due to:

• Lack of data on solar activity
• Errors in modelling the upper atmosphere
• Errors in modelling the attitude of the object re-entering, which is often considered to have a spherical geometry

In practical terms, an estimation error of 10% means a ground footprint of 40,000 km, ten hours before the date of re-entry.

During atmospheric re-entry by a space vehicle, it will be subjected to considerable thermomechanical stresses. These stresses will lead to the fragmentation and sometimes the explosion of the re-entering object.

Once the re-entering object has broken up, generating a number of fragments, these will in turn be subjected to significant thermomechanical stresses, given that these fragments will be passing through increasingly dense layers of the atmosphere.

Some of these fragments will literally burn up and never reach the ground, while components designed to withstand high temperature and pressure stresses (e.g. combustion chambers, propellant tanks, etc.), may reach the ground.

Figure 2
Pressurised tank which fell in South Africa (Source: Argus/Enver Essop)

If the re-entering object has a sufficient quantity of energy, controlled re-entry can be performed by a series of thrusts enabling it to re-enter in a predetermined geographical region. Furthermore, in the case of controlled re-entry, the attitude at the beginning of the re-entry phase is controlled, which makes it easier to estimate the “footprint” or the geographical region in which fragments of the re-entering object will fall (see Fig. 3).

Figure 3
Controlled re-entry strategy for the ATV (Automated Transfer Vehicle) and footprint

In the case of an uncontrolled re-entry, correct estimation of the “footprint” becomes even more complicated, because the re-entering object is not accurately identified and uncertainty over the actual date of re-entry is significant.

For the time being, nobody has yet been killed by a space debris impact. The probability of an individual being struck by space debris is estimated at one in a thousand billion. Comparatively, the probability of being struck by lightning is estimated at one in a million.