Orbital debris mean danger on the ground

Most satellites remain in orbit for many years (even centuries), however, the decay, or re-entry, of satellites occurs on a regular basis.

Furthermore, during satellite launches, the last or final stage of the launch vehicle will usually orbit the Earth briefly before decaying and safely burning up in the atmosphere. But there are occasions when the destruction of these objects is not completed by the heat of re-entry. And satellites which did not reach the correct orbit or satellites that have finally been affected by atmospheric drag will often reach the ground with a very high terminal velocity – potentially causing destruction.

There are two reasons why Europeans are interested in knowing when – and where – space debris could potentially come back down to Earth.

The first of this is for the obvious reason of protection of life and property.

The second reason is one of responsibility. Just as European satellite owners and operators are legally accountable for any damage their assets inflict in orbit, they are also responsible for any harm caused by a re-entry event. It is, therefore, Europe's responsibility to be able to predict re-entry events so as to minimise or mitigate any possible inflicted by European-owned assets.

Predicting reentries

We can predict that most satellites will remain in orbit for many years, decades or even centuries. However, it is no surprise that the decay, or re-entry of satellite occurs on a regular basis. During satellite launches, the last or final stage of the launch vehicle will orbit the Earth briefly before decaying; safely burning up in the atmosphere.

However, there are occasions when the destruction of these objects is not completed by the heat of re-entry. Satellites which did not reach the correct orbit or satellites that have finally been snared by the drag caused by Earth’s atmosphere will often reach the ground with a very high terminal velocity – and hence a potentially destructive energy.

The ability to provide accurate decay predictions relies on an accurate data set or catalogue of orbiting objects, in a very similar fashion to the detection of in-orbit collisions. In reality the sensors, infrastructure and data are almost identical, although the data is processed through specific software that then predicts the impact point.

For high precision predictions, it is also necessary to possess the ability to image objects in orbit. With an image, one can determine the attitude of the object and hence attempt to determine how it will react when encountering the denser regions of the atmosphere.

Certain European institutions possess this ability, as shown in Figure 3, below (from the TIRA radar in Wachtberg, Germany). Currently this is done an experimental basis rather than for routine operations.

To ensure compliance with routine operational requirements, three main elements are required. The first of these is equipment that can be tasked with short notice to perform tracking of objects as they begin the decay. This is vital for accurate orbit predictions – especially as the conditions are changing rapidly when an object starts to feel to full effects of the atmosphere’s drag.

The second is the ability to image an object as described above. Knowing the actual orientation of the re-entry subject has large effects on the accuracy of the decay prediction.

The final requirement is to have a system of governance and data management to ensure that the right people receive correct information in a timely fashion. Without a rapid warning of decay, the rest of the system loses its power to aid and assist governments, security services as well as the necessary emergency services.


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Last update: 31 May 2010

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