Satellite re-entry prediction products for civil protection applications

Currently, approximately 70% of the re-entries of intact orbital objects are uncontrolled, corresponding to about 50% of the returning mass, i.e. ~100 metric tons per year. On average, there is one spacecraft or rocket body uncontrolled re-entry every week, with an average mass around 2000 kg. Even though a detailed demise analysis is available only occasionally, in many cases the alert casualty expectancy threshold of 1:10,000 is probably violated. Re-entry predictions are affected by various sources of inevitable uncertainty and, in spite of decades of efforts, residual lifetime mean relative errors around 20% should typically be expected. Due to the fact that re-entering satellites in nearly circular orbit complete a full revolution around the Earth in just less than 90 minutes, even a few days before orbital decay a re-entry uncertainty window still includes many revolutions, overflying most of the planet. This also means that even predictions issued just 3 hours before re-entry may be affected by an along-track uncertainty of 40,000 km (corresponding to one full orbital path), possibly halved during the last hour, if further tracking data are available. This kind of information is not much useful and manageable for civil protection applications, often resulting in confusion and misunderstandings regarding its precise meaning and relevance. In order to overcome these problems, since 2003 specific approaches and procedures were developed at ISTI/CNR in Pisa, Italy, producing output products tailored for civil protection applications by the national authorities. The experience gained during several re-entry campaigns was very positive and it was possible to verify in real case situations the accuracy and appropriateness of the developed products. This paper will present in detail the work carried out so far for the Italian territory and air space. In particular, a detailed description of the followed approaches and procedures will be given, with product examples taken from real re-entries occurred during the last decade. Moreover, prescriptions will be provided on how our methods and strategies might be extended and applied to wider areas of the planet, possibly the entire world, in order to supply more accurate early alerts to aircraft and on the ground, in terms of reasonably slim risk time windows and re-entry tracks for any location possibly affected by the re-entering debris.