Uncontrolled re-entries of sizable spacecrafts and rocket bodies: a potential threat in the airspace and on the ground

More than 24,000 cataloged orbiting objects have re-entered so far into the Earth's atmosphere since the beginning of the space age. The associated returning mass, close to 30,000 metric tons, was mainly concentrated in intact objects, i.e. spacecraft, platforms and spent upper stages, accounting for nearly 29% of the re-entered objects. Over the last ten years, from 2008 to 2017, almost 440 large intact objects (80 spacecraft and 360 rocket bodies) have re-entered without control, with a total mass of about 900 metric tons (150 and 750 metric tons for spacecraft and spent upper stages, respectively). Therefore, approximately 90 metric tons belonging to large intact objects would have, on average, re-entered the Earth's atmosphere each year, and a mass fraction between 5% (4500 kg) and 15% (13,500 kg) might have crossed the airspace and hit the ground. In spite of a not negligible amount of mass suspected to have survived re-entry, and of at least fifteen events, from 2008 to 2017, in which spacecraft and rocket bodies components were retrieved, trivial damages to property occurred and no case of personal injury was confirmed. However, even if the risk related to the re-entry of manmade space objects is still extremely low, it cannot be commonly accepted as being inevitable. Moreover, it cannot be excluded that uncontrolled re-entries of sizable space objects might become of greater concern in the future, as a consequence of the increased use of space and growing population density on the ground. After an overview of the most critical historic re-entries events, the attention will be focused on the re-entries of massive objects occurred without control during the last decade, highlighting: 1) the relative relevance of spacecraft versus rocket bodies (typically, the re-entry of large spacecraft catches more media and people attention than the re-entry of an equally, or even more massive, upper stage); 2) the re-entry frequency; 3) the distribution in inclination; 4) the potential risk due to a large returning mass (such as the NASA's Upper Atmosphere Research Satellite, the Russian cargo ship Progress M-27M, the Russian-Ukrainian second stage of the Zenit-3 launcher), or the presence on board of a significant amount of highly toxic propellants (as was the case of the Phobos-Grunt probe). For a number of re-entry events eyewitnesses sightings were reported and/or pieces of debris were recovered (as for debris from the Russian-Ukrainian Zenit-3 second stage 2015-074C, landed in Vietnam at the beginning of January 2016, or the Composite Overwrapped Pressure Vessel of the Vega's upper stage AVUM 2012-006K, recovered near Oddanchatram, India, in November 2016). Hence, lessons learned from past uncontrolled re-entries of enough massive bodies suggest that, even if still small compared to other commonly accepted risks related to the lifestyle, or workplace and household safety, the risk for aircraft, or people and property on the ground, of being hit by falling orbital debris cannot be absolutely neglected. Moreover, it should not be ignored that also space vehicles intended for a controlled re-entry at the end of their mission may sometimes suffer failures, compromising the success of the planned de-orbiting strategy. This is, for instance, the case of the Chinese space station Tiangong-1. There are also large upper stages, with masses of about 4 metric tons or more, which still mostly re-enter without control.