Criticality assessment of the Italian Non-Maneuverable satellites in Low Earth Orbit

Looking at the situation in low Earth orbit (LEO), i.e. below 2000 km, on September 9, 2017, there were 21 Italian satellites, one spent upper stage, crossing most of the region, and one debris related to the OptSat 3000 classified mission, probably a mission related object (MRO). They represented just 0.18% of the objects cataloged in LEO by the US Space Surveillance Network (SSN). Moreover, since the release, in 2002, of the first version of the Mitigation Guidelines compiled by the Inter-Agency Space Debris Coordination Committee (IADC), there had been a significant improvement in the level of compliance with debris mitigation requirements for the Italian satellites, included the so called "25-year" rule, i.e. the limitation of the post-mission disposal lifetime to less than 25 years. Several objects had been, however, launched before the introduction of the IADC Mitigation Guidelines, and also before the more than doubling of the debris population below 1000 km caused, in 2007, by the Fengyun 1C anti-satellite test and, in 2009, by the accidental collision between Iridium 33 and Cosmos 2251. In this paper, the environmental criticality of the Italian satellites in LEO, from the orbital debris point of view, was evaluated addressing the following issues: o Their residual lifetime; o The probability of impact with other objects; o The probability of catastrophic fragmentation; o The number of expected fragments in case of catastrophic collisional breakups; o The possible interference with the orbit of the International Space Station; o An overall ranking, using a criticality index developed by the authors. The results obtained not only represented an updated assessment for the Italian objects launched until the summer of 2017, but also a guide for planning and conducting future missions in LEO in a way as safe and sensible as possible.