## 6 Conclusions  
This paper presents some recent research activities conducted by the University of Padova in the framework of Space Debris. The presence of a hypervelocity impact facility, with a high shot frequency up to ten experiments per day, makes it possible to perform experimental campaigns on a wide number of targets, from advanced shields to smart structures and sensors. With respect to larger hypervelocity impact facilities, the LGG available at the University of Padova has a maximum velocity of 5.5 km/s for projectiles with a mass of up to 100 mg; investigating higher velocities requires numerical simulations, as long as their validation can be performed within the LGG operative range. The development and installation of an upgraded LGG is under evaluation; in parallel, the formal collaboration with other laboratories which complements operational parameters will allow the enhancement of the available ranges of projectiles masses and velocities. The performed experimental activities spanned from materials and structures survivability testing to the investigation of the fragmentation processes. The results from the test campaigns were in line with data collected from other institutions; in particular, fragmentation distributions can be compared with experimental results for both simple and complex targets.  
In parallel, the development of the Collision Simulation Tool Solver allowed us to study complex orbital collision scenarios with large impactors and full details included. The CSTS is therefore a powerful tool for investigating catastrophic impacts and improving current spacecraft fragmentation models. The current challenges in CSTS are the extension of the number of available macroscopic elements, to include electronic boards, solar panels, and pressurized tanks; material libraries are also still limited to the most common materials employed in the space sector.  
To overcome these limitations, current experimental activities focus on material fragmentation, in the framework of commercial contracts with enterprises and institutions; numerical investigations are similarly dedicated to updating the CSTS breakup models and to upgrading the catastrophic impact threshold formulation.  
Figure 15: Simulated collision scenarios involving ENVISAT and a 100 kg small sat (**left**) or a 3-ton rocket stage (**right**).  
**Author Contributions:** Conceptualization, L.O., data curation, C.G. and S.L.; writing--original draft preparation, L.O. and S.L.; writing--review and editing, C.G. and A.F.; supervision, A.F.; project administration. All authors have read and agreed to the published version of the manuscript.  
The IMBEMUS project was supported by the CARIPARO foundation. Results presented in Section 3.2.3 were funded through the European Commission Horizon 2020, Framework Programme for Research and Innovation (2014-2020), under the ReDSHIFT project (grant agreement no. 687500). The CADETS project was partially funded by the Malta Council for Science and Technology. The CSTS development and the fragmentation tests were supported by the European Space Agency through contracts No. 4000119143/16/NL/BJ/zk, \"Numerical simulations for spacecraft catastrophic disruption analysis\", and No. 4000133656/20/D/SR, \"Exploiting numerical modelling for the characterisation of collision break-ups\". Part of this work is supported by the Italian Space Agency, in the framework of the ASI-INAF Agreement \"Supporto alle attivita' IADC e validazione pre-operativa per SST (N. 2020-6-HH.0)\".  
**Data Availability Statement:** Not applicable.  
The authors declare no conflict of interest.