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In terms of cataloged debris produced, the anti-satellite test carried out by Russia, in November 2021, at an altitude of about 480 km, leading to the destruction of the old satellite Cosmos 1408, was the second worst to date and represented the third worse fragmentation in orbit. It generated more than 1/4 of the cataloged debris produced over 55 years by all such tests and almost twice as many as were produced by all previous Soviet tests. After placing this event in its historical context, this paper analyzes in detail how the evolution of the Cosmos 1408 debris cloud impacted the environment below 600 km in the first seven months, focusing on the two operational space stations and the Starlink mega-constellation of satellites. For the International Space Station, the Cosmos 1408 cloud of fragments increased the flux of cataloged objects, on average, by nearly 80%, while for the Tiangong Space Station the increase was almost 65%. Immediately after the test, the Starlink mega-constellation saw as well an increase in the flux of cataloged objects, of the order of 20% on average. Some orbital planes, the "counter-rotating" ones with respect to the Cosmos 1408 debris cloud, were more affected than others, and the affected planes gradually changed over time, due to the differential precession of cloud and constellation nodes. However, being the Starlink constellation 70 km higher up, the flux of Cosmos 1408 cataloged debris steadily decreased over the period analyzed, due to the cloud orbital decay, more than halving after seven months.

Following the observational discovery of a significant number of objects characterized both by high eccentricity geosynchronous orbits and by extremely high values of their area-to-mass ratios (A/M), whose origin can probably be traced back to the degradation and detachment of the very light specific mass layers used to protect the surfaces of geostationary spacecraft, a thorough investigation was carried out since 2008 to explore the long-term dynamical evolution of fictitious high A/M objects released, with a negligible Delta-V, in each of the six orbital planes used by Global Positioning System (GPS) satellites. As for the objects observed and studied in near synchronous trajectories, also in this case long lifetime orbits, with mean motions of about 2 revolutions per day, were found possible even for bodies with extremely high A/M. The behavior details, affected by the complex interplay of solar radiation pressure, geopotential and luni-solar resonances, obviously depend on the initial conditions. However, objects with A/M as high as 20-40 m2/kg can avoid orbital decay for at least one century or more, with semi-major axes close to the semi-synchronous values, with maximum eccentricities between 0.3 and 0.7, and with significant orbit pole precessions (faster and wider for increasing values of A/M), leading to inclinations between 30° and more than 90°. For particularly high values of A/M, in fact, the transition from prograde (inclination < 90°) to retrograde (inclination > 90°) motion, and vice versa, would be possible.

The SNO+ experiment is located 2 km underground at SNOLAB in Sudbury, Canada. A low background search for neutrinoless double beta ($0������$) decay will be conducted using 780 tonnes of liquid scintillator loaded with 3.9 tonnes of natural tellurium, corresponding to 1.3 tonnes of $^{130}$Te. This paper provides a general overview of the SNO+ experiment, including detector design, construction of process plants, commissioning efforts, electronics upgrades, data acquisition systems, and calibration techniques. The SNO+ collaboration is reusing the acrylic vessel, PMT array, and electronics of the SNO detector, having made a number of experimental upgrades and essential adaptations for use with the liquid scintillator. With low backgrounds and a low energy threshold, the SNO+ collaboration will also pursue a rich physics program beyond the search for $0������$ decay, including studies of geo- and reactor antineutrinos, supernova and solar neutrinos, and exotic physics such as the search for invisible nucleon decay. The SNO+ approach to the search for $0������$ decay is scalable: a future phase with high $^{130}$Te-loading is envisioned to probe an effective Majorana mass in the inverted mass ordering region., 61 pages, 23 figures, 4 tables

This report integrates a variety of methods to find internationally agreed list of the most dangerous space debris objects in low Earth orbits. These methods each generate different results based on diverse hypotheses and approaches. A state-of-the-art model consolidation approach was applied for the integration of these reputable models. The significance of the results of this effort is noteworthy -- 19 experts from 13 countries/organizations had their 11 individual assessments aggregated into a list of the 50 statistically-most-concerning (SMC) objects for debris generation. Upon examination of the original 11 lists, it is noted that all of them had between ~40 to 60% objects in common between some other top-list. In addition, it is also important that, even though only two of the 11 approaches specifically disregarded payloads, 39 of the top 50 SMC are derelict rocket bodies; only 11 are non-operational payloads.

In this paper we introduce a new index for evaluating the propensity of a volume of space in low Earth orbit (LEO) to catastrophic collisions, that is to accidental collisions leading to the complete destruction of intact objects. The proposed index is therefore not intended to assess the criticality of individual objects or missions, but rather to estimate the global impact of space activities on a given region of space. Moreover, the new index was conceived to be objective, as simple as possible, built from easily accessible data, as well as smoothly reproducible by third parties. Named "volumetric collision rate index", it was developed starting from the analytical equations that express the collision rate as a function of the fluxes of intact objects and cataloged debris pieces. The application of reasonable simplifying assumptions and approximations finally allowed to define a dimensionless index that explicitly depends only on the spatial densities of intact objects and cataloged debris pieces. It was therefore applied to the LEO environment, analyzing its evolution from mid-2008 to mid-2020, a crucial period characterized by an impressive change of space activity patterns, with the launch of lots of small satellites and mega-constellations. We also discuss how the index can be further improved, taking into account the maneuverable satellites that do not contribute to the collision rate and the increasing number of cubesats, which in many respects would be more similar to debris, finally presenting a preliminary analysis in this direction.

Questo documento è il terzo deliverable dell'accordo di collaborazione tra ASI e INAF in ambito "Detriti Spaziali- Supporto alle attività IADC e SST 2019-2021".

Introduction to the draft report of AI 36.3 "Disposal of high inclination geosynchronous spacecraft and orbital stages"

The semi-empirical atmospheric density models widely used by the space community were mainly developed taking into account satellite drag measurements and other observations, either in situ and ground based, acquired at relatively low altitudes, mostly below 500-600 km, and in general below 1000 km. The launch of the Italian geodetic satellite LARES, in 2012, at the altitude of about 1450 km and with an inclination of 70 degrees, offered however the rare possibility of probing the atmosphere at such height. This spherical satellite, fully covered with corner-cube laser retro-reflectors, has the highest area-to-mass ratio of any artificial object launched so far, being therefore not well suited for detecting small non-gravitational forces, like atmospheric drag. However, the very high accuracy of its orbit determinations, made possible by the laser tracking technique, more than compensated its unfavorable area-to-mass ratio, and the signature of atmospheric drag was extremely evident in the measured semi-major axis decay. Such decay, observed since 2012, was therefore used to infer the neutral atmosphere drag at the height of LARES during a 7-year span of solar cycle 24, covering the solar maximum, the declining phase and the beginning of the minimum. These measurements were compared with the predictions of six semi-empirical density models (JR-71, MSIS-86, MSISE-90, NRLMSISE-00, GOST-2004, and JB2008), employed well outside of their typical application ranges. In general, their predictions resulted quite satisfactory, with uncertainties not so far from those already known at lower altitudes. This study was also supplemented by the simultaneous analysis of another spherical geodetic satellite, the Japanese Ajisai, just 50 km higher, but with an area-to-mass ratio nearly 20 times greater than that of LARES and a smaller inclination of 50 degrees. An attempt was also made to estimate the physical drag coefficients of both satellites, in order to derive the mean density biases of the models. None of them could be considered unconditionally the best, the specific outcome depending on solar activity and on the regions of the atmosphere crossed by the satellites. Moreover, during solar maximum conditions, an additional density bias, probably linked to the different high latitudes overflown by the satellites, was detected.

Since the definition of a Low Earth Orbit (LEO) protected region, at the beginning of the 2000s, most of the attention of the space debris mitigation community was focused on heights greater than 600 km. In such orbital regimes, in fact, the average residual lifetimes of inert satellites and rocket bodies become typically higher than 20 years and the highest concentrations of functional satellites and space debris were found. The low LEO region, below 600 km, is however extremely important for space applications. In fact, since the last Apollo mission to the Moon, all human spaceflight was carried out there and also extremely important scientific missions, like the Hubble Space Telescope (HST), are orbiting in this volume of space. In recent years there was a dramatic increase in the launch rate of small satellites and cubesats in low LEO, boosting the number of potentially risky objects to be tracked and monitored. But the most dramatic development currently going on is the deployment of mega-constellations of satellites, with almost 10,000 spacecraft planned only in low LEO in the coming years. Even though any failed satellite of the planned systems will decay from orbit in much less than 25 years, therefore formally complying with current international space debris mitigation guidelines, it is realistic to expect a relatively high number of failures, considering the experimental nature of spacecraft tested in space, and in great numbers, for the first time. The short- and medium-term consequences for the satellite operations in low LEO might therefore be far from negligible, not to mention the negative effects on astronomy and the observation of the night sky. In the coming decade, a significant increase of close approaches and collision avoidance maneuvers in low LEO should be expected mainly due to: 1) The great number of non-maneuverable nanosats launched below 650 km; 2) The failed satellites of the mega-constellations launched there; 3) The disposed satellites of the mega-constellations launched in high LEO, either failed or with reduced maneuverability and/or operability; 4) A greater number of disposed satellites not belonging to mega-constellations, in order to comply with space debris mitigation guidelines or standard. The aim of this presentation is mainly to review the new challenges to be faced by spacecraft and space operations in low LEO due to these quite recent developments and to the current launch forecasts.

The substantial space traffic changes occurred since the 2010s are having measurable repercussions even at the altitudes used for human spaceflight. These changes were mainly driven by the deployment of thousands of small satellites and cubesats below 600 km. After having evaluated how the situation evolved, from 2008 to 2021, at the altitudes of the International and Chinese space stations, and discussed the main aspects of the problem that may have an operational impact, the attention was focused on what might be expected in the 2020s, whether the current deployment plans for mega-constellations will be realized in whole or in part. Finally, the rationale for the introduction of a "human spaceflight protected region", with the associated space traffic management recommendations, is presented and discussed.

We present a new experiment called SaToR-G (Satellites Tests of Relativistic Gravity) which mainly concerns on verifying the gravitational interaction beyond the predictions of General Relativity, looking for possible effects connected with new physics, and foreseen by different alternative theories of gravitation. SaToR-G exploits the improvement of the dynamical model of the two LAGEOS and of LARES satellites performed within the previous research program called LAser RAnged Satellites Experiment (LARASE: 2013-2019) and funded by the Italian INFN (Istituto Nazionale di Fisica Nucleare). Within LARASE we achieved a new measurement of the Lense-Thirring precession with an accuracy better than 2%. To reach the objectives foreseen by SaToR-G, we need to provide a precise orbit determination of a set of laser-ranged satellites, such as the two LAGEOS, LARES, and the forthcoming LARES-2, whose launch is expected before the end of this year. The state-of-the-art regarding the modelling improvements currently reached with LARASE will be presented together with the main objectives of SaToR-G in the fields of relativistic measurements and space geodesy.

At the beginning of 2018 (Tiangong-1) and 2019 (Electron Second Stage 2018-010D) we had the occasion of participating to a couple of international reentry prediction test campaigns, carried out during a low minimum of solar activity, characterized by a nearly stable solar flux at 10.7 cm of about 70 standard flux units. With these environmental conditions, the accuracy of the reentry predictions was significantly affected by the more or less predictable occurrence of minor - G1 class - geomagnetic storms, and the operational impact of these uncertainties, in particular for events with civil protection implications, as that involving the Chinese space station, will be presented. In the case of Tiangong-1, due to the relevance of the object, we started systematic reentry predictions at the beginning of 2017, i.e. at an altitude (> 350 km) much higher than the initial one generally found in typical reentry prediction campaigns. This offered the occasion to "probe" the behavior of some atmospheric density models - in particular between 350 km and 250 km, where Tiangong-1 spent more than 1 year - in conditions of quite low solar activity and cold thermosphere. Estimates of the residual lifetime during 2017 and until February 2018 were performed using the NRLMSISE-00 model. Since 1 February 2018, also the GOST2004 model was employed for comparison reasons and for checking the mutual consistency of the forecasts. Concerning the predictions carried out with NRLMSISE-00 during the year 2017 and until the beginning of February 2018, they systematically obtained a nominal re-entry epoch earlier than the actual one. Therefore, a post-reentry analysis was performed to investigate whether the cause of the systematic earlier predicted reentry times was due to a bias in the density model used, or to errors in the forecasts of solar and geomagnetic activity. The second possibility was first verified by replacing the predicted values of the solar and geomagnetic indices with the observed ones, and hence by re-propagating the trajectory under true environmental conditions. In this case, still using NRLMSISE-00, the re-computed nominal reentry times were indeed delayed with respect to the original predictions. Therefore, a "true" representation of the environment would have led to an improvement of the forecasts, by obtaining nominal reentry times closer to the actual one. However, all the revised predictions adopting the real space weather conditions were still systematically anticipated with respect to the actual reentry epoch. The other test consisted in changing the density model used for the thermosphere. For such a posteriori analysis, also JB2008 and GOST2004 were considered, in addition to NRLMSISE-00, to re-calculate the nominal reentry times. Nevertheless, the three density models behaved in a similar way, with a maximum discrepancy, in terms of residual lifetime, of about 9%. In other words, all three found a residual lifetime systematically shorter than the actual one. A possible explanation might be that, under the environmental conditions encountered by Tiangong-1 during its uncontrolled decay (i.e. a deep minimum of the solar activity cycle), each atmospheric model analyzed was probably affected by an intrinsic density bias, with a negative derivative with respect to the geodetic altitude, at least down to 270 km. This lead to a percentage increase in the estimation of the atmospheric density as the spacecraft height decreased, neutralizing the absorption of the bias at higher altitudes through the previous ballistic parameter calibrations and causing the systematic underestimation of the residual lifetime recorded up to the beginning of February 2018.

Questo documento ha lo scopo di descrivere le attività svolte nell'ambito del progetto "Sviluppo di SW a supporto dei servizi SST per lo studio di detriti spaziali" relativamente ai quattro pacchi di lavoro in cui l'attività è stata suddivisa. Nello specifico, il presente documento illustra nel dettaglio: o l'architettura e i risultati della validazione dei software sviluppati nell'ambito dei pacchetti di lavoro WP2000 e WP3000, inerenti all'analisi di frammentazioni e alla simulazione di reti di sensori; o l'analisi e la validazione delle procedure di rientro eseguita nell'ambito del pacchetto di lavoro WP4000. Dopo una descrizione degli obiettivi del lavoro e della Work Breakdown Structure riportati nella Sezione 2, la Sezione 3 e la Sezione 4 dettagliano l'architettura e la validazione dei software. La Sezione 5 è invece dedicata all'analisi e alla validazione delle procedure di rientro. Infine, la Sezione 6 fornisce la lista dei deliverables del progetto.

Since the definition of a Low Earth Orbit (LEO) protected region, up to the altitude of 2000 km, at the beginning of the 2000s, most of the attention of the space debris mitigation community was focused on heights greater than 600 km. In such orbital regimes, in fact, the average residual lifetimes of inert satellites and rocket bodies are typically greater than 20 years and the highest concentrations of functional satellites and space debris were historically found. The low LEO region, below 600 km, is however extremely important for space applications. In fact, since the last Apollo mission to the Moon, in December 1972, all human spaceflight was carried out there, the International Space Station, the planned large Chinese space station, the Indian crewed program and possible human tended private havens are making or will make use of this region of space, and also absolutely critical missions, like the Hubble Space Telescope, are orbiting below 600 km. In recent years there was a dramatic increase in the launch rate of small satellites and cubesats in low LEO, boosting the number of potentially risky objects to be tracked and monitored. But the most dramatic development currently going on is the deployment of large satellite constellations, with almost 10,000 spacecraft planned only in low LEO in the coming years. Even though any failed satellite of the planned mega-constellations will decay from orbit in less than 25 years, therefore formally complying with current international space debris mitigation guidelines, it is realistic to expect a relatively high number of failures, considering the experimental nature of spacecraft tested in space, and in great numbers, for the first time. The short- and medium-term consequences for the satellite operations in low LEO might therefore be far from negligible. The aim of this paper is therefore to present a preliminary review of the new challenges to be faced by spacecraft and space operations below 600 km, in the coming years, due to the deployment of large constellations of small satellites.

This paper describes a process for identifying the intact objects in orbit that (a) pose the greatest debris-generating potential risk to operational satellites or (b) would reduce the risk the most if they were removed or prevented from colliding with each other (i.e., remediated). To accomplish this, a number of diverse, international space organizations were solicited to contribute their lists of the 50 statistically-most-concerning objects. The results of the multiple algorithms are compared, a composite ranked list is provided, and the significance of the consolidated list is presented including critical assumptions and key factors in determining this "hit list." It is found that the four primary factors used in these processes are mass, encounter rates, orbital lifetime, and proximity to operational satellites. This cooperative international assessment provides a useful ranking of the most hazardous massive derelicts in low Earth orbit as a prioritized list for remediation to (1) enhance space safety and (2) assure long-term space sustainability. This will hopefully catalyze international action in debris remediation.

This presentation outlines the final results of the LARASE experiment and the on-going activities of the SaToR-G experiment.

The evolution of cataloged orbital debris in low Earth orbit (LEO) over the last quarter of century was analyzed in detail, to gather insights on the development of space activities, on the effectiveness of the debris mitigation measures recommended in the meantime, and on the environmental impact of fragmentations, in particular collisions, both intentional and accidental. The main conclusion was that the observed evolution matched on the whole the predictions of the unmitigated business-as-usual scenarios simulated twenty years ago, and that the benefits caused by the progressive worldwide adoption of mitigation measures were unfortunately offset by a couple of catastrophic collisions and prolonged weak solar activity. Nevertheless, and despite the dramatic increase of launched satellites since the mid-2010s, nowhere have the signs of more than linear increases been revealed so far, aside from a few sizable, but circumscribed, fragmentation events. Even though above 700 km the debris population may be intrinsically unstable in the long run, the situation can still be managed and controlled. Therefore, even though the overall picture has worsened during the last 25 years, nothing of irremediable has been done so far. This does not mean that extreme care is not required in planning and conducting new space activities from now on, especially in a phase of increased and ever more rapid exploitation of space, to ensure its long-term sustainability and full utilization. In order to assess the sustainability of space activities, especially in the short and medium term, that is over the next 10-30 years, several environmental criticality indexes have been introduced and discussed, estimating their current values in LEO, as well as their upper limits - sometimes considered tolerable, others not - associated with specific scenarios of debris growth. They could provide simple tools for evaluating the relative and absolute impact on the debris environment, either in LEO as a whole or in specific altitude shells, of new spacecraft deployments and operations, as in the case of mega-constellations of satellites.

This presentation outlines the role played by CNR-ISTI in the LARASE and SaToR-G experiments, in particular regarding the modeling of atmospheric drag.

Questo documento è il primo deliverable dell'accordo di collaborazione tra ASI e INAF in ambito "Detriti Spaziali- Supporto alle attività IADC e SST 2019-2021". Sono riportate le attività svolte, dai vari enti coinvolti, nei primi 6 mesi dall'avvio dell'accordo, suddivisi per WP (Work Package).

Questo documento ha lo scopo di dettagliare lo stato di avanzamento delle attività del progetto "Sviluppo di SW a supporto dei servizi SST per lo studio di detriti spaziali" relativamente ai quattro pacchi di lavoro in cui l'attività è stata suddivisa. Nello specifico, il presente documento si riferisce alle attività svolte fra la prima riunione di avanzamento (RA1 - 16 ottobre 2020) e la seconda riunione di avanzamento (RA2 - 30 novembre 2020). Dopo un richiamo agli obiettivi del lavoro e alla Work Breakdown Structure riportati nella Sezione 2, la Sezione 3 dettaglia l'avanzamento delle attività per ogni pacco di lavoro. La Sezione 4 descrive lo stato del progetto in relazione alla pianificazione presentata al Kick-off Meeting, mentre la Sezione 5 descrive lo stato dei deliverables.

Introduction to the LARASE and SaToR-G experiments and review of the LARASE results.