## 2 Fragmentation Events
### Fragmentation Characteristics of Individual Events
(**a**) NASA breakup model approach for the Fengyun-1C event as of 2018; (**b**) simulated Fengyun-1C event for a different number of TLE fragments. These include the Chinese anti-satellite test regarding Fengyun-1C [12], fragments from Cosmos-2251, which was involved in the Cosmos-Iridum collision in 2009 [13] and the explosions of DMSP-F13 [14] and NOOA-16 [15]. Therein, the shaded areas show the time from the last number update until today. In particular, the numbers of TLE fragments for the Fengyun-1C and Cosmos-2251 event have been significantly increasing over the last 6-7 years. After the Fengyun-1C event, the number of cataloged TLE objects was constantly increasing for almost one year. During the MASTER-2009 population update, the numbers had increased to around 2000 and have now stabilized to 3438 objects [4]. At each population update, the underlying clouds were generated based on the most recent available data. The evolution of the number-size-distribution for the different number of TLE objects for the Fenyun-1C event is shown in Figure 4b. Over a time frame of 10 years, the total number of objects larger than the minimum trackable diameter, which is approximately 10 cm, increased by almost a factor of 3.5. This has a direct consequence on the object numbers in the 1 cm regime. Table 2 gives an overview over the total number change over the years. The increasing factor for objects \(d>10\) cm is also fully applied on the smaller diameter regime. This results in a number increase of objects larger than 1 cm from 60,000 to 204,000. All fragmentation events undergo the same number change according to their update on cataloged fragments. The process is done for all events but poses some challenges regarding recent or relatively new events. The number of TLE fragments for the DSMP-F13 and NOAA-16 event have increased since the initial tracking of the cloud (cf. Figure 5c,d). Since the events occurred in the beginning  
Figure 5: Temporal evolution on total number of cataloged debris for selected events in monthly discretization using data from [4]. (**a**) Fengyun-1C; (**b**) Cosmos-2251; (**c**) DMSP-F13; (**d**) NOAA-16. of and late 2015, and the fragmentation altitude for both events were between 800 km and 900 km, it is expected that these events will lead to more detected fragments in the future with a long-term contribution to the space debris environment. Validating this modeling approach is done by comparing measurement data from dedicated space debris observation campaigns to the MASTER model population. These so-called \"beam-park\" experiments usually have a lower minimum detection diameter of around 1 cm in Low-Earth Orbit (LEO), which allows for a more refined calibration of the population. Comparing measurement data with the model data is performed with PROOF and is a crucial step in the validation phase [16; 17; 18]. One optional step in the model verification is to visualize the initial cloud evolution of each event, as shown in Figure 6.  
These visualizations are created with a tool called DOCTOR (Display of Objects Circulating in Terrestrial Orbits) and help to identify major modeling issues, data integrity as well as supporting illustrative publications [19; 20; 21]. One major benefit in modeling the space debris environment source wise is that every object can be related to a specific event and source and visualized using a source specific color code. Especially when interpreting measurement data, the MASTER model can be used to check the integrity of dedicated space debris observation campaigns.