## 2 Fragmentation Events
### Event History  
To establish an accurate space debris model, not only recent events have to be considered, but also the complete history of on orbit fragmentations have to be maintained on a regular basis. Currently, there are 261 considered fragmentations and each of them is modeled as an individual event. There are some additional events that were categorized as unknown anomalies; however, they are neglected in the fragmentation modeling process [6]. All considered events are categorized as explosion or (non-)catastrophic collision including detailed information on the event characteristics such as the involved mass or collision velocity. Out of these 261 events, 247 are categorized as explosions (\(\approx\)95%) [6]. The remaining 14 events are categorized as confirmed (e.g., the Fengyun-1C Anti-Satellite Test) and assumed collisions. However, looking at the total amount of tracked debris by the Space Surveillance Network (SSN), there are twice as many explosion fragments than collision fragments. Figure 2a shows the number of fragmentations over the last 60 years.  
The mean number of fragmentations per year accounts to 4.9 (\(SD=2.8\)) with no sign of regression or fundamental change. However, there was a slight decrease in fragmentations for three years after the introduction of the Inter-Agency Space Debris Coordination Committee (IADC) Space Debris Mitigation Guidelines in 2010. An examination of the fragmented mass over the years is shown in Figure 2b. It is shown that most of the objects that experienced a breakup had a mass less than 500 kg. Looking at the other extreme of the histogram, there are two events with a fragmentation mass of 26,000 kg and 30,000 kg. These events happened in the late 1960s during the Apollo program, but, due to their low event altitude of below 300 kg, almost all fragments re-entered into Earth's atmosphere and consequently do not contribute to the space debris environment in the long term.  
Figure 2: Evaluation on history of breakup events. (**a**) number of on-orbit breakups per year; (**b**) histogram data on number of objects per fragmentation mass.  
Looking at the difference between the event fragmentation (breakup) epoch and the initial launch of the corresponding payload, the age of the object can be obtained. A histogram covering all fragmentations showing the object's age is shown in Figure 3.  
It is evident that most breakups occur during the first year of operation in space. The exact reasons for the breakups are very difficult to uncover, but there are a few likely possibilities such as propulsion- or electrical-related events. Over 80% of the objects experienced a breakup during the first 10 years of operation. However, there are also single events that happened over 30 years after launch. They are mostly old and non-passivated upper stages with some stored rest energy. The events with the biggest generation of trackable debris up to date are shown in Table 1.  
The Fengyun-1C event is still by far the most prominent event to date. Due to the relatively high event altitude, most of the objects are still in orbit and consequently could collide with other spacecraft. The second largest event was the deliberate destruction of USA-193. However, due to its low event altitude, no objects pose a long-term thread for other spacecraft [8]. These two events have a deliberate cause of their fragmentation, whereas the Cosmos-2251 and the Breeze-M fragmentations have not. There is a trend in the number of generated objects in a way that deliberately destroyed objects yield a higher number of objects.