## 3 History of Operation  
### Mission History  
In this section, the operational history of ALE-1 and SDOM is discussed using images from the TOF (Time-of-Flight) camera onboard ALE-1. The mission history is shown in Figure 4, and the corresponding TLE history of ALE-1 is shown in Figure 5.  
The separated SDOM sail is unlikely to generate additional space debris, as its area-to-mass ratio is large enough to de-orbit itself for a very short orbital period, ultimately being burned up during the re-entry. One reference for this technology is the 1U-sized CubeSat FREEDOM, which was operated in 2017 by Nakashima Engineering Works, Ltd. and Tohoku University [17]. FREEDOM carried a DOM with a film size of 1.5 m \(\times\) 1.5 m. Based  
Figure 4: Mission history of ALE-1 and SDOM.  
Figure 5: Orbital altitude history of ALE-1 based on TLE: Two-Line Element. The semi-major axis of the orbit is plotted together with the mission history.  
Figure 3: Operational phases of SDOM.  
on the orbit history of FREEDOM, it took about 22 days to re-enter the Earth's atmosphere from an altitude of 400 km [13]. According to this background, the separated part of the SDOM is expected to re-enter the atmosphere within a few days.  
A detailed description of the mechanisms and satellite subsystems of ALE-1 can be found in [7; 18; 19]. Each phase transition is initiated by stored commands that are uplinked from the ground station after the required status checks for each phase transition have been verified [11].  
After ALE-1 entered orbit on 18 January 2019, the satellite was operated and underwent orbital functional verification. The SDOM mission started on 11 June 2019 when its lid was opened. Shortly thereafter, the boom extension was initiated, but it could not be confirmed as intended. After 5 months of regular observations, the boom extension was finally confirmed on 6 November 2019, which initiated phase 2. The deployment of the thin film was postponed for an additional 50 days to allow for the detailed evaluation of the gravity gradient effects acting on the satellite system. The film was then deployed on 25 December 2019 [5].  
Based on the initial estimation, ALE-1 with the SDOM deployed was expected to descend to the ISS orbital altitude in about 650 days [18]; in contrast with the estimation, however, the deployed SDOM experienced a series of problems. On 28 December 2019, one of the film's four corner connections was found to be damaged, which left only half the area of the film effective for drag. Approximately two weeks later, another corner connection was lost, effectively changing the film from a drag sail to a drag flag. This issue is discussed in more detail in the next section. In addition, between 20 April 2020 and 27 May 2020, a gas leak occurred from a tank installed for the artificial shooting star mission; the gas leak was caused by a malfunction of the gas output control system, and the malfunction was resolved when the control system was restarted. After that, operations went smoothly, and on 27 July 2022, when the orbital altitude was confirmed to be below 400 km, the sail section of SDOM was separated from ALE-1. The SDOM separation was observed by the DMC (DOM Monitoring Camera) as illustrated in Figure 6. ALE-1 completed its descent, albeit about a year later than predicted, and is still flying today, maintaining its orbital altitude.