Mission analysis of the space-based optical observation for orbital debris

Space situation especially in low earth orbit (LEO) is serious to require precise tracking of smaller debris as one of the most important technologies in debris counter measures. However, there is insufficiency in size limitation for smaller debris to be tracked. This paper introduces a space-based optical system as new observatory for LEO debris. Since such new system requires specific mission design procedure to be realized, we developed a new mission and its capability analysis tool. This tool incorporates observation criterion calculation and optimization in altitude, orbital plane and observation direction of the observatory. The observation criteria analysis is based on conversion between apparent magnitude and photon count on CCD array. And this criterion also considers pixel displacement of each observation in the form of photon count density. Altitude optimization part gives two candidates as 450 km and 850 km. Candidates in the orbital plane optimization can be assumed as zero and six o'clock in local time of the ascending node because the observatory should be placed in sun-synchronous orbit due to its homeostatic sunlight condition. We compare potential observation opportunity in four orbit candidates and then optimal orbit is given. The observatory with optimal condition calculated with the tool virtually observed approximately 6.2% of total debris within one year, which account for 30% of objects greater than approximately 10 cm, and 0.2% of objects between 1 cm and 10 cm. The final goal of the mission is small debris tracking, so that multi-observation is important and essential for better orbit estimation. A virtual observation result indicates that 80% of observed objects can be observed twice or more. This paper also introduces two alternative mission scenarios. One is using telephoto lens therefore this scenario has a longer range but a narrower field of view (FOV). Although far objects have superiority in observation opportunity because these objects travel slower in FOV than near objects do, telephoto scenario observed less number of objects, compared to the original scenario, because of disadvantages in duration and smaller observation cone due to the narrower FOV. However, observation opportunity increases non-proportionally and this increase indicates that this scenario would give a difference result with longer range limitation in simulation. Another alternative is using observatory constellation. This scenario has effectiveness in observation frequency and smaller debris observation opportunity improvement. Since larger objects are relatively easy to be observed even for single observatory, increase in smaller debris observation opportunity is outstanding. Outcome of the two scenarios conclude that optimal mission scenario should be a wide FOV optical system on multi-observatory.