Mitigation of Collision Hazard for the International Space Station (iss) from Globally Launched Objects

For the International Space Station (ISS), it can take 6 to 24 hours to reliably catalog a newly disposed upper stage and up to 33 hours to plan and execute an avoidance maneuver. This creates a gap in the existing collision risk protection for newly launched vehicles, which covers the period when these launched objects are still under propulsive control; specifically, upper stage separation plus 100 minutes for most missions. This gap results in a vulnerability of the ISS from the end of current “Launch Collision Avoidance (COLA)” protection until approximately launch plus 56 hours. In order to help mitigate this gap, conjunction analyses are being developed that identify launch times when the disposed upper stage could violate safe separation distances from the ISS. Launch window cut-out times can be determined from the analysis and implemented to protect the ISS. The COLA Gap is considered to be a risk to ISS operations and vehicle safety. Methods can be used to mitigate the risk, but the criteria and process need to be established and developed in order to reduce operational disruptions and potential risk to ISS vehicle. New requirements and analytical methods can close the current COLA gap with minimal impact to typical launch windows for Geo-Transfer Orbit (GTO) and direct injection missions. Also, strategies can be established to produce common standards in the U.S. and the world to close the current Launch COLA gap. 1. WHAT IS THE HAZARD? Launched objects, their disposed upper stage(s), and/or payloads may have orbits containing potential conjunctions (i.e. near misses) with the ISS. This condition is highly dependent on the trajectories (geometry) of the two vehicles and their respective uncertainties as shown in Fig. 1. An example of this hazard can be seen in Fig. 2, an illustration of the geometry for both the ISS and the 2 nd and 3 rd upper stages of a launched vehicle during the 56-hour COLA gap interval. Figure 1: Orbital geometry between ISS and 2 nd Stage of a launched object. Figure 2: Illustration of geometry for both ISS and launched upper stage. There are two scenarios that cause concern for the ISS. The first, and less likely, is the object actually colliding with the ISS. The more imperative concern is that of the object passing within the same vicinity of the ISS, thereby disrupting Ground Control operations and/or ISS Crew operations. The U.S. Joint Space Operations Center (JSpOC) has stated approximately six (6) to twenty-four (24) hours are required after launch to acquire an orbital track and calculate possible conjunctions with the ISS. Once a conjunction warning is received, NASA ISS operations typically requires a minimum of thirty to thirty-two hours to plan and execute an ISS evasive maneuver depending on vehicle configuration, flight planning, and on-board operational activities. Launch Collision Avoidance (Launch COLA) analysis is performed for all United States (U.S.) launches: military, civilian, and commercial. The current Launch COLA safety requirements for all three types of launches are designed to protect against collision/conjunction with orbiting assets that are occupied or can be occupied by human beings. The time period covered by launch COLA extends from launch to approximately end of spacecraft/upper stage separation plus 100 minutes. This duration is by convention, but there are some practical limits associated with large uncertainty in predicting the position of a typical liquid fueled upper stage and payload after burn out. Conjunctions are often determined between launched objects (upper stages and their payloads) and all objects in the U.S. catalog of resident objects for mission assurance purposes. Miss distance and probability are computed and compared to thresholds for each opportunity during the launch window. As a result, a “COLA Gap” occurs between current Launch COLA protection and prior to routine ISS onorbit conjunction assessment capability following the new object cataloging. The ISS is vulnerable during this COLA Gap from the end of the current Launch COLA until ~L+36 to ~L+56 hours. Even though there is a small likelihood of an actual collision, the consequences would be catastrophic for both the launcher and ISS. 2. CURRENT LAUNCH COLA METHODS Two independent yet similar geometric assessment techniques have been developed for analyzing spacecraft launches during the identified gap. The Aerospace Corporation and the Launch Services Program (LSP) respectively analyze Nodal Separation (Radial) and Argument of Latitude (In-track) distances between upper stage/payload and the ISS orbits in support of The U.S. Air Force and NASA. The analyses first determine the minimum nodal separation (Radial) distance between orbit traces over a 56 hour interval, for each launch window over a range of launch dates as shown in Fig. 2. Nodal Separation analysis is performed primarily to reveal which launch dates might have an ISS/upper stage COLA Gap issue. This initial look determines the minimum nodal distance between orbit traces over the analyzed interval. If Nodal Separation is outside the mission specific uncertainty region, then the ISS will not pass through the upper stage’s dispersion cloud, and collision would not be possible. If Nodal Separation is within a specified uncertainty region, there could be a COLA Gap issue. Figure 2: Illustration of Nodal (Radial) separation obtained from Kennedy Space Center (KSC) Launch Services Program (LSP). Even if the Nodal Separation check fails--meaning there is a good chance of a conjunction--there is still the possibility that the objects will remain far apart (i.e., phased over different regions of the Earth, even though the full orbit traces come within close range of each other). An additional analysis using the Argument of Latitude (In-track) position of both objects (Depicted in Fig. 3) is used to confirm close proximity location of both the ISS and second stage. Figure 3: Illustration of Argument of Latitude (Intrack) separation courtesy of LSP. At each intersection point, both Radial and In-track miss distances are computed. These miss distances are compared to initial screening criteria based on position uncertainties to determine if a conjunction is likely. The analysis uses nominal trajectory and a miss distance threshold encompassing 3-sigma dispersions. This analysis is performed for a range of launch days at launch window points to determine if there is effectively a zero or extremely remote chance of a conjunction. 3. ISS ACCEPTABILITY These analyses define the distribution of Radial and Intrack positions between the launcher and the ISS, which can be used to determine if there is essentially a zero or extremely low chance of an actual collision. However, these analyses do not consider the operational impacts of an occupied orbiting asset. There is no real identification of the risk that the upper stage will pass close enough to the ISS to be within established thresholds that trigger a potential debris avoidance maneuver, thus disrupting on-board and ground operations. Ground operators, JSpOC Orbital Safety Analyst (OSA) and NASA’s Johnson Space Center (JSC) Trajectory Operations Officers (TOPO) must cooperatively manage any close approach passing through the ISS notification volume. In such gap situations, where there is not sufficient notification time, the ISS does not have time to plan and execute a standard evasive maneuver. In these situations, the crew is required have to stop all onboard operations and shelter-in-place. 4. PROPOSED PROBABILISTIC PROCESS Specialists at JSC are working with Aerospace Corporation and Kennedy Space Center’s (KSC’s) Launch Services Program (LSP) to develop a more formal risk evaluation process that is consistent with existing ISS Program on-orbit collision avoidance techniques. The goal is to establish a set volume that balances protecting ISS collision and limiting the disruptions to ground and Crew operations with an acceptable confidence level from the launcher. In order to provide adequate protection for the ISS and based on current operational threshold criteria, NASA has proposed using a 1x10 -8 probability of actually hitting the ISS to warrant closing out a launch opportunity during a window. In order to meet the proposed probability of collision, launchers would have to ensure their spacecraft and/or second stage will remain outside a 20 km x 100 km “COLA Box” centered about the ISS. Launchers would have to verify that the probability of either object entering this volume is 1 in 1000 or less. Then, the probability of entering the current ISS Notification Box (4 km x 50 km) will be 1 in 10,000. Ideally, up to 100,000 Monte Carlo cases would be necessary to verify no penetration of the smaller ISS Notification Box. The larger COLA Box requires less computational resources, yet still maintains the desired level of protection. Given a 1 in 1000 chance of entering the COLA Box and the ratio between COLA Box area and ISS area, this further reduces to a 1x10 -8 chance of hitting the station. The complete reduction is shown in Fig. 4. Figure 4: Reduction of proposed COLA Box to the probability of hitting the ISS A recent study was conducted to determine if there would be an appreciable impact to launch windows by using the NASA proposed probability threshold for ISS protection. Two previous missions that had the potential to be affected by the COLA Gap were chosen for the study. The results generally indicated that there was no significant impact to the launch window by using a probabilistic method. The methods described earlier are aimed at protecting the ISS from U.S. launched objects. However, this problem is not constrained to just the U.S. nor only to the ISS. Launched objects around the world could impact any other global space assets including existing international Low Earth Orbit (LEO) satellites. It is imperative that launch safety organizations around the world collectively use resources to understand and develop the best method to mitigate the risk of on-orbit collisions because such collisions can dramatically increase the threat from orbital debris impacts on all other on-orbit assets. U.S. launchers do not have a formal documented requirement to conduct COLA Gap analysis for protecting ISS operations. Currently, the U.S. is using a letter from the NASA Associate Administrator of Human Exploration and Operations as the justification 4km 50km Notification Box 100km