## 3 Close-Proximity Concepts Design and Analysis
### Corridors  
Corridors are spatial and dynamic envelopes to be followed by the servicer in a specific zone. Their definition and sizing are mission specific, and a violation triggers an active safety action. This safety action is associated with the concepts of Abort and Cancel, which are hereby defined as follows:  
* Abort is the operation to safely terminate the operations if mission safety cannot be ensured. It consists of a maneuver that places the servicer in a passively safe trajectory.
* Cancel is the sequence of maneuvers that interrupts the operations if approach conditions are violated, and initiates a passively safe trajectory towards a safe state, from which the operations can be resumed.  
Therefore, Abort is triggered when safety is at risk, being quite aggressive in terms of action, whereas Cancel is a retreat due to a non-hazardous violation of approach conditions, that still allows for a more controlled withdrawal. Any of these contingency maneuvers are designed to place the servicer in a reference point (Cancel) or in passively safe trajectories (Abort) from which recovery can be performed and nominal operations resumed.  
The Abort and Approach corridors are closely related to the Abort and Cancel actions. For CS-1, an illustration of the Abort and Approach corridors' location within the KOZ is presented in Figure 6. The corridors were dimensioned in such a way to leave enough room for the servicer to control its position and attitude, and to accommodate a margin for the Cancel or Abort maneuvers to be performed without any physical contact with the client, as follows:  
* Abort Corridor:
* A Spatial and dynamical envelope within the KOZ, which ensures safety within its limits, and outside which mission safety is at risk (e.g., collision).
* The Abort corridor covers distances between the entrance in the KOZ and up to the Point of No Return (PNR), after which it is no longer possible to trigger the Abort.
* Violating this corridor results in an Abort, e.g., a Collision Avoidance Maneuver (CAM), driving the servicer to a safe trajectory, from which recovery can be initiated through a safe sequence.
* Approach Corridor:
* A Spatial and dynamical envelope to be followed by the servicer, from the entry into the KOZ until the capture of the client, that ensures mission success.
* Violating the Approach Corridor (blue area) within the Abort Corridor (green area in Figure 6), triggers a Cancel towards a recovery point, usually FKP. Otherwise, an Abort is initiated.  
The fact that the Abort corridor does not extend to the Point of No Return for CS-1 can be explained though the nature of the Abort and Cancel maneuvers themselves. Indeed, as we get closer to the PNR, and thus to the target, the more aggressive the recovery maneuver needs to be. In the case of CS-1, the Cancel is considered not safe enough when very close to the PNR, and any violation of the approach corridor (blue area in Figure 6) leads to an Abort in that vicinity.  
It is relevant to remark that the design of the Abort and Cancel allows them to be commanded autonomously or by ground command, thus still meeting their purpose when triggered from within the corridors.  
The zones are further validated with Monte Carlo results, as presented in Section 5.