Technical Challenges  The thermal environment in Low Earth Orbit presented one of the key technical challenges to the docking  system design. The temperature of hardware in space varies from extreme heat when in direct sunlight to  extreme cold when facing deep space. Even with the protection afforded by insulating blankets, the linear  actuators must perform over a wide temperature range. In order to perform the delicate capture operation,  the actuators must control their force output accurately. The NDS is able to accomplish this feat without the  use of load sensing because of its highly efficient ball screws. As a result, the current feedback from the  motors is able to provide a reliable indication of actuator force output.  The risk of collision between the chaser vehicle and the Space Station was another key technical challenge  faced by the NDS designers. Because of the direct electric design of the soft capture system, a power loss  or avionics failure during docking results in loss of ability to control vehicle relative motion. The high  efficiency ball screw actuators present little resistance when unpowered, and therefore introduce the risk  that the chaser vehicle could drift into the Space Station after a failure, with catastrophic consequences. To  mitigate this risk, a redundant string of avionics was included in the NDS design, which can be activated in  a time-critical failure scenario during docking. The redundant string is able to slow the vehicle’s motion  enough to give it time to open the capture latches and perform a controlled abort, after which the docking  can be reattempted. To protect against the unlikely event that both avionics strings fail during docking, a  secondary capture latch release, independent of both avionics strings, was also included in the design. The  secondary release uses a non-explosive separation mechanism that, while irreversible, is fast enough to  separate the soft capture interface and allow the incoming vehicle to escape without collisions and return  home safely.  System Performance  The Six Degree of Freedom (6DoF) test system at Johnson Space Center provides a sophisticated  hardware-in-the-loop simulation of the docking event. As shown in Figure 8, the target vehicle side is  represented by a qualification-fidelity test article of the Space Station’s docking adapter, mounted to a  motion table. The motion table is driven by a computer simulation of the relative motion of the two spacecraft  in zero gravity. Although the active docking system test article is mounted to fixed structure in the test, the  simulated motion of both vehicles is incorporated into the table’s motion. The 6DoF facility is able to  introduce misalignments and off-axis rates to simulate the inaccuracies of the spacecraft GN&C systems.   In order to characterize the NDS capture performance in a statistically representative manner, the test  cases were randomly distributed within limits that envelope the expected GN&C performance of the target  and chaser vehicles. The initial contact condition limits are shown in Table 1. The simulated chaser vehicle  masses were selected to bound the lightest and heaviest cargo and crew vehicles planned to dock to the  Space Station (10 t and 18 t). At the outset of the NDS design, it was established that missed captures are  acceptable in up to 1% of misalignment cases, provided that the system is capable of safely recovering for  a second docking attempt. Despite this allowance, the NDS successfully captured all misalignment cases  during the 6DoF test, while remaining within load and vehicle relative motion limits. For practical reasons,  the 6DoF test was performed in the ambient air environment. As a result, the final verification of capture  performance comes from an analysis that incorporates the temperature-dependent component data into a  test-correlated computer simulation.  NASA/CP—2018-219887 478