At the moment, if the user wants to change the _target_, _spacecraft_ or _instrument_ property of the `Trajectory` object, it should be done with the `__getitem__()` function: ```python new_traj_janus = traj_sc['JANUS'] # new observer => JUICE_JANUS new_traj_juice = traj_inst['JUICE'] # new observer => JUICE new_traj_jupiter = traj['JUPITER'] # new target => Earth ``` Internally, a **new trajectory object** is created and return to the user with the updated parameter. The ways the new trajectory is created can be a bit confusing. It is not clear which property is updated and in some case we even observed name collisions (`JANUS` can be `JUICE_JANUS` instrument and `JANUS` the minor moon of Saturn). @lpenasa also noticed that it is possible to edit the `Trajectory` property directly and in that case, the properties already computed are kept in the cache: ```python >>> traj.dist # Distance to Ganymede for example [1, 2, 3, 4, ...] >>> traj.target = 'Earth' >>> traj.dist [1, 2, 3, 4, ...] # Cached propertied from the old target ``` This manual property change should not be allowed or at least should purge the cache if it appends (and raise a warning message). Here we propose to add an explicit method to create/edit trajectory properties: ```python traj.new_target('Earth') traj.new_spacecraft('Europa Clipper') traj.new_instrument('JANUS') ``` The current `__getitem__()` should be depreciated (with a warning) and the new syntax should be reported in the docs and its usage encourage. These methods can also be promoted to the `TourConfig` object as well. **Note:** We currently support shortcuts like `traj['SUN']` and `traj['SS']` to retrieve the sub-solar points coordinates. These shortcuts will also be depreciated and the user is encourage to use the `traj.ss` or `traj.sun_lonlat` property instead.