In other languages (e.g. German) you can "fine-tune" the order and hierarchy of the attributes with a comma.
private, robotic spacecraft (with comma) -> private and robotic are attributes to spacecraft, the order could be changed but as in english you have an order that is more likely used
private robotic spacecraft (without comma) -> the attribute private refers to a robotic spacecraft
If you would like to emphasize that this is a private (and not public) robotic spacecraft you would use the version without comma.
private, robotic spacecraft (with comma) -> private and robotic are attributes to spacecraft, the order could be changed but as in english you have an order that is more likely used
private robotic spacecraft (without comma) -> the attribute private refers to a robotic spacecraft
If you would like to emphasize that this is a private (and not public) robotic spacecraft you would use the version without comma.