The airport comparison breaks at the first hold. An aircraft waiting on a taxiway costs its operator time; a vehicle waiting near Gateway costs propellant, because, as the phys.org account puts it, there is no tarmac and nothing stays still [14]. Diane Davis of Texas A&M, one of the study's authors, calls it a Goldilocks zone: parked vehicles have to sit far enough apart to be safe and close enough to their destination that resources are not wasted [10].
That band is not a fixed volume. Gateway's orbit comes within 1,000 miles of the lunar north pole and swings nearly 40,000 miles past the south pole [4], so the scale of the neighborhood changes by a factor of 40 over a single revolution [15], and at the far end the holding area sits roughly a sixth of the way back to Earth [16]. Any separation rule expressed as one number has to survive both ends of that trip, and a hold that is comfortable near the pole may not be comfortable out past the other one.
Davis describes the orbit as nearly stable and cheap to maintain, with an uninterrupted line of sight to Earth [3], and for one station that is sufficient: occasional thruster burns keep it on course [6]. Cheap station-keeping for a single object buys nothing for the relative geometry of several. The definition the team works from is the revealing part. Loitering means holding a spacecraft relative to a specific orbit or trajectory without executing an immediate maneuver [8]. Clearance is therefore issued against a reference trajectory rather than a place, in an orbit no crewed vehicle has used before [5]. That is harder to write into a flight rule than a keep-out sphere, and harder for a crew to check by eye.
The ordering matters. Gateway is still a planned spaceport, with its busy period projected across the next two decades [12], while the algorithms have already been run through thousands of simulations carrying realistic navigation error [11]. Whatever error budget those runs assumed becomes, in practice, a requirement on hardware nobody has finished building: a lander that cannot hold position to that standard earns a wider standoff, a longer wait, or both. And the institutions doing the writing, as described by phys.org, are Texas A&M, NASA's Johnson Space Center and Purdue [1]. The operators of the cargo ships and landers that will queue for a port are not among them. Davis says the future of lunar exploration depends as much on traffic management as on the rocket science [13]. Read narrowly, that is a statement about which document the vehicles will be argued against, and it is being drafted while the arguing parties are still on paper.
