Left: evolution of the latitudes ϕ of the periapsis and apoapsis (blue dotted lines) of MPO and of the observation limit at h = 1050 km (light gray line) of BELA in the northern hemisphere. The upper panel of the left plot shows the descending portion of the orbit, while the lower panel shows the ascending portion. Only observations below 1050 km are considered in this study (green), while BELA observations might be possible at higher altitudes (red). After 2028 April, observations are missing before and after the orbit passes the north pole (black dashed–dotted line), inducing an observational polar gap. Right: two examples before (2027 July ) and after (2029 January) the apparition of a polar gap.