Why then can a rocket be “on the schedule” today and off the pad the next? The brief answer is that launch time is not a convenient time slot. It is a physics slot. Listings on public launches are commonly in the form of a straightforward calendar, although the time next to each mission is often the tip of a far narrower engineering challenge that orbital geometry and weather and readiness of the vehicle and the place it is heading provide.
That is why a late-March launch line-up can appear heavy-handed and fluid at the same time. In Norway, Isar Aerospace had been aiming at a window opening in 4 p.m. EDT at Andøya Spaceport as part of the Norwegian “Onward and Upward” mission of Spectrum, a flight carrying the CubeSats and technology payloads of the European Space Agency Boost program. However schedule keepers announced the second attempt miss once more following previous delays due to strong wind and the previous malfunction of a valve, as one report said the launch attempt was aborted. The mission has more than a single countdown: Spectrum is an element of the European effort to expand the availability of sovereign small-launch capability.
The pressure of time occurring at that same time is also totally different in missions of low earth orbit. It may seem that SpaceX missions launching Starlink satellites off California and Florida may be more lenient than a lunar or interplanetary flight, even those missions are limited by the orbit they must achieve and how to recover the first-stage booster. A Falcon 9 launch with 25 Starlink V2 Mini Optimized satellites onboard, which was Vandenberg bound, was included on the Spaceflight Now list, and another mission was planned Cape Canaveral bound with 29 more. Their missions are not arbitrary, but frequent.
The earth is rotating at a speed of approximately 1000 miles per hour at the equator and instead of struggling against it, the launch teams utilize the rotation. The educational explanation of the launch windows by NASA brings the concept down to a helpful fact: a rocket is launched as a moving object on a moving planet towards another moving object. In certain missions, the appropriate time extends to hours. To other users, particularly rendezvous flights or missions which require a specific orbital plane, the available margin may be reduced to minutes or even a point of time.
That is the more important reason why the launch pages are always changing. A “launch period” may take days or weeks, whereas a launch window is just the usable slice of one day, and the window might be as short as a second. Weather can close that slice. It can be shut by upper-level winds. It can be closed by a minor sensor problem. It can also be shut down by range availability, downrange sea conditions, and when a booster droneship should land. When a mission is off the window, it will not simply re-initiate the countdown at the same clock time since the earth has rotated and the geometry has changed with it. It is also due to that shifting geometry that launch schedules are not becoming more predictable, they are becoming crowded.
The latest mission of Falcon 9 is represented on reference trackers of late March and early April with stacked missions of Falcon 9, Atlas V, Electron, Vega C and Artemis II. The next ESA navigation flight by the rocket Lab is scheduled towards the first two satellites in Celeste LEO-PNT, the project is intended to experiment how positioning satellites in low-orbit could supplement the hitherto available navigation networks. Increased rockets are being launched, through increased spaceports, to more dedicated orbits. To the viewers, that will translate to increased opportunities to watch a launch. To the launch operators, it implies that all the published times are temporary until the last minutes when physics, weather and hardware finally come to an agreement.