What determines if a launch has to be done instantaneously or has a launch window of minutes or hours? To what parts is it about celestial and ground issues?

Just one example of alleged switching between a window and instantaneously, Falcon 9 planned launch of multiple communication satellites to LEO in 2015-12-19:

That window was expected to be 20:02-23:02 Eastern. However, that has since been updated to an instantaneous window of 20:20 Eastern.


4 Answers 4


In the case of Falcon 9 / Orbcomm OG2 launch from SLC-40, Cape Canaveral Air Force Station, FL that's now scheduled for Dec. 20th at 8:29 p.m. EST (01:29 a.m. UTC), according to Spaceflight Now:

Sources said only an instantaneous launch opportunity is available Sunday due to restrictions imposed by the Federal Aviation Administration, which may be hesitant to re-route holiday week air traffic around the Cape Canaveral launch base for several hours in case of launch delays.

So in this case it's range imposed restrictions. On top of other answers in this thread, here are some others that are related:

So to recap, and adding a few points to what has already been previously discussed, possible restrictions could be:

  • Range: Launch times have to be coordinated with other local and sometimes international air and maritime authorities to redirect traffic along the route that a launch vehicle would take, and safety exclusion zone has to be cordoned off for debris and returning stages along its ground track. This requires a lot of coordination and scrambling of hardware that has to happen prior to launch and for the duration of its launch window and then some until the range is cleared after the last launch opportunity.
  • Tracking: Time of onsite and downrange tracking facilities needs to be allocated, and temporary permissions to use frequencies used for telemetry and other broadcasts need to be acquired. Since these are shared resources, operators and authorities might only allocate short time slots to support the needs of your operation. This might also depend on availability and accessibility of orbital facilities, like TDRS.
  • Target: Oftentimes a launched spacecraft will either chase another vehicle in orbit for rendezvous and both vehicles' orbital phases have to align over the launch site, or the vehicle will target a specific local time-dependent orbit like SSO or specific orbital plane on interplanetary trajectory as the Earth rotates on its axis. Suborbital launches might also target specific phenomena like auroras as they occur.
  • Weather: Some launch sites (and especially Florida) have predictable weather cycles and specific times of the day or night could be targeted for launch to maximize the chance that weather cooperates and meets launch criteria. Depending on the time of the year, there might also be only a limited period of time when environmental constraints (like temperature, humidity) stay within safety threshold.
  • Payload: Some payloads (e.g. small live cargo) can only be supported on the pad for limited periods of time, especially those that in any way rely on limited life support provisions or internal power and for whichever reason can't be connected with umbilicals externally for power and supplies, or the fairing sufficiently ventilated to prevent hazard of bled off propellants, and similar.
  • Launch hardware: Some facilities and the launch vehicle itself might only be capable of continuous support on the pad and once fueled for a very limited period of time, like e.g. high pressure pumps, cryogenic storage, ability to purge bled off propellants, ice condensing on cryogenic surfaces, and so on.
  • Collision avoidance: Launched object will intersect air and orbital space of other aircraft, satellites and orbital debris, so conjunction assessment (CA), collision avoidance (COLA), and likely launch collision avoidance (LCOLA) procedures will have to be done to avoid that. Since that requires awareness of flight paths, activities and orbits of classified objects, it is done (in US) on request through Joint Space Operations Center, part of Joint Functional Component Command for Space of US Strategic Command. See AFI91-217 (PDF) for more info. These analyses may give recommendations and limits with regard to launch window and assumed trajectory, but they won't specify reasons for them, so you might not know why your launch window might be instantaneous even if you'd otherwise be ready to launch during a longer launch window.
  • Other: Eventually, people will want to go home and return to their families, and sitting in constant readiness for hours on end isn't easy. It might be more prudent to split launch opportunities into smaller chunks of up to some number of hours per day, than to increase risk of human factor affecting safety. Theirs and the launch vehicle's.

Additionally, in our specific case and if I'm allowed a bit of conjecture, SpaceX is with Orbcomm OG2 launch both returning to flight after a RUD (rapid unscheduled disassembly) event during the CRS-7 launch, and they're doing this with an upgraded launch vehicle (Falcon 9 v1.2) that uses higher propellant density and deep cryogenic liquid oxygen (which, according to Elon Musk himself, is a bit of a challenge). They might have estimated that if they'll have to hold the launch for whichever reason, there's a good chance they would have to scrub it for the day and have only requested a very short opportunity to launch. If they're working with some quota (by any authority or service, from FAA, FCC, USCG, MARAD, AST, JSpOC, Space wing, TDRS, Bermuda downrange tracking station,...), and their customer isn't pushing for launch as soon as possible, it makes sense not to upset the deities with unreasonable demands that might not be met, or indeed needed.

Just for reference, here are the safety zones (Launch Hazard Area and Airspace Closure Area, respectively) and their schedule for Dec. 20th launch attempt, announced by 45th Space Wing, Patrick Air Force Base:

Launch Hazard Area Airspace Closure Area

Interestingly, issued documents list launch window of 26 minutes for 20th:

  • 20 Dec 2015 8:29pm - 8:55pm
  • 21 Dec 2015 0129Z – 0155Z

That's the same launch window, just in local EST (Eastern Standard Time) time zone followed by "Zulu" time (today, just a military way of saying UTC really). Just announced launch webcast is also an hour long, ending 31 minutes after the instantaneous launch on 8:29 p.m. EST (1:29 UTC), which would be unusually long for SpaceX hosted webcasts if they really stayed on-air for that long after liftoff. Unless it was a CRS launch to ISS for NASA, they would usually wrap up minutes after second stage separation. I'm not exactly sure what to make of this, but all that talk of instantaneous launch might have been for naught. I couldn't find any official word on it, not from SpaceX and neither from Elon Musk that tweeted all the updates.

26 minutes long launch window would permit for up to two countdown clock recycles of 13 minutes that were usual during previous SpaceX Falcon 9 launches with vehicle's terminal countdown of 10 minutes, so possibly up to three attempts to launch on that same day, assuming no issues are worked on that extend the hold (like issues with the vehicle or a boat breaching the safety exclusion zone which did happen before and even resulted in a parody Twitter account).

Just another quick update that an announcement of tomorrow's launch now appeared also on SpaceX pages and it reads (emphasis mine):

SpaceX is currently aiming for a December 20th launch of the Falcon 9 rocket, carrying 11 satellites for ORBCOMM. The launch is part of ORBCOMM's second and final OG2 Mission and will lift off from SpaceX's launch pad at Cape Canaveral Air Force Station in Florida. This mission also marks the first time SpaceX will attempt to land the first stage of the Falcon 9 rocket on land. The landing of the first stage is a secondary test objective.

The launch webcast is targeted to begin at 5:05pm PT with liftoff at 5:29pm PT. The launch webcast can be viewed at spacex.com/webcast. For updates, visit www.spacex.com and www.orbcomm.com.

So here's another explanation, that the launch might still be instantaneous and those 26 minutes are now allocated for both launch of Falcon 9R from SLC-40 and landing of its first stage at Landing Complex 1. How cool is that?

  • $\begingroup$ That's a great overview of reasons! But how come, as in my example, one and the same launch can be switched from a time window to an instantaneous launch? Except for potentially weather, your other proposals seem mute in at least that case. $\endgroup$
    – LocalFluff
    Dec 19, 2015 at 13:49
  • 3
    $\begingroup$ Speculating a bit more, it's likely that the FAA was a bit POed that they rescheduled / rerouted lots of flights on one of the busiest commercial air flight travel days of the year for a launch that didn't happen. To top things off, SpaceX asked for a reschedule on the very next day. I'm surprised the FAA didn't say "Bah humbug! You can launch on Christmas day!" $\endgroup$ Dec 19, 2015 at 17:13
  • $\begingroup$ Given what people already put up with for holiday travel, what's a few extra hours of delay for a scheduled rocket launch? $\endgroup$
    – Vikki
    Jul 3, 2019 at 23:52

A constraint not mentioned is the launch vehicle capability to target a variable azimuth. The Delta II launch vehicle did not have a variable launch azimuth capability. As a result, Mars missions on the Delta II (there were several) would generally have two instantaneous launch windows each day at two azimuths. Two were considered sufficient to address possible range issues.

It took half an hour to reload and verify the parameters on the Delta II, so the distance between the two had to be at least that. The launch vehicle could only wait for 83 minutes before the risk of freezing the feed system, so the distance had to be less than that. The windows for MER ended up about 37 to 44 minutes apart.

The windows were not technically instantaneous, but actually one second long, so the guy on the button didn't have to be perfect. (Yes, there was a guy with a button.)


The target is usually the key, although the range might have a say as well. For instance, an ISS mission must be lined up to get to the ISS, requiring a small window. For the purpose of LEO communication satellites, the key is to get to an orbit with a time of day that is particularly useful. For most of the Falcon 9 launches, this has been the case. There have been some very long windows, for instance. It could be restricted on either the rocket side (Very unlikely), launch site support, or blocking Air Traffic in the area, all of which might require a smaller window.

It seems to me that in this case, it's probably not the customer. One might recall that ORBCOMM previously had a 2 hour 33 minute launch window. As the goal seems to be 4 planes with the two launches, most of the satellites are going to have to do some traveling to achieve the goal. I suspect it has more to do with SpaceX, or more likely, FAA regulating the time of their launch during a fairly busy travel season.


Length of launch windows is determined by many things but one that hasn't been mentioned yet is performance margin of the booster. As the Earth rotates away from the desired orbital plane, the booster has to provide extra performance to change the plane. This effect was often a launch window length constraint for Shuttle rendezvous missions. If the vehicle has little performance margin an instantaneous window is likely.


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