Words have meaning: (Launch) Window

Word today that WISE launch is now scheduled for Monday, December 14, 2009 (6:09:33 – 6:23:51 a.m. PST ). For those of you paying attention, you may notice that while the date changes, the time of the launch is still the same. This time range is the “launch window.”

The timing for the launch window can be as short as minutes, and as long as weeks. It all depends on where you are trying to put your spacecraft. For any mission that must operate in a particular orientation with respect to the Earth (this would be pretty much anything that directly orbits the Earth), the window is on the order of minutes. That’s because the Earth turns relatively quickly, rotating 15 degrees per hour. The more precise the needed orbit, the shorter the window becomes. It’s hard to compensate for an orbital offset once you’re in space.

Missions with more distant targets, like the moon, other planets, comets, or one of the Lagrangian points, have a better chance of compensating for changes in launch time. Small perturbations of the path early on translate into much larger shifts in position after they’ve travelled millions of miles. This means the timing of the launch window is typically a function of the relative positions of bodies in the solar system. In fact, more important than knowing where the planets/moons/etc. are now, is knowing where they WILL be at the time you want the spacecraft to rendezvous with them.

After all, if they miss, there’s no turning back…

(Hat tip to palmerin for the ability to sleep in!)


5 Responses to “Words have meaning: (Launch) Window”

  1. I bet Ned Wright is chewing on his shoes right about now…
    I’ve never seen a PI with a personal webpage about their mission however he has one on WISE:

  2. Does anyone know why WISE is being put into a polar orbit? While polar orbits tend to have great advantages for Earth observing, for sky viewing they’re not particularly advantageous. Interestingly, the launch windows for Saturday and Monday were the same local time, so the aspect to the sun is slightly different on the two days.

    I suppose it’s time for a little googling to figure it out.

    • I can think of two possible advantages for the polar orbit (outside of the thermal one you suggest below). Both relate to the fact that WISE is an Explorer-class mission, and thus had a very small amount of money to work with (compared with some of the more visible missions).

      1) A polar orbit allows the observatory to utilize a stationary high-gain antenna to send science data to the ground using the NASA-funded (and thus essentially free) polar ground stations at Svalbard, Norway and Poker Flat, Alaska. The cost increase necessary to use a steerable high-gain antenna is due to both the mechanism itself being more expensive, and the increased planning complexity requiring more manpower. Ground stations are very easy to plan for, especially if the spacecraft maintains a consistent orientation with respect to the Earth’s surface. A steering mechanism on the antenna also increases risk by adding single-point failure possibilities.

      2) Since WISE is an all-sky observer, it is likely that the orbit is being used to generate the sky-coverage. That is, rather than changing its orientation regularly to observe the whole sky like some missions (Fermi is an example of this), the polar orbit will allow WISE to gain that coverage without requiring any orientation changes. Unfortunately, the polar orbit also means the all-sky coverage will require about six months to achieve, as the orientation of the observatory’s orbit with respect to the background stars will only change due to the Earth’s movement around the Sun. It definitely reduces cost, but here risk is increased. If there is a failure in that first six months of full operations, the all-sky map will be incomplete.

      • I definitely agree on point 2. (And point 1, too, of course.)

        It is definitely true that this zenith pointing is a cheaper way to get full sky coverage, but as you say, it’s at the cost of taking 6 months to complete the observation.

        I think you’ve got it.

  3. For the moment, I’m going to suspect that it has to do with creating a stable thermal environment for the telescope.

    (I like the seasonal snow flakes. If only it were possible to have such control over snowfall and direction IRL.)

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