Hi! Aaron Brown <asb@CSUA.Berkeley.EDU> said: >I noticed that when the current VAB launch has been delayed, it has been >delayed for exactly 24 hours. My question is why? It would seem that if >they have a specific orbit to get the satellite into, then the launch >window would shift, as it has when they have delayed the current shuttle >launch. What is it that dictates that the VAB launch window does not >shift? I'm not 100% sure of this. You have been warned :-) If the orbit's plane remained fixed in space (with respect to distant quasars or whatever you consider 'fixed' enough), a 24h delay would really mean a 23h 56min 4s delay (the lenght of the sidereal day, or more exactly, the time the Earth takes to complete one revolution, which is not the same!). But Terra is to be launched on a sun synchronous orbit. That means that its orbit's plane precesses eastward about one degree each day due to the Earth's oblateness (the same angle as the Sun moves in our sky during that period, that's where the name comes from) How much will it take until the Earth rotates that extra degree? 3 min 56 s. So we have 23h56min4s + 3min56s = 24h exactly (surprise!) because the sun-sychronous orbit has that property BY DEFINITION. So they don't need to "accomodate" the next launch in the launch window as someone has said before. The launch will take place at the same local time if everything goes OK. However, in the STS-Hubble case, each "24 h" delay has a monster shift because 1) Hubble of course goes form West to East, so precession goes West, and you need an earlier launch each day. 2) Hubble orbits closer to the Earth than typical heliosynchronic satellites so effects are greater and 3) Its inclination is lower, and precession is maximum with inclination zero and null in polar orbits. Just my 2 cents, as you say there in the US. If there's any specialist out there, correct me if I am wrong. I'm not a rocket scientist although you may know it by now anyway. - Ruben Velasco <heston@arrakis.es> 37.3877N 6.0008W +39m WGS84 PGP KeyID 37219E45 Fingerprint = 96575B8713370081 1BC5D43D324B3F4D ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html