Halfway through our exam week! Today is all dedicated to preparing our upcoming final sims: Russian segment tomorrow and Soyuz on Friday. Can’t believe that in little more than two days we’ll be completely done!
In the meantime, on Monday Terry, Anton and I passed our Soyuz flight program exam. A specialist came from Mission Control Moscow to grind us about our knowledge of the procedures: what happens when, what do we do if this or that goes wrong and we can not proceed with the nominal plan… stuff like that.
For example, a nominal undocking is always planned on orbit 15 to land in Kazakhstan on orbit 1 of the next day. But if we end up being late for whatever reason, we can still land in Kazakhstan on the two subsequent orbits, number 2 and number 3.
OK, OK… what does that mean? Well, the ISS completes one orbit every 93 minutes (roughly), which means that there are about 15.5 orbits per day. We have a conventional numbering of those orbits, from orbit 1 to orbit 15 (mostly) or 16 (once in a while, to catch up). Since the Earth rotates Eastward beneath the ISS, the ground track of the orbit moves towards the West. So, let’s say you passed over your town at 7am; after one orbit, at 8:33, you won’t pass over your town any more, because your town has moved to the East in the meantime! How much? Roughly 23 degrees of longitude. You can get an idea fromt he picture, that I took from ISS-Tracker (www.isstracker.com)
All this to say that, if you want to land in Kazakhstan, you have to plan to descent on orbit 1, 2 or 3: on orbit 4 Kazakhstan will be already too far
East and, you guessed it, you would have to wait until orbit 1, 2 or 3 of the next day. Btw, hitting Kazakhstan does not qualify for a precise landing: the country is about as big as Europe! To make sure that there will be a rescue team waiting for us at the landing site, we have to do quite a bit better. There is a lot that plays into it, starting with a very precise timing of the deorbit burn.
The fine tuning is done after atmospheric reentry: the computer flies a trajectory to bring us to the nominal parachute opening point. To be able to calculate the correct control inputs, it has to know the center of mass of the vehicle: that’s why loading of return cargo is so important. So important that we had a dedicated lesson about it on Monday.
https://twitter.com/AstroSamantha/status/526822816185085952
So, fast forward to next spring: about two weeks before landing we will get a very long radiogram (those are the Russian instructions) with all the details about stowing return cargo, so that the center of mass of the vehicle will be well known!
29/10/2014