Twisst is sending out lots of tweets again! A big thank you to everyone who told me how glad they are to receive the alerts again.
Everything down here seems to be working smoothly. The only thing messing things up... is the ISS itself :-)
No need for alarm though. On Thursday, the space station was boosted into a slightly higher orbit. This happens every once in a while. The ISS gets a push from a visiting space ship (in this case, the unmanned cargo vessel ATV-3) to counteract the slight atmospheric drag. If not for these pushes, the ISS at some point would slowly but surely fall back to earth. As you can see in this graph from Heavens-Above.com, the result is a frequent change in altitude:
Every time the ISS gets a push into a higher orbit, any predictions about passes made before the boost are no longer valid. The pass times will be different because the ISS is going faster and higher.
The Twitter alerts are queued up to three days in advance, so after a boost some of them can be off. Some people are confused by the differences they observe between Twisst data and other sources. These discrepancies can be accounted for by the boosts. Usually we should all agree within a margin of a couple of seconds, but after a boost passes may differ up to a few minutes. The data on your personal page on this website is updated at least once day, so it is accurate almost all the time.
In the meantime, we are working to make Twisst even better. Your response to our recent troubles gives us a lot of motivation to keep going :-) So @michelschep is changing our system to create the alerts less far in advance (making them even more accurate) and @adam_horn and I are working to allow you to set some preferences, like filters.
P.s.: I just read about 'night glider mode' on Wikipedia. The solar array on the space station is usually turned towards to the sun to generate the most electricity. But when in the shadow of the Earth, it is rotated to align with the Earth. That way, the large panels catch less 'wind' and the drag of the ISS is reduced by 30 percent! How cool is that! I didn't know that!