Now I know why there is no tanking test today. CBS’ status report (a long reading with many details) provides the reason: it is to preserve the capability to do an extra EVA (“extravehicular activity”, also known as a “spacewalk”) on the STS-122 mission.
Each launch attempt draws at least a bit from the internal consumables. Let me quote the relevant part from CBS’s page:
In the meantime, NASA managers ruled out a fueling test Friday and decided instead to top off the shuttle’s onboard supply of liquid hydrogen to power the ship’s electricity producing fuel cells. That would permit launch attempts Saturday and Sunday and still provide enough on-board supplies for a two-day mission extension and the addition of a fourth spacewalk.
This makes an awful lot of sense if you take the problems with the international space station’s solar array rotary joint into account. While that poses no problem for Columbus, I think the Kibo module can not be attached (or at least not sufficiently be powered) if the SARJ issues has not been solved until then.
NASA is hoping for a two-day mission extension to do an extra spacewalk in support of the SARJ issue. However, this is only possible if consumables permit. Having full tanks on launch is a number one requirement to achieve that goal…