Well, nuts. Here I was all psyched up for tonight's anticipated launch of STS-119. The launch has already been delayed four times hence I held no lusty expectation it would leave "on time," still I can't help but miss that electric feel space-loving land-lubbers know when one of our impressive orbiters is hard at work.

Discovery's mission is to finalize construction of the giant solar arrays powering the International Space Station. Her crew was set to install the final S6 truss segment completing the "backbone of the station" leaving it ready to provide enough power for the eventual demands of a six-person crew. In addition the mission involves a crew swap, bringing Japanese member Koichi Wakata aboard and returning Flight Engineer Sandra Magnus.

Discovery has suffered this nagging string of similar delays since scrapping its original launch date back in the middle of February. The problem relates to suspect valves that vent excess hydrogen from the main engines. One of the valves from Endeavour's last flight survived with a thumbnail-sized wedge of metal shaken free. (This shouldn't escape notice as an example, perhaps underemphasized, of the importance of returning hardware. Testing is good but experience is better.)

Things otherwise go well on Alpha. Yesterday saw an uneventful, successful spacewalk for commander Mike Fincke and flight engineer Yury Lonchakov. The five hour excursion allowed them to clean up a docking target used by automated craft, mount some experimental equipment, and also take a look at how the structure is doing after surviving ten years in one of nature's harshest environments.

The launch of Discovery (and fourteen day mission) has been scrubbed until no earlier than tomorrow night. Then and Friday are two reasonable chances provided discomfort associated with the valves subsides. She has until the end of the week before a launch of the next scheduled ISS supply ship takes priority. It seems like forever since shuttles have flown (this will be the first flight of the year!). Any significant launch slip risks a ripple in subsequent flights. Not good.

Reports recently announced that debris from the satellite collision of a few weeks ago has begun to fall from the sky. That's a disturbing coincidence. Of the two parts of a rocket interpreted as a "controlled explosion," the first part ("controlled") always presents the harder problems. These engines are enormously complex devices and this issue must be understood. Better to have no headlines than banner headlines for the wrong reason, but still better to present steady progress to a pacing public.