Discovery Launch Scrubbed

Image credit: NASA

eWeek summarizes the problem:

A leaking gaseous hydrogen vent line forces NASA to scratch planned March 11 launch. Depending on what repairs are needed, NASA said it was keeping the option open for a March 12 night launch to deliver the International Space Station's fourth and final set of solar array wings, completing the station's truss, or backbone.

NASA Scrubs Space Shuttle Discovery Launch

There was concern about a failure of this part on the last shuttle mission, rather than a detected failure on this one, according to a NASA handout on the problem:

During space shuttle Endeavour’s STS-126 mission in November 2008, flight controllers identified that [Gaseous Hydrogen (GH2)] was flowing from one of the shuttle’s engines at a higher than normal rate. To compensate, the other two gaseous hydrogen flow control valves reduced the amount of their flow and there were no issues during launch. After landing, the main propulsion system was inspected and engineers discovered the GH2 flow control valve poppet on the suspect line was cracked and a small piece was missing.

The poppet on the valve acts like a pop-up on a sprinkler to let the GH2 flow. The damaged valve was removed from Endeavour and shipped to the vender (Vacco) for disassembly. It was then sent to the Boeing Co. in Huntington Beach, Calif., where engineers determined that the crack was caused by fatigue.

The concerns are whether a failed poppet or poppets could cause:

1) a rupture in the gaseous hydrogen line, resulting in loss of pressure to the external tank's hydrogen tank. This could result in a main engine shutdown.

2) an over pressurization of the hydrogen tank, forcing open a vent line that could expel hydrogen into an oxygen-filled area.

STS 119 Flow Valve Fact Sheet (PDF)

They're waiting for some tests to be completed on this valve before going ahead with the new mission.

As most of us remember, hydrogen gas in the vicinity of oxygen can be very bad. Discretion is definitely the better part of valor here, I'd say.

As the quote indicates, this mission is another International Space Station construction mission. The mission is to change ISS from the way it looks in the photo above to this:
Image credit: NASA. Click for larger image.

The blurry bits are already on the ISS. The part that's most visible is the new solar array and the S6 truss that it's connected to.

NASA describes this new addition:

The International Space Station’s power generating capacity is set to get a boost early next year with the installation of the starboard 6, or S6, truss segment and solar arrays. The S6 is the final piece of the station's football-field-long backbone.
...
Each set of solar arrays has a wingspan of 240 feet. Like the other three already installed on the station, the S6 solar array wing has two arrays with 32,800 solar cells. The solar panels convert sunlight to DC power, which is routed to the batteries and either sent on to power the station’s equipment or stored for use when the panels have no sunlight to generate power. Rotation joints steer the solar panels so that they can track the sun as the station moves in its orbit around Earth.

Photovoltaic radiator panels are attached at a right angle to the arrays to permit circulation of cooling fluid that reduces battery and electrical system temperatures.

More Power To Them

This power increase will support increasing the size of the crew that can stay on the ISS from three to six. Once the remainder of the Kibo is added in May, six person crews should be possible. According to NASA's schedule, there will be seven more flights to add more parts to the ISS, five by the shuttle.

Unless it was done on the Mir station, I don't think we've ever had that many people on one spacecraft for an extended period. The ISS has now been in space for ten years (PDF), and so it has now definitely surpassed that old record of Mir's.

The effort and expenditure necessary to gain even this small foothold in space show how far we have to go before we can consider working or living there.