The international space station's $190 million mobile transporter, a motorized flat car designed to carry the lab's robot arm to various work sites on the station's huge solar array truss, began inching its way to a work site on the far end of the new P1 truss in prapration for today's spacewalk. But after crossing over onto P1, the transporter unexpectedly stopped about 10 feet short of its destination. Engineers on the ground are evaluating the situation.

The mobile transporter's 1.2-inch-per-second creep began around 11:21 a.m. EST.

Playing it safe, the mobile transporter, or MT, is not carrying the arm for this first move from work site 4 on the central S0 truss to work site 7 on the far end of the P1 truss some 55 feet away. The robot arm currently is locked onto a grapple fixture atop the U.S. Destiny lab module.

Once the MT reaches work site 7, the arm's free end will latch onto a power-and-data-grapple fixture atop the MT. The other end of the arm then will release the grapple fixture on the Destiny module to complete an inchworm-like "walkoff" to the MT.

Today's spacewalk by John Herrington and Michael Lopez-Alegria is scheduled to begin after the arm is plugged into its power-and-data socket atop the MT.

The MT/robot arm relocation procedure was expected to take about two hours to complete. But that could change depending on what sort of troubleshooting is required to get the MT moving again.