On sol 156, Spirit roved 42 meters (138 feet) closer to a vantage point where it could observe the hill outcrops. Some of the images that Spirit sent back revealed a small and unusual rock that piqued scientists' interest and was informally named "End-of-Rainbow."
Part of the sol 157 plan was to observe End-of-Rainbow and use the alpha particle X-ray spectrometer, Mossbauer spectrometer and microscopic imager to study the "Shredded" soil target. However, the command load for sol 157 never made it to Spirit. Further analysis indicated that the problem had to do with the frequency drift associated with the colder temperatures on Mars as the planet moves into its southern winter season. This was an anticipated problem, and the rover team has already imposed some strategies that will help to prevent the problem in the future.
So, Spirit got a break on sol 157 and began sol 158 with nicely charged batteries. She executed the activities originally planned for sol 157, and then began to drive a bit closer to End-of-Rainbow. Although the direct path to End-of-Rainbow would be only a 4-meter (13 feet) drive, it was deemed too steep and hazardous by the rover team, so they planned a multi-stepped drive that could get the rover to the target safely. On sol 158, the first part of the drive was completed, putting Spirit a little farther away from the End-of-Rainbow target, but with a straight shot to the rock for sol 159.