These two radar images show the majestic Yellowstone National Park, Wyoming, the oldest national park in the United States and home to the world's most spectacular geysers and hot springs. The region supports large populations of grizzly bears, elk and bison. In 1988, the park was burned by one of the most widespread fires to occur in the northern Rocky Mountains in the last 50 years. Surveys indicated that 793,880 acres of land burned. Of that, 41 percent was burned forest, with tree canopies totally consumed by the fire; 35 percent was a combination of unburned, scorched and blackened trees; 13 percent was surface burn under an unburned canopy; 6 percent was non-forest burn; and 5 percent was undifferentiated burn. Six years later, the burned areas are still clearly visible in these false-color radar images obtained by the Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar on board the space shuttle Endeavour. The image at the left was obtained using the L-band radar channel, horizontally received and vertically transmitted, on the shuttle's 39th orbit on October 2, 1994. The area shown is 45 kilometers by 71 kilometers (28 miles by 44 miles) in size and centered at 44.6 degrees north latitude, 110.7 degrees west longitude. North is toward the top of the image (to the right). Most trees in this area are lodgepole pines at different stages of fire succession. Yellowstone Lake appears as a large dark feature at the bottom of the scene. At right is a map of the forest crown, showing its biomass, or amount of vegetation, which includes foliage and branches. The map was created by inverting SIR-C data and using in situ estimates of crown biomass gathered by the Yellowstone National Biological Survey. The map is displayed on a color scale from blue (rivers and lakes with no biomass) to brown (non-forest areas with crown biomass of less than 4 tons per hectare) to light brown (areas of canopy burn with biomass of between 4 and 12 tons per hectare). Yellow indicates areas of canopy burn and mixed burn with a biomass of between 12 to 20 tons per hectare; light green is mixed burn and on-burn forest with a biomass of 20 to 35 tons per hectare; and green is non-burned forest with a biomass of greater than 35 tons per hectare. Forest recovery from the fire seems to depend on fire intensity and soil conditions. In areas of severe canopy burn and poor soil conditions, crown biomass was still low in 1994 (indicated by the brown areas at the center left), whereas in areas of mixed burn with nutrient-rich soils, seen west of Yellowstone Lake, crown biomass has increased significantly in six years (indicated by the yellow and light green areas). Imaging fire-affected regions with spaceborne radar illustrates SIR-C/X-SAR's keen abilities to monitor regrowth after a fire. Knowing the amount of carbon accumulated in the atmosphere by regenerating forest in the 20 to 50 years following a fire disturbance is also a significant factor in understanding the global carbon cycle. Measuring crown biomass is necessary to evaluate the effects of past and future fires in specific regions. Spaceborne Imaging Radar-C and X-Synthetic Aperture Radar (SIR-C/X-SAR) are part of NASA's Mission to Planet Earth. The radars illuminate Earth with microwaves, allowing detailed observations at any time, regardless of weather or sunlight conditions. SIR-C/X-SAR uses three microwave wavelengths: L-band (24 cm), C-band (6 cm), and X-band (3 cm). The multi-frequency data will be used by the international scientific community to better understand the global environment and how it is changing. The SIR-C/X-SAR data, complemented by aircraft and ground studies, will give scientists clearer insights into those environmental changes that are caused by nature and those changes that are induced by human activity.