The final two will happen on the day of reentry. The satellite is being destroyed because mission guidelines dictate that, should one of the spacecraft's three gyroscopes fail, as happened last December 19th, the mission would be terminated. The spacecraft could still be controlled with two gyroscopes but would be uncontrollable should a second unit fail. In March, Edward J. Weiler, NASA's Associate Administrator for the Office of Space Science, explained that there was a 10 percent chance that a second gyro would fail during the next three years. CGRO's mission was nominally for 5 years, but it has provided valuable data for nearly 9 years.
North is up.
Courtesy Boston University and Boston Museum of Science . Click on image for larger view. The proximity of Mercury to the Sun makes its surface difficult to view from Earth, and even though Mariner 10 made three flybys during the 1970s, roughly half of the planet could not be photographed at close range. This scheme allowed the planet to be observed somewhat higher in the sky, which minimized the degrading atmospheric effects present close to the horizon. To reveal details on the planet's surface, the best images were then added together by computer, resulting in composite views relatively free of the blurring effects of Earth's atmosphere.