Recently, the Fermi team announced that the spacecraft dodged a very large bullet in the form of a defunct Soviet spy satellite: http://www.nasa.gov/mission_pages/GLAST/news/bullet-dodge.html. The close encounter with Cosmos 1805 was reminder that even though space is very large, there are some real threats to our invaluable telescopes that are in orbit.
Credit: NASA's Goddard Space Flight Center
While Chandra's orbit keeps it far away from most orbital debris like the Cosmos 1805 satellite, it doesn't protect it from other hazards including meteors.
Every year, as the Earth passes through the wreckage of comets that have long been destroyed, we experience the wonders of annual meteor showers here on the ground. In space, however, these relatively tiny bits of rock can pose real threats to spacecraft like Chandra. In order to minimize this possibility, the staff at Chandra's Operation Control Center undertakes major preparations. You can read about these efforts in this Chandra Chronicle article: http://chandra.harvard.edu/chronicle/0104/leonidhit/
We are thankful that Fermi remains safe and healthy after its recent near miss. We hope that the rest of telescopes in space that help us better understand the Universe can stay out of harm's way as well – no matter what the danger.
-Megan Watzke, CXC