This is a good article on the crash of the two satellites recently.
How and Why Did Two Satellites Collide This Week? | Universe Today: “The collision this week involving an active U.S. commercial Iridium satellite and an inactive Russian Cosmos 2251 satellite in low Earth orbit has, if nothing else, raised public awareness of the growing problem of space debris. But how and why did this collision happen? If NORAD, the U.S. Air Forces’s Space Surveillance Network, NASA’s Orbital Debris Program Office and other entities are tracking space debris, did anyone know the collision was going to occur? Those who analyze data and track satellites say predicting collisions is difficult because of changes in satellite orbits which occur due to solar radiation and the gravitational effects of the Moon and Earth. Therefore, the orbit analysis is only as good as the data, which may be imprecise. ‘The main problem here is the data quality for the data representing the satellites locations,’ said Bob Hall, Technical Director of Analytical Graphics, Inc. (AGI), the company that released video and images on Thursday recreating the collision event. ‘Given the uncertainty in the accuracy of the TLE orbital data, I do not believe anyone was predicting or necessarily expecting an event.'”