This animation shows the distribution of space debris created by the Chineese anti satellite test in January 2007 over a time frame of more than 1 year. The animated plot shows apogee and perigee altitudes versus orbit periode of the more than 2000 individual fragments.
One can clearly identify the original orbit of the Fengyun 1C at about 850km, 102 min period. Each fragment is displayed as a vertical line connecting a blue (apogee) and a purple (perigee) dot.
The particles of the debris cloud received a spatially distributed velocity change relative to the original satellite. Depending on the direction of the delta V, the apogee or perigee altitudes or both changed respectively. This results in the particular x-shape as seen in the plot.
As time progresses particles in lower orbit are slowed down due to increasing drag in the upper atmosphere. The will first significantly decrease the apogee altitude until the orbit is more or less circular. At this point the particle begins a spiraling decent and eventually disappears from the plot.
As this can be observed throughout the animatio for particles orbiting at lower orbits, higher particle orbits remain unchanged. Atmospheric drag effects are orders of magnitudes smaller at higher altitude which means that the majority of debris particles will remain in orbit for a very long time, persumably decades or even centuries.

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