Cloud of debris surrounding the spacecraft?

Assuming a minimal impetus of ejection of say 1 mile per hour the debris should have dispersed to a distance of 10 miles in 10 hours. Include course changes of the spacecraft and anyone want to speculate on how the debris continued to remain in the vicinity of the ship in such density as to obscure the navigational 'scope? According to my understanding of celestial mechanics the ship should have been thousands of miles clear of the debris field by the time of the second and third burns.

SAMU