We do not know much about the Oort Cloud so it is hard to speculate on where some of its members might come from. Presumably, the vast majority of these 5-100 km sized cometary nuclei, literally trillions of them, have always been hanging around the Sun since the time the Sun formed 4.5 billion years ago. Every once in a while, though, they may get jostled by a passing star and make a plunge into the inner solar system. This is about like shooting at a basket ball from a distance of 10 miles! Note, in the cartoon above, the edge of the actual cloud is not this sharp!
At the scale of a few kilometers, we simply have no idea just what interstellar space contains, whether it is empty except for a few atoms per cubic centimeter, or whether it is filled with many stray cometary nuclei. For more details, visit the web page at the University of Oregon
Copyright 1997 Dr. Sten Odenwald
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