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Link here. The author attempts to make a back of the envelope calculation of the number of abandoned alien interstellar spacecraft in the Milky Way (like our Voyager and Pioneer spacecraft), and from this calculate the number that might actually be drifting through the solar system. His conclusions?
Wondering if there’s any alien goodies in our solar system?
Well, the distance inner edge of the Oort cloud is estimated to be 4,000 A.U. This would make the volume of the solar system = 201 million cubic A.U.s, and the chance of an alien artifact adrift in our solar system (other than our own) is less than 1 in a 1,000,000. Using the outer edge distance for the Oort cloud at 50,000 AU = gives the volume of the solar system at 31.4 billion, with a slightly better than 1-in-45 chance.
But the nearest star system, Alpha Centauri is 4.4 light years distant, which equals a sphere 243 cubic light years in volume, with lots of elbow room for alien space junk!
The author also admits that these calculations depend on many assumptions, and should not be taken very seriously. Nonetheless, they are intriguing, and fun to consider.