(Before It's News)
There were lots more asteroids to be discovered orbiting Sol. Pairs of digitalized photographs could be assembled, maybe hours apart. Multiple images could also be collected over a series of days &/or from different orbiting telescopes. A pair of such images could be compared. Bodies in orbits around Sol would appear positioned differently above the stable starry far background. A combined stereo view of an asteroid could then seem to float it drifting above the starry background, nicely letting wanted asteroid distances get computed.
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Source:
http://www.prophet666.com/2014/01/discovery-of-more-asteroids-in-future.html