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Charon, a moon of Pluto, was discovered in 1978. Today we know that Pluto has three additional moons, but they were discovered 3 decades later? Also, Hubble Space Telescope was in operation since 1990 - how could it miss for 15 years that a planet has so many other moons?

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This would likely be a better suited question for our sister site Astronomy. BTW, Pluto has 4 additional moons on top of Charon (that we know of). Charon is by far the largest, and other 4 are more than 10 times smaller in diameter. Also, Charon is large enough for the Pluto-Charon barycenter to be outside of Pluto, so it would be easier detectable simply because of the wobble and oscillation in measured albedo as they move in opposition. So without looking for references, it should be because of size and required sensitivity / exposure. –  TildalWave Mar 13 at 2:51
    
@TildalWave I assumed that since the question is related to the discover or something, it better fits to this site. –  Zoltán Schmidt Mar 13 at 15:58
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It was just a suggestion, it is IMO not strictly off-topic here, so it's your choice where you'd rather receive answers for it. ;) –  TildalWave Mar 13 at 16:02
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This is the 1978 image of the Pluto system that led to the discovery of Charon. This is a negative, so the big black blob in the middle is Pluto and Charon. Charon? It's the little bump on the upper right of that blob. You can barely make out Charon. Additional satellites? No.


This is a 1990 image of Pluto and Charon taken by the Hubble:

This was before multiple upgrades to Hubble's optics, and multiple upgrades of the ground software that processes Hubble imagery. The light gathering quality and the resolution just weren't there in 1990 to resolve those small moons.


This is a sequence of Hubble photographs from 2002 and 2005.

Notice the marked improvement in the quality of the images compared to 1990. However, it wasn't until overexposed images were taken in 2005 that enough light was gathered to suggest that Pluto-Charon system had additional moons.

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Presumably it's also partly a matter of Hubble not spending much time looking at Pluto. Hubble time is a scarce resource, and as interesting as Pluto is, there's a lot of other cool stuff out there. Including actual planets. 8-)} –  Keith Thompson Mar 13 at 22:26
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@KeithThompson - That's certainly the case for the differences between the 2002 and 2005 images. The 1990 images? More time most likely wouldn't have helped. The pre-Hubble 1978 images? More time wouldn't have helped one iota. –  David Hammen Mar 13 at 22:45
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