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Once they knew it was there,the astronomers were able to calculate an orbit for it, and then go back and look for it in older Cassini images. They found the clump in more than 100 such pictures! But in most cases it’s so faint and difficult to see that it was completely overlooked before. It appears brighter at certain viewing angles, which is why it was so obvious in the discovery image. The earliest it was seen was in May 2012, but before then Cassini was not in a good orbit to detect the clump, so there’s no way to really know how old it is.

But then things get weirder. In images taken before January 2013 there’s only a single object, but in later images, just around the time the object was discovered, a second one appeared! They’re obviously related, but it’s not clear whether the main object broke up due to a collision with something else, or whether Saturn’s tides (the change in the force of gravity over distance) pulled it apart. It may also be due to some other mechanism entirely.

I dug up another of the images showing the clumps, taken on June 22, 2013:

I’ve indicated their locations; you can just see a gap between them and the main A ring. Once I knew what to look for they were obvious, but I can see why they could’ve been missed before. They’re hard to spot. I’ll note that in other images they found what may be a third object as well, but it’s difficult to know what it’s relationship is to the other two (if any).

And we’re still not done. Not long after the discovery image, Object 2 disappeared! Due to complex interactions with the ring particles, an object the size of the small moon can migrate outward, away from Saturn, over time. Once it leaves the rings it would become essentially invisible. An alternate explanation is that it disintegrated; if its orbit was a bit eccentric, a bit stretched, compared to the particles in the rings it would have been continually bombarded by them, and could have been obliterated.