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Figure 1. The outer satellites with well determined orbits and large inner satellites. Symbol size represents the Log of the satellite size. a once-larger satellite that helped form a retrograde cluster through collisions. Prograde Carpo has similar retrograde collision probabilities as S/2016 J2, it being bigger but closer in. Retrograde collisions with Himalia members are also possible (Nesvorn´yet al. 2003). Though retrograde-prograde moon-moon collisions are unlikely individually, taken together, a retrograde-prograde collision has likely occurred between the outer satellites of Jupiter. We observed every well known outer satellite of Jupiter in 2017 and 2018 and rediscovered many of the 2003 and 2011 satellites that had poorly determined orbits (Jacobson et al. (2012), Brozovi´c& Jacobson (2017)). We also found many unknown satellites over 24th mag that were too faint to follow and obtain good orbits. Two of these lost satellites appear to be in the Himalia group and the rest retrogrades. Some are likely the faint low quality orbit 2003 satellites yet to be linked. This confirms Sheppard & Jewitt (2003) that Jupiter has about a hundred satellites larger than 1 km. The smallest satellites in the orbital groups are still abundant and not dispersed towards Jupiter. This suggests the collisions that created them occurred after the era of planet formation as significant gas and dust would preferentially drag the smaller satellites inward (Jewitt & Sheppard 2005). There are no obvious retrograde satellite groups around the other giant planets, except possibly Neptune (Sheppard et al. 2006). Saturn’s progrades show inclination clustering but with less stringent semi-major axis cluster- ing (Gladman et al. (2001), Holt et al. (2018)). 3

This project used DECam, which was constructed by the Dark Energy Survey collaboration. Observations were partly obtained at NOAO and Gemini that are operated by AURA, under cooperation with NSF. This work was partly funded by NASA planetary grant NN15AF44G. 4

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