Kepler didn’t use the ablative form, as far as I know. What he wrote in his 1611 Dioptrice was:
Atq[ue] en inventum Iovi satellitium seniculo vero decrepito duos servos, qui incessum illius adjutent, nunquam a lateribus illius discedentes.
(But behold, the discovery of satellites by Jupiter, like two servants of a feeble old man, which help his march, never departing from his sides.)
At least that’s the use I found. The scan I have isn’t searchable because of OCR errors. He may use the word elsewhere in the book.
I should probably use satelles or satellitium (the genitive plural) when referring to what Kepler wrote.