The sentence in question: 八王子隕石は落下中に爆発したと考えられ、距離にして１０キロほどの範囲に、少なくとも１０個以上のかけらが落下したことが、当時の日記や随筆など多くの文献に記されています。
My attempt at translation: "Concerning Hachioji meteorite, it is assumed/thought that it exploded midair (=in the middle of the fall) and in contemporary literature like diaries and essays it is written down, that in a scope of around 10 kilometers, at least not less than 10 fragments fell down."
I don't understand what exactly 距離にして does here and especially I dont know how it functions. "Making it into range/distance" is how I would translate it literally. It isn't much of an obstacle for me in understanding what the sentence wants to tell me, but since it adds up with １０キロほどの範囲に it feels redundant, since in this phrase it is already indicated that we are talking about a spatial entity.