Where does space begin?
Space begins at the top of the atmosphere. As you go higher, the air gets thinner. At a height of about 100 miles the air is so thin that there is virtually none left. This is where space begins. (Random House)
as you go higher,…・・・高く上がるにつれ、〜
100miles・・・160km
so thin that…・・・あまりにも薄いので〜
virtually none left・・・ほとんど残っていない
this is where…..・・・関係副詞の “where” は先行詞が “the place” の場合は省略可能です（書かなくても分かるので）。つまり元の形は This is the place where space begins. となります。
