2002’s Voices of a Distant Star is a standalone original video animation by Makoto Shinkai. It was later the basis of a 2004 manga by Sumomo Yumeka.
Noboru Terao, obviously (although inarticulately) smitten with fellow student Mikako Nagamine, expects to spend his years in high school mooning after Mikako. Unbeknownst to Noboru, Mikako has volunteered to join the UN Space Army. Mikako will not be attending high school. She will be travelling across the solar system and beyond.
They are determined to remain in contact. Physics will not be their friend in this matter.