Voices of a Distant Star

Voices of a Distant Star is a Japanese science fiction original video animation (OVA) short film created and animated by Makoto Shinkai. It follows the lives of two close childhood friends, a boy and a girl, who are separated when the girl is sent into space to fight in a war against aliens. As the 15-year-old girl goes deeper and deeper into space, the texts she sends take longer to reach the Earth; the film simultaneously follows her battles and the boy's life as he receives her texts over the years.

Synopsis from Wikipedia, CC BY-SA 4.0. Everything below written here.

Poster © the film's rights holders.