When Asumi was just a baby, Japan's first manned space rocket, the Lion, malfunctioned and crashed into her hometown of Yuigahama. The impact killed many of the townspeople and critically wounded the girl's mother, who lay in a coma for several years before passing away, having never regained consciousness. With the help and friendship of Lion-san, a mask-wearing ghost who claims to have come from the rocket, Asumi grew up focusing on her dream – to someday travel to space. Now a young adult, she is determined to beat the odds and attend the newly-formed Tokyo National Space School to achieve her lifelong goal. Alongside Lion-san's guidance and several new, dear friends, Asumi will try her hardest to touch the stars.
This recommendation is for the MANGA OF THESE TITLES, SPECIFICALLY. Both deviate to certain degrees from their anime counterparts, and are not as applicable. If you read either of these MANGA - continue on. Planetes' sci fi is far more realized, while Twin Spica's timeline takes place when man isn't traveling into space as heavily. Regardless, both manga are highly introspective, have gorgeous artwork (including full, multi-page shots of the starry sky - or space itself), and explore the relationships of the main character for better or worse. There's tragedy in both as well. Call it a gut feeling beyond that, but I'm pretty much certain that if you liked one of these manga, you'd consider the other to be equally moving.
Lonely Kaoru Nishimi has lost count of how many times he has moved schools thanks to his father's naval career. Newly landed in Kyushu, he finds most of his classmates instantly antagonistic towards him and suspicious of his elite background. Only two other students seem not to mind who he is – the infamously abrasive Sentarou and the gentle Ritsuko. Unbeknownst to Kaoru they have a passion for jazz, and they're about to pull him out of his stiff, joyless world into theirs of loose, swinging self-expression. Through friendship and frustration, love and despair, Kaoru will learn that life has a wealth of experiences in store for him!
Yeah, one of these is about outer space and the other is about jazz, but despite that they have very similar themes and atmosphere. Both are mainly about a character that used to be a loner making lasting friendships, and throwing themselves wholeheartedly into their dreams. Both also feel very nostalgic and wistful.
Plot-wise, Saturn Apartments is far closer to Planetes. But it's akin to Twin Spica in another, just as important way - the tone, feel and art style. Both are very 'cute' for seinen works, yet are equally lighthearted, somber/melancholy at times, and emotionally moving. Both are tales of a young person who's suffered a great loss, but has determination to succeed. If you're into sci fi or seinen at all, or want a fantastic tale of friendship, these are an excellent match.
When Mutta and Hibito were children, they made a promise to become astronauts together after spotting a UFO one night. Now adults, the duo's path couldn't have diverged more – Hibito is about to travel to the moon with NASA to help simulate the future exploration of Mars, and Mutta is unemployed, having recently headbutted his boss at an auto company. Still, the man can't shake his desire to surpass his younger brother, and soon, he becomes an applicant for Japan's JAXA space program. His ultimate goal, to get one step ahead of Hibito and go to Mars. But the path to becoming an astronaut is long and fraught with tests and challenges. Will Mutta and newfound friends Kenji and Serika manage to persevere and achieve their dream?
Two sci-fi/slice-of-life manga about charaters' quests to become astronauts. The main characters of each have dreamed of space since they were young, so flashbacks are fairly common. Both are mostly realistic, yet very optimistic about the future of space travel.
The year is 2076, and the world is ready to launch its first manned space probe to Jupiter. Onboard is Kousei Amami, a friend of Yuuto's family, and personal hero to the boy, who dreams of following Kousei into space as an astronaut someday - even promising his friend that when he returns from his mission, Yuuta will surely follow his dream. But when a terrible accident occurs and the ship is seemingly lost in the depths of space, Yuuto decides that now more than ever, he will make his way into the astronaut program in hopes of someday traveling to Jupiter, in the hope that Kousei is still alive...
Star Children copies Twin Spica pretty heavily, specifically a drawn out isolation test in the space training school. While SC does take place partially in space and TS is contained on Earth, these two are very similar titles that will appeal to the same audience. Just keep in mind SC isn't that great and has a rushed/unsatisfying ending, and doesn't come close to the masterpiece of TS.