It's pretty incredible how similar Saturn Apartments and Planetes are - each follows a group of people who hold dangerous yet needed jobs in space, and examines their feelings and experiences in a psychological, slow and touching manner. Saturn Apartments is definitely more cutesy (based on visual style alone), but these are hands down two of the best sci fi manga I've read.
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...
There's no mistaking the fact that SC was influenced by Planetes, at one point discussing the issue of garbage in space and how it can kill people. While not similar besides that in plot, both are sci fi titles about people who work their hardest to be astronauts - fans of one will appreciate the other, but Planetes is the far superior of the two.
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.
For years, paranoid Satou Tatsuhiro has shut himself away in his apartment for days on end, with barely any social contact whatsoever. Then one day when he answers the door, he is greeted by an older woman and a girl called Misaki, who are going from door-to-door to inform people of the recent social problems of hikikomoris. In a fit of depression about his way of living, Satou decides that the only way to escape his current life cycle is to muster up his courage, face his fears, and go outside to find a job. However, instead of employment, he finds Misaki waiting for him; she intends to make Satou her ‘project' so that he can reconnect with society. Now with the help of his friend Yamazaki and Misaki's evening meetings, will Satou be able to escape his hikikomori lifestyle, or will he simply fall deeper into the clutches of conspiracy and his own demons?