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.
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.
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.