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.
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.