Complex sentence: Space shuttle "Atlantis" had been scheduled to visit the International Space Station (ISS) again on STS-114 mission in March 2003, however, due to the shuttle Columbia disaster all space shuttles including Atlantis were temporarily grounded.
Split into simpler sentences: Space shuttle "Atlantis" was supposed to visit the International Space Station (ISS) again on STS-114 mission in March 2003. However, due to the shuttle Columbia disaster all space shuttles including Atlantis were temporarily grounded.