Complex sentence: Originally EOM-1 Spacelab mission, cancelled in December 1984 due to planned combining with EOM-2 mission and re-manifested as STS-61-K which was then cancelled due to the "Challenger" disaster.
Split into simpler sentences: Originally EOM-1 Spacelab mission, cancelled in December 1984 due to planned combining with EOM-2 mission. Later re-manifested as STS-61-K which was then cancelled due to the "Challenger" disaster.