Complex sentence: Stardust was a 300-kilogram robotic space probe launched by NASA on February 7,1999 to study the asteroid 5535 Annefrank, and to collect samples from the coma of comet Wild 2, as well as samples of cosmic dust, and return these to Earth.
Split into simpler sentences: Stardust was a 300-kilogram robotic space probe launched by NASA on February 7,1999. Its primary mission was to study the asteroid 5535 Annefrank and to collect samples from the coma of comet Wild 2, as well as samples of cosmic dust, and return these to Earth.