Complex sentence: It is the first NASA exploratory mission to use ion propulsion to enter orbits; previous multi-target missions using conventional drives, such as the "Voyager" program, were restricted to flybys.
Split into simpler sentences: It is the first NASA exploratory mission to use ion propulsion, which enabled it to enter multiple orbits. Previous multi-target missions using conventional drives, such as the "Voyager" program, were restricted to flybys.