Delta II

Delta II was the descendant of the Thor intermediate-range ballistic missile, carrying forward a rocket lineage that began in the 1950s. Boeing (through its heritage company Douglas Aircraft) refined the design into one of the most reliable launch vehicles in history.

The rocket''s versatility made it NASA''s preferred launch vehicle for mid-size science missions. It placed the Pathfinder lander and Sojourner rover on Mars, launched the twin Mars Exploration Rovers Spirit and Opportunity, deployed the GPS constellation that revolutionized navigation, and carried the Kepler space telescope to its planet-hunting orbit.

Delta II flew 155 times between 1989 and 2018, with only two failures -- a success rate of 98.7%. Its retirement in 2018, after launching the ICESat-2 Earth observation satellite, ended a rocket family whose lineage spanned six decades and over 400 flights across all Delta variants.