NASA SLS display model. Space Launch System model (abbreviated as SLS) is an American super heavy-lift expendable launch vehicle under development by NASA since 2011. It replaces the Ares I and Ares V launch vehicles, which were cancelled along with the rest of the Constellation program. The SLS is intended to become the successor to the retired Space Shuttle as the primary launch vehicle of NASA's deep space exploration plans through the 2020s and beyond. Crewed lunar flights are planned as part of the Artemis program, leading to a possible human mission to Mars. The SLS is being developed in three major phases with increasing capabilities: Block 1, Block 1B, and Block 2. As of August 2019, SLS Block 1 launch vehicles are to launch the first three Artemis missions. Five subsequent SLS flights are planned to use Block 1B, after which all flights will use Block 2.