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NASA's Space Launch System (SLS) is a super-heavy-lift launch vehicle designed to support human exploration activities beyond Earth orbit. Begun in 2011, the program is a hold-over from NASA's Constellation Program, which was canceled in 2010. Leveraging NASA investments in Space Shuttle and Constellation Program hardware, the SLS launch vehicle comprises an 8.4-meter-diameter core stage, which is the same diameter as the Space Shuttle external tank. The core stage is flanked by two five-segment solid rocket boosters modified from the four-segment SRBs used on Shuttle. Core stage main propulsion will be provided by four RS-25E Space Shuttle main engines repurposed for the SLS application. The upper stage comprises a Delta (IV) Cryogenic Second Stage (DCSS) with a single RL10 engine, called the Interim Cryogenic Propulsion Stage (ICPS). The ICPS will eventually be replaced by a four-engine upper stage Advanced Cryogenic Evolved Stage (ACES). The initial capability of the "Block 1A" vehicle (scheduled to launch by mid-2020) is 70,000 kg to low-Earth orbit. later versions of the vehicle are slated to carry up to 105,000 kg (Block 1B) and 130,000 kg (Block 2). The first launch of SLS will be an unmanned Orion spacecraft, which will perform autonomous maneuvers around the Moon (including a Distant Retrograde Orbit) before returning to Earth for splashdown.