WASHINGTON — NASA has finalized the contract under which Boeing Space Exploration of Houston will deliver a pair of Space Launch System rocket cores needed for missions to lunar orbit scheduled for 2018 and 2021, the agency announced July 2.

On June 30, NASA and Boeing signed a six-and-a-half-year, $2.8 billion SLS Stages Contract that runs through 2021 and calls for the company to deliver two SLS cores, including hydrogen and oxygen tanks, and avionics.

The total includes money Boeing has already received through an undefinitized contract that allowed the company to continue SLS-applicable work started in 2007 under the Constellation Moon-exploration program, NASA spokeswoman Rachel Kraft said.

Since 2007, when Boeing began work on the upper stage and avionics systems for the Ares 1 crew launcher that was canceled along with Constellation in 2010, Boeing has received $1.66 billion for work related to the SLS core stage, NASA spokeswoman Kimberly Henry said.

Besides work on the rocket’s core stage, the new SLS Stages Contract also authorizes Boeing to begin studying a new SLS upper stage — which NASA and Boeing call the Exploration Upper Stage — to replace the Boeing-provided Interim Cryogenic Propulsion Stage the rocket will use for its first two missions, Kraft said.

Counting the SLS Stages Contract just signed and an Interim Cryogenic Propulsion Stage contract that with options could be worth up to $307 million, Boeing stands to receive roughly $3 billion through 2021 for work delivering the core  and upper stages for the first two SLS rockets.

Both of the first two SLS missions, known as Exploration Mission 1 and Exploration Mission 2, will boost the Lockheed Martin-built Orion crew capsule to the same distant lunar retrograde orbit where NASA has proposed redirecting a small asteroid for astronauts to visit by 2025. Only Exploration Mission 2 will be crewed.

In May, Virginia Barnes, Boeing’s SLS program manager, said the baseline concept for the Exploration Upper Stage includes four Aerojet Rocketdyne RL10 hydrogen-fueled engines, rather than the single Aerojet Rocketdyne J-2X NASA envisioned using for deep-space missions when it unveiled SLS in 2011.

The core stage of the SLS variant set for flight in 2018 and 2021 will be able to boost 70 metric tons to low Earth orbit using four Aerojet Rocketdyne RS-25 engines left over from the space shuttle program, plus a pair of five-segment solid-rocket motors ATK of Magna, Utah, is building under a $1.19 billion contract that calls for delivery of two flight sets. Like Boeing’s SLS Stages Contract, ATK’s SLS booster contract was adapted from a Constellation award.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.