Boeing‘s [BA] first core stage for the NASA Space Launch System (SLS) arrived at the agency’s Kennedy Space Center in Florida on Apr. 29, the company said.

The 212-foot core stage will be stacked with a Boeing/United Launch Alliance (ULA) Interim Cryogenic Upper Stage, two solid rocket boosters, a Launch Vehicle Stage Adapter, and the Orion spacecraft. ULA is a partnership between Boeing and Lockheed Martin [LMT].

This year, SLS is to launch Orion on the Artemis I uncrewed mission to the moon and back–a mission that is to test the operation of Orion and SLS as an integrated system before crewed flights to the moon for sustained exploration.

Boeing said that it is linking major elements for the Artemis II core stage at NASA’s Michoud Assembly Facility in New Orleans; building core stage elements for Artemis III; and working on evolvable rocket capabilities, such as the Exploration Upper Stage, which is beginning production at Michoud.

NASA announced the Artemis program in May 2019 with a goal of returning American astronauts to the moon by 2024. Artemis II is to be the first crewed mission for SLS and plans to orbit the Moon. The Artemis III mission in late 2024 is to land astronauts on the moon’s south pole to begin creating a sustainable human lunar presence.

In 2011 and 2012, NASA contracted with Boeing, Aerojet Rocketdyne [AJRD], and Northrop Grumman [NOC]—to develop the major elements of SLS, which NASA then planned to use for deep space exploration. Boeing is to provide the SLS core stage, upper stage, and avionics.

Aerojet Rocketdyne is to provide the RS-25 engines and Northrop Grumman the solid rocket boosters that help power SLS.

“NASA has made significant progress with the Artemis missions including stacking the SLS solid rocket boosters onto the mobile launcher, delivering Orion to Kennedy for final integration, and stabilizing future launch manifests,” per a report this month by the NASA Inspector General (IG).

“Nonetheless, the agency faces significant challenges that we believe will make its current plan to launch Artemis I in 2021 and ultimately land astronauts on the moon by the end of 2024 highly unlikely,” the report said. “Following delivery of the core stage to Kennedy, NASA will shift its focus to its first-time integration of the SLS, Orion, and EGS [Exploration Ground Systems] for the Artemis I launch. Integration and final systems testing is a complex and time-consuming process that often discovers issues in need of costly rework. Given all of these factors, a planned 2021 Artemis I launch is in jeopardy of slipping to 2022, a delay that would cascade and push back the launch of Artemis II into at least the third quarter of 2023, ultimately impacting the launch date for Artemis III.”

NASA said that it has spent $37.2 billion on Artemis so far, including $17.2 billion for SLS, $2 billion more than budgeted. NASA said that it projects it will spend another $11.3 billion on SLS through fiscal 2025.

This month, NASA also said that SpaceX is to develop and build the Starship HLS lunar lander.

“Although the new [Biden] administration has publicly expressed support for the Artemis missions, it has not weighed in on the agency’s current plans for a lunar landing by the end of 2024,” per the NASA IG report this month. “Nonetheless, achieving any date close to this ambitious goal—and reaching Mars in the 2030s—will require strong, consistent, sustained leadership from the president, Congress, and NASA, as well as stable and timely funding.”