NASA’s Space Launch System (SLS) rocket has left the Vehicle Assembly Building at the Kennedy Space Center in Florida and completed its 4.2-mile (6.7-km) journey to Launch Complex 39B.

The multi-hour rollout process resulted in a sunrise arrival at the pad. The rollout is the last major milestone ahead of launch, which will differ from most recent missions in that the rocket’s needed azimuth — or flight path — will continuously change through each day’s launch window.

Launching into a rendezvous orbit with a satellite or station in low Earth orbit can be relatively simplified as needing to launch directly into the plane – and therefore the same orbital inclination – of the target’s orbit.

For example, when launching to the International Space Station from Florida, the azimuth the rocket follows is 44.98°. This does not change based on when within the daily window liftoff occurs.

However, the same is not true when trying to launch into an intercept trajectory with the Moon.

As related by Artemis 1 Ascent/Entry Flight Director Judd Frieling to NASASpaceflight during Artemis Day events in Mission Control at the Johnson Space Center, the Moon’s motion in its orbit coupled with its constantly-changing relative inclination to the launch site complicates the needed launch azimuth for SLS.

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On each launch day, the azimuth SLS must fly moves incrementally, second-by-second, throughout the window to match the movement of the Moon relative to the Earth for the translunar injection (TLI) burn.

According to NASA, for SLS and Artemis 1, the azimuth at the opening of the window on all three launch attempts on August 29, September 2, and September 5 is 62°, resulting in a 38° inclination orbit.

At the end of each window, the azimuth flown would be 108° into a 32° inclination orbit.

But before SLS can be readied for its roll onto course on launch day, it must first arrive at the pad.

The Artemis 1 launch rollout marks the first time since May 31, 2011, that a vehicle has emerged from the Vehicle Assembly Building (VAB) at the Kennedy Space Center for launch operations.

SLS and Orion at LC-39B during preparations for the WDR (Credit: Julia Bergeron for NSF/L2)

As it has twice already for its wet dress rehearsal campaigns, the SLS rocket for Artemis 1 will make the journey to LC-39B atop crawler-transporter 2, one of two crawler-transporters owned by NASA and the only one modified to carry the full stack Artemis/SLS vehicle to the pad.

The upgrades were necessary due to the crawler’s age and the increased mass of the SLS vehicle with its combined Mobile Launcher (ML).

The combined SLS/ML weight is approximately 15 million pounds (6.8 million kg) and is significantly heavier than the previous record holder in the Space Shuttle at 12 million pounds (5.4 million kg).

Upgrades included a rating to handle 18 million pounds (8.1 million kg), a 50% greater load than was originally envisioned, as well as a new 1,500-kilowatt electrical power generator, parking and service brakes, redesigned and upgraded roller bearings, and several other modifications for the Artemis program.

Like the crawlers, their purpose-built road, the crawlerway, also underwent upgrades between Shuttle and SLS.

Beginning in 2013, the crawlerway’s foundations were repaired with new lime rock to return them to their original condition and ready them for the Block 1B SLS, presently scheduled for later this decade, which will be heavier than the Block 1 SLS used for Artemis 1.

The 15 million pound SLS and ML on LC-39B during Wet Dress Rehearsal. (Credit: Nathan Barker for NSF)

Additionally, 30,000 tons of new Alabama river rock were added to return the crawlerway to its optimal depth.

For  Launch Complex 39B , which was used for Apollo, Skylab, Apollo-Soyuz, Space Shuttle, and Ares I-X missions, the pad was slowly modified in stages, beginning in the final years of the Shuttle program, into a clean pad configuration with three, 600-foot (183 m) lightning towers connected with catenary wires.

The clean pad is without the Shuttle-era fixed and rotating service structures that serviced the Shuttle stack.

The sound suppression system, flame trench, cabling, and other systems were also upgraded during the transition to SLS. Work on Pad 39B has also included a new 1.25 million gallon liquid hydrogen tank, though this is not yet complete and will not be used for Artemis 1.

Pad 39B’s clean pad configuration was designed to be able to handle different types of rockets as part of a multi-user spaceport emphasis. To date, only Northrop Grumman expressed interest in the pad share for their now-canceled OmegA rocket.

Artemis 1 is scheduled to spend 13 days at Pad 39B after the August 16 rollout. During this time, the ML will be hooked up to the plumbing servicing the rocket with liquid oxygen, liquid hydrogen, helium, and liquid nitrogen.

Crawler-Transporter-2 (CT-2) during rollout testing. (Credit: NASA)

Other round systems required for the launch will also be activated while teams conduct system checks on the SLS and Orion. Should all go well, the stage will be set for the 60th overall launch — and the second flight to the Moon after Apollo 10 — from Pad 39B.

The Artemis 1 countdown is currently scheduled to begin with Call To Stations at 9:53 AM EDT (13:53 UTC) on August 27. Fueling would begin early in the morning of August 29 for a two-hour launch window opening at 8:33 AM EDT (12:33 UTC).

Overall,  Artemis 1 has 25 days to launch after the flight termination system (FTS)  testing on the launch vehicle was completed on August 12.

Should Artemis 1 not be able to launch on August 29, launch windows for September 2 and 5 are available.

The two-hour September 2 launch window starts at 12:48 PM EDT (16:48 UTC) while the September 5 window lasts for 90 minutes, starting at 5:12 PM EDT (21:12 UTC).

Should Artemis 1 not be able to make any of the launch windows, crawler-transporter 2 would return to Pad 39B to roll the stack back to the VAB for FTS replacement and any other work the vehicle or ML might need before the next available launch window, most likely October 17 through 31.

Wide-angle photo showing the enormous Vehicle Assembly Building, standing 525 ft. tall. The crawler-transporter 2 is now in place under @NASA_SLS and @NASA_Orion and ready for tomorrow’s roll out to Launch Pad 39B. pic.twitter.com/fCDGS0wouf

— NASA's Exploration Ground Systems (@NASAGroundSys) August 15, 2022

Together, the first two SLS/Orion Artemis missions will pave the way for the first human lunar landing since 1972 on Artemis 3, currently scheduled for no earlier than late 2025.

Artemis 3 will use the SLS and Orion to ferry astronauts to lunar orbit, where a waiting SpaceX Starship lander procured under the  HLS contract  will transport them to and from the surface near the Moon’s south pole.

Just under 50 years after humanity last left the Moon in December 1972, Artemis 1 stands ready to begin our return journey. This time, to stay.