NASA Considers Adding Rendezvous to Artemis 2 Mission Objectives
NASA recently announced that the organization was actively studying the addition of an Orion rendezvous demonstration during the Artemis 2 test flight. Now, if you’re unfamiliar with what “rendezvous” means, rendezvous is the maneuvering of two spacecraft to arrive at the same orbit and approach to a very close distance. Rendezvous of the Orion spacecraft is a “mission critical” functionality since the spacecraft needs to be able to rendezvous with Gateway, the future lunar space station, and potentially the landing vehicles that will take astronauts down to the surface of the Moon. Oh, and it needs to be able to do this around the Moon. Needless to say, this change to mission architecture is a big deal.
The addition of rendezvous testing to the mission would serve to reduce risks and test objectives that were originally left to the Artemis 3 mission, the mission that will land astronauts on the Moon by the end of 2024 for the first time in over 50 years. NASA is still working on the trade-offs for this change, studying the risks involved with requiring this rendezvous test on the first crew mission of the Orion spacecraft.
Moving the rendezvous tests to Artemis 2 would not require an extensive rework of the current mission plan. The current mission architecture for Artemis 2 revolves around the Orion spacecraft performing a flyby of the Moon with crew in 2023 (no official date yet). This mission is designed to fully test Orion’s functionality before the big Moon landing mission. NASA has not made any official decisions or announcements as there is still much to consider with this change. Crucial questions need to be answered such as what will the Orion spacecraft rendezvous with and where will this rendezvous test occur during the Artemis 2 mission.
The Current Artemis 2 Mission
The current Artemis 2 mission plan follows a similar path to Apollo 8 and Apollo 10 missions. Artemis 2 is designed to be the first crew flight of the new Orion spacecraft mission following a series of Low Earth Orbit (LEO) to High Earth Orbital (HEO) maneuvers before the Orion capsule launches itself into a Lunar Flyby trajectory around the moon.
Until 2017, Artemis 2 (then known as Exploration Mission-2) was planned to be launched on the Space Launch System (SLS) in its more powerful “Block 1b” configuration. This launch configuration would utilize the powerful Exploration Upper Stage (EUS) to push the Orion spacecraft from LEO into its HEO. After completing this orbit, the Orion Spacecraft would separate from the EUS and ignite its engine sending four astronauts in the first crewed Orion capsule into a lunar flyby for a maximum of 21 days. Overall, the Orion spacecraft would orbit Earth twice while periodically burning its engines to push it gradually toward the Moon before looping back to Earth on a Free Return Trajectory.
Since then Artemis 2, has undergone a significant change to the plan: SLS Block 1b is unavailable meaning the Interim Cyrogenic Propulsion Stage (ICPS) will take over for the EUS. Although Artemis 2 still looks the same on paper, the overall parameters of the mission were extended creating an excellent opportunity to test rendezvous.
The change from the original plan to using ICPS also brought changes to the orbit design. The HEO orbit was extended significantly changing the highest point in the orbit to over 59000 nautical miles (109268 km); this was achieved by including a much longer ICPS burn changing the orbital period (the time it takes to orbit Earth once) from 24 hours to 42 hours! With the goal of reducing Orion’s needed propellant usage to transfer to the Moon, this change opens up a perfect opportunity to test Orion’s rendezvous capability.
Testing Rendezvous on Artemis 2
The rendezvous demonstration for the Orion spacecraft was originally planned to be introduced on Artemis 3. In fact, Orion’s flight software wasn’t even planned to be fully capable of supporting rendezvous as the Artemis 2 Mission would not fly with all of the needed sensors or equipment to perform the close proximity operations with a target.
Although very possible, moving the rendezvous testing objectives to Artemis 2 would require extensive hardware and software development acceleration. Orion’s flight software would need to be accelerated by almost a year to bring rendezvous capabilities to the Artemis 2 Orion spacecraft. Additional hardware would need to be built, tested, and certified a year before the originally planned date. On top of that, adding the rendezvous test objectives creates a few other questions that need to be heavily considered and ultimately answered: when will Orion perform the rendezvous tests and what will it use as its target? Although still looking into options, NASA has a plan.
Apollo performed several rendezvous and docking tests throughout the early Apollo missions. Almost all of these missions tested rendezvous and docking using large “rideshare” payloads such as the Lunar Module (LM) mock-up. The goal of the Artemis 2 rendezvous test is to determine Orion’s maneuvering capability and to verify the flight software performance. NASA has considered just about every option and has looked into launching a Gateway Logistics Services vehicle (like SpaceX’s Dragon XL) or another large system to be used as the target spacecraft. This is very, very difficult since the high orbit Artemis 2 launches on creates almost no gap or leniency in launch schedule, and the logistics for launching the two separate vehicles would be a nightmare with high risks to mission success.
Luckily, since the Apollo era, technology has come a long way. NASA could potential utilize some additional minor cargo that would “rideshare” with the ICPS and Orion spacecraft to act as the target. In fact, several industry studies have considered using CubeSats or very small PicoSat transponders to act as rendezvous targets. These systems are no bigger than a shoe box or iPhone. One other very attractive option is to just use what Orion is already bringing with it: the ICPS. After Orion separates from the upper stage, it could maneuver away to a predetermined distance before engaging the rendezvous maneuvers. Of course, even this attractive option comes with its problems that need a solution. Due to the nature of the ICPS as a propulsion stage, it must always manage its internal pressure and temperature so it must vent and maneuver to keep everything in balance. This would require careful planning when performing the close approach rendezvous test.
Conclusions
Overall, the idea to accelerate the rendezvous demonstration to Artemis 2 is sound as it reduces the already hefty test objective load involved with landing humans on the surface of the Moon with Artemis 3. Of course, with this alteration, a significant amount of software and hardware development will need to be accelerated by a year or more. This will require additional funding and hardly leaves any room for error in an already packed development timeline.
NASA will need to carefully consider all of the different mission impacts deciding the best course of action for when and where this rendezvous should take place. Ultimately, performing the rendezvous test in the HEO segment of the mission presents the lowest risk to both astronauts and the mission itself as waiting until after this segment means the astronauts would be in the Lunar Transfer Orbit with no quick way back if something goes wrong.
Other critical decisions must be made when considering the exact test objectives. It is one thing to say “test rendezvous capabilities” but even this simple statement leaves a lot to be desired. Rendezvous maneuvers are often split into multiple segments based on approach distance and each segment requires different hardware. Do you test all approaches? Are all of the systems ready or will they be ready to test by launch?
Even though there is still a lot of work to do before an Artemis 2 rendezvous demonstration can be solidified, I have no doubts that NASA will be able to determine and plan a best course of action enabling additional Orion capabilities to be tested before the moon landing flight in 2024. We will just have to wait and see.
