Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment
NASA’s CAPSTONE mission is a whole deep-space tech demonstrator packed into approximately 730 cubed inches. Even its acronym, CAPSTONE (Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment), does a whole lot in a small space. From leading our return to the moon to demonstrating exciting low-energy trajectories let’s explore CAPSTONE!
Illustration by NASA/Daniel Rutter
What is CAPSTONE’s Mission?
Capstone is a 12U Cubesat (small enough to fit into my freezer if I clear it out a bit), whose goal is to validate the power and propulsion requirements to maintain an object in a specified Near Rectilinear Halo Orbit (NRHO) about the Earth-Moon’s first Lagrange point. This blog has gone over the 3-body dynamics that cause Lagrange points to emerge as well as explored the periodic orbits about these Lagrange points, so I won’t repeat that discussion here, but the reason NASA is so interested in this specific NRHO, is that, as part of the Artemis program, NASA intends to place a Moon-focused space station in this orbit. This station called Gateway would both support NASA’s goal of long-term human activity on the moon, as well as serve as a staging point for future deep space exploration. Unfortunately, no spacecraft has ever been put into this orbit, so before NASA places a whole space station there, CAPSTONE is being used to characterize the robot and validate that NASA’s plans for it are viable. The Lagrange points and their orbits are not stable in the traditional sense, so one particular item CAPSTONE will be validating is the expected stationkeeping costs of maintaining the orbit. In addition to its main mission of validating the NRHO, CAPSTONE is also set to demonstrate the Cislunar Autonomous Positioning System (CAPS), which is a method for determining the spacecraft orbital position using only crosslink measurements to NASA’s LRO spacecraft (or in the future other spacecraft). Most spacecraft determine their position using information from Earth-based ground stations, but the farther you are from Earth, the larger the ground station is needed. As Lunar space becomes more crowded due to the increased interest over the coming decade the number of large Earth-based ground stations would need to increase substantially. CAPS would mitigate this problem by removing the need for these increased number of Earth-based ground stations and instead allow spacecraft to determine their positions relative to other Lunar spacecraft in a method that not only scales but improves as more lunar spacecraft arrive and use CAPS.
Getting to the Moon
CAPSTONE isn’t just about the destination, it’s also about the journey. In order to deliver the most mass to the NRHO, CAPSTONE will be utilizing a low-energy transfer. In short, a low energy transfer is one where the spacecraft uses a highly elliptical trajectory, with an apogee past the Moon, to get to a region of space where the perturbations from the Sun are significant. Utilizing the Sun’s perturbations allow delta-V’s of 1/5 to 1/3 of those required for a traditional Lunar trajectory. There are no free lunches in astrodynamics, and while a traditional Lunar trajectory has a time of flight of 3 days, a low-energy transfer can take approximately 3 months. These long times of flight rule them out for use by humans, but will likely become far more utilized in the future for non-perishable cargo. Below is a video from Advanced Space showing one of these low-energy transfers, along with the instantaneous 2-body trajectory in white.
Who’s Building CAPSTONE?
Normally I stay focused on the missions and the spacecraft themselves, but for CAPSTONE let’s take a step back and take a look at the company and a few of the people behind it, Advanced Space of Colorado. I’ve been lucky to have been following Advanced Space for a while, as their CEO, Bradley Cheetham, completed his undergraduate degree at the same department I completed mine (Mechanical and Aerospace Engineering at the University of Buffalo), and the department would often invite him back to talk about Advanced Space (What’s more, he helped found the SEDS chapter at UB that I was heavily involved in, thanks Brad!). Advanced space also has some very talented aerodynamicists on their staff like Jeff Parker and Nathan (Parrish) Ré. Jeff, previously at JPL, quite literally wrote the book on Low-Energy Lunar Trajectory Design. Now, with CAPSTONE, he brings the design techniques from his book into reality. Finally, the last highlight for this post is Nathan Ré. For their dissertation, Nathan talked about enabling real-time optimal control through the use of a neural network to map the spacecraft state to the co-states, which are then used to calculate if the spacecraft should be thrusting and in what direction. If this sounds familiar, it’s because it’s similar to my first paper Neural Network Optimal Control in Astrodynamics where I tried to map the spacecraft state directly to the control outputs, and cite Nathans’s work quite heavily. I believe they will implement some of Nathan’s dissertation research to enable autonomous operations on CAPSTONE! Nathan and Jeff’s dedication to turning astrodynamics theory into practice is one reason why I find Advanced Space to be such a cool company!
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