CAPSTONE (Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment)
CAPSTONE is the pathfinder mission for NASA’s Artemis program. The overall mission will include collaboration with the Lunar Reconnaissance Orbiter (LRO) operations team at NASA Goddard Space Flight Center to demonstrate inter-spacecraft ranging between the CAPSTONE spacecraft and LRO and with the NASA Gateway Operations team at NASA Johnson Space Center to inform the requirements and autonomous mission operations approach for the eventual Gateway systems. Critical success criteria for CAPSTONE in this demonstration are a transfer to and arrival into an NRHO (Near Rectilinear Halo Orbit), semi-autonomous operations and orbital maintenance of a spacecraft in an NRHO, collection of inter-spacecraft ranging data, and execution of the CAPS navigation software system on-board the CAPSTONE spacecraft. 1)
Scheduled to launch on a Rocket Lab Electron in October 2021, the Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment (CAPSTONE) mission will leverage a 12U CubeSat to demonstrate both the core software for the Cislunar Autonomous Positioning System (CAPS) as well as a validation of the mission design and operations of the NRHO (Near Rectilinear Halo Orbit) that NASA has baselined for the Artemis Lunar Gateway architecture. Currently being developed in a Phase III of NASA’s SBIR program, our CAPS software will allow missions to manage themselves and enable more critical communications to be prioritized between Earth and future cislunar missions without putting these missions at increased risk.
The CAPSTONE mission is a pathfinder for NRHO operations that will be vital for future Lunar Gateway activities. Awarded in mid-2019 to Advanced Space LLC of Westminster, CO, the mission has since completed its key development and implementation milestones and is in final system assembly and test in preparation for launch in October 2021 on a Rocket Lab Electron from Mahia, NZ. Partnering with Tyvak Nanosatellite Systems of Irvine, CA, Stellar Exploration, and Rocket Lab as well as working closely with NASA through multiple centers, the CAPSTONE mission represents not only the first steps to laying the foundation of the Artemis Program but also a different way of collaboration. By utilizing the best features of commercial and governmental capabilities, this technology demonstration is being executed in a rapid and low-cost mission not typically seen in more traditional NASA small-mission contracts.
In addition to the orbital and operational focuses of this demonstration, the mission will also serve as a technology demonstration of an Advanced Space navigation product developed via a NASA SBIR that recently concluded its Phase II period of performance and has been extended into Phase III for this demonstration mission. The Cislunar Autonomous Positioning System (CAPS) is Advanced Space’s solution to the lunar congestion predicted to dramatically increase in the coming years as government, commercial, and international interests bring more missions into the cislunar environment in and about the Moon. By demonstrating this absolute navigation capability on the CAPSTONE mission, it will also rapidly raise the Technology Readiness Level (TRL), commercialization, and infusion of CAPS in preparation of the cislunar traffic to come.
1) Validate and demonstrate NRHO / highly dynamic Earth-Moon Operations. The first mission objective is focused on mitigating technical uncertainties associated with operating in the uniquely beneficial and challenging orbital regime defined as NRHO (Near Rectilinear Halo Orbits). This objective will include demonstrating navigation capabilities and validating stationkeeping strategies and operational simulations. This objective will directly support future missions through dissemination of operational information and by obtaining operational experience in this unique orbital regime.
2) Inform future lunar exploration requirements and operations. The second CAPSTONE mission objective is focused on building experience operating in complex lunar orbital regimes to inform future lunar exploration requirements and operations, including human exploration flights with lower risk thresholds and higher certainty of success requirements. This will include the establishment of commercially available capacity to support NASA, commercial, and international lunar missions in the future. This objective also seeks to demonstrate the capacity of innovative NASA-industry approaches to rapidly bring capabilities to the Moon and challenge current expectations for cost and schedule.
3) Demonstrate and accelerate the infusion of the CAPS (Cislunar Autonomous Positioning System). The third objective is focused on demonstrating core technical components of CAPS in an orbital demonstration. This objective will include collaboration with the operations team at NASA Goddard Space Flight Center to demonstrate inter-spacecraft ranging between the CAPSTONE spacecraft and the Lunar Reconnaissance Orbiter (LRO) currently in operation at the Moon. In addition to demonstrating key inter-spacecraft navigation in cislunar space, CAPSTONE will also enhance the technology readiness level of the CAPS software. This accelerated maturation will permit it to be available to support near term flight plans to the Moon and to be more widely adopted by future lunar missions and thus increase the value of this peer-to-peer navigation capability.
The CAPSTONE spacecraft is a 12U CubeSat (mass of ~25 kg) that has been designed and is being built by Tyvak Nanosatellite Systems based on their exiting, commercial 12U satellite bus. In addition to the CAPS payload flight board, CAPSTONE also includes a color commercial CMOS imager for generating images of the Earth and Moon, two communication systems (X-band and S-band) as well a CSAC for generating additional 1-way navigation data. The X-band system will be used for two-way communicate with the ground as well as part of the 1-way ranging experiment, while the S-band system will perform radiometric measurements with LRO to gather data for the CAPS payload using a S-band patch array antenna. The X-Band system will be used to communicate with the ground, while the S-band system will perform radiometric measurements with LRO to gather data for the CAPS payload.
Figure 1: The CAPSTONE spacecraft in a deployed (top) and stowed (bottom) configuration ( image credit: Tyvak)
The spacecraft hosts a monopropellant hydrazine propulsion system, delivered by Stellar Exploration, Inc. of San Luis Obispo, providing over 200 m/s of total ΔV with eight 0.25-Newton thrusters. Four will be used for translational maneuvers and attitude control, and four will be used for attitude control and momentum desaturation.
Operations Overview: The mission is scheduled to launch in October 2021, have an approximate three-month, low energy deep space transfer with several maneuvers leading up to the NRHO insertion and then perform its primary and enhanced mission over the next eighteen months. From the launch deployment state, the spacecraft will traverse a low-energy Ballistic Lunar Transfer (BLT), which is a very energy-efficient means of transferring from the LEO environment to an orbit near or about the Moon. The spacecraft will perform up to six Trajectory Correction Maneuvers (TCMs) to clean up launch injection, navigation, and maneuver execution errors prior to arriving at its NRHO insertion point. The spacecraft will then perform a relatively small maneuver to insert into the NRHO; this maneuver is referred to as the NRHO Insertion Maneuver (NIM). Two small clean-up maneuvers are planned to ensure that the spacecraft is successfully inserted into the NRHO: Insertion Correction Maneuvers 1 and 2 (ICM-1 and ICM-2). From there, small stationkeeping maneuvers, known as Orbit Maintenance Maneuvers (OMMs) in combination with momentum desaturation, are performed on a weekly cadence to remain in the NRHO. Finally, a small Decommissioning Maneuver (DM) is performed when the mission is complete to safely decommission the spacecraft to either deep space or to a predetermined designated impact point on the lunar surface.
Figure 2: TCM placement in the Earth-centered EME2000 frame (image credit: CAPSTONE Team)
Trajectory Correction Maneuvers: CAPSTONE must perform multiple TCMs (Trajectory Correction Maneuvers) in order to clean up launch vehicle errors, target the insertion maneuver timing to achieve an Earth-eclipse free NRHO, as well as correct for navigation and maneuver execution errors. Given that the CAPSTONE spacecraft must achieve both a specific state and epoch after insertion to achieve an Earth-eclipse free NRHO, the TCMs and insertion maneuver are designed using a three-burn optimization, rather than a simple one or two-burn targeter. Each correction maneuver is designed to minimize the sum of the ΔVs of the current TCM, the next TCM, and the insertion maneuver to achieve a state along the reference NRHO (and corresponding epoch). The final TCM is not redesigned, but rather executed as it is designed in the previous TCM’s optimization. The insertion maneuver initial epoch is also part of the optimization, while the TCMs are fixed in time.
Insertion into NRHO: Given the objective of demonstrating NRHO stationkeeping and operations to inform future lunar exploration, CAPSTONE will operate in the same sized orbit targeted by the Lunar Gateway: a 9:2 resonant, southern L2 NRHO. For every two lunar synodic periods, approximately 29.53 days each, CAPSTONE will complete 9 revolutions in its NRHO.
There are four families of NRHOs, distinguished by the location of their apolune. The apolune may be stretched towards either the L1 or L2 Earth-Moon Lagrange point, and may be either above (northern) or below (southern), the lunar orbital plane. A 9:2 resonant NRHO only exists in the L2 family, and the southern trajectory was chosen to maximize the Lunar Gateway’s time over the lunar south pole, an area especially of interest to lunar exploration.
The NRHO Reference Orbit (NRO) is specifically designed to avoid Earth induced solar eclipses for the duration of the primary and enhanced mission (18 months), as well as align the epoch of the first perilune with the low-energy transfer to minimize the ΔV required to achieve the NRHO. Although periodic in the circular restricted three-body problem, a multi-shoot method is used to build an Earth-eclipse free, 90-revolution NRO with minimal discontinuities in the ephemeris model.
Figure 3: NRHO reference orbit in the Earth-Moon rotating (left) and Sun-Earth Rotating (right) frames (image credit: CAPSTONE Team)
As the spacecraft arrives at the Moon it targets the reference NRHO periapse passage, and the spacecraft must precisely insert into the NRHO. Planetary missions are often robust to large maneuver execution errors upon insertion into a capture orbit because the capture orbit simply must be safe and otherwise a good transition to the science orbit via additional maneuvers.
CAPSTONE’s insertion is quite
different from this model. First, the NRHO Insertion Maneuver (NIM) is
on the order of 20 m/s, quite a small maneuver compared with
conventional planetary orbit insertions. Second, the maneuver must very
accurately target the NRHO. If the post-NIM state, mapped to the
nearest periapse, is more than 5 m/s different from the NRO periapse
state, the resulting orbit is far different from the NRHO, and the
mission must spend a significant amount of fuel to return to the
reference NRHO. One way in which CAPSTONE’s orbit insertion is
similar to conventional planetary insertions is that the NIM is
considered a critical maneuver: it must be performed approximately
CAPSTONE introduces two maneuvers following NIM: ICM-1 is performed one day later to clean up NIM execution errors; ICM-2 is performed three days after ICM-1 to complete the insertion process into the reference NRHO. The two Insertion Cleanup Maneuvers offer six degrees of freedom to achieve a six-state at the time of ICM-2, i.e., they compose a two-burn transfer from the post-NIM orbit to the reference NRHO and must be performed relatively soon after NIM in order to avoid exponentially increasing delta-V cost. Some separation is desirable between the two ICMs to give the spacecraft time to drift from the post-NIM orbit to the reference NRHO, but it is desirable to achieve the transfer prior to the next periapse passage since periapse passages are very sensitive to orbital errors. A full summary of all TCM’s, ICM’s and OMM’s their timing, and purpose are outlined below.
Table 1: CAPSTONE's planned maneuvers
Daily Operations: The orbit maintenance strategy for CAPSTONE is both informing and responding to projected lunar Gateway requirements. CAPSTONE’s OMM (Orbit Maintenance Maneuvers) are based on the “X-axis crossing control” strategy—a low-cost, robust method of maintaining NRHOs for long durations.3-5 The maneuver is designed to achieve a target 6.5 revolutions downstream (that is, at the seventh perilune crossing), where the target is the X-velocity of the reference orbit (NRO) in the Earth-Moon rotating frame at that perilune. This strategy is low-cost and effective at maintaining the NRHO-like motion, but the controlled spacecraft drifts away from the reference over time. This drift is realized not only in the position and velocity states of the spacecraft, but also the phasing. Such a drift is undesirable because the NRHO for CAPSTONE was designed to avoid Earth induced solar eclipses by targeting strict phasing.
Different approaches to correct this drift have been developed. 2) 3) Some of these strategies implement a two-burn sequence to correct the phasing, while others implement small augmentations to the traditional X-axis crossing algorithm. The current OMM approach baselined for CAPSTONE supplements the traditional X-axis crossing control algorithm by adding an epoch constraint to the maneuver targets. This constraint requires the perilune passage time of the controlled spacecraft to match the perilune passage time of the reference spacecraft to some tolerance.
Other OMM augmentations were considered as well. These augmentations incorporated different aspects of phase-control strategies presented in literature, such as where in the NRHO the OMM is performed. In these cases, where the location of the maneuver was changed, the same epoch constraint as discussed in the CAPSTONE baseline OMM strategy are still implemented. The additional maneuver locations chosen were 160º and 200º osculating true anomaly (in the Earth-Moon rotating frame).
Current Mission Status
Currently, the CAPSTONE mission is in its final stages of development and implementation. The spacecraft has completed assembly and initial test and is currently in environmental testing at Tyvak in preparation for delivery in early September 2020. The Rocket Lab Lunar Photon upper stage has completed system test and is in final preparation for delivery for integration with the CAPSTONE spacecraft at LC-1 in Mahia, NZ. Currently, launch is planned within a launch period of October 20-31 with no operational constraints on the daily launch window related to post-TLI ΔV requirements or NRHO insertion geometry. All operational systems are in place at Tyvak (for spacecraft and overall mission operations support), Advanced Space (for navigation and flight dynamics support), the Deep Space Network (for tracking and communications support) as well as the support functions for the LRO spacecraft operations once CAPSTONE is in the NRHO and ready for the CAPS demonstration.
Launch: August 16, 2021: Rocket Lab won a $9.95 million NASA contract in February 2020 to launch the CAPSTONE mission aboard the company’s Electron rocket, with an extra boost from Rocket Lab’s Photon propulsion platform to send the small spacecraft toward the moon. 4)
At the time, NASA and Rocket Lab said CAPSTONE would take off from a new Electron launch pad at Wallops Island, Virginia, in early 2021. 5) Rocket Lab announced Aug. 6 that CAPSTONE is now slated to launch from the company’s operational launch base on Mahia Peninsula in New Zealand in the fourth quarter of 2021. — Delays in NASA’s certification of the Electron rocket’s new autonomous flight safety system have kept Rocket Lab from beginning service from the Virginia launch base.
The 12U CAPSTONE CubeSat (25 kg) will ride Rocket Lab’s two-stage Electron launcher on its initial climb into space. Rocket Lab’s Photon spacecraft platform, which also serves as an upper stage, will perform a series of orbit-raising burns and a final trans-lunar injection maneuver to send the CAPSTONE spacecraft toward the moon.
CAPSTONE will fly on a low-energy ballistic trajectory to the moon after separating from the Photon upper stage about a week after launch. The journey will take three-to-four months, according to NASA, before the spacecraft maneuvers itself into the planned near rectilinear halo orbit, passing as close as 1,000 miles (~1,500 km) and as far as 43,500 miles (~70,000 km) from the moon.
The elongated orbit’s advantages include the relative ease of entering and exiting the orbit. A spacecraft in a near rectilinear halo orbit also has a continuous view of Earth, ensuring a constant communications link with ground controllers. The orbit also gives lunar landers access to the moon’s south pole.
CAPSTONE will be the first lunar mission launched by Rocket Lab. - CAPSTONE’s tech demo mission will last about six months.
The next steps in preparing for CAPSTONE’s launch include final assembly of the spacecraft, and shipment of the satellite from the United States to New Zealand in late September for integration with Rocket Lab’s Photon upper stage and Electron launcher.
Orbit: CAPSTONE is expected to be the first spacecraft to operate in a NRHO (Near Rectilinear Halo Orbit) around the Moon. In this unique orbit, the CubeSat will rotate together with the Moon as it orbits Earth and will pass as close as 1,000 miles and as far as 43,500 miles from the lunar surface. 6)
Figure 4: Highly elliptical, a near rectilinear halo orbit around the Moon takes advantage of a precise balance point in the gravities of Earth and the Moon and creates a stability that is ideal for long-term missions like Gateway (image credit: Advanced Space)
The CAPS navigation software will be hosted on a separate flight computer from the spacecraft’s primary redundant flight computer boards and will operate distinctly from one another. Being hosted on a separate board allows for the CAPS software to experience updates and modifications throughout the demonstration as observations are returned, without disrupting the operations of the primary flight computer(s).
CAPS (Cislunar Autonomous Positioning System): CAPS is a unique innovation that operationalizes, and leverages investments made in algorithms, flight computers, and radios over the past decade. At its foundation, CAPS starts with the algorithms and logic of automated navigation layered on top of an innovative approach to absolute orbit determination that requires only relative radiometric ranging and Doppler measurements. In its most streamlined implementation, CAPS will be a software innovation that can be incorporated on any future spacecraft.
SBIR Development: From 2017 to 2020 the CAPS development was supported via NASA SBIR contract through Goddard Space Flight Center. In this time the software was developed and tested in a lab environment that readied it for further integration and ultimately flight testing. With the Phase II concluded in mid-2020, the CAPS research and development was funded for continuation through a Phase II-e and Phase III SBIR awards. The intent for these awards is the ongoing development and support of the software as it approaches demonstration on the CAPSTONE mission. Part of these funding extensions is also to expand the data types ingestible by CAPS, thereby widening its navigation capabilities in the cislunar environment.
Summary: As the Moon becomes the focus of more entities – whether governmental, commercial, or international – it is becoming imperative that there is a stronger understanding of the nuances and difficulties faced by future spacecraft as they enter this new environment. Not only is CAPSTONE providing rapid feedback and experience on operational challenges soon to be faced by the future Lunar Gateway and other Artemis Program missions, but it is doing so in a way that is expedited via commercial strengths in schedule and cost management. It is the hope and goal of the Advanced Space team that this technology demonstration will not only support Artemis operations and CAPS maturation but will also model a new way of commercial partnerships for NASA and other agencies to pursue in support of their directives.
1) Thomas Gardner, Brad Cheetham, Alec Forsman, Cameron Meek, Ethan Kayser, Jeff Parker, Michael Thompson , Tristan Latchu, Rebecca Rogers, Brennan Bryant, Tomas Svitek, ”CAPSTONE: A CubeSat Pathfinder for the Lunar Gateway Ecosystem,” 35th Annual Small Satellite Conference, August 7-12, 2021, paper: SSC21-II-06 , https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=5016&context=smallsat
2) Diane C. Davis,Fouad S. Khoury, Kathleen C. Howell, Daniel J. Sweeney , “Phase Control and Eclipse Avoidance in Near Rectilinear Halo Orbits,” AAS Guidance Navigation and Control Conference, Breckenridge, CO, 2019. AAS 20-047, URL: https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Conferences/2020_AAS_DavKhoHowSwe.pdf
3) Nathan L. Parrish, Matthew J. Bolliger, Ethan Kayser, Michael R. Thompson, Jeffrey S. Parker, Bradley W. Cheetham, Diane C. Davis and Daniel J. Sweeney”Near Rectilinear Halo Orbit Determination with Simulated DSN Observations,” AIAA Scitech 2020 Forum, 6-10 January 2020, Orlando, FL, https://doi.org/10.2514/6.2020-1700
4) Stephen Clark, ”Launch of lunar CubeSat moved from Virginia to New Zealand,” Spaceflight Now, 16 August 2021, URL: https://spaceflightnow.com/2021/08/16/
”NASA Awards Contract to Launch CubeSat to Moon from
Virginia,” NASA Contract Release C20-005, 14 February 2020, URL: https://www.nasa.gov/press-release/
6) ”NASAfunds CubeSat Pathfinder mission to unique lunar orbit,” LPI (Lunar and Planetary Institute), 2019, URL: https://www.lpi.usra.edu/publications/newsletters/
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).