Capstone’s journey starts with a launch on a Rocket Lab Electron rocket from the company’s Launch Complex 1 on Mahia Peninsula in New Zealand. The Electron rocket will place Capstone into an elliptical transfer orbit with a low point, or perigee, of approximately 500 km and a high point, or apogee, of over 35,000 km after separating from the rocket’s second stage.
From this initial transfer orbit, Capstone will use its onboard electric propulsion system to gradually increase its orbit over several months. The spacecraft is equipped with a Hall effect thruster powered by kW-class solar electric propulsion. Hall thrusters accelerate ions using electric and magnetic fields to produce thrust efficiently over long periods of time with minimal propellant requirements. This propulsion method allows Capstone to slowly spiral its orbit outward through low-thrust maneuvers without needing chemical propellant burns common to traditional chemical rockets.
Once separated from the rocket, Capstone’s solar panels will deploy and begin recharging its onboard batteries to power the electric thruster. Over the course of several months, the spacecraft will make a series of short thruster burns to raise the low point of its orbit each revolution. During the first few weeks, the thruster will fire as needed to circularize the transfer orbit to approximately 1,000 km altitude. From this vantage point, mission controllers will check out the spacecraft and electric propulsion system in detail.
With the checkouts complete, a series of about 140 thruster burns over the next 3-4 months will systematically raise Capstone’s apogee to match the target lunar orbit altitude. The duration of each individual burn ranges from a few minutes to a couple hours with breaks in between as the spacecraft travels around the Earth. The increasing apogee altitude efficiently increases the overall orbital energy through these low-thrust maneuvers without requiring a high output chemical engine. By late 2022, the final apogee raise maneuvers will achieve the target altitude of over 54,000 km to complete the Earth orbital phase.
At the point when Capstone’s elliptical orbit passes through the location of the Moon’s orbit once per revolution, known as the orbital resonance point, the electric thruster will fire to perform the lunar orbit insertion burn. This multi-hour burn executed near the Moon’s location will change the orbit plane and reduce velocity just enough for lunar gravity to capture the spacecraft. After orbital insertion, Capstone will be in an elliptical lunar orbit approximately 500 km by 80,000 km, similar to the target rectilinear halo orbit but with higher perigee and apogee distances.
Over the following month, frequent but short electric thruster burns will fine tune the orbit, systematically decreasing both perigee and apogee altitudes to precisely match the target near rectilinear halo orbit parameters. The complex 6-dimensional orbital elements of inclination, right ascension of the ascending node, argument of perigee, mean anomaly, semimajor axis, and eccentricity must all be adjusted in tandem through coordinated thruster firings. Telemetry from Capstone will be closely monitored during orbit adjustment to precisely hit the desired orbital parameters.
When complete, Capstone will be in a halo orbit around the Earth-Moon L1 Lagrange point with a nominal altitude of just 10 km from the target orbit. At this point in late 2022, the technology demonstration mission objectives will be considered achieved with the spacecraft positioned in its optimum vantage point to characterize the dynamics and environment of this unique orbit. Capstone will then begin on-orbit operations to gather data for at least 6 months to validate the viability and performance of smallsat operations in cislunar space.
This ambitious but efficient trajectory allows a small spacecraft like Capstone to reach the first stable halo orbit around the Moon’s nearest Lagrange point using nothing but sunlight and low-thrust electric propulsion. The step-by-step process of raising unique transfer and intermediate orbits systematically injects just the right amount of orbital energy to place the probe into its destination six months after launch. The trajectory was optimized through extensive mission design and modeling to fulfill the technology demonstration goals while minimizing propellant mass and launch vehicle capability requirements. If successful, Capstone will pave the way for extended missions in cislunar space using similar propulsion strategies.