Yuri Shimane, Georgia Institute of Technology; Kento Tomita, Georgia Institute of Technology; Koki Ho, Georgia Institute of Technology
Keywords: Low Energy Transfers, Cislunar SDA, Astrodynamics
Abstract:
Low-energy transfers (LETs), also called ballistic lunar transfers (BLTs), are a class of translunar trajectories that leverage the third-body effect of the Sun to its advantage to reduce the arrival Delta-V cost in cislunar space. In exchange, LETs require longer times of flight typically ranging around 3 months. This makes LETs unsuitable for crewed campaigns, but they are still attractive for robotic lunar missions. While the history of spacecraft flying along LET goes back to JAXA’s Hiten mission in 1991 and NASA’s GRAIL mission in 2011, their use in frequent years between flown and planned missions has dramatically increased; in the past year alone, NASA’s CAPSTONE mission, KARI’s Danuri mission, and ispace’s M1 mission all utilized LETs. Among the multiple NASA Commercial Lunar Payload Service (CLPS) missions planned over the next few years together with the roll-out of more Artemis missions, the traffic along LETs to cislunar space is expected to grow rapidly.
With the increase in cislunar activities by both commercial and governmental players, interest in cislunar space domain awareness (SDA) has grown rapidly as well; several studies have considered both monitoring vast cislunar regions, as well as tracking specific translunar and cislunar trajectories of interest. For example, these include regions near the Earth-Moon L1 and L2 points, as well as the near-rectilinear halo orbit (NRHO), which is the planned hosting orbit for the Lunar Gateway.
In contrast, the scenario for monitoring spacecraft on LETs have yet to be studied in detail. Crucially, compared to other translunar trajectories such as direct transfers and low-thrust transfers, LETs spend the majority of their time at high apogees typically extending to 1.5 million km, well beyond the Moon’s orbit. This would make typical cislunar SDA architectures tasked with observing regions up to around Earth-Moon L2 incapable of providing meaningful coverage over the entire transfer. If the LET is heading to a libration point orbit (LPO), it may still be possible to observe them through these architectures upon the asymptotic approach phase along the stable manifold. On the other hand, if the LET is heading to a low lunar orbit (LLO), the transit through the vicinity of Earth-Moon L2 occurs through a wide variety of geometries, hence requiring special considerations for monitoring.
Through this work, we propose an analysis for monitoring spacecraft flying along LETs to LLOs as they approach the Moon’s vicinity. We leverage a database of LETs generated in the bicircular restricted four-body problem (BCR4BP) and prune the trajectories based on mission operations criteria to identify a subset of LETs that are most likely to be flown frequently. Then, leveraging the periodic nature of LET launch windows, this work aims to identify time-dependent strategic regions from which SDA activities could be conducted.
There are two periodicities associated with launch and arrival time windows with LETs. At the longer time scale, there is a monthly periodicity for any LET in the BCR4BP, as the Earth, Moon, and Sun come back to the exact same orientation every synodic period. Note that when transitioned to the full-ephemeris model, this relationship becomes only quasi-periodic, as effects of the eccentricities of the Earth-Moon system as well as the inclination difference of the Earth-Moon and Sun-Earth orbital planets, among other minor discrepancies, start to kick in. The second periodicity occurs at roughly 14.5 days or roughly half of the synodic period. This is because the Sun’s gravity gradient may be leveraged on both the Sun-Earth L1 or Sun-Earth L2 side to raise the perigee of the spacecraft to the Moon’s orbit, thus achieving the “low-energy” effect. These inherent periodicities translate to observation opportunities for LETs and their associated monitoring regions also being periodic.
The regions of interest for monitoring LETs depend not only on the LETs themselves but also on the application of the monitoring activity as well as the limitation that arises from where an observing spacecraft or constellation may be placed. Examples of activity may be the detection of a transiting, non-cooperative target, or the orbit determination of a cooperative target. Depending on the application, there is a trade-off between the volume which must be covered and the quality of the observation that may be conducted. In addition to these considerations, we must keep in mind that the location of the observer is not unrestricted; just as we cannot place a “geostationary” satellite above non-equator regions of the Earth, the dynamics inhibit us from placing an observing satellite in any arbitrary location. Rather, the satellite must lie on a periodic orbit, either about the Moon or the Earth-Moon libration points. Keeping these in mind, key ideas relating to defining the regions of interest are the Earth-Moon L2 neck region, the time-dependent illumination conditions, and possible observer locations. Taking these into consideration, we expect to be able to define a time-dependent set of target observation directions for monitoring LETs.
Date of Conference: September 19-22, 2023
Track: Cislunar SDA