arXiv subjects

Roberto Flores

Publications and source records attributed to Roberto Flores.

16 recordsLinked to original sources

arXiv2026

Double Gravity-Assist Rendezvous Trajectory to Halley's Comet Using Deep-Space Low Thrust

The perihelion of comet 1P/Halley in 2061 is an excellent chance to revisit this object of outstanding scientific and cultural relevance. During its 1986 approach to the Sun, it was targeted by several flyby missions. Due to its retrograde, highly-inclined orbit, the relative velocities during the encounter were large, limiting the scientific return due to the short time spent inside the coma. A rendezvous trajectory would overcome this limitation, but the design is challenging due to the limitations of current propulsion technology. Given the lead times of spacecraft development and the long duration of the interplanetary transfer to the comet, it is imperative to start mission planning as soon as possible. We present a novel rendezvous strategy, combining unpowered Jupiter and Saturn gravity-assists with deep-space low-thrust arcs. It minimizes launch energy and propellant budget, constraining the arrival to occur before the onset of high activity. The double flyby strategy reduces the otherwise prohibitive cost of the plane change maneuver. Closed expressions for the optimal flyby geometry, together with an explicit low-thrust transcription technique, reduce the number of design parameter to three, improving computational efficiency. Crucially, this is the first rendezvous mission concept achievable with well-proven technology (standard radioisotope thermoelectric generators and a Hall-effect thruster) and compatible with several existing launchers. We describe the trajectory optimization strategy and perform a comprehensive exploration of the design space. Finally, we present two promising proof-of-concept trajectories in detail.

astro-ph.EP↗

arXiv2026

Massive Star Population in the Sextans A Dwarf Galaxy from HST UV Photometry

We build a catalog of massive (M>$8~$M$_\odot$) main sequence stars in the \mbox{metal-poor} ($\sim0.1~$Z$_\odot$) dwarf irregular galaxy Sextans A. HST WFC3 UV photometry in the 275 and 336 nm wideband filters is arranged in a Color-Magnitude Diagram (CMD), and overlaid on top of stellar evolutionary tracks from the MIST library. The star properties (mass, age, etc.) are computed with a Finite Element (FE) interpolation of the stellar tracks. The FE method, originally developed for solid mechanics problems, provides a general framework for interpolating fields inside domains of complex geometry. Besides the interpolated properties, the algorithm computes their gradients with respect to the photometry. These sensitivities provide a direct an efficient estimate of the associated uncertainties. Our catalog contains 655 stars, with the most massive one estimated at $58\pm11~$M$_\odot$. A comparison with a ground-based spectroscopic census of OB stars yields only 8 matches, evidencing the minimal overlap between both datasets. The mass estimates derived from the UV CMD and the spectral classification are in good agreement for the majority of O-type stars found in both datasets. Our catalog provides an extensive list of candidates for followup spectroscopic observation, which could improve our understanding of the early evolutionary stages of massive \mbox{low-metallicity} stars.

astro-ph.GA↗

arXiv2025

Uncontrolled geostationary satellites: mapping periodic transitions to chaos with Lagrangian Descriptors

Uncontrolled geostationary satellites abandoned near an unstable equilibrium point of the equator experience irregular transitions between dynamical states (continuous circulation, long and short libration). They are caused by the interaction between the longitudinal dynamics, governed by the tesseral harmonics of the geopotential, and the orbital precession forced by Earth's oblateness and lunisolar perturbations. The transitions are extremely sensitive to small perturbations, making the long-term evolution unpredictable. Recently, a Monte Carlo analysis of trajectories starting in the immediate vicinity of the 165 degrees E unstable equilibrium point, revealed that the evolution to chaos is not gradual. It occurs via sudden episodes of disorder at specific points of the precession cycle, when the orbital inclination is minimal. Due to the high cost of the statistical analysis, the results were limited to a single initial longitude. This paper applies modified versions of the diameter Lagrangian descriptor to reduce the computational burden. This enables mapping the dynamical behavior over the complete range of longitudes where transitions between modes of motion are possible, considering both unstable equilibrium points (165 degrees E and 15 degrees W). It is found that the episodes of chaos remain linked to the orbital inclination cycle, but their timing depends on the initial spacecraft longitude. As the initial position moves farther away from the unstable point, the transitions take place at higher values of the orbital inclination. The longitudes where the transitions occur at maximum inclination correspond to the boundaries of the chaotic region.

nlin.CD↗

arXiv2025

Design of a low-thrust gravity-assisted rendezvous trajectory to Halley's comet

Comets are the most pristine planetesimals left from the formation of the Solar System. They carry unique information on the materials and the physical processes which led to the presence of planets and moons. Many important questions about cometary physics, such as origin, constituents and mechanism of cometary activity, remain unanswered. The next perihelion of comet 1P/Halley, in 2061, is an excellent opportunity to revisit this object of outstanding scientific and cultural relevance. In 1986, during its latest approach to the Sun, several flyby targeted Halley's comet to observe its nucleus and shed light on its properties, origin, and evolution. However, due to its retrograde orbit and high ecliptic inclination, the quality of data was limited by the large relative velocity and short time spent by the spacecraft inside the coma of the comet. A rendezvous mission like ESA/Rosetta would overcome such limitations, but the trajectory design is extremely challenging due to the shortcomings of current propulsion technology. Given the considerable lead times of spacecraft development and the long duration of the interplanetary transfer required to reach the comet, it is imperative to start mission planning several decades in advance. This study presents a low-thrust rendezvous strategy to reach the comet before the phase of intense activity during the close approach to the Sun. The trajectory design combines a gravity-assist maneuver with electric propulsion arcs to maximize scientific payload mass while constraining transfer duration. A propulsive plane change maneuver would be prohibitive. To keep the propellant budget within reasonable limits, most of the plane change maneuver is achieved via either a Jupiter or a Saturn flyby. The interplanetary low-thrust gravity-assisted trajectory design strategy is described, followed by the presentation of multiple proof-of-concept solutions.

astro-ph.EP↗

arXiv2025

Preparing for the 2061 return of Halley's comet -- A rendezvous mission with an innovative imaging system

The return of Comet 1P/Halley will promote a wide interest for ground and space observations of a celestial body of outstanding scientific and cultural interest. In addition to remote observations, space will open the possibility of in situ science similarly to the passage of 1986. In this paper, we first discuss the scientific motivations for a rendezvous mission, capable to overcome the limitations of the flyby missions that took place at that time. In the second part, we describe an example of a rendezvous trajectory that can be carried out with existing power and propulsion technologies. The transfer is made possible by the gravitational assistance of a giant planet. The resulting mission will be capable to reach the comet beyond the distance of Saturn, when the sublimation of super-volatile species will be ongoing, and well before the onset of the sublimation of water (4 AU). After rendezvous, the spacecraft will accompany the comet for several years before, around and after perihelion (July 2061). Our concept mission does not foresee the implementation of solar panels. In this way, operations can occur even inside the dense dust coma at short distance from the nucleus. In the third part of the paper, an innovative imaging system is proposed, with a very large field of view (100{\deg}) capable to record on the same frame details on the surface and the surrounding space, in order to follow for several degrees the trajectories of chunks and clouds ejected by pits or fractures, crucial to the understanding of the cometary activity. A concerted effort is needed in the current decade to plan and approve a rendezvous mission to 1P. Indeed, the scenario here described requires launching before 2040, less than 15 years from now. Later launches imply a severe loss of scientific knowledge, because the spacecraft will not be able to reach the comet before the onset of water sublimation.

astro-ph.EP↗

arXiv2024

Direct low-energy trajectories to Near-Earth Objects

Near-Earth Objects (NEOs) are asteroids, comets and meteoroids in heliocentric orbits with perihelion below 1.3 au. Similarly to the population of the Main Asteroid Belt, NEOs are primordial bodies and their study can improve our understanding of the origins of the Solar System. With a catalog of over 30~000 known asteroids and approximately 100 listed short-period comets, the NEO population represents an inventory of exploration targets reachable with significantly lower cost than the objects of the Main Asteroid Belt. In addition, the materials present in these bodies could be used to resupply spacecraft en route to other destinations. The trajectories of past missions to NEOs have been designed with the patched-conics technique supplemented by impulsive and/or low-thrust maneuvers and planetary gravity assist. The transfer times range from some months to a few years, and the close-approach speeds relative to the target have been as high as 10 km/s. The design technique described in this work leverages the invariant structures of the circular restricted three-body problem (CR3BP) to connect the vicinity of the Earth with NEOs in low-eccentricity, low-inclination orbits. The fundamental building blocks are periodic orbits around the collinear points L$_1$ and L$_2$ of the Sun-Earth CR3BP. These orbits are used to generate paths that follow the associated hyperbolic invariant manifolds, exit the sphere of influence of the Earth and reach NEOs on nearby orbits. The strategy is simple, can be applied to depart either a libration point orbit or the vicinity of the Earth, and offers attractive performance features.

astro-ph.EP↗

arXiv2024

Orbital perturbation coupling of primary oblateness and solar radiation pressure

Solar radiation pressure can have a substantial long-term effect on the orbits of high area-to-mass ratio spacecraft, such as solar sails. We present a study of the coupling between radiation pressure and the gravitational perturbation due to polar flattening. Removing the short-period terms via perturbation theory yields a time-dependent two-degree-of-freedom Hamiltonian, depending on one physical and one dynamical parameter. While the reduced model is non-integrable in general, assuming coplanar orbits (i.e., both Spacecraft and Sun on the equator) results in an integrable invariant manifold. We discuss the qualitative features of the coplanar dynamics, and find three regions of the parameters space characterized by different regimes of the reduced flow. For each regime, we identify the fixed points and their character. The fixed points represent frozen orbits, configurations for which the long-term perturbations cancel out to the order of the theory. They are advantageous from the point of view of station keeping, allowing the orbit to be maintained with minimal propellant consumption. We complement existing studies of the coplanar dynamics with a more rigorous treatment, deriving the generating function of the canonical transformation that underpins the use of averaged equations. Furthermore, we obtain an analytical expression for the bifurcation lines that separate the regions with different qualitative flow.

astro-ph.EP↗

arXiv2024

Computing Classical Orbital Elements with Improved Efficiency and Accuracy

This paper reviews the standard algorithm for converting spacecraft state vectors to Keplerian orbital elements with a focus on its computer implementation. It analyzes the shortcomings of the scheme as described in the literature, and proposes changes to the implementation to address orbits of arbitrary eccentricity and inclination in a simple and robust way. It presents two coding strategies that simplify the program structure while improving the accuracy and speed of the transformation on modern computer architectures. Comprehensive numerical benchmarks demonstrate accuracy improvements by two orders of magnitude, together with a 40% reduction of computational cost relative to the standard implementation.

physics.comp-ph↗

arXiv2023

A new automated strategy for optimizing inclined interplanetary low-thrust trajectories

This study proposes a new automated strategy for designing and optimizing three-dimensional interplanetary low-thrust (LT) trajectories. The method formulates the design as a hybrid optimal control problem and solves it using a two-step approach. In Step 1, a three-dimensional model based on generalized logarithmic spirals is used with heuristics in combination with a gradient-based solver to perform an automated multi-objective global search of trajectories and optimize for parameters defining the spirals, the launch date, as well as the number, sequence and configuration of the planetary flybys. In Step 2, candidate solutions from Step 1 are refined by further optimization with a direct method. Results show that, compared to similar algorithms based on two-dimensional models, the strategy implemented in Step 1 leads to better estimates of the optimal trajectories, especially when the orbits of the involved bodies are inclined with respect to the ecliptic plane. The proposed approximate method (Step 1) yields better agreement with high-fidelity solutions (Step 2) in terms of launch, flyby and arrival dates, in-plane and out-of-plane average LT accelerations and propellant consumption, leading to improved convergence when the Step 1 trajectories are employed to initiate the search in Step 2.

math.OC↗

arXiv2023

End-to-end trajectory concept for close exploration of Saturn's Inner Large Moons

We present a trajectory concept for a small mission to the four inner large satellites of Saturn. Leveraging the high efficiency of electric propulsion, the concept enables orbit insertion around each of the moons, for arbitrarily long close observation periods. The mission starts with a EVVES interplanetary segment, where a combination of multiple gravity assists and deep space low thrust enables reduced relative arrival velocity at Saturn, followed by an unpowered capture via a sequence of resonant flybys with Titan. The transfers between moons use a low-thrust control law that connects unstable and stable branches of the invariant manifolds of planar Lyapunov orbits from the circular restricted three-body problem of each moon and Saturn. The exploration of the moons relies on homoclinic and heteroclinic connections of the Lyapunov orbits around the L$_1$ and L$_2$ equilibrium points. These science orbits can be extended for arbitrary lengths of time with negligible propellant usage. The strategy enables a comprehensive scientific exploration of the inner large moons, located deep inside the gravitational well of Saturn, which is unfeasible with conventional impulsive maneuvers due to excessive fuel consumption.

astro-ph.EP↗

arXiv2023

Higher-order composition of short- and long-period effects for improving analytical ephemeris computation

The construction of an analytic orbit theory that takes into account the main effects of the Geopotential is notably simplified when splitting the removal of periodic effects in several stages. Conversely, this splitting of the analytical solution into several transformations reduces the evaluation efficiency for dense ephemeris output. However, the advantage is twofold when the different parts of the mean-to-osculating transformation are composed into a single transformation. To show that, Brouwer's solution is extended to the second order of the zonal harmonic of the second degree by the sequential elimination of short- and long-period terms. Then, the generating functions of the different transformations are composed into a single one, from which a single mean-to-osculating transformation is derived. The new, unique transformation notably speeds up the evaluation process, commonly improving evaluation efficiency by at least one third with respect to the customary decomposition of the analytical solution into three different parts.

physics.class-ph↗

arXiv2023

Statistical Study of Uncontrolled Geostationary Satellites Near an Unstable Equilibrium Point

The growth of the population of space debris in the geostationary ring and the resulting threat to active satellites require insight into the dynamics of uncontrolled objects in the region. A Monte Carlo simulation analyzed the sensitivity to initial conditions of the long-term evolution of geostationary spacecraft near an unstable point of the geopotential, where irregular behavior (e.g., transitions between long libration and continuous circulation) occurs. A statistical analysis unveiled sudden transitions from order to disorder, interspersed with intervals of smooth evolution. There is a periodicity of approximately half a century in the episodes of disorder, suggesting a connection with the precession of the orbital plane, due to Earth's oblateness and lunisolar perturbations. The third-degree harmonics of the geopotential also play a vital role. They introduce an asymmetry between the unstable equilibrium points, enabling the long libration mode. The unpredictability occurs just in a small fraction of the precession cycle, when the inclination is close to zero. A simplified model, including only gravity harmonics up to degree 3 and the Earth and Moon in circular coplanar orbits is capable of reproducing most features of the high-fidelity simulation.

astro-ph.EP↗

arXiv2021

A Method for Accurate and Efficient Propagation of Satellite Orbits: A Case Study for a Molniya Orbit

Fast and precise propagation of satellite orbits is required for mission design, orbit determination and payload data analysis. We present a method to improve the computational performance of numerical propagators and simultaneously maintain the accuracy level required by any particular application. This is achieved by determining the positional accuracy needed and the corresponding acceptable error in acceleration on the basis of the mission requirements, removing those perturbation forces whose effect is negligible compared to the accuracy requirement, implementing an efficient and precise algorithm for the harmonic synthesis of the geopotential gradient (i.e., the gravitational acceleration) and adjusting the tolerance of the numerical propagator to achieve the prescribed accuracy level with minimum cost. In particular, to achieve the optimum balance between accuracy and computational performance, the number of geopotential spherical harmonics to retain is adjusted during the integration on the basis of the accuracy requirement. The contribution of high-order harmonics decays rapidly with altitude, so the minimum expansion degree meeting the target accuracy decreases with height. The optimum degree for each altitude is determined by making the truncation error of the harmonic synthesis equal to the admissible acceleration error. This paper presents a detailed description of the technique and test cases highlighting its accuracy and efficiency.

astro-ph.EP↗

arXiv2021

Long-Term Orbit Dynamics of Decommissioned Geostationary Satellites

In nominal mission scenarios, geostationary satellites perform end-of-life orbit maneuvers to reach suitable disposal orbits, where they do not interfere with operational satellites. This research investigates the long-term orbit evolution of decommissioned geostationary satellite under the assumption that the disposal maneuver does not occur and the orbit evolves with no control. The dynamical model accounts for all the relevant harmonics of the gravity field at the altitude of geostationary orbits, as well as solar radiation pressure and third-body perturbations caused by the Moon and the Sun. Orbit propagations are performed using two algorithms based on different equations of motion and numerical integration methods: (i) Gauss planetary equations for modified equinoctial elements with a Runge-Kutta numerical integration scheme based on 8-7th-order Dorman and Prince formulas; (ii) Cartesian state equations of motion in an Earth-fixed frame with a Runge-Kutta Fehlberg 7/8 integration scheme. The numerical results exhibit excellent agreement over integration times of decades. Some well-known phenomena emerge, such as the longitudinal drift due to the resonance between the orbital motion and Earth's rotation, attributable to the J22 term of the geopotential. In addition, the third-body perturbation due to Sun and Moon causes two major effects: (a) a precession of the orbital plane, and (b) complex longitudinal dynamics. This study proposes an analytical approach for the prediction of the precessional motion and shows its agreement with the orbit evolution obtained numerically. Moreover, long-term orbit propagations show that the above mentioned complex longitudinal dynamics persists over time scales of several decades. Frequent and unpredictable migrations toward different longitude regions occur, in contrast with the known effects due only to the J22 perturbation.

astro-ph.EP↗

arXiv2020

Accurate and Efficient Propagation of Satellite Orbits in the Terrestrial Gravity Field

Fast and precise propagation of satellite orbits is required for mission design, orbit determination in support of operations and payload data analysis. This demand must also comply with the different accuracy requirements set by a growing variety of scientific and service missions. This contribution proposes a method to improve the computational performance of orbit propagators through an efficient numerical integration that meets the accuracy requirements set by the specific application. This is achieved by appropriately tuning the parameters of the numerical propagator (relative tolerance and maximum time step), establishing a threshold for the perturbing accelerations (Earth's gravitational potential, atmospheric drag, solar radiation pressure, third-body perturbations, relativistic correction to gravity) below which they can be neglected without altering the quality of the results and implementing an efficient and precise algorithm for the harmonic synthesis of the geo-potential and its first-order gradient. In particular, when performing the harmonic synthesis, the number of spherical harmonics to retain (i.e., the expansion degree) is determined by the accuracy requirement. Given that higher-order harmonics decay rapidly with altitude, the expansion degree necessary to meet the target accuracy decreases with height. To improve the computational efficiency, the number of degrees to retain is determined dynamically while the trajectory is being computed. The optimum expansion degree for each altitude is determined by ensuring that the truncation error of the harmonic synthesis is below the threshold acceleration. The work is a generalization to arbitrary orbits of a previous study that focused on communication satellites in geosynchronous inclined orbits. The method is presented and a set of test cases is analysed and discussed.

astro-ph.EP↗

arXiv2020

Reduction of Saturn Orbit Insertion Impulse using Deep-Space Low Thrust

Orbit insertion at Saturn requires a large impulsive manoeuver due to the velocity difference between the spacecraft and the planet. This paper presents a strategy to reduce dramatically the hyperbolic excess speed at Saturn by means of deep-space electric propulsion. The interplanetary trajectory includes a gravity assist at Jupiter, combined with low-thrust maneuvers. The thrust arc from Earth to Jupiter lowers the launch energy requirement, while an ad hoc steering law applied after the Jupiter flyby reduces the hyperbolic excess speed upon arrival at Saturn. This lowers the orbit insertion impulse to the point where capture is possible even with a gravity assist with Titan. The control-law algorithm, the benefits to the mass budget and the main technological aspects are presented and discussed. The simple steering law is compared with a trajectory optimizer to evaluate the quality of the results and possibilities for improvement.

astro-ph.EP↗