Basic targeting strategies for rendezvous and flyby missions to the near-Earth asteroids
Introduction
In more than 30 years of planetary exploration, the experience gained in carrying out deep space missions has allowed to lower considerably their cost and their complexity. The success of the NASA Discovery program (e.g. Kicza and Vorder Bruegge, 1995) has pushed both international and national programs to endorse low-cost interplanetary mission studies of a scientific as well as of a technological character. The ESA SMART mission concept and the ambitious Japanese planetary exploration program witness the advances obtained so far.
Among the possible targets, the Near Earth Asteroids (NEAs), whose dynamical characteristics allow close approaches with our planet, are gaining an increasing importance in many respects: science, technology, and the commercial exploitation of space. These celestial bodies are scientifically relevant as dynamically and physically evolved primitive bodies of the solar system, technologically challenging for their possible future exploitation as extraterrestrial resources, while the recent issues devoted to protect our planet from cosmic impacts has brought them inside the broader topic of risk hazard assessment. Furthermore, from the point of view of mission analysis, their periodic proximity to our planet and the possibility of remaining well within the inner planetary region with the consequent advantages on thermal and electrical power requirements, allows to consider them as favourable targets for both, rendezvous and flyby missions. The NEAR mission (Farquhar, 1995), presently orbiting around 433 Eros, shows the actual feasibility of a highly sophisticated interplanetary mission with a first-class scientific target, at a reasonably low cost and spacecraft and operation complexity. On the other hand, the recently proposed NEAP mission by a private space enterprise (Benson, 1998), aimed to land on 4660 Nereus, shows that Near-Earth Asteroid are possibly to become the first targets for commercially available deep space missions.
When selecting the target for a space mission, one tries to maximize the ratio between its scientific return and its cost — the last parameter being, in the last decade, an increasingly important factor. Applying this simple criterion to NEAs, it appears that, with the notable exception of Eros — mainly due to its large dimensions and already visited by the aforementioned NEAR mission — the choice is rather open. Usually, the scientific community indicates a number of candidates solely on the basis of their scientific relevance, which must then be examined by the technical counterpart in order to check the feasibility of a mission, eventually undergoing cost estimates. It often happens that not only the “best choices” from a scientific point of view, but also the “intermediate ones” do not match safe engineering and management plans.
Some of the most frequent constraints influencing the target selection can be briefly summarized as: (a) rendezvous missions are obviously preferred allowing close and extended observations and measurements, but in general they are rather demanding in terms of energy requirements; (b) in spite of their periodic proximity to our planet most NEAs move on highly eccentric and/or inclined orbits, a fact that has nontrivial dynamical implications; (c) advanced propulsion systems, such as low-thrust electric propulsion, increase the overall energy budget of a mission but need longer periods of time to be fully exploited; (d) the spacecraft mass, the launch scenario and its timing, needed for Earth phasing, represent crucial parameters but are often defined rather late within the mission study.
In what follows, we have tried to give a comprehensive approach to the problem in order to provide a method which takes into consideration both, the scientific and the technical constraints. The possibility of defining quickly the accessibility of an object might prove especially useful when treating NEAs targeting, since only 20% of the population is presently known and with the recent operation of wide-field, high-sensitivity telescopes, the number of new discoveries is growing steadily with time.
After characterizing from a dynamical point of view the NEAs population and describing some basic flight dynamics tools, rendezvous missions are introduced. Through a comparison between the scientific relevance of each possible target and the corresponding estimate of the energy needed for a “best case” mission scenario, a subset of candidates is identified. These results are compared with the preferences expressed by the scientific community and the mission profiles developed for the ESA SMART-1 Asteroid Rendezvous Option (Barucci et al., 1998); although not selected (the mission is now aimed to the Moon) this study proved to be extremely useful in addressing the general topic of NEAs target selection. In fact, several high-priority targets for science, in particular the high- inclination ones, appeared definitely out of reach at the present technological level when considering basic rendezvous missions (i.e. no gravity-assisted trajectories are foreseen).
In order to increase the superposition among scientifically appealing targets and realistic mission profiles, flyby trajectories have been investigated too. In particular, the possibility of increasing the scientific return when using nodal resonant-flyby strategies is proposed. Finally, the results are discussed within the framework of launch and propulsion system scenarios.
Section snippets
The NEAs population
NEAs are generally believed to be dynamically evolved fragments of main-belt asteroids entering the inner solar system on chaotic orbits. Orbital resonances represent the leading mechanism for delivering matter from the main belt: thus most NEAs share the orbital paths of meteorites and their final fates, either colliding with a terrestrial planet, being ejected from the solar system on hyperbolic orbits, or melting into the Sun (Farinella et al., 1994). Yet a significant fraction of NEAs — up
Transfer trajectories
The problem of finding the trajectory in space allowing a spacecraft to reach a given target can be solved in many different ways, depending on the level of approximation needed. Sophisticated optimal transfer algorithms, modelling not only the gravitational environment but also the navigation and propulsion systems on board the spacecraft, are used for operational purposes. In general, being based on minimization procedures, they are not easy to use and the solutions found may still be
Rendezvous missions
The possibility of inserting a spacecraft in orbit around a small body has obvious advantages for science: close observations extended in time would allow detailed investigation on the composition and the morphology of its surface, on the shape of the object and on its rotational properties. Through the analysis of the spacecraft trajectory it is also possible to measure its mass and obtain indications on the internal structure. Furthermore, a rendezvous mission profile represents the first
Nodal flyby missions
Apart from representing an intermediate scenario, the main motivation for performing an asteroid flyby instead of a rendezvous is that the target requires a much too high ΔV, which in the case of NEAs, as shown in Fig. 8, Fig. 9, is mainly due to the high inclination of the orbit or, to a minor extent, when a high eccentricity causes a rather distant aphelion to occur.
If we constrain ourselves to simple fly-by mission profiles, NEAs are intrinsically accessible from Earth: their orbital
Resonant flyby missions
Although a nodal flyby mission may offer the only chance for encountering a given NEA, it has the disadvantage of lasting only a very short time if compared to the overall mission duration. Moreover, the encounter geometry and the relative velocity between the spacecraft and the target may not allow accurate observations, which are essential in order to support the funding of an interplanetary mission. As a matter of fact, main-belt and near-earth asteroid flybys are considered at present only
Launch scenario
An important parameter influencing the feasibility of a mission is represented by the launch scenario, and in particular by the performances of the selected rocket, by its ability of supporting an upper stage, and by the existence and the technical characteristics of on-board propulsion systems. Whether the launcher is going to leave the spacecraft still within the gravitational field of our planet — thus needing an additional energy source to escape into interplanetary space — or deliver it
Discussion and conclusions
The attempt of finding an overall accessibility criterion for Near-Earth Asteroids always requires some level of approximation in order to reduce the number of free parameters (the orbital characteristics of the target, Earth phasing, the total energy budget, the different mission profiles, the launch scenario, etc.). As an example, Lau and Hulkower (1987) introduced a measure of accessibility as the global minimum total ΔV for a two-impulse rendezvous mission profile (Hulkower et al., 1984).
Acknowledgments
The authors would like to thank Paolo Farinella and Richard P. Binzel for the many suggestions during the SMART-1 NEAs selection phase, and Elisabetta Dotto for her useful comments on the early developments of this paper. The work of E.P. at the Observatoire de Paris is supported by the “Giuseppe Colombo Research Fellowship” of the European Space Agency.
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