1 Introduction

Asteroid exploration missions attract many scientists’ interest, because asteroids hold key clues to the understanding of the origin of our solar system and the formation of the planets. Near-Earth asteroids (NEAs) comprises a population sub-class composed of objects that move in orbits, which may present a significant hazard to human civilization. These objects, whose dynamic characteristics allow close approaches to our planet, are gaining an increasing importance in many respects: science, technology and low cost (e.g. lower velocity increments, smaller launch vehicle, etc.). 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.

To select a suitable target and analyze the accessibility of NEAs, some studies have been carried out in the past. Lau and Hulkower (1987) introduced a measure of accessibility, which is taken to be the global minimum total ΔV for a two-burn impulse rendezvous mission profile; and they presented a ranking of accessibility of NEAs. Perozzi, Rossi, & Valsecchi (2001) discussed the global characterization of the NEAs population on the basis of their dynamics, physical properties and flight dynamic considerations, investigated some basic targeting strategies for rendezvous and nodal and resonant fly missions to the NEAs, and presented a group of candidates, whose scientific motivations and some relevant orbital parameters were reported. A similar procedure was used by Binzel et al. (2004). They brought together an analysis of both Near-Earth Objects accessibility and preliminary assessments of their compositions. Using an H-plot analysis, they identified 234 currently known Near-Earth Objects that are accessible for rendezvous with a “best case” ΔVof less than 7.0 km/s. In order to assess the accessibility of celestial bodies, the Hohmann strategy was used. These studies proved to be extremely useful in addressing the general topic of NEAs target selection. However, it is worthwhile noting that, in previous literature, the classical Hohmann transfer strategy and Lambert problem were used to estimate the minimum energy for rendezvous missions. Some targets in orbits similar to that of the Earth are easier to reach by using these classical transfer strategies. Yet, some high priority targets for science, in particular the large semimajor or eccentric ones, appear definitely out of reach given the present technological level when considering basic rendezvous missions (i.e. no gravity-assisted trajectories are foreseen). These high priority targets for science are extremely likely to be abandoned by a mission designer.

In what follows, we have primarily aimed this study at two aspects. Firstly, from the point of view of selecting scientifically interesting targets, we summarize some of the frequent factors influencing target selection on the basis of their physical properties and orbital characteristics. When selecting the target for a space mission, these factors can be regarded as the scientific motivations. Secondly, when analyzing the accessibility of targets, instead of the classical transfer strategies in previous literature, we calculate transfer trajectories to objects using the planetary swingby techniques. There are several reasons that lead us to adopt this approach. The main reason is that planetary swingby can reduce the total velocity increments and launch energy effectively. After calculating the transfer trajectory, an estimate of the energy needed for a “best case” mission scenario is presented. Finally, through a comparison between the scientific relevance of each possible target and the corresponding estimate of energy needed for rendezvous missions, the ranking of candidates is identified.

2 Target selection

NEAs are generally believed to be dynamically evolved fragments of main belt asteroids entering the inner solar system on chaotic orbits. Dynamical calculations show that the life spans for NEAs are typically a few million years, eventually meeting their doom by crashing into the Sun, being ejected from the solar system, or impacting a terrestrial world (Morbidelli, Bottke, Froeschlé, & Michel, 2002). With such short lifetimes, the NEAs observed today cannot be residual bodies that have continued orbiting among the inner planets since the beginning of the solar system. Understanding the sources and mechanisms of their evolution is one of the fundamental scientific goals for NEAs studies. The orbital characteristic, peculiar physical properties, in particular the taxonomic and mineralogical characterization of NEAs, may be extremely useful information for pinpointing the source regions of NEAs and their dynamical evolution. So it is very possible that the NEAs with peculiar orbital and physical properties become high scientific priority targets for exploration missions. The scientifically significant target will play an important role in understanding the origin and formation of the solar system and the planets. Of course, it is impossible and unwise to sum up all scientifically significant targets on the basis of our present lack of knowledge concerning their physical nature. Therefore, we will sum up some frequent factors influencing scientifically significant target selection on the basis of their physical properties and orbital characteristics. When selecting the target for a space mission, these factors can be regarded as the scientific motivations.

2.1 Physical properties

When analyzing and summarizing the frequent factors that influence scientifically significant target selection, we focus on those properties that give the best indication of the origin and dynamical evolution. We particularly focus on taxonomy, the relationships of NEAs to comets and ordinary-chondrite meteorites and the peculiar properties of NEAs such as shapes, rotations, optical properties and so on.

Taxonomic classes are from the system defined by Tholen (1984) and extended to include the additional designations developed by Bus (1999) and Bus and Binzel (2002). Almost all the taxonomic classes of main-belt asteroids are represented among classified NEAs, including the P-types and D-types most commonly found in the outer asteroid belt, between the Hilda and Trojan asteroid, or possibly among comet nuclei (Barucci, Cruikshank, Mottola, & Lazzarin, 2002; Weissman, Bottke, & Levison, 2002). Unique taxonomic classifications and mineralogical interpretations do show evidence for specific ties to main-belt sources. In particular, E-type asteroids appear both compositionally and dynamically related to the Hungaria region (high-inclination objects) of the inner asteroid belt (Gaffey et al., 1992). The C-type bodies would represent a sample of the pristine material characterizing the outer asteroid main belt. From the point of view of hazard-assessment and resource utilization, perhaps the objects of most practical interest are the M-types that may be highly metallic in composition (Tedesco & Gradie, 1987). V-type asteroids are also interesting, because they are widely believed to be fragments of the basaltic (pyroxene rich) surface of the large main belt asteroid 4 Vesta and are thought to be possible parent bodies of the Howardite Eucrite Diogenite meteorites (Binzel & Xu, 1993; Cruikshank, Tholen, Hartmann, Bell, & Brown, 1991; Migliorini et al., 1997; Thomas et al., 1997). So unique taxonomic classifications and mineralogical interpretations are the significant factor influencing scientifically significant target selection.

From the point of view of dynamical evolution, NEAs are widely believed to be dynamically evolved fragments of main belt asteroids entering the inner solar system. Taxonomic and mineralogic characterizations of NEAs also provide confident links to main-belt origins. Yet a fraction of NEAs might be composed by extinct cometary nuclei (Gladman et al., 1999). For instance, the taxonomic classification for 3200 Phaethon and 4015 Wilson–Harrington appears consistent with primitive solar system materials presumed to dominate in comets. D-type asteroids such as 3552 Don Quixote and 1997 SE5 do add to the list of NEAs having taxonomic characteristics that make them extinct comet candidates (Hicks, Buratti, Newburn, & Rabinowitz, 2000). These asteroids are well-known examples of this kind. So these objects, which might be cometary candidates, fragments of the main belt asteroid, meteor parent bodies etc., are potentially high value targets for science.

Though the taxonomic classes of NEAs show close similarities to main belt asteroids and the corresponding meteorites, the physical properties of NEAs may differ from those of main belt asteroids and meteorites. According to radar observations and the study of photometric light curves, some NEAs show the occurrence of highly elongated shapes, binary system, fast rotators and so on. These peculiar properties are valuable for the studies of origin and evolution of NEAs. For example, studies of amplitude brightness variation indicate that elongated shapes may provide some suggestions, when combined with dynamical and compositional factors, for discerning NEAs as having a cometary origin. Similarly, Binzel, Xu, Bus, and Bowell (1992) found that slower rotations might also indicate cometary candidates. Rotation and shape can help us understand these problems, which we focus on, notwithstanding the fact that rotation and shape alone are not sufficient by themselves to conclusively reveal a cometary origin for an individual NEA. Therefore, some peculiar properties such as exotic rotation station states, binary system, elongated shape, low albedo and so on, will also be significant factors influencing scientific target selection.

2.2 Orbital characteristics

The orbital characteristics of objects directly influence their accessibility for rendezvous mission, especially the semimajor axis a, eccentricity e and inclination i. A larger semimajor axis or inclination will make a challenge for rendezvous mission. On the other hand, NEAs moving on highly eccentric or inclined orbits have nontrivial dynamical implications. For example, most Amor asteroids are generally believed to be dynamically evolved fragments of main belt asteroids and short period comets. The inclination poses severe constraint on mission profiles, especially as far as rendezvous missions are concerned, in that out of plane maneuvers are in general rather demanding in terms of energy changes. According to the available estimates, there should be about more than half of NEAs having i > 10°. A few objects may reach 50° or more. These larger inclination objects always attract many scientists’ attention because some of the larger inclination objects provide extremely useful information. For instance, among them, 1580 Betulia, whose inclination is 52.1°, D-class giant object 3552 Don Quixote asteroid, whose i is 30.8°, and 2,102 Tantalus (i = 64.0°) are well-known examples of this kind (Perozzi et al., 2001). Therefore, the orbital characteristics, which have nontrivial dynamic implications, are also an important factor, which impacts on target selection.

With the development and application of high sensitivity telescope techniques, the number of new asteroid discoveries increases dramatically with time. Scientific interest in the new individual targets is not very clear at present. So it is impossible to summarize all of the scientifically significant targets. On the other hand, a rather subjective parameter or finding a rigorous criterion to access the scientific relevance of an asteroid is also an elusive endeavor. Whereas, according to influencing factors mentioned above, the “scientific objects” can be easily updated and supplemented. In previous literature, Perozzi et al. (2001) presented the set of 60 scientifically significant targets. In this paper, we supplement 18 new candidates listed in Table 1. For each of them, the scientific motivations, and some relevant orbital and physical parameters are reported.

Table 1 New “scientific object” sample
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When selecting the target for a space mission, it usually happens that some objects are the “best candidates” from a scientific point of view, but they don’t satisfy the technical requirements. Therefore, when choosing a suitable target for exploration mission, the scientific and technical information about the target asteroid should be taken in account. In the next section, we will discuss the accessibility of NEAs.

3 Analyzing the accessibility of NEAs

We know the rendezvous missions would be in favor of investigating some object details such as composition, the morphology of its surface, shape, rotational properties, measuring its mass and obtaining indications of the internal structure and so on. The primary mission design consideration for a NEA rendezvous is usually the delta-V budget. This is a function of the velocity increment needed at the point of departure to insert the spacecraft into the transfer path and the change required to cancel the relative velocity between spacecraft and target at arrival. In previous literature, the direct transfer was used for analyzing the accessibility of NEAs. For example, the Hohmann transfer trajectories and the Lambert problem were used for giving a reference on the accessibility of NEAs (Binzel et al., 2004; Perozzi et al., 2001); the Gauss algorithm with boundary condition was used for calculating the transfer trajectory for NEAs (Christou, 2003). The global minimum total ΔVfor a two-impulse transfer was used to measure the accessibility (Helin, Hulkower, & Bender, 1984; Hulkower, Lau, & Bender, 1984; Lau et al., 1987). Asteroids in orbits similar to that of the Earth have better accessibility by using these classical transfer strategies. However, some high priority targets for science, in particular the large semimajor and eccentric ones, appear definitely out of reach at the present technological level when considering basic rendezvous missions. Therefore, in this paper, the planetary swingby techniques will be applied to generate transfer trajectories when measuring the accessibility of candidates. This approach can reduce the launch energy and total velocity increments and present a significant reference for the mission designer when selecting the maximized superposition between a scientifically significant and a technically feasible target for a space mission.

3.1 Transfer trajectory generation

The problem of finding the trajectory in space allowing a spacecraft to reach a given target can be solved in many different ways. For the purpose of this paper, namely to give a quick reference on the accessibility of NEAs, the approach based on Keplerian motion will suffice. In particular, the method of optimum two-impulsive transfer for preliminary interplanetary trajectory design (Hulkower et al., 1984) and the approach of Earth gravity-assist with deep-space maneuver (ΔV-EGA) (Farquhar, Dunham, & McAdams, 1995; Sim et al., 1997) will be used. ΔV-EGA refers to the use of a relatively small deep space maneuver to modify the excess hyperbolic velocity at a body. This maneuver, in conjunction with a gravity assist at the body, reduces the launch energy requirements and the total velocity increments for a mission.

3.1.1 Direct transfer

Asteroids in orbits similar to that of the Earth are easier to reach by using direct transfer profiles. So we adopt the optimum two-impulse transfer profiles. The method for numerically determining the optimum two-impulse transfer between two positions in two different heliocentric orbits was described (Hulkower et al., 1984). In this paper, it will not be reviewed. The optimum transfers obtained are “time-open” and the trajectories are computed using the patched-conic method. The minima are found by varying the semilatus rectum for each fixed pair of mean anomalies on the grid. The contours of minimum total ΔVare plotted on axes of mean anomaly of the launch body at launch and mean anomaly of the target body at arrival. In the case of the 1998 KY26 Asteroid, the entire space of optimum rendezvous trajectories from Earth to target is displayed in Fig. 1 (It assumes that the Earth parking orbit is circular and of 200 km altitude). Through global search algorithms, the exact value of the global minimum total ΔV is found. The flight path is shown in Fig. 2.

Fig. 1
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Contours of minimum total ΔV for a two-impulse transfer trajectory to rendezvous with the 1998 KY26 asteroid

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Fig. 2
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The flight trajectory to rendezvous with the 1998 KY26 asteroid

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Some of the “scientifically significant” targets in orbits similar to that of the Earth are calculated by using optimum two-impulse transfer profiles; and the trajectory parameters are displayed in Table 2 [The total ΔVfor two-impulse transfer consists of the launch from an Earth parking orbit (circular, 200 km altitude) ΔV L and the impulse for rendezvous ΔV a].

Table 2 Global minimum two-impulsive trajectory parameters for rendezvous with target asteroid
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3.1.2 Earth swingby transfer

Candidates moving on large eccentric or semimajor orbits require too much launch energy and total velocity increments for rendezvous mission by using the direct transfer strategies. In order to reduce further the dynamical requirements and avoid time-dependency, the gravity-assisted approach of Earth with deep-space maneuver (ΔV-EGA) will be used. First, it determines the two-impulse transfer trajectory from the Earth to asteroid. Then, through adjusting the mean anomaly of Earth at launch and matching the C 3, where C 3 defines the hyperbolic excess velocity squared \({V_\infty ^2}\) , it adds the ΔV-EGA to the beginning of the two-impulse transfer trajectory to reduce the required launch energy and the total ΔV for exploration missions. The flight path of the ΔV-EGA transfer profile is described by using the mean anomaly at launch M L, the mean anomaly at swingby M s, and the mean anomaly at rendezvous M a , is the “time-open” or “ephemeris-free” solution. It is suitable for evaluating the accessibility of NEAs (Qiao, Cui, & Cui, 2006). The detailed design characteristics will not be reviewed.

The frequency of feasible opportunities is also an important factor when considering an asteroid for interplanetary mission. In previous literature, some studies have been carried out (Lau & Hulkower, 1987; Perozzi et al., 2001); in particular, Lau and Hulkower (1987) listed the frequency of feasible opportunities and average time for rendezvous with an asteroid. Although these frequencies were for the 21-years period, they were representative of what could be expected for these asteroids in general. Here, it is worthwhile noting that the ΔV-EGA allows extension of the classical two-impulse transfer strategy. The frequency of feasible opportunities for the ΔV-EGA strategy is larger than that of the two-impulse strategy, because there are several ΔV-EGA trajectories (such as 2:1 (±) ΔV-EGA etc.) that would correspond to a two-impulse trajectory.

In what follows, taking the 1627 Ivar asteroid, whose aphelion and inclination is 2.602 AU and 8.4° respectively, as an example, we will provide the ΔV-EGA strategy for analyzing its accessibility and predict the feasible opportunities from 2006 to 2016 years.

First, the two-impulse transfer trajectory of the global minimum total ΔV for a rendezvous mission is found. It assumes that the Earth parking orbit is circular and of 200 km altitude. The contour of minimum total ΔV for two-impulse transfer is shown in Fig. 3. The exact value of the global minimum total ΔV is found by using the global search algorithms. The flight path is shown in Fig. 4.

Fig. 3
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Contours of minimum total ΔV for 1627 Ivar asteroid

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Fig. 4
figure 4

Flight path of two-impulse transfer for Ivar asteroid

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The mean anomaly of the Earth at launch for this two-impulse transfer trajectory can be obtained. It is regarded as the mean anomaly of the Earth at swingby for the ΔV-EGA transfer orbit. According to the characteristics of the Ivar asteroid orbit, we select the 2:1 type for ΔV-EGA transfer (Qiao et al., 2006; Sims, Longuski, & Staugler, 1997). Then, we search a mean anomaly of the Earth at launch for the ΔV-EGA transfer orbit and propagate the orbit to the aphelion. The deep-space maneuver can be performed at aphelion; and the maneuver enables the Earth to be used as a gravity-assisted body. After swingby, the spacecraft flies to the target asteroid. The 2:1(±) ΔV-EGA transfer trajectory for rendezvous with Ivar asteroid is shown in Fig. 5 and Fig. 6, respectively.

Fig. 5
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The 2:1(+) ΔV-EGA transfer trajectory for Ivar

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Fig. 6
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The 2:1(–) ΔV-EGA transfer trajectory for Ivar asteroid

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The trajectory parameters of optimum two-impulse profiles and 2:1(±) ΔV-EGA profiles for rendezvous with the 1627 Ivar asteroid are listed in Table 3.

Table 3 A comparison of the optimum two-impulse and the 2:1 ΔV-EGA trajectory parameters for the Ivar asteroid rendezvous mission
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In Table 3, the M L, M s and M a stand for Mean Anomaly of Earth at Launch, swingby and that of asteroid at arrival, respectively. For the swingby transfer, the total ΔV consists of the launch from an Earth parking orbit (circular, 200 km altitude) ΔV L, the deep-space maneuver ΔV m at aphelion and the impulse for rendezvous ΔV a. The post-launch velocity increments ΔV p is the sum of ΔV m and ΔV a. The C 3 and T are launch energy and time of flight, respectively. From Table 3, we can see that the total ΔV and launch energy C 3, compared with optimum two-impulse profiles, can be reduced 0.719 km/s and 24.237 km2/s2, respectively, by using 2:1(−) ΔV-EGA profile. In addition, we also see that one two-impulse trajectory allows extension of two ΔV-EGA trajectories. It also shows that the frequency of feasible opportunities for ΔV-EGA strategy will be more than that of a two-impulse strategy. According to Table 3, we can predict the feasible opportunities from 2006 to 2016 years. These feasible opportunities are listed in Table 4.

Table 4 The feasible opportunities for rendezvous with the Ivar asteroid from 2006 to 2016 years
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It should be noted that there is a difficulty in that the relative geometry of the celestial bodies and flight time needed is consistent with the ideal case because of the ephemeris constraints. So the parameters of these opportunities have changed slightly. Although these opportunities are not optimal, they have a good initial value for a trajectory optimizer.

4 Results

The ΔV-EGA transfer trajectory technique can reduce launch energy requirements and total velocity increments at the cost of a deep-space maneuver and increasing flight time. This approach is suitable for a target with large-eccentricity or a large-semimajor. By using these classical two-impulse transfer strategies, some asteroids in orbits similar to that of the Earth are usually the easiest to reach [e.g. 1998 KY26 (a = 1.232 AU, e = 0.201), 1999 SF10 (a = 1.278 AU, e = 0.253), etc.]. However, to the object with large-eccentricity or large-semimajor [e.g. 4015 Wilson–Harrington (a = 4.258 AU, e = 0.623), 4179 Toutatis (a = 4.122 AU, e = 0.635), etc.], it requires so much launch energy and total ΔV that it appears to be out of reach (i.e. no gravity-assisted or low-thrust trajectories are foreseen). So in this case, the gravity-assisted strategy should be considered. In Table 5, we show the comparison of accessibility of some targets between using the two-impulse and gravity-assisted strategy. (It assumes that the Earth parking orbit is circular and of 200 km altitude).

In Table 5, the Q and e stand for the aphelion distance and eccentricity of asteroid, respectively. The total ΔV and launch energy C 3 required for rendezvous with large-eccentricity asteroids are reduced obviously by using the ΔV-EGA transfer technique, especially for candidates such as 4015 Wilson–Harrington (compared with the optimal two-impulse profile, the total ΔV and launch energy C 3 can be reduced by 1.168 km/s and 41.489 km2/s2, respectively), 4179 Toutatis (compared with the optimal two-impulse transfer, the total ΔV and launch energy of the ΔV-EGA transfer decreased by 17.32% and 60.36%, respectively), 6489 Golevka (the ΔV-EGA transfer profile has 1.053 km/s and 37.46 km2/s2 decrease in total ΔV and launch energy, respectively), and so on.

Table 5 A comparison of the accessibility of some targets between using the classic two-impulse strategy and the gravity-assisted strategy
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It is worthwhile noting that the classical two-impulse strategy was used for some asteroids in orbits similar to that of the Earth, when evaluating the accessibility of these objects. There are several reasons which lead us to adopt this approach. Perhaps the two most important ones are that (a) they display a very low launch energy and total ΔV: they have already appeared reasonably accessible by using the two-impulse transfer strategy; (b) if the ΔV-EGA strategy is used, it should need to add a heliocentric orbit with a period slightly greater than an integer number of years and a perihelion radius equal to the heliocentric orbit radius of Earth to the beginning of the two-impulse transfer trajectory. The heliocentric orbit not only needs some launch energy C 3 (e.g. for the 2:1 ΔV-EGA, it usually requires 25 ∼ 26 km2/s2), but also adds a minor deep-space maneuver. So if the launch energy C 3 is less than 26 km2/s2 by using the classic two-impulse transfer, the ΔV-EGA strategy will not usually be considered.

5 Discussion and conclusions

When selecting the target for a space mission, besides potential scientific value, we need to take into account the technical feasibility of the mission. So the choice of a suitable target, which both involves scientific relevance and takes into account mission design considerations, is often a difficult task because of the limited launch energy and limited total energy budget at disposal. In this paper, we provide some approaches to resolving these problems. In this section, the ranking of accessibility for potential scientific value NEAs is completed. These scientifically significant targets include 60 objects (Perozzi et al., 2001) and 18 objects are listed in Table 1. A summary of these results is presented in Table 6.

Table 6 The ranking of accessibility for potential scientific value NEAs
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In Table 6, we consider some of the most frequent constraints for mission design: (1) Total velocity increments ΔV total: this is a crucial parameter influencing the feasibility of a mission; this condition should ensure that the mission could be accomplished with a small or medium class launch vehicle; (2) Launch energy C 3: the launch energy has impact on the choice of rocket, with the current trend towards cheaper missions, the rocket may often represent the largest cost faced by a mission designer; therefore reducing the launch energy and choosing the smallest possible launcher able to accomplish a desired mission becomes crucial; (3) Post-launch velocity increments ΔV p: after the spacecraft escapes into interplanetary space by supporting an upper stage of the rocket, the post-launch velocity increments will be offered by the spacecraft on-board propulsion system; so the post-launch velocity increments have an impact on the technical characteristics of the on-board propulsion systems, the dimensioning of the spacecraft and payload and the launch vehicle; (4) Physical property: the physical properties of a NEA influence its scientific value from a mission operational aspect and its overall engineering feasibility for a rendezvous mission. For example, NEAs belonging to specific taxonomic classes may be preferred as mission targets for scientific reasons. The size of the asteroid combined with its albedo and rotation rates have impact on the navigation system of the spacecraft. Some parameters about these constraints are also listed in Table 6.

We know that the NEAR mission (Farquhar et al., 1995) showed 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. However, the choice of a suitable target is a difficult task, because according to NASA instructions, Discovery mission must have well-focused scientific objectives as well as strict limits on project costs and development time. In addition, Discovery missions must use launch vehicles of the Delta class or smaller. According to these requirements, some constraints were determined. For example, the total ΔV requirements should be less than 6.0 km/s and the post-launch ΔV requirements should not exceed 2.00 km/s and so on. Considering these conditions, we found that 4 candidates out of 60’s scientifically significant targets can satisfy them on the basis of Perozzi et al. (2001) estimates. However, by using the Earth swingby technique, 19 candidates can satisfy with these constraints, in particular several high priority targets for science such as 4015 Wilson–Harrington, 4179 Toutatis, 3551 Verenia, 6489 Golevka and 887 Alinda. In what follows, these scientifically relevant asteroids as listed in Table 6 can be grouped for discussion according to the most important topics that a space mission could help investigate.

We know the difficulties involved in trying to reach cometary candidates, because their dynamical characteristics have already been described. Among these strong cometary candidates, 4015 Wilson–Harrington appears to have reasonable accessibility in that the total velocity increments (ΔV total = 5.623 km/s) and post-launch velocity increments (ΔV p = 1.316 km/s) are not only lower than other cometary candidates, but can also satisfy some of the requirements of the NEAR mission.

Meteor parent bodies are interesting because they possibly hold primitive information about planetary evolution. Among these candidates, 6178 1986 DA, that is generally believed to be the iron meteorites parent body, requires a high total velocity for rendezvous mission (ΔV total = 9.1 km/s, Perozzi et al., 2001); yet by using Earth swingby profiles, the ΔV total is reduced to 6.355 km/s. Giant NEAs are not particularly accessible for rendezvous, apart from 433 Eros, because of the high inclinations involved and the long revolution periods.

Some objects that have some peculiar properties such as exotic rotation station states, a binary system, elongated shape, low albedo and so on, are also regarded as primary targets. Among them, asteroid 4179 Toutatis, extensively imaged by radar and possibly in a peculiar rotation state, exhibits some favorable accessibility. The total velocity increments and post-launch velocity increments required for rendezvous mission are 5.092 and 0.796 km/s, respectively. In addition, 887 Alinda, 3288 Seleucus, 1995 HM, 1997 BR, 1998 KY26 that have exotic rotation station states, also show extremely favorable accessibility.

V-type objects are also regarded as primary targets, because of their supposed origin as fragments of the basaltic surface of Vesta. The 3361 Orpheus, whose period is 1.33 years and inclination is 2.68° shows the lower rendezvous energy requirements (ΔV total = 5.280 km/s, ΔV p = 1.223 km/s). For the others the situation is worsened by the distant aphelia, which in the case of 6489 Golevka may exceed 4 AU, is also found. Although the distant aphelia of 6489 Golevka exceeds 4 AU, the asteroid is approachable because of the low total velocity increments (ΔV total = 5.447 km/s) and post-launch velocity increments (ΔV p = 1.095 km/s).

It should, however, be kept in mind that these considerations refer only to the basic transfer trajectory. If multiple planetary swingby will be foreseen in the mission design process, even the most difficult cases may become feasible; however, the two-impulse and Earth swingby transfer trajectory should be computed starting from the actual transfer orbit found.

In conclusion, the purpose of this paper was to give a basic reference for the target selection and the analysis of the accessibility of a rendezvous mission. Some frequent factors influencing target selection were summarized. According to these factors, more and more potentially scientifically valuable asteroids will be found and updated. When measuring the accessibility of candidates, the planetary swingby techniques have been applied to the transfer path generation. It can effectively reduce the launch energy and total velocity increments for rendezvous mission and avoid the possibility that some high priority targets for science will be discarded due to requiring too much launch energy and total velocity increments.