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Dawn visits both Vesta and Ceres, thanks to its patient solar electric ion propulsion. AFAIK more xenon and perhaps a little more hydrazine for the attitude thrusters, would've sufficed to give it power to visit even more targets.

But what is the geography of a multi asteroid trip like? Looking at the diagrams below, asteroid families are identified with similar semi-major axis, inclination and eccentricity. Could a SEP spacecraft like Dawn+ visit several asteroids a year within such a family? Massalia and Koronis look pretty well concentrated. And aren't most NEA thought to come from one single family with low inclination in the inner part of the Main Belt and wouldn't it be interesting and easier to visit them out there than trying to catch them at perihelion where they are more spread out due to differences in eccentricities and higher velocities?

Couldn't Dawn make even one single near passage of any other asteroid during its long trip from Vesta to Ceres? Several other spacecrafts, like Rosetta and Chang'e 3 (upper stage) and New Horizons have done asteroid flybys without even being Main Belt missions.

To understand the scale in these diagrams, 0.1 AU is about 250 times the distance Earth-Moon. The distance between the perihelion Dwarf-Ceres is 0.5 AU which is about the distance between Earth and Mars when at the closest. The inclination of the Moon is about 1.5 degrees.

Inclination and semi-major axis: Inclination and semi-major axis Inclination and eccentricity: Inclination and eccentricity

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Those charts don't tell the whole story. Keep in mind that inclination, eccentricity, and SMA are only three of six orbital elements that define the position of an orbiting body. Two bodies in the exact same orbital path (inclination, eccentricity, SMA, argument of periapsis, and LAN) could be in different positions on that orbit, on opposite sides of the sun.

According to Wikipedia, the plan is to leave Dawn in Ceres orbit at the conclusion of the mission, rather than continue on to other targets.

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  • $\begingroup$ But if a family has the same inclination and the same semi major axis, then a circular orbit in their midst will still encounter them regardless of argument of periapsis and longitude of ascending node. $\endgroup$
    – LocalFluff
    Feb 9, 2015 at 19:39
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    $\begingroup$ Given enough time.... $\endgroup$
    – Erik
    Feb 10, 2015 at 5:48
  • $\begingroup$ That is a good point, given enough time. I estimated the time for the Gefion group with the member Ceres. This group is from 2.7 to 2.8 AU and Ceres is at 2.768 AU. The time of one full orbit is 4.436 to 4.685 years, 4.605 years for Ceres. In more than 124 years there are 27 orbits for an asteroid at 2.7 AU and 28 orbits for Ceres. For asteroids at 2.8 AU there are 58 orbits in more than 271 years while Ceres does 59 orbits. Pretty long time to do one orbit more or one less. $\endgroup$
    – Uwe
    Jan 11, 2017 at 13:33
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CASTAway (PDF) is a mission proposed for ESA that would flyby 10 asteroids in the Main Belt and observe tens thousands telescopically to determine their composition.

enter image description here

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