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increase of brightness occurs even at large heliocentric distances. In addition to comet Schwassmann-Wachmann 1, 10 other cases with at least one outburst are known. The frequency of the outbursts increases with the decrease of brightness amplitudes, and the nonperiodic changes in magnitudes of very faint starlike comets with asteroidal appearance might be expected as very typical. The search for old but still partly active cometary nuclei among very faint asteroids would appear to be a worthwhile program, especially among the asteroids of the Palomar-Leiden survey. Because of the differences in cometary and asteroidal orbits, one can assume that some families and streams (in the sense of Alfvén's streams) of long-life comets with asteroidal appearance exist and can be distinguished from asteroids not only by occasional brightness flares but also by the grouping of orbital elements.

REFERENCES

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Adsorption in the Snows of the Nucleus. Planet. Space Sci. 18, 717-730.
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Mém. Soc. Roy. Sci. Liège Ser. 4, vol. 18, pp. 124-132.
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Motion, Evolution of Orbits and Origin of Comets, IAU Symp. 45. D. Reidel.
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PART ||

POSSIBLE SPACE MISSIONS AND FUTURE WORK

ARGUMENTS FOR A MISSION TO AN ASTEROID

H. Alfvév and G. ARRF/EN/US
University of California, San Diego

GROUPS OF ASTEROIDS

With respect to their orbital parameters, the known asteroids fall into different groups:

(1) Main belt asteroids orbit between Mars and Jupiter. Most of them have semimajor axes a in the range 2.0 < a < 3.5 AU.

(2) Asteroids outside the main belt form a number of different groups, such as the Trojans, Hildas, etc.

(3) Mars-crossing asteroids are bodies with low a values and perihelia inside the Martian orbit; in several cases they are inside Earth's orbit. They are the closest neighbors in space of the Earth-Moon system. A list of them is given by Marsden, table II." We select from these the ones with eccentricities in the range 0.2 to 0.4; they will be referred to as the Eros group, named after its largest member. The arguments for a mission to an asteroid refer especially to the Eros group as a first target.

SPECIFIC INTEREST OF ASTEROID EXPLORATION

Access

Of the celestial bodies yet discovered, our closest neighbors in space, except for the Moon, are members of the Apollo and Amor groups. One of them, Icarus, passed very close to Earth in 1968.

A flyby mission to members of these groups would be relatively simple, but probably not very rewarding because their relative velocities when close to Earth are very high, on the order of 30 km/s. A rendezvous and soft landing is for this reason a technically difficult project.

Of all the translunar celestial bodies, the Eros asteroids are the easiest to reach and therefore would be more favorable objects for investigation. They have eccentricities that are definitely lower than the other Apollo and Amor asteroids (although still rather high), and their relative velocities when close to

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