#24 April 1989
Section 18.104.22.168.024.of the Artemis Data Book
Q&A COLONIST'S I.Q. QUIZ
 True or False. The asteroids are all that remains of a lost planet that once circled the Sun between the orbits of Mars and Jupiter, and which disintegrated in some unknown catastrophe.
 True or False. The Asteroid Belt is thin like Saturn's rings.
 True or False. The main portion of the Asteroid Belt is so thick that from the surface of any one asteroid, many others should be visible to the naked eye.
 True or False. All asteroids are irregular in shape much like little Phobos and Deimos, the small moons of Mars, themselves possibly former asteroids.
 True or False. There is a correlation between the types of meteorites we've found an Earth and the various classes of asteroids.
 The asteroids being so small and far away, how is it possible that we know anything about them at all beyond their orbital statistics?
 Are any asteroids important enough in their own right to be more than chance targets of opportunity?
 How many asteroids have so far been discovered in "Earth-Trojan" orbits, i.e. preceding and following Earth in its orbit in the stable L4 and L5 Earth-Sun Lagrange points?
 The theme of several science fiction stories, this is one of those romantic notions (much like that of Atlantis) that dies hard. The overwhelming evidence and the mainstream opinion is that no single planet was able to form at this distance from the Sun because of the disruptive effects of Jupiter's gravity, evident today in the several gaps (e.g. Kirkwood gap) or zones of avoidance in the belt where the orbital period would not be a simple harmonic of Jupiter's. The total mass of the known (about 4,000) AND conjectured asteroids together is much less than that of the Moon, which would have made for a small planet indeed. While most people now reject the Lost Planet theory, many still cling inadvertently to another theory that stands or falls with it, namely that before the hypothetical Lost Planet broke up, it had become differentiated with heavy metals towards the core, stone and water to the outside, and that same fragments must be incredibly rich pure metal as a result. Instead it seems certain that denser, stonier asteroids formed nearer the Sun, water-rich carbonaceous ones further from the Sun. The original family of asteroids has probably all been broken up and reassembled by mutual impacts in the 4+ billion years since formation. But it is unlikely that any of the original number, Proto-Vesta being the sole possible exception, was ever large enough or hot enough to undergo differentiation.
 False. The range of orbital inclinations to the mean plane (represented by the Earth-chauvinistic ecliptic or much better by Jupiter's orbital plane) is quite high, averaging about 10 degrees with some stragglers inclined as much as 43 degrees (#944 Hidalgo). The greater the inclination of an orbit, the harder (more delta V) it will be to reach from Earth.
 False. The image of a packed belt may set a mean stage for science fiction stories and movies, but the 'Belt' is mostly empty space. Take a thickness of 100-200 million miles, a depth about the same, and a circumference on the order of 600-1000 million miles and sprinkle with a few thousand asteroids and they will all be pretty lonely. Occasionally asteroids will pass one another close enough to be seen with the naked eye. But Earth, Mars, Jupiter, and Saturn will thoroughly dominate asteroid skies.
 In theory, any body of 250 miles diameter or more will have sufficient mass to shape itself 'in time' into a body with an equipotential surface: a sphere. The evidence from Voyager views of the small moons of Saturn fits this well (Hyperion, obviously suffering from a geologically recent major impact, being the exception). In fact, Ceres, by far the largest, may be the only asteroid that fits this spherical mold.
 True. The spectral analysis of light from various asteroids seems to coordinate very closely with meteorite classes, signifying that the Belt is their source.
 As we have just indicated, the asteroid's light can be passed through a spectrograph which, acting like a prism breaks up the light into its component colors, gives a clue to the surface composition. Photometric studies of light fluctuations can tell us about a body' s rotation rate and even its polar inclination. Polarization and albedo (light reflectance) studies can tell us more about composition and granulation. Only a few asteroids have been thoroughly studied in this way (a neglected field crying for amateur attention) and only one, Vesta, has been crudely mapped by the new process of speckle interferometry [see page 12].
 Vesta seems far and away the most interesting for several reasons. Ceres, the largest, and Pallas to a lesser degree, are inviting targets - see the articles that follow. The rest all fall into well-known classes any of whose members will probably yield as much information as any other. All the asteroids so far mentioned for flyby encounters on the road to Jupiter are targets of opportunity, no more.
 Despite careful searches that should have turned up anything larger than a couple of kilometers in diameter, no such Earth-coorbital bodies have yet been detected. As such bodies would provide continuous travel opportunities at very low delta-V, this is rather disappointing.
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