[Condensed] We search 0.02 deg^2 for trans-Neptunian objects (TNOs) with m<=29.2 (diameter ~15 km) using the ACS on HST. Three new objects are discovered, roughly 25 times fewer than expected from extrapolation of the differential sky density Sigma(m) of brighter objects. The ACS and other recent TNO surveys show departures from a power law size distribution. Division of the TNO sample into ``classical Kuiper belt'' (CKB) and ``Excited'' samples reveals that Sigma(m) differs for the two populations at 96% confidence. A double power law adequately fits all data. Implications include: The total mass of the CKB is ~0.010 M_Earth, only a few times Pluto's mass, and is predominately in the form of ~100 km bodies. The mass of Excited objects is perhaps a few times larger. The Excited class has a shallower bright-end size distribution; the largest objects, including Pluto, comprise tens of percent of the total mass whereas the largest CKBOs are only ~2% of its mass. The predicted mass of the largest Excited body is close to the Pluto mass; the largest CKBO is ~60 times less massive. The deficit of small TNOs occurs for sizes subject to disruption by present-day collisions, suggesting extensive depletion by collisions. Both accretion and erosion appearing to have proceeded to more advanced stages in the Excited class than the CKB. The absence of distant TNOs implies that any distant (60 AU) population must have less than the CKB mass in the form of objects 40 km or larger. The CKB population is sparser than theoretical estimates of the required precursor population for short period comets, but the Excited population could be a viable precursor population.Comment: Revised version accepted to the Astronomical Journal. Numerical results are very slightly revised. Implications for the origins of short-period comets are substantially revised, and tedious material on statistical tests has been collected into a new Appendi

The origin of the highly eccentric, inclined, and resonance-locked orbit of Pluto has long been a puzzle. A possible explanation has been proposed recently [Malhotra, 1993, Nature, 365, 819] which suggests that these extraordinary orbital properties may be a natural consequence of the formation and early dynamical evolution of the outer solar system. A resonance capture mechanism is possible during the cleating of the residual planetesimal debris and the formation of the Oort Cloud of comets by planetesimal mass loss from the vicinity of the giant planets. If this mechanism were in operation during the early history of the planetary system, the entire region between the orbit of Neptune and approximately 50 AU would have been sweptby first-order mean motion resonances. Thus, resonance capture could occur not only for Pluto, but quite generally for other trans-Neptunian small bodies. Some consequences of this evolution for the present-day dynamical structure of the trans-Neptunian region are (i) most of the objects in the region beyond Neptune and up to --50 AU exist in very narrow zones located at orbital resonances with Neptune (particularly the 3:2 and the 2:1 resonances); and (ii) these resonant objects would have significantly large eccentricities. The distribution of objects in the Kuiper Belt as predicted by this theory is presented here.

New insights into the history of the inner solar system are derived from the impact cratering record of the Moon, Mars, Venus and Mercury, and from the size distributions of asteroid populations. Old craters from a unique period of heavy bombardment that ended ∼3.8 billion years ago were made by asteroids that were dynamically ejected from the main asteroid belt, possibly due to the orbital migration of the giant planets. The impactors of the past ∼3.8 billion years have a size distribution quite different from the main belt asteroids, but very similar to the population of near-Earth asteroids.The Moon and all the terrestrial planets were resurfaced during a period of intense impact cratering that occurred between the time of their accretion, ∼ 4.5 Ga, and ∼ 3.85 Ga. The lunar cratering record and the radiometrically dated Apollo samples have shown that the intense bombardment of the Moon ended at ∼ 3.85 Ga; the impact flux since that time to the present has been at least an order of magnitude smaller. The 3.85 Ga epoch might represent the final end of an era of steadily declining large impacts (the tail end of the accretion of the planets). However, it has also been argued that only a sudden injection of impacting objects into the terrestrial planet zone accounts for the abrupt end of the intense bombardment; thus, this event has been named the Late Heavy Bombardment (LHB), or sometimes the Lunar Cataclysm, to distinguish it from the prior final accretion of the planets at 4.5 Ga. Specifically, the lunar cataclysm hypothesis (1,2) postulates that the intense bombardment of the Moon lasted only a very short period of time, 20-200 My (2-5). Recent results on the impact ages of lunar meteorites (which represent a much broader region of the lunar surface than the Apollo samples) support this hypothesis (6-8). Furthermore, the impact-reset ages of meteoritic samples of asteroids (9-10) and the shock-metamorphosing at 3.92 Ga of the only known sample of the heavily cratered highlands of Mars, meteorite Allan Hills 84001 (11), indicate that the LHB affected the entire inner Solar System, not just the Moon.Identifying the sources of planetary impactors has proven elusive. Dynamical models invoking both geocentric and heliocentric debris and both asteroidal and cometary reservoirs have been

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