The Size Distribution of Trans-Neptunian Bodies

[Condensed] We search 0.02 deg^2 for trans-Neptunian objects (TNOs) withm<=29.2 (diameter ~15 km) using the ACS on HST. Three new objects arediscovered, roughly 25 times fewer than expected from extrapolation of thedifferential sky density Sigma(m) of brighter objects. The ACS and other recentTNO surveys show departures from a power law size distribution. Division of theTNO sample into ``classical Kuiper belt'' (CKB) and ``Excited'' samples revealsthat Sigma(m) differs for the two populations at 96% confidence. A double powerlaw adequately fits all data. Implications include: The total mass of the CKBis ~0.010 M_Earth, only a few times Pluto's mass, and is predominately in theform of ~100 km bodies. The mass of Excited objects is perhaps a few timeslarger. The Excited class has a shallower bright-end size distribution; thelargest objects, including Pluto, comprise tens of percent of the total masswhereas the largest CKBOs are only ~2% of its mass. The predicted mass of thelargest Excited body is close to the Pluto mass; the largest CKBO is ~60 timesless massive. The deficit of small TNOs occurs for sizes subject to disruptionby present-day collisions, suggesting extensive depletion by collisions. Bothaccretion and erosion appearing to have proceeded to more advanced stages inthe Excited class than the CKB. The absence of distant TNOs implies that anydistant (60 AU) population must have less than the CKB mass in the form ofobjects 40 km or larger. The CKB population is sparser than theoreticalestimates of the required precursor population for short period comets, but theExcited 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