Paul R. Weissman

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of the objects in the scattered disk remain after 4 billion years, and that 6 ϫ 10 8 comets are currently required. Thus, we require an initial population of only 6 ϫ 10 10 comets [ϳ0.4 Earth masses (23)] on Neptune-encountering orbits. Because planet formation is unlikely to have been 100% efficient, the original disk could have resulted from the(More)
The Oort cloud is the primary source of the " nearly isotropic " comets, which include new and returning long-period comets and Halley-type comets. We focus on the following topics: (1) the orbital distribution of known comets and the cometary " fading " problem; (2) the population and mass of the Oort cloud, including the hypothetical inner Oort cloud; (3)(More)
On 4 July 2005, many observatories around the world and in space observed the collision of Deep Impact with comet 9P/Tempel 1 or its aftermath. This was an unprecedented coordinated observational campaign. These data show that (i) there was new material after impact that was compositionally different from that seen before impact; (ii) the ratio of dust mass(More)
Context. Near-Earth asteroid 162173 (1999 JU3) is a potential flyby and rendezvous target for interplanetary missions because of its easy to reach orbit. The physical and thermal properties of the asteroid are relevant for establishing the scientific mission goals and also important in the context of near-Earth object studies in general. Aims. Our goal was(More)
The Oort cloud of comets was formed by the ejection of icy planetesimals from the region of giant planets--Jupiter, Saturn, Uranus and Neptune--during their formation. Dynamical simulations have previously shown that comets reach the Oort cloud only after being perturbed into eccentric orbits that result in close encounters with the giant planets, which(More)
Earth-and space-based observations provide synergistic information for space mission encounters by providing data over longer timescales, at different wavelengths and using techniques that are impossible with an in situ flyby. We report here such observations in support of the EPOXI spacecraft flyby of comet 103P/Hartley 2. The nucleus is small and dark,(More)