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Images taken by the Stardust mission during its flyby of 81P/Wild 2 show the comet to be a 5-kilometer oblate body covered with remarkable topographic features, including unusual circular features that appear to be impact craters. The presence of high-angle slopes shows that the surface is cohesive and self-supporting. The comet does not appear to be a rubble pile, and its rounded shape is not directly consistent with the comet being a fragment of a larger body. The surface is active and yet it retains ancient terrain. Wild 2 appears to be in the early stages of its degradation phase as a small volatile-rich body in the inner solar system.

We interpret the nucleus properties and jet activity from the Stardust spacecraft imaging and the onboard dust monitoring system data. Triangulation of 20 jets shows that 2 emanate from the nucleus dark side and 16 emanate from sources that are on slopes where the Sun's elevation is greater than predicted from the fitted triaxial ellipsoid. Seven sources, including five in the Mayo depression, coincide with relatively bright surface spots. Fitting the imaged jets, the spikelike temporal distribution of dust impacts indicates that the spacecraft crossed thin, densely populated sheets of particulate ejecta extending from small sources on the rotating nucleus, consistent with an emission cone model.

New osculating orbits are presented for 110 nearly parabolic comets. Combining these with selected orbit determinations from other sources, we consider a total of 200 orbits where the available observations yield a result of very good (first class) or good (second class) quality. For each of these, the original and future orbits (referred to the barycenter of the solar system) are calculated. The Oort effect (a tendency for original 1/a values to range from 0 to-f-lOOXlO-6 AU-1) is clearly seen among the first-class orbits but not among the second-class orbits. Modifications in original 1/a values due to the effects of nongravitational forces are considered.

[1] Stardust, the 4th Discovery mission launched in February 1999, will collect coma samples from the recently deflected comet 81P/Wild 2 on 2 January 2004 and return them to Earth on 15 January 2006 for detailed laboratory analyses. Stardust will be the first mission to bring samples back to Earth from a known comet and also the first to bring back contemporary interstellar particles recently discovered. These samples should provide important insights into the nature and amount of dust released by comets, the roles of comets in planetary systems, clues to the importance of comets in producing dust in our zodiacal cloud as well as circumstellar dust around other stars, and the links between collected meteoritic samples with a known cometary body. Samples are collected in newly invented continuous gradient density silica aerogel. Stardust is facilitated by a magnificent trajectory designed to accomplish a complex and ambitious flyby sample return mission within the Discovery program restrictions. The remaining science payload, which provides important context for the captured samples, includes a time-of-flight spectrometer measuring the chemical and isotopic composition of dust grains; a polyvinylidene fluoride dust flux monitor determining dust flux profiles; a CCD camera for imaging Wild 2 coma and its nucleus; a shared X band transponder providing two-way Doppler shifts to estimate limits to Wild 2 mass and integrated dust fluence; and tracking of the spacecraft's attitude sensing for the detection of large particle impacts. The graphite composite spacecraft brings the collected sample back to Earth by a direct reentry in a capsule.

We describe the physical and orbital properties of C/2011 W3. After surviving the perihelion passage, the comet was observed to undergo major physical changes. The permanent loss of the nuclear condensation and the formation of a narrow spine tail was observed first at Malargue, Argentina, on December 20 and then systematically at Siding Spring, Australia. The process of disintegration culminated with an outburst (terminal fragmentation event) on December 17.6 UT. The postperihelion tail, observed for ∼3 months, was the product of activity over <2 days. Because of the delayed response to the hostile environment in the immediate proximity of the Sun, the nucleus' breakup and crumbling was probably caused by thermal stress due to the penetration of intense heat pulse deep into the nucleus' interior after perihelion. The same mechanism may be responsible for cascading fragmentation of sungrazers at large heliocentric distances. The observed behavior is at odds with the rubble-pile model, since the residual mass of the nucleus after perihelion, estimated at ∼10 12 g (a sphere ∼150-200 m across), still possessed significant cohesive strength. The spine tail -the product of the terminal outburst -was a synchronic feature, whose brightest part contained submillimeter-sized dust particles, released at velocities not exceeding 30 m s −1 . The loss of the nuclear condensation prevented an accurate orbital-period determination by traditional techniques. Since the missing nucleus must have been located on the synchrone, whose orientation and sunward tip have been measured, we compute the astrometric positions of this missing nucleus as the coordinates of the points of intersection of the spine tail's axis with the lines of forced orbital-period variation, derived from the orbital solutions based on high-quality preperihelion astrometry from the ground. The resulting orbit gives 698 ± 2 years for the osculating orbital period, which proves that C/2011 W3 is the first major member of the expected new, 21st-century cluster of bright Kreutz-system sungrazers, whose existence was predicted by these authors in 2007. From the spine tail's evolution, we determine that its measured tip, populated by dust particles 1-2 mm in diameter, receded antisunward from the computed position of the missing nucleus. The bizarre appearance of the comet's dust tail in images taken only hours after perihelion with the coronagraphs on board the SOHO and STEREO spacecraft is readily understood. The disconnection of the comet's head from the tail released before perihelion and an apparent activity attenuation near perihelion have a common cause -sublimation of all dust at heliocentric distances smaller than about 1.8 solar radii. The tail's brightness is strongly affected by forward scattering of sunlight by dust. From an initially broad range of particle sizes, the grains that were imaged the longest had a radiation-pressure parameter β ≃ 0.6, diagnostic of submicron-sized silicate grains and consistent with the existence of the dust-free zone around ...

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We present X = 3-5 and 8-13 pm images and long-slit spectra of comet Hale-Bopp (C/1995 01) taken in February-April 1997, near the time of perihelion passage. In agreement with other infrared observations, we find that the 3-13 pm spectral energy distribution was dominated by a 10 pm silicate feature with a strong underlying continuum. In the nuclear region, the feature's peak flux-to-continuum ratio varied from 2.8 on February 17 to 3.3 on April 19, and the color temperature of the 3-5 pm continuum was N 1.8 times the blackbody temperature. Both quantities are larger than in any previous comet and indicate a high abundance of submicron dust particles. Spectral structure within the silicate feature reveals the presence of crystalline olivine and pyroxene grains. The physical properties of the grains, as evidenced by their infrared emission, correlated with the morphology. Within the visible jets and halos the silicate feature was up to 15% stronger, and the 8-13 pm color temperature-1.1 times higher, than in the regions between the halos. Therefore, the halos were enriched in submicron particles relative to the background, as expected from the higher outflow velocities of these small grains. Within the halos, there was no measureable variation in the strength of the silicate feature or the color temperature with distance from the nucleus; that is, we detect no time-dependent evolution of dust particle properties (such as fragmentation) on a time scale of one rotational cycle (between N 2 and 13 h after ejection). The spectral shape of the silicate feature was essentially the same everywhere in the inner coma in all of our 1997 spectra. Monte Carlo simulation of dust particle motions in the coma indicates that the observed patterns of jets and halos were dominated by submicron-sized grains. Modeling of the thermal emission from small grains demonstrates that the principal features of the 3-13 pm continuum and the 8-13 pm silicate feature can be synthesized from a mixture of amorphous carbon and amorphous and crystalline silicates. The overall shape of the silicate feature resembled that in other bright comets, indicating a similar mineral mix. However, the 10.0 pm peak was sharper in Hale-Bopp near perihelion than in any previously observed comet except comet Mueller (C/1993 Al) at 2 AU. Subject headings: comets: individual (Hale-Bopp C/1995 01)-comets: generalmineralogyinterplanetary medium ~ interplanetary dust particles