We used the UltraViolet-Optical Telescope on board Swift to systematically follow the dynamically new comet C/2013 A1 (Siding Spring) on its approach to the Sun. The comet was observed from a heliocentric distance of 4.5 AU pre-perihelion to its peri
helion at 1.4 AU. From our observations, we estimate that the water production rate during closest approach to Mars was 1.5 +/- 0.3 x 1E28 molecules/s, that peak gas delivery rates were between 4.5-8.8 kg/s, and that in total between 3.1-5.4 x 1E4 kg cometary gas was delivered to the planet. Seasonal and evolutionary effects on the nucleus govern the pre-perihelion activity of comet Siding Spring. The sudden increase of its water production between 2.46-2.06 AU suggests the onset of the sublimation of icy grains in the coma, likely driven by CO2. As the comet got closer to the Sun, the relative contribution of the nucleus water production increased, while CO2 production rates decreased. The changes in the comets activity can be explained by a depletion of CO2, but the comets high mass loss rate suggests they may also reflect primordial heterogeneities in the nucleus.
Asteroid (596) Scheila was the first object for which the immediate aftermath of an inter-asteroidal collision was observed. In Dec. 2010, the 113 km-sized asteroid was impacted by a smaller asteroid of less than 100 m in diameter. The scale of the i
mpactor was established by observations of fading ejecta plumes. Comparison of the light curves obtained before and after the impact allowed us to assess how much of Scheilas surface was altered. Cratering physics based on the impactor size suggests that the size of the affected area is larger than expected, (effective radii of 3.5 -- 10 km depending on the change in the albedo of the surface). Similar but more localized albedo changes have been observed on Vesta and the Martian moons, but are not understood. Empirical laws describing ejecta blankets however indicate that at distances between 3.5 -- 10 km from the crater, Scheilas surface would be covered by a thin layer 2 mm to 2 cm thick. This dusting, possibly mixed with bright impactor material may be enough to explain to observed brightness increase. Our results show that sub-critical impacts may play a significant role in processing the surfaces of asteroids. The large effect of small impacts on asteroidal light curves complicate shape modeling.
The coma of comet 103P/Hartley 2 has a significant population of large particles observed as point sources in images taken by the Deep Impact spacecraft. We measure their spatial and flux distributions, and attempt to constrain their composition. The
flux distribution of these particles implies a very steep size distribution with power-law slopes ranging from -6.6 to -4.7. The radii of the particles extend up to 20 cm, and perhaps up to 2 m, but their exact sizes depend on their unknown light scattering properties. We consider two cases: bright icy material, and dark dusty material. The icy case better describes the particles if water sublimation from the particles causes a significant rocket force, which we propose as the best method to account for the observed spatial distribution. Solar radiation is a plausible alternative, but only if the particles are very low density aggregates. If we treat the particles as mini-nuclei, we estimate they account for <16-80% of the comets total water production rate (within 20.6 km). Dark dusty particles, however, are not favored based on mass arguments. The water production rate from bright icy particles is constrained with an upper limit of 0.1 to 0.5% of the total water production rate of the comet. If indeed icy with a high albedo, these particles do not appear to account for the comets large water production rate. production rate. Erratum: We have corrected the radii and masses of the large particles of comet 103P/Hartley 2 and present revised conclusions in the attached erratum.
We present the detection of new cometary X-ray emission lines in the 1.0 to 2.0 keV range using a sample of comets observed with the Chandra X-ray observatory and ACIS spectrometer. We have selected 5 comets from the Chandra sample with good signal-t
o-noise spectra. The surveyed comets are: C/1999 S4 (LINEAR), C/1999 T1 (McNaught-Hartley), 153P/2002 (Ikeya-Zhang), 2P/2003 (Encke), and C/2008 8P (Tuttle). We modeled the spectra with an extended version of our solar wind charge exchange (SWCX) emission model (Bodewits et al. 2007). Above 1 keV, we find Ikeya-Zhang to have strong emission lines at 1340 and 1850 eV that we identify as being created by solar wind charge exchange lines of Mg XI and Si XIII, respectively, and weaker emission lines at 1470, 1600, and 1950 eV formed by SWCX of Mg XII, Mg XI, and Si XIV, respectively. The Mg XI and XII and Si XIII and XIV lines are detected at a significant level for the other comets in our sample (LS4, MH, Encke, 8P), and these lines promise additional diagnostics to be included in SWCX models. The silicon lines in the 1700 to 2000 eV range are detected for all comets, but with the rising background and decreasing cometary emission, we caution these detections need further confirmation with higher resolution instruments.
Aims. Rotation periods of cometary nuclei are scarce, though important when studying the nature and origin of these objects. Our aim is to derive a rotation period for the nucleus of comet C/2004 Q2 (Machholz). Methods. C/2004 Q2 (Machholz) was monit
ored using the Merope CCD camera on the Mercator telescope at La Palma, Spain, in January 2005, during its closest approach to Earth, implying a high spatial resolution (50km per pixel). One hundred seventy images were recorded in three different photometric broadband filters, two blue ones (Geneva U and B) and one red (Cousins I). Magnitudes for the comets optocentre were derived with very small apertures to isolate the contribution of the nucleus to the bright coma, including correction for the seeing. Our CCD photometry also permitted us to study the coma profile of the inner coma in the different bands. Results. A rotation period for the nucleus of P = 9.1 +/- 0.2 h was derived. The period is on the short side compared to published periods of other comets, but still shorter periods are known. Nevertheless, comparing our results with images obtained in the narrowband CN filter, the possibility that our method sampled P/2 instead of P cannot be excluded. Coma profiles are also presented, and a terminal ejection velocity of the grains v_gr = 1609 +/- 48 m/s is found from the continuum profile in the I band.
