Water Ice in 2060 Chiron and its Implications for Centaurs and Kuiper Belt Objects

We report the detection of water ice in the Centaur 2060 Chiron, based on near-infrared spectra (1.0 - 2.5 micron) taken with the 3.8-meter United Kingdom Infrared Telescope (UKIRT) and the 10-meter Keck Telescope. The appearance of this ice is correlated with the recent decline in Chiron's cometary activity: the decrease in the coma cross-section allows previously hidden solid-state surface features to be seen. We predict that water ice is ubiquitous among Centaurs and Kuiper Belt objects, but its surface coverage varies from object to object, and thus determines its detectability and the occurrence of cometary activity.Comment: 18 pages, 3 figures, accepted by ApJ Letter

Population of the Scattered Kuiper Belt

We present the discovery of three new Scattered Kuiper Belt Objects (SKBOs) from a wide-field survey of the ecliptic. This continuing survey has to date covered 20.2 square degrees to a limiting red magnitude of 23.6. We combine the data from this new survey with an existing survey conducted at the University of Hawaii 2.2m telescope to constrain the number and mass of the SKBOs. The SKBOs are characterized by large eccentricities, perihelia near 35 AU, and semi-major axes > 50 AU. Using a maximum-likelihood model, we estimate the total number of SKBOs larger than 100 km in diameter to be N = 3.1 (+1.9/-1.3) x 10^4 (1 sigma) and the total mass of SKBOs to be about 0.05 Earth masses, demonstrating that the SKBOs are similar in number and mass to the Kuiper Belt inside 50 AU.Comment: 15 pages, 3 figure

The rotation and coma profiles of comet C/2004 Q2 (Machholz)

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 monitored 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 comet's 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.Comment: 11 pages, 9 figures, accepted by A&

Exploring methodological innovation in the social sciences: the body in digital environments and the arts

In this paper we examine methodological innovation in the social sciences through a focus on researching the body in digital environments. There are two strands to our argument as to why this is a useful site to explore methodological innovation in the social sciences. First, researching the body in digital environments places new methodological demands on social science. Second, as an area of interest at the intersection of the social sciences and the arts, it provides a focus for exploring how social science innovation can be informed by engagement with the arts, in this instance how the arts work with the body in digital environments and take up social science ideas in novel ways. We argue that social science engagement with the arts and the relatively unmapped terrain of the body in digital environments has the potential to open up spaces for innovative social science questions and methods: spaces, questions and methods that have potential for more general social science methodological innovation. We draw on the findings of the Methodological Innovation in Digital Arts and Social Sciences (MIDAS) project a multi-site ethnography of the research ecologies of the social sciences and the arts related to the body in digital environments. We propose a continuum of methodological innovation that attends to how methods are moved across research contexts and disciplines, in this instance the social sciences and the digital arts. We illustrate and discuss the innovative potential of expanding and re-situating methods across the social sciences and the arts, the transfer of methods and concepts across disciplinary borders and the interdisciplinary generation of new methods. We discuss the catalysts and challenges for social science methodological innovation in relation to the digital and the arts, with attention to how the social sciences might engage with the arts towards innovative research

The Orbit, Mass, and Albedo of Transneptunian Binary 1999 RZ253

We have observed 1999 RZ253 with the Hubble Space Telescope at seven separate epochs and have fit an orbit to the observed relative positions of this binary. Two orbital solutions have been identified that differ primarily in the inclination of the orbit plane. The best fit corresponds to an orbital period, P=46.263 +0.006/-0.074 days, semimajor axis a=4,660 +/-170 km and orbital eccentricity e=0.460 +/-0.013 corresponding to a system mass m=3.7 +/-0.4 x10^18 kg. For a density of rho = 1000 kg m^-3 the albedo at 477 nm is p = 0.12 +/-0.01, significantly higher than has been commonly assumed for objects in the Kuiper Belt. Multicolor, multiepoch photometry shows this pair to have colors typical for the Kuiper belt with a spectral gradient of 0.35 per 100 nm in the range between 475 and 775 nm. Photometric variations at the four epochs we observed were as large as 12 +/-3% but the sampling is insufficient to confirm the existence of a lightcurve

Migration of Jupiter-family comets and resonant asteroids to near-Earth space

We estimated the rate of comet and asteroid collisions with the terrestrial planets by calculating the orbits of 13000 Jupiter-crossing objects (JCOs) and 1300 resonant asteroids and computing the probabilities of collisions based on random-phase approximations and the orbital elements sampled with a 500 yr step. The Bulirsh-Stoer and a symplectic orbit integrator gave similar results for orbital evolution, but sometimes give different collision probabilities with the Sun. A small fraction of former JCOs reached orbits with aphelia inside Jupiter's orbit, and some reached Apollo orbits with semi-major axes less than 2 AU, Aten orbits, and inner-Earth orbits (with aphelia less than 0.983 AU) and remained there for millions of years. Though less than 0.1% of the total, these objects were responsible for most of the collision probability of former JCOs with Earth and Venus. Some Jupiter-family comets can reach inclinations i>90 deg. We conclude that a significant fraction of near-Earth objects could be extinct comets that came from the trans-Neptunian region.Comment: Proc. of the international conference "New trends in astrodynamics and applications" (20-22 January 2003, University of Maryland, College Park

Photometry of cometary nuclei: Rotation rates, colours and a comparison with Kuiper Belt Objects

We present time-series data on Jupiter Family Comets (JFCs) 17P/Holmes, 47P/Ashbrook-Jackson and 137P/Shoemaker-Levy 2. In addition we also present results from `snap-shot' observations of comets 43P/Wolf-Harrington, 44P/Reinmuth 2, 103P/Hartley 2 and 104P/Kowal 2 taken during the same run. The comets were at heliocentric distances of between 3 and 7 AU at this time. We present measurements of size and activity levels for the snap-shot targets. The time-series data allow us to constrain rotation periods and shapes, and thus bulk densities. We also measure colour indices (V-R) and (R-I) and reliable radii for these comets. We compare all of our findings to date with similar results for other comets and Kuiper Belt Objects (KBOs). We find that the rotational properties of nuclei and KBOs are very similar, that there is evidence for a cut-off in bulk densities at ~ 0.6 g cm^{-3} in both populations, and the colours of the two populations show similar correlations. For JFCs there is no observational evidence for the optical colours being dependant on either position in the orbit or on orbital parameters.Comment: 15 pages, 19 figures, accepted for publication in MNRA

The nucleus of 103P/Hartley 2, target of the EPOXI mission

103P/Hartley 2 was selected as the target comet for the Deep Impact extended mission, EPOXI, in October 2007. There have been no direct optical observations of the nucleus of this comet, as it has always been highly active when previously observed. We aimed to recover the comet near to aphelion, to a) confirm that it had not broken up and was in the predicted position, b) to provide astrometry and brightness information for mission planning, and c) to continue the characterisation of the nucleus. We observed the comet at heliocentric distances between 5.7 and 5.5 AU, using FORS2 at the VLT, at 4 epochs between May and July 2008. We performed VRI photometry on deep stacked images to look for activity and measure the absolute magnitude and therefore estimate the size of the nucleus. We recovered the comet near the expected position, with a magnitude of m_R = 23.74 \pm 0.06 at the first epoch. The comet had no visible coma, although comparison of the profile with a stellar one showed that there was faint activity, or possibly a contribution to the flux from the dust trail from previous activity. This activity appears to fade at further epochs, implying that this is a continuation of activity past aphelion from the previous apparition rather than an early start to activity before the next perihelion. Our data imply a nucleus radius of \le 1 km for an assumed 4% albedo; we estimate a ~6% albedo. We measure a colour of (V-R) = 0. 26 \pm 0.09.Comment: 5 pages, 4 figures, accepted for publication in A&