arXiv subjects

David Jewitt

Publications and source records attributed to David Jewitt.

At least 73 records · Page 4Linked to original sources

arXiv2017

A binary main belt comet

The asteroids are primitive solar system bodies which evolve both collisionally and through disruptions due to rapid rotation [1]. These processes can lead to the formation of binary asteroids [2-4] and to the release of dust [5], both directly and, in some cases, through uncovering frozen volatiles. In a sub-set of the asteroids called main-belt comets (MBCs), the sublimation of excavated volatiles causes transient comet-like activity [6-8]. Torques exerted by sublimation measurably influence the spin rates of active comets [9] and might lead to the splitting of bilobate comet nuclei [10]. The kilometer-sized main-belt asteroid 288P (300163) showed activity for several months around its perihelion 2011 [11], suspected to be sustained by the sublimation of water ice [12] and supported by rapid rotation [13], while at least one component rotates slowly with a period of 16 hours [14]. 288P is part of a young family of at least 11 asteroids that formed from a ~10km diameter precursor during a shattering collision 7.5 million years ago [15]. Here we report that 288P is a binary main-belt comet. It is different from the known asteroid binaries for its combination of wide separation, near-equal component size, high eccentricity, and comet-like activity. The observations also provide strong support for sublimation as the driver of activity in 288P and show that sublimation torques may play a significant role in binary orbit evolution.

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arXiv2017

A Comet Active Beyond the Crystallization Zone

We present observations showing in-bound long-period comet C/2017 K2 (PANSTARRS) to be active at record heliocentric distance. Nucleus temperatures are too low (60 K to 70 K) either for water ice to sublimate or for amorphous ice to crystallize, requiring another source for the observed activity. Using the Hubble Space Telescope we find a sharply-bounded, circularly symmetric dust coma 10$^5$ km in radius, with a total scattering cross section of $\sim$10$^5$ km$^2$. The coma has a logarithmic surface brightness gradient -1 over much of its surface, indicating sustained, steady-state dust production. A lack of clear evidence for the action of solar radiation pressure suggests that the dust particles are large, with a mean size $\gtrsim$ 0.1 mm. Using a coma convolution model, we find a limit to the apparent magnitude of the nucleus $V >$ 25.2 (absolute magnitude $H >$ 12.9). With assumed geometric albedo $p_V$ = 0.04, the limit to the nucleus circular equivalent radius is $<$ 9 km. Pre-discovery observations from 2013 show that the comet was also active at 23.7 AU heliocentric distance. While neither water ice sublimation nor exothermic crystallization can account for the observed distant activity, the measured properties are consistent with activity driven by sublimating supervolatile ices such as CO$_2$, CO, O$_2$ and N$_2$. Survival of supervolatiles at the nucleus surface is likely a result of the comet's recent arrival from the frigid Oort cloud.

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arXiv2017

Split Active Asteroid P/2016 J1 (PANSTARRS)

We present a photometric and astrometric study of the split active asteroid P/2016 J1 (PANSTARRS). The two components (hereafter J1-A and J1-B) separated either $\sim$1500 days (2012 May to June) or 2300 days (2010 April) prior to the current epoch, with a separation speed $V_{\mathrm{sep}} = 0.70 \pm 0.02$ m s$^{-1}$ for the former scenario, or $0.83 \pm 0.06$ m s$^{-1}$ for the latter. Keck photometry reveals that the two fragments have similar, Sun-like colors which are comparable to the colors of primitive C- and G-type asteroids. With a nominal comet-like albedo, $p_{R} = 0.04$, the effective, dust-contaminated cross sections are estimated to be 2.4 km$^{2}$ for J1-A, and 0.5 km$^{2}$ for J1-B. We estimate that the nucleus radii lie in the range $140 \lesssim R_{\mathrm{N}} \lesssim 900$ m for J1-A and $40 \lesssim R_{\mathrm{N}} \lesssim 400$ m, for J1-B. A syndyne-synchrone simulation shows that both components have been active for 3 to 6 months, by ejecting dust grains at speeds $\sim$0.5 m s$^{-1}$ with rates $\sim$1 kg s$^{-1}$ for J1-A and 0.1 kg s$^{-1}$ for J1-B. In its present orbit, the rotational spin-up and devolatilization times of 2016 J1 are very small compared to the age of the solar system, raising the question of why this object still exists. We suggest that ice that was formerly buried within this asteroid became exposed at the surface, perhaps via a small impact, and that sublimation torques then rapidly drove it to break-up. Further disintegration events are anticipated due to the rotational instability.

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arXiv2016

Non-Gravitational Acceleration of the Active Asteroids

Comets can exhibit non-gravitational accelerations caused by recoil forces due to anisotropic mass loss. So might active asteroids. We present an astrometric investigation of 18 active asteroids in search of non-gravitational acceleration. Statistically significant (signal-to-noise ratio (SNR) $> 3$) detections are obtained in three objects: 313P/Gibbs, 324P/La Sagra and (3200) Phaethon. The strongest and most convincing detection ($>$7$σ$ in each of three orthogonal components of the acceleration), is for the $\sim$1 km diameter nucleus of 324P/La Sagra. A 4.5$σ$ detection of the transverse component of the acceleration of 313P/Gibbs (also $\sim$1 km in diameter) is likely genuine too, as evidenced by the stability of the solution to the rejection or inclusion of specific astrometric datasets. We also find a 3.4$σ$ radial-component detection for $\sim$5 km diameter (3200) Phaethon, but this detection is more sensitive to the inclusion of specific datasets, suggesting that it is likely spurious in origin. The other 15 active asteroids in our sample all show non-gravitational accelerations consistent with zero. We explore different physical mechanisms which may give rise to the observed non-gravitational effects, and estimate mass-loss rates from the non-gravitational accelerations. We present a revised momentum-transfer law based on a physically realistic sublimation model for future work on non-gravitational forces, but note that it has little effect on the derived orbital elements.

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arXiv2016

Fragmentation Kinematics in Comet 332P/Ikeya-Murakami

We present initial time-resolved observations of the split comet 332P/Ikeya-Murakami taken using the Hubble Space Telescope. Our images reveal a dust-bathed cluster of fragments receding from their parent nucleus at projected speeds in the range 0.06 to 3.5 m s$^{-1}$ from which we estimate ejection times from October to December 2015. The number of fragments with effective radii $\gtrsim$20 m follows a differential power law with index $γ$ = -3.6$\pm$0.6, while smaller fragments are less abundant than expected from an extrapolation of this power-law. We argue that, in addition to losses due to observational selection, torques from anisotropic outgassing are capable of destroying the small fragments by driving them quickly to rotational instability. Specifically, the spin-up times of fragments $\lesssim$20 m in radius are shorter than the time elapsed since ejection from the parent nucleus. The effective radius of the parent nucleus is $r_e \le$ 275 m (geometric albedo 0.04 assumed). This is about seven times smaller than previous estimates and results in a nucleus mass at least 300 times smaller than previously thought. The mass in solid pieces, $2\times10^9$ kg, is about 4% of the mass of the parent nucleus. As a result of its small size, the parent nucleus also has a short spin-up time. Brightness variations in time-resolved nucleus photometry are consistent with rotational instability playing a role in the release of fragments.

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arXiv2016

Hubble Space Telescope Observations of Active Asteroid 324P/La Sagra

Hubble Space Telescope observations of active asteroid 324P/La Sagra near perihelion show continued mass loss consistent with the sublimation of near-surface ice. Isophotes of the coma measured from a vantage point below the orbital plane are best matched by steady emission of particles having a nominal size $a \sim$ 100 $μ$m. The inferred rate of mass loss, $dM_d/dt \sim$0.2 kg s$^{-1}$, can be supplied by sublimation of water ice in thermal equilibrium with sunlight from an area as small as 930 m$^2$, corresponding to about 0.2\% of the nucleus surface. Observations taken from a vantage point only 0.6\degr~from the orbital plane of 324P set a limit to the velocity of ejection of dust in the direction perpendicular to the plane, $V_{\perp} <$ 1 m s$^{-1}$. Short-term photometric variations of the near-nucleus region, if related to rotation of the underlying nucleus, rule out periods $\le$ 3.8 hr and suggest that rotation probably does not play a central role in driving the observed mass loss. We estimate that, in the previous orbit, 324P lost about 4$\times$10$^7$ kg in dust particles, corresponding to 6$\times$10$^{-5}$ of the mass of a 550 m spherical nucleus of assumed density $ρ$ = 1000 kg m$^{-3}$. If continued, mass loss at this rate would limit the lifetime of 324P to $\sim$1.6$\times$10$^4$ orbits (about 10$^5$ yr). To survive for the 100 Myr to 400 Myr timescales corresponding, respectively, to dynamical and collisional stability requires a duty cycle $2\times 10^{-4} \le f_d \le 8\times 10^{-4}$. Unless its time in orbit is over-estimated by many orders of magnitude, 324P is revealed as a briefly-active member of a vast population of otherwise dormant ice-containing asteroids.

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arXiv2015

Hubble and Keck Telescope Observations of Active Asteroid 288P/300163 (2006 VW139)

We present Hubble Space Telescope and Keck 10 meter telescope observations of active asteroid 288P/300163 (2006 VW139) taken to examine ejected dust. The nucleus is a C-type object with absolute magnitude $H_V$ = 17.0$\pm$0.1 and estimated diameter $\sim$2.6 km (for assumed visual geometric albedo $p_V$ = 0.04). Variations in the brightness of the nucleus at the 10% to 15% level are significant in both 2011 December and 2012 October but we possess too few data to distinguish variations caused by activity from those caused by rotation. The dust scattering cross-section in 2011 December is $\sim$40 km$^2$, corresponding to a dust mass $\sim$9$\times$10$^6$ kg (88 $μ$m mean particle radius assumed). The full width at half maximum of the debris sheet varies from $\sim$100 km near the nucleus to $\sim$1000 km 30arcsec (40,000 km) east of it. Dust dynamical models indicate ejection speeds between 0.06 and 0.3 m s$^{-1}$, particle sizes between 10 and 300 $μ$m and an inverse square-root relation between particle size and velocity. Overall, the data are most simply explained by prolonged, low velocity ejection of dust, starting in or before 2011 July and continuing until at least 2011 October. These properties are consistent with the sublimation of near-surface ice aided by centrifugal forces. The high spatial resolution of our HST images (52 km per pixel) reveals details that remained hidden in previous ground-based observations, such as the extraordinarily small vertical extent of the dust sheet, ejection speeds well below the nucleus escape speed, and the possibility of a binary nucleus.

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arXiv2015

Physical Observations of (196256) 2003 EH$_{1}$, Presumed Parent of the Quadrantid Meteoroid Stream

The near-Earth asteroid (196256) 2003 EH1 has been suggested to have a dynamical association with the Quadrantid meteoroid stream. We present photometric observations taken to investigate the physical character of this body and to explore its possible relation to the stream. We find no evidence for on-going mass-loss. A model fitted to the point-like surface brightness profile at 2.1 AU limits the fractional contribution to the integrated brightness by near-nucleus coma to $\leq$ 2.5 %. Assuming an albedo equal to those typical of cometary nuclei ($\it p_{\rm R}$=0.04), we find that the effective nucleus radius is $r_e$ = 2.0$\pm$0.2 km. Time-resolved ${\it R}$-band photometry can be fitted by a two-peaked lightcurve having a rotational period of 12.650$\pm$0.033 hr. The range of the lightcurve, $Δm_{\rm R}$= 0.44 $\pm$ 0 .01 mag, is indicative of an elongated shape having an axis ratio $\sim$1.5 projected into the plane of the sky. The asteroid shows colors slightly redder than the Sun, being comparable with those of C-type asteroids. The limit to the mass loss rate set by the absence of resolved coma is $\lesssim$ 2.5$\times$ 10$^{-2}$ kg ${\rm s^{-1}}$, corresponding to an upper limit on the fraction of the surface that could be sublimating water ice $f_A$ $\lesssim$ 10$^{-4}$. Even if sustained over the 200-500 yr dynamical age of the Quadrantid stream, the total mass loss from 2003 EH1 would be too small to supply the reported stream mass ($10^{13}$ kg), implying either that the stream has another parent or that mass loss from 2003 EH1 is episodic.

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arXiv2015

Nucleus and Mass Loss from Active Asteroid 313P/Gibbs

We present Hubble Space Telescope observations of active asteroid 313P/Gibbs (formerly P/2014 S4) taken over the five month interval from 2014 October to 2015 March. This object has been recurrently active near perihelion (at 2.4 AU) in two different orbits, a property that is naturally explained by the sublimation of near surface ice but which is difficult to reconcile with other activity mechanisms. We find that the mass loss peaks near 1 kg s$^{-1}$ in October and then declines over the subsequent months by about a factor of five, at nearly constant heliocentric distance. This decrease is too large to be caused by the change in heliocentric distance during the period of observation. However, it is consistent with sublimation from an ice patch shadowed by local topography, for example in a pit like those observed on the nuclei of short-period comet 67P/Churyumov-Gerasimenko. While no unique interpretation is possible, a simple self shadowing model shows that sublimation from a pit with depth to diameter ratio near 1/2 matches the observed rate of decline of the activity, while deeper and shallower pits do not. We estimate the nucleus radius to be 700$\pm$100 m (geometric albedo 0.05 assumed). Measurements of the spatial distribution of the dust were obtained from different viewing geometries. They show that dust was ejected continuously not impulsively, that the effective particle size is large, $\sim$50 $μm$, and that the ejection speed is $\sim$2.5 m s$^{-1}$. The total dust mass ejected is $\sim$10$^7$ kg, corresponding to $\sim$10$^{-5}$ of the nucleus mass. The observations are consistent with partially shadowed sublimation from $\sim$10$^4$ m$^2$ of ice, corresponding to $\sim$0.2\% of the nucleus surface. For ice to survive in 313P for billion-year timescales requires that the duty cycle for sublimation be $\lesssim$10$^{-3}$.

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arXiv2015

Fast Rotation and Trailing Fragments of the Active Asteroid P/2012 F5 (Gibbs)

While having a comet-like appearance, P/2012 F5 (Gibbs) has an orbit native to the Main Asteroid Belt, and physically is a km-sized asteroid which recently (mid 2011) experienced an impulsive mass ejection event. Here we report new observations of this object obtained with the Keck II telescope on UT 2014 August 26. The data show previously undetected 200-m scale fragments of the main nucleus, and reveal a rapid nucleus spin with a rotation period of 3.24 $\pm$ 0.01 hr. The existence of large fragments and the fast nucleus spin are both consistent with rotational instability and partial disruption of the object. To date, many fast rotators have been identified among the minor bodies, which, however, do not eject detectable fragments at the present-day epoch, and also fragmentation events have been observed, but with no rotation period measured. P/2012 F5 is unique in that for the first time we detected fragments and quantified the rotation rate of one and the same object. The rapid spin rate of P/2012 F5 is very close to the spin rates of two other active asteroids in the Main Belt, 133P/Elst-Pizarro and (62412), confirming the existence of a population of fast rotators among these objects. But while P/2012 F5 shows impulsive ejection of dust and fragments, the mass loss from 133P is prolonged and recurrent. We believe that these two types of activity observed in the rapidly rotating active asteroids have a common origin in the rotational instability of the nucleus.

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arXiv2015

Disappearance of Comet C/2010 X1 (Elenin): Gone with a Whimper, not a Bang

We examine the rise and sudden demise of comet C/2010 X1 (Elenin) on its approach to perihelion. Discovered inbound at 4.2 AU, this long-period comet was predicted to become very bright when near perihelion, at 0.48 AU on 2011 September 10. Observations starting 2011 February (heliocentric distance $\sim$3.5 AU) indeed show the comet to brighten by about 11 magnitudes, with most of the increase occurring inside 1 AU from the Sun. The peak brightness reached $m_R$ = 6 on UT 2011 August 12.95$\pm$0.50, when at $\sim$0.83 AU from the Sun. We find that most of the surge in brightness in mid-August resulted from dust particle forward-scattering, not from a sudden increase in the activity. A much smaller ($\sim$3 magnitudes) brightening reached a maximum on UT 2011 August 30$\pm$1 (at 0.56 AU), and reflects the true break-up of the nucleus. This second peak was matched by a change in the morphology from centrally condensed to diffuse. The estimated cross-section of the nucleus when at 1 AU inbound was $\sim$1 km$^2$, corresponding to an equal-area circle of radius 0.6 km. No surviving fragments were found to a limiting red magnitude $r'$ = 24.4, corresponding to radii $\lesssim$40 m (red geometric albedo = 0.04 assumed). Our observations are consistent with disintegration of the nucleus into a power law size distribution of fragments with index $q$ = 3.3$\pm$0.2 combined with the action of radiation pressure. We speculate about physical processes that might cause nucleus disruption in a comet when still 0.7 AU from the Sun. Tidal stresses and devolatilization of the nucleus by sublimation are both negligible at this distance. However, the torque caused by mass loss, even at the very low rates measured in comet Elenin, is potentially large enough to be responsible by driving the nucleus to rotational instability.

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arXiv2015

The Active Asteroids

Some asteroids eject dust, producing transient, comet-like comae and tails; these are the active asteroids. The causes of activity in this newly-identified population are many and varied. They include impact ejection and disruption, rotational instabilities, electrostatic repulsion, radiation pressure sweeping, dehydration stresses and thermal fracture, in addition to the sublimation of asteroidal ice. These processes were either unsuspected or thought to lie beyond the realm of observation before the discovery of asteroid activity. Scientific interest in the active asteroids lies in their promise to open new avenues into the direct study of asteroid destruction, the production of interplanetary debris, the abundance of asteroid ice and the origin of terrestrial planet volatiles.

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arXiv2014

New Active Asteroid 313P/Gibbs

We present initial observations of the newly-discovered active asteroid 313P/Gibbs (formerly P/2014 S4), taken to characterize its nucleus and comet-like activity. The central object has a radius $\sim$0.5 km (geometric albedo 0.05 assumed). We find no evidence for secondary nuclei and set (with qualifications) an upper limit to the radii of such objects near 25 m, assuming the same albedo. Both aperture photometry and a morphological analysis of the ejected dust show that mass-loss is continuous at rates $\sim$0.2 to 0.4 kg s$^{-1}$, inconsistent with an impact origin. Large dust particles, with radii $\sim$50 to 100 $μ$m, dominate the optical appearance. At 2.4 AU from the Sun, the surface equilibrium temperatures are too low for thermal or desiccation stresses to be responsible for the ejection of dust. No gas is spectroscopically detected (limiting the gas mass loss rate to $<$1.8 kg s$^{-1}$). However, the protracted emission of dust seen in our data and the detection of another episode of dust release near perihelion, in archival observations from 2003, are highly suggestive of an origin by the sublimation of ice. Coincidentally, the orbit of 313P/Gibbs is similar to those of several active asteroids independently suspected to be ice sublimators, including P/2012 T1, 238P/Read and 133P/Elst-Pizarro, suggesting that ice is abundant in the outer asteroid belt.

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arXiv2014

Peculiar Near-Nucleus Outgassing of Comet 17P/Holmes During Its 2007 Outburst

We present high angular resolution Submillimeter Array observations ofthe outbursting Jupiter family comet 17P/Holmes on 2007 October 26-29, achieving a spatial resolution of 2.5", or ~3000 km at the comet distance. The observations resulted in detections of the rotational lines CO 3-2, HCN 4-3, H$^{13}$CN 4-3, CS 7-6, H$_2$CO 3$_{1,2}$-2$_{1,1}$, H$_2$S 2$_{2,0}$-2$_{1,1}$, and multiple CH$_3$OH lines, along with the associated dust continuum at 221 and 349 GHz. The continuum has a spectral index of 2.7$\pm$0.3, slightly steeper than blackbody emission from large dust particles. From the imaging data, we identify two components in the molecular emission. One component is characterized by a relatively broad line width (~1 km s$^{-1}$ FWHM) exhibiting a symmetric outgassing pattern with respect to the nucleus position. The second component has a narrower line width (<0.5 km s$^{-1}$ FWHM) with the line center red-shifted by 0.1-0.2 km s$^{-1}$ (cometocentric frame), and shows a velocity shift across the nucleus position with the position angle gradually changing from 66 to 30 degrees within the four days of observations. We determine distinctly different CO/HCN ratios for each of the components. For the broad-line component we find CO/HCN <7, while in the narrow-line component, CO/HCN = 40$\pm$5. We hypothesize that the narrow-line component originates from the ice grain halo found in near-nucleus photometry, believed to be created by sublimating recently released ice grains around the nucleus during the outburst. In this interpretation, the high CO/HCN ratio of this component reflects the more pristine volatile composition of nucleus material released in the outburst.

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arXiv2014

Episodic Ejection from Active Asteroid 311P/PANSTARRS

We examine the development of the active asteroid 311P/PANSTARRS (formerly, 2013 P5) in the period from 2013 September to 2014 February using high resolution images from the Hubble Space Telescope. This multi-tailed object is characterized by a single, reddish nucleus of absolute magnitude $H \ge$ 18.98$\pm$0.10, corresponding to an equal-area sphere of radius $\le$200$\pm$20 m (for assumed geometric albedo 0.29$\pm$0.09). We set an upper limit to the radii of possible companion nuclei at $\sim$10 m. The nucleus ejected debris in nine discrete episodes, spread irregularly over a nine month interval, each time forming a distinct tail. Particles in the tails range from about 10 $μ$m to at least 80 mm in radius, and were ejected at speeds $<$1 m s$^{-1}$. The ratio of the total ejected dust mass to the nucleus mass is $\sim$3$\times$10$^{-5}$, corresponding to a global surface layer $\sim$2 mm thick, or to a deeper layer covering a smaller fraction of the surface. The observations are incompatible with an origin of the activity by impact or by the sublimation of entrapped ice. This object appears to be shedding its regolith by rotational (presumably YORP-driven) instability. Long-term fading of the photometry (months) is attributed to gradual dissipation of near-nucleus dust. Photometric variations on short timescales ($<$0.7 hr) are probably caused by fast rotation of the nucleus. However, because of limited time coverage and dilution of the nucleus signal by near-nucleus dust, we have not been able to determine the rotation period.

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arXiv2014

Outbursting Comet P/2010 V1 (Ikeya-Murakami): A Miniature Comet Holmes

Short-period comet P/2010 V1 (Ikeya-Murakami, hereafter V1) was discovered visually by two amateur astronomers. The appearance of the comet was peculiar, consisting of an envelope, a spherical coma near the nucleus and a tail extending in the anti-solar direction. We investigated the brightness and the morphological development of the comet by taking optical images with ground-based telescopes. Our observations show that V1 experienced a large-scale explosion between UT 2010 October 31 and November 3. The color of the comet was consistent with the Sun (g'-RC=0.61+-0.20, RC-IC=0.20+-0.20, and B-RC=0.93+-0.25), suggesting that dust particles were responsible for the brightening. We used a dynamical model to understand the peculiar morphology, and found that the envelope consisted of small grains (0.3-1 micron) expanding at a maximum speed of 500+-40 m/s, while the tail and coma were composed of a wider range of dust particle sizes (0.4-570 micron) and expansion speeds 7-390 m/s. The total mass of ejecta is ~5x10^8 kg and kinetic energy ~5x10^12 J. These values are much smaller than in the historic outburst of 17P/Holmes in 2007, but the energy per unit mass (1x10^4 J/kg) is comparable. The energy per unit mass is about 10% of the energy released during the crystallization of amorphous water ice suggesting that crystallization of buried amorphous ice can supply the mass and energy of the outburst ejecta.

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arXiv2014

Disintegrating Asteroid P/2013 R3

Splitting of the nuclei of comets into multiple components has been frequently observed but, to date, no main-belt asteroid has been observed to break-up. Using the Hubble Space Telescope, we find that main-belt asteroid P/2013 R3 consists of 10 or more distinct components, the largest up to 200 m in radius (assumed geometric albedo of 0.05) each of which produces a coma and comet-like dust tail. A diffuse debris cloud with total mass roughly 2x10^8 kg further envelopes the entire system. The velocity dispersion among the components is about V = 0.2 to 0.5 m/s, is comparable to the gravitational escape speeds of the largest members, while their extrapolated plane-of-sky motions suggest break-up between February and September 2013. The broadband optical colors are those of a C-type asteroid. We find no spectral evidence for gaseous emission, placing model-dependent upper limits to the water production rate near 1 kg/s. Breakup may be due to a rotationally induced structural failure of the precursor body.

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arXiv2014

Hubble Space Telescope Investigation of Main-Belt Comet 133P/Elst-Pizarro

We report new observations of the prototype main-belt comet (active asteroid) 133P/Elst-Pizarro taken at high angular resolution using the Hubble Space Telescope. The object has three main components; a) a point-like nucleus, b) a long, narrow antisolar dust tail and c) a short, sunward anti-tail. There is no resolved coma. The nucleus has a mean absolute magnitude H_V = 15.70+/-0.10 and a lightcurve range 0.42 mag., the latter corresponding to projected dimensions 3.6 x 5.4 km (axis ratio 1.5:1), at the previously measured geometric albedo of 0.05+/-0.02. We explored a range of continuous and impulsive emission models to simultaneously fit the measured surface brightness profile, width and position angle of the antisolar tail. Preferred fits invoke protracted emission, over a period of 150 days or less, of dust grains following a differential power-law size distribution with index 3.25 < q < 3.5 and having a wide range of sizes. Ultra-low surface brightness dust projected in the sunward direction is a remnant from emission activity occurring in previous orbits, and consists of the largest (>cm-sized) particles. Ejection velocities of one micron-sized particles are comparable to the ~1.8 m/s gravitational escape speed of the nucleus, while larger particles are released at speeds less than the gravitational escape velocity. The observations are consistent with, but do not prove, a hybrid hypothesis in which mass loss is driven by gas drag from the sublimation of near-surface water ice, but escape is aided by centripetal acceleration from the rotation of the elongated nucleus. No plausible alternative hypothesis has been identified.

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