Comet 133P/Elst–Pizarro is the first known and currently best-characterized member of the main-belt comets, a recently identified class of objects that exhibit cometary activity but which are ...dynamically indistinguishable from main-belt asteroids. We report here on the results of a multiyear monitoring campaign from 2003 to 2008, and present observations of the return of activity in 2007. We find a pattern of activity consistent with seasonal activity modulation. Additionally, recomputation of phase function parameters using data in which 133P was inactive yields new IAU parameters of HR= 15.49 ± 0.05 mag and GR= 0.04 ± 0.05, and linear parameters of mR(1, 1, 0) = 15.80 ± 0.05 mag and β= 0.041 ± 0.005 mag deg−1. The comparison between predicted magnitudes using these new parameters and the comet's actual brightnesses during its 2002 and 2007 active periods reveals the presence of unresolved coma during both episodes, of the order of ∼0.20 of the nucleus cross-section in 2002 and ∼0.25 in 2007. Multifilter observations during 133P's 2007 active outburst yield mean nucleus colours of B−V= 0.65 ± 0.03, V−R= 0.36 ± 0.01 and R−I= 0.32 ± 0.01, with no indication of significant rotational variation, and similar colours for the trail. Finally, while 133P's trail appears shorter and weaker in 2007 than in 2002, other measures of activity strength such as dust velocity and coma contamination of nucleus photometry are found to remain approximately constant. We attribute changes in trail strength to the timing of observations and projection effects, thus finding no evidence of any substantial decrease in the activity strength between 2002 and 2007.
Around the time of its perihelion passage the observability of 67P/Churyumov-Gerasimenko from Earth was limited to very short windows each morning from any given site, due to the low solar elongation ...of the comet. The peak in the comet's activity was therefore difficult to observe with conventionally scheduled telescopes, but was possible where service/queue scheduled mode was possible, and with robotic telescopes. We describe the robotic observations that allowed us to measure the total activity of the comet around perihelion, via photometry (dust) and spectroscopy (gas), and compare these results with the measurements at this time by Rosetta's instruments. The peak of activity occurred approximately two weeks after perihelion. The total brightness (dust) largely followed the predictions from Snodgrass et al. (2013), with no significant change in total activity levels from previous apparitions. The CN gas production rate matched previous orbits near perihelion, but appeared to be relatively low later in the year.
The peculiar object P/2010 A2 was discovered in January 2010 and given a cometary designation because of the presence of a trail of material, although there was no central condensation or coma. The ...appearance of this object, in an asteroidal orbit (small eccentricity and inclination) in the inner main asteroid belt attracted attention as a potential new member of the recently recognized class of main-belt comets. If confirmed, this new object would expand the range in heliocentric distance over which main-belt comets are found. Here we report observations of P/2010 A2 by the Rosetta spacecraft. We conclude that the trail arose from a single event, rather than a period of cometary activity, in agreement with independent results. The trail is made up of relatively large particles of millimetre to centimetre size that remain close to the parent asteroid. The shape of the trail can be explained by an initial impact ejecting large clumps of debris that disintegrated and dispersed almost immediately. We determine that this was an asteroid collision that occurred around 10 February 2009.
Abstract
We have monitored the Didymos–Dimorphos binary system in imaging polarimetric mode before and after the impact from the Double Asteroid Redirection Test mission. A previous ...spectropolarimetric study showed that the impact caused a dramatic drop in polarization. Our longer-term monitoring shows that the polarization of the post-impact system remains lower than the pre-impact system even months after the impact, suggesting that some fresh ejecta material remains in the system at the time of our observations, either in orbit or settled on the surface. The slope of the post-impact polarimetric curve is shallower than that of the pre-impact system, implying an increase in albedo of the system. This suggests that the ejected material is composed of smaller and possibly brighter particles than those present on the pre-impact surface of the asteroid. Our polarimetric maps show that the dust cloud ejected immediately after the impact polarizes light in a spatially uniform manner (and at a lower level than pre-impact). Later maps exhibit a gradient in polarization between the photocentre (which probes the asteroid surface) and the surrounding cloud and tail. The polarization occasionally shows some small-scale variations, the source of which is not yet clear. The polarimetric phase curve of Didymos–Dimorphos resembles that of the S-type asteroid class.
Abstract The NASA Double Asteroid Redirection Test (DART) spacecraft impacted the secondary body of the binary asteroid (65803) Didymos on 2022 September 26 and altered its orbit about the primary ...body. Before the DART impact, we performed visible and mid-infrared observations to constrain the pre-impact thermophysical properties of the Didymos system and to model its Yarkovsky effect. Analysis of the photometric phase curve derives a Bond albedo of 0.07 ± 0.01, and a thermophysical analysis of the mid-infrared observations derives a thermal inertia of 320 ± 70 J m −2 K −1 s −1/2 and a thermal roughness of 40° ± 3° rms slope. These properties are compatible with the ranges derived for other S-type near-Earth asteroids. Model-to-measurement comparisons of the Yarkovsky orbital drift for Didymos derives a bulk density of 2750 ± 350 kg m −3 , which agrees with other independent measures based on the binary mutual orbit. This bulk density indicates that Didymos is spinning at or near its critical spin-limit at which self-gravity balances equatorial centrifugal forces. Furthermore, comparisons with the post-impact infrared observations presented in Rivkin et al. indicate no change in the thermal inertia of the Didymos system following the DART impact. Finally, orbital temperature simulations indicate that subsurface water ice is stable over geologic timescales in the polar regions if present. These findings will be investigated in more detail by the upcoming ESA Hera mission.
OGLE III and MOA-II are discovering 600–1000 Galactic bulge microlens events each year. This stretches the resources available for intensive follow-up monitoring of the light curves in search of ...anomalies caused by planets near the lens stars. We advocate optimizing microlens planet searches by using an automatic prioritization algorithm based on the planet detection zone area probed by each new data point. This optimization scheme takes account of the telescope and detector characteristics, observing overheads, sky conditions and the time available for observing on each night. The predicted brightness and magnification of each microlens target are estimated by fitting to available data points. The optimization scheme then yields a decision on which targets to observe and which to skip, and a recommended exposure time for each target, designed to maximize the planet detection capability of the observations. The optimal strategy maximizes detection of planet anomalies, and this must be coupled with rapid data reduction to trigger continuous follow-up of anomalies that are thereby found. A web interface makes the scheme available for use by human or robotic observers at any telescope. We also outline a possible self-organizing scheme that may be suitable for coordination of microlens observations by a heterogeneous telescope network.
Abstract
We have observed the Didymos-Dimorphos binary system with the MUSE integral field unit spectrograph mounted at the Very Large Telescope before and after DART impact and captured the ensuing ...ejecta cone, debris cloud, and tails at subarcsecond resolutions. We targeted the Didymos system over 11 nights from 2022 September 26 to October 25 and utilized both narrow- and wide-field observations with and without adaptive optics, respectively. We took advantage of the spectral–spatial coupled measurements and produced both white-light images and spectral maps of the dust reflectance. We identified and characterized numerous dust features, such as the ejecta cone, spirals, wings, clumps, and tails. We found that the base of the sunward edge of the wings, from October 3 to 19, is consistent with maximum grain sizes on the order of 0.05–0.2 mm and that the earliest detected clumps have the highest velocities, on the order of ;10 m s
−1
. We also see that three clumps in narrow-field mode (8″ × 8″) exhibit redder colors and slower speeds, around 0.09 m s
−1
, than the surrounding ejecta, likely indicating that the clump is composed of larger, slower grains. We measured the properties of the primary tail and resolved and measured the properties of the secondary tail earlier than any other published study, with first retrieval on October 3. Both tails exhibit similarities in curvature and relative flux; however, the secondary tail appears thinner, which may be caused by lower-energy ejecta and possibly a low-energy formation mechanism such as secondary impacts.
•We analyze the activity of 89 comets observed with the Spitzer Space Telescope.•We conclude that at least 24% of Jupiter-family comets are active at 3–7AU.•Comets are more likely to be active post ...perihelion.•We find that all survey targets comets with perihelion inside of 1.8AU are inactive.
We present an analysis of comet activity based on the Spitzer Space Telescope component of the Survey of the Ensemble Physical Properties of Cometary Nuclei. We show that the survey is well suited to measuring the activity of Jupiter-family comets at 3–7AU from the Sun. Dust was detected in 33 of 89 targets (37±6%), and we conclude that 21 comets (24±5%) have morphologies that suggest ongoing or recent cometary activity. Our dust detections are sensitivity limited, therefore our measured activity rate is necessarily a lower limit. All comets with small perihelion distances (q<1.8AU) are inactive in our survey, and the active comets in our sample are strongly biased to post-perihelion epochs. We introduce the quantity ∊fρ, intended to be a thermal emission counterpart to the often reported Afρ, and find that the comets with large perihelion distances likely have greater dust production rates than other comets in our survey at 3–7AU from the Sun, indicating a bias in the discovered Jupiter-family comet population. By examining the orbital history of our survey sample, we suggest that comets perturbed to smaller perihelion distances in the past 150yr are more likely to be active, but more study on this effect is needed.
Abstract
The NASA’s Double-Asteroid Redirection Test (DART) was a unique planetary defence and technology test mission, the first of its kind. The main spacecraft of the DART mission impacted the ...target asteroid Dimorphos, a small moon orbiting the asteroid Didymos (65803), on 2022 September 26. The impact brought up a mass of ejecta which, together with the direct momentum transfer from the collision, caused an orbital period change of 33 ± 1 minutes, as measured by ground-based observations. We report here the outcome of the optical monitoring campaign of the Didymos system from the Danish 1.54 m telescope at La Silla around the time of impact. The observations contributed to the determination of the changes in the orbital parameters of the Didymos–Dimorphos system, as reported by Thomas et al., but in this paper we focus on the ejecta produced by the DART impact. We present photometric measurements from which we remove the contribution from the Didymos–Dimorphos system using an
H
–
G
photometric model. Using two photometric apertures we determine the fading rate of the ejecta to be 0.115 ± 0.003 mag day
−1
(in a 2″ aperture) and 0.086 ± 0.003 mag day
−1
(5″) over the first week postimpact. After about 8 days postimpact we note the fading slows down to 0.057 ± 0.003 mag day
−1
(2″ aperture) and 0.068 ± 0.002 mag day
−1
(5″). We include deep-stacked images of the system to illustrate the ejecta evolution during the first 18 days, noting the emergence of dust tails formed from ejecta pushed in the antisolar direction, and measuring the extent of the particles ejected Sunward to be at least 4000 km.
We present the results of a search for the reactivation of active asteroid 176P/LINEAR during its 2011 perihelion passage using deep optical observations obtained before, during, and after that ...perihelion passage. Deep composite images of 176P constructed from data obtained between 2011 June and 2011 December show no visible signs of activity, while photometric measurements of the object during this period also show no significant brightness enhancements similar to that observed for 176P between 2005 November and 2005 December when it was previously observed to be active. An azimuthal search for dust emission likewise reveals no evidence for directed emission (i.e., a tail, as was previously observed for 176P), while a one-dimensional surface brightness profile analysis shows no indication of a spherically symmetric coma at any time in 2011. We conclude that 176P did not in fact exhibit activity in 2011, at least not on the level on which it exhibited activity in 2005, and suggest that this could be due to the devolatization or mantling of the active site responsible for its activity in 2005.
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