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Register a new userThe pre-perihelion activity of dynamically new comet C/2013 A1 (Siding Spring) and its close encounter with Mars
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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 perihelion 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.
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The Mars flyby of C/2013 A1 (Siding Spring) represented a unique opportunity for imaging a long-period comet and resolving its nucleus and rotation period. Because of the small encounter distance and the high relative velocity, the goal of successfully observing C/2013 A1 from the Mars orbiting spacecrafts posed strict accuracy requirements on the comets ephemerides. These requirements were hard to meet, as comets are known for being highly unpredictable: astrometric observations can be significantly biased and nongravitational perturbations affect comet trajectories. Therefore, even prior to the encounter, we remeasured a couple of hundred astrometric images obtained with ground-based and Earth-orbiting telescopes. We also observed the comet with the Mars Reconnaissance Orbiters High Resolution Imaging Science Experiment (HiRISE) camera on 2014 October 7. In particular, these HiRISE observations were decisive in securing the trajectory and revealed that out-of-plane nongravitational perturbations were larger than previously assumed. Though the resulting ephemeris predictions for the Mars encounter allowed observations of the comet from the Mars orbiting spacecrafts, post-encounter observations show a discrepancy with the pre-encounter trajectory. We reconcile this discrepancy by employing the Rotating Jet Model, which is a higher fidelity model for nongravitational perturbations and provides an estimate of C/2013 A1s spin pole.
We observed Comet C/Siding Spring using the Hubble Space Telescope (HST) during its close approach to Mars. The high spatial resolution images obtained through the F689M, F775W, and F845M filters reveal the characteristics of the dust coma. The dust production rate of C/Siding Spring, quantified by $Afrho$, is 590$pm$30, 640$pm$30, and 670$pm$30 cm in a 420 km-radius aperture at 38$^circ$ solar phase angle through the three filters, respectively, consistent with other observations at similar time and geometry, and with model predictions based on earlier measurements. The dust expansion velocity is ~150-250 m s$^{-1}$ for micron-sized dust grains, similar to the speeds found for other comets. The coma has a color slope of (5.5$pm$1.5)%/100 nm between 689 and 845 nm, similar to previous HST measurements at comparable aperture sizes, consistent with the lack of color dependence on heliocentric distance for almost all previously observed active comets. The rotational period of the nucleus of C/Siding Spring is determined from the periodic brightness variation in the coma to be 8.00$pm$0.08 hours, with no excited rotational state detected. The dust coma shows a broad and diffuse fan-shaped feature in the sunward direction, with no temporal morphological variation observed. The projected orientation of the dust feature, combined with the previous analysis of the coma morphology and other characteristics, suggests secular activity evolution of the comet in its inner solar system passage as one previously observed active region turns off whereas new regions exposed to sunlight due to seasonal illumination change.
Comet C/2013 A1 (siding Spring) will experience a high velocity encounter with Mars on October 19, 2014 at a distance of 135,000 km +- 5000 km from the planet center. We present a comprehensive analysis of the trajectory of both the comet nucleus and the dust tail. The nucleus of C/2013 A1 cannot impact on Mars even in the case of unexpectedly large nongravitational perturbations. Furthermore, we compute the required ejection velocities for the dust grains of the tail to reach Mars as a function of particle radius and density and heliocentric distance of the ejection. A comparison between our results and the most current modeling of the ejection velocities suggests that impacts are possible only for millimeter to centimeter size particles released more than 13 au from the Sun. However, this level of cometary activity that far from the Sun is considered extremely unlikely. The arrival time of these particles spans a 20-minute time interval centered at October 19, 2014 at 20:09 TDB, i.e., around the time that Mars crosses the orbital plane of C/2013 A1. Ejection velocities larger than currently estimated by a factor >2 would allow impacts for smaller particles ejected as close as 3 au from the Sun. These particles would reach Mars from 43 to 130 min after the nominal close approach epoch of the purely gravitational trajectory of the nucleus.
The close encounter of Comet C/2013 A1 (Siding Spring) with Mars on October 19, 2014 presented an extremely rare opportunity to obtain the first flyby quality data of the nucleus and inner coma of a dynamically new comet. However, the comets dust tail potentially posed an impact hazard to those spacecraft. To characterize the comet at large heliocentric distances, study its long-term evolution, and provide critical inputs to hazard modeling, we imaged C/Siding Spring with the Hubble Space Telescope when the comet was at 4.58, 3.77, and 3.28 AU from the Sun. The dust production rate, parameterized by the quantity Af$rho$, was 2500, 2100, and 1700 cm (5000-km radius aperture) for the three epochs, respectively. The color of the dust coma is 5.0$pm$0.3$%$/100 nm for the first two epochs, and 9.0$pm$0.3$%$/100 nm for the last epoch, and reddens with increasing cometocentric distance out to ~3000 km from the nucleus. The spatial distribution and the temporal evolution of the dust color are most consistent with the existence of icy grains in the coma. Two jet-like dust features appear in the north-northwest and southeast directions projected in the sky plane. Within each epoch of 1-2 hour duration, no temporal variations were observed for either feature, but the PA of the southeastern feature varied between the three epochs by ~30$^circ$. The dust feature morphology suggests two possible orientations for the rotational pole of the nucleus, (RA, Dec) = (295$^circpm$5$^circ$, +43$^circpm$2$^circ$) and (190$^circpm$10$^circ$, 50$^circpm$5$^circ$), or their diametrically opposite orientations.
Long period comet C/2021 A1 (Leonard) will approach Venus to within 0.029 au on 2021 December 18 and may subsequently graze the planet with its dust trail less than two days later. We observed C/2021 A1 with the Lowell Discovery Telescope on 2021 January 13 and March 3, as well as with the Palomar Hale Telescope on 2021 March 20, while the comet was inbound at heliocentric distances of r=4.97 au, 4.46 au, and 4.28 au, respectively. Tail morphology suggests that the dust is optically dominated by ~0.1-1 mm radius grains produced in the prior year. Neither narrowband imaging photometry nor spectrophotometry reveal any definitive gas emission, placing 3-sigma upper bounds on CN production of <1e23 molec/s at both of the latter two epochs. Trajectory analysis indicates that large (>1 mm) grains ejected at extremely large heliocentric distances (r>30 au) are most strongly favored to reach Venus. The flux of such meteors on Venus, and thus their potential direct or indirect observability, is highly uncertain as the comets dust production history is poorly constrained at these distances, but will likely fall well below the meteor flux from comet C/2013 A1 (Siding Spring)s closer encounter to Mars in 2014, and thus poses negligible risk to any spacecraft in orbit around Venus. Dust produced in previous apparitions will not likely contribute substantially to the meteor flux, nor will dust from any future activity apart from an unlikely high speed (>0.5 km/s) dust outburst prior to the comet reaching r~2 au in 2021 September.
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