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We quantified eight parent volatiles (H₂O, C₂H₆, HCN, CO, CH₃OH, H₂CO, C₂H₂, and CH₄) in the Jupiter-family comet Tempel 1 using high-dispersion infrared spectroscopy in the wavelength range 2.8 to 5.0 micrometers. The abundance ratio for ethane was significantly higher after impact, whereas those for methanol and hydrogen cyanide were unchanged. The abundance ratios in the ejecta are similar to those for most Oort cloud comets, but methanol and acetylene are lower in Tempel 1 by a factor of about 2. These results suggest that the volatile ices in Tempel 1 and in most Oort cloud comets originated in a common region of the protoplanetary disk.

Asteroids with diameters less than about 5 km have complex histories because they are small enough for radiative torques (that is, YORP, short for the Yarkovsky–O’Keefe–Radzievskii–Paddack effect) 1 to be a notable factor in their evolution 2 . (152830) Dinkinesh is a small asteroid orbiting the Sun near the inner edge of the main asteroid belt with a heliocentric semimajor axis of 2.19  au ; its S-type spectrum 3 , 4 is typical of bodies in this part of the main belt 5 . Here we report observations by the Lucy spacecraft 6 , 7 as it passed within 431 km of Dinkinesh. Lucy revealed Dinkinesh, which has an effective diameter of only 720 m, to be unexpectedly complex. Of particular note is the presence of a prominent longitudinal trough overlain by a substantial equatorial ridge and the discovery of the first confirmed contact binary satellite, now named (152830) Dinkinesh I Selam. Selam consists of two near-equal-sized lobes with diameters of 210 m and 230 m. It orbits Dinkinesh at a distance of 3.1 km with an orbital period of about 52.7 h and is tidally locked. The dynamical state, angular momentum and geomorphologic observations of the system lead us to infer that the ridge and trough of Dinkinesh are probably the result of mass failure resulting from spin-up by YORP followed by the partial reaccretion of the shed material. Selam probably accreted from material shed by this event. Observations from the Lucy spacecraft of the small main-belt asteroid (152830) Dinkinesh reveals unexpected complexity, with a longitudinal trough and equatorial ridge, as well as the discovery of the first contact binary satellite.

The Lucy spacecraft successfully performed the first of two Earth Gravity Assist maneuvers on October 16th 2022, flying 360 km above the Earth’s surface at 11:04 UT. The flyby was essential for the Lucy mission design, but also provided a wealth of data for scientific, calibration, and public engagement purposes. The Earth and Moon provided excellent calibration targets, being large, bright, and well-characterized, though instrument saturation was sometimes an issue, as the instruments are designed for operation 5 AU from the sun. Calibration data of the Earth and/or Moon were taken with all Lucy instruments, improving knowledge of instrument alignment, stray light characteristics, and sensitivity to resolved targets. In addition, Lucy obtained scientifically valuable thermal emission spectra of the Moon, and extensive images of the DART mission impact into the Didymos system, from a unique geometry, 20 days before the Earth flyby.

The composition of ices in comets may reflect that of the molecular cloud in which the Sun formed, or it may show evidence of chemical processing in the pre-planetary accretion disk around the proto-Sun. As carbon monoxide (CO) is ubiquitous in molecular clouds 1 , 2 , its abundance with respect to water could help to determine the degree to which pre-cometary material was processed, although variations in CO abundance may also be influenced by the distance from the Sun at which comets formed 3 , 4 , 5 . Observations have not hitherto provided an unambiguous measure of CO in the cometary ice (native CO). Evidence for an extended source of CO associated with comet Halley was provided by the Giotto spacecraft 6 , 7 , 8 , 9 , but alternative interpretations exist 10 . Here we report observations of comet Hale–Bopp which show that about half of the CO in the comet comes directly from ice stored in the nucleus. The abundance of this CO with respect to water (12 per cent) is smaller than in quiescent regions of molecular clouds, but is consistent with that measured in proto-stellar envelopes 11 , suggesting that the ices underwent some processing before their inclusion into Hale–Bopp. The remaining CO arises in the coma, probably through thermal destruction of more complex molecules.

The saturated hydrocarbons ethane (C$_2$H$_6$) and methane (CH$_4$) along with carbon monoxide (CO) and water (H$_2$O) were detected in comet C/1996 B2 Hyakutake with the use of high-resolution infrared spectroscopy at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. The inferred production rates of molecular gases from the icy, cometary nucleus (in molecules per second) are 6.4 × 10$^{26}$ for C$_2$H$_6$, 1.2 × 10$^{27}$ for CH$_4$, 9.8 × 10$^{27}$ for CO, and 1.7 × 10$^{29}$ for H$_2$O. An abundance of C$_2$H$_6$ comparable to that of CH$_4$ implies that ices in C/1996 B2 Hyakutake did not originate in a thermochemically equilibrated region of the solar nebula. The abundances are consistent with a kinetically controlled production process, but production of C$_2$H$_6$ by gas-phase ion molecule reactions in the natal cloud core is energetically forbidden. The high C$_2$H$_6$/CH$_4$ ratio is consistent with production of C$_2$H$_6$ in icy grain mantles in the natal cloud, either by photolysis of CH$_4$-rich ice or by hydrogen-addition reactions to acetylene condensed from the gas phase.

Comets provide a valuable window into the chemical and physical conditions at the time of their formation in the young solar system. We seek insights into where and when these objects formed by comparing the range of abundances observed for nine molecules and their average values across a sample of 29 comets to the predicted midplane ice abundances from models of the protosolar nebula. Our fiducial model, where ices are inherited from the interstellar medium, can account for the observed mixing ratio ranges of each molecule considered, but no single location or time reproduces the abundances of all molecules simultaneously. This suggests that each comet consists of material processed under a range of conditions. In contrast, a model where the initial composition of disk material is `reset', wiping out any previous chemical history, cannot account for the complete range of abundances observed in comets. Using toy models that combine material processed under different thermal conditions we find that a combination of warm (CO-poor) and cold (CO-rich) material is required to account for both the average properties of the Jupiter-family and Oort cloud comets, and the individual comets we consider. This could occur by the transport (either radial or vertical) of ice-coated dust grains in the early solar system. Comparison of the models to the average Jupiter-family and Oort cloud comet compositions suggest the two families formed in overlapping regions of the disk, in agreement with the findings of A'Hearn et al. (2012) and with the predictions of the Nice model (Gomes et al. 2005, Tsiganis et al. 2005).

Comet 73P in pieces The 2006 visit of comet 73P/Schwassmann-Wachmann 3 (comet 73P) took it quite close to Earth. It had split a decade earlier into at least five fragments and by 2006, more than 60 fragments were orbiting the Sun. The comet's disruption had exposed fresh material that had been formerly buried deep within the comet's original nucleus. This provided ideal conditions for a natural equivalent to NASA's Deep Impact mission, digging even deeper into the comet than the probe that struck comet Tempel 1. Observation of fragments B and C of comet 73P showed them to be remarkably similar in composition. This contrasts to the marked variation in composition between different comets, and contradicts previous assumptions that short-period comets would have strong compositional variation with depth. A report of the chemical composition of two distinct fragments of comet 73P/Schwassmann–Wachmann 3. The fragments are similar in composition, in contrast to the chemical diversity of comets and contrary to the expectation that short-period comets should show strong compositional variation with depth. The remarkable compositional diversity of volatile ices within comets 1 , 2 , 3 can plausibly be attributed to several factors, including differences in the chemical, thermal and radiation environments in comet-forming regions, chemical evolution during their long storage in reservoirs far from the Sun 4 , and thermal processing by the Sun after removal from these reservoirs. To determine the relevance of these factors, measurements of the chemistry as a function of depth in cometary nuclei are critical. Fragmenting comets expose formerly buried material, but observational constraints have in the past limited the ability to assess the importance of formative conditions and the effects of evolutionary processes on measured composition 5 , 6 , 7 , 8 . Here we report the chemical composition of two distinct fragments of 73P/Schwassmann–Wachmann 3. The fragments are remarkably similar in composition, in marked contrast to the chemical diversity within the overall comet population and contrary to the expectation that short-period comets should show strong compositional variation with depth in the nucleus owing to evolutionary processing from numerous close passages to the Sun. Comet 73P/Schwassmann–Wachmann 3 is also depleted in the most volatile ices compared to other comets, suggesting that the depleted carbon-chain chemistry seen in some comets from the Kuiper belt reservoir is primordial and not evolutionary 1 .

We present high spectral resolution optical spectra obtained with the ARCES instrument at Apache Point Observatory showing detection of the [OI]6300 A line in interstellar comet 2I/Borisov. We employ the observed flux in this line to derive an H\\(_2\\)O production rate of (6.3\\(\\pm\\)1.5)\\(\\times\\)10\\(^{26}\\) mol s\\(^{-1}\\). Comparing to previously reported observations of CN, this implies a CN/H\\(_2\\)O ratio of \\(\\sim\\)0.3-0.6%. The lower end of this range is consistent with the average value in comets, while the upper end is higher than the average value for Solar System comets, but still within the range of observed values. C\\(_2\\)/H\\(_2\\)O is depleted, with a value likely less than 0.1%. The dust-to-gas ratio is consistent with the normal value for Solar System comets. Using a simple sublimation model we estimate an H\\(_2\\)O active area of 1.7 km\\(^2\\), which for current estimates for the size of Borisov suggests active fractions between 1-150%, consistent with values measured in Solar System comets. More detailed characterization of 2I/Borisov, including compositional information and properties of the nucleus, is needed to fully interpret the observed H\\(_2\\)O production rate.

Since its launch in 1990, the Hubble Space Telescope (HST) has served as a platform with unique capabilities for remote observations of comets in the far-ultraviolet region of the spectrum. Successive generations of imagers and spectrographs have seen large advances in sensitivity and spectral resolution enabling observations of the diverse properties of a representative number of comets during the past 25 years. To date, four comets have been observed in the far-ultraviolet by the Cosmic Origins Spectrograph (COS), the last spectrograph to be installed in HST, in 2009: 103P/Hartley 2, C/2009 P1 (Garradd), C/2012 S1 (ISON), and C/2014 Q2 (Lovejoy). COS has unprecedented sensitivity, but limited spatial information in its 2.5 arcsec diameter circular aperture, and our objective was to determine the CO production rates from measurements of the CO Fourth Positive system in the spectral range of 1400 to 1700 A. In the two brightest comets, nineteen bands of this system were clearly identified. The water production rates were derived from nearly concurrent observations of the OH (0,0) band at 3085 A by the Space Telescope Imaging Spectrograph (STIS). The derived CO/H2O production rate ratio ranged from ~0.3% for Hartley 2 to ~22% for Garradd. In addition, strong partially resolved emission features due to multiplets of S I, centered at 1429 A and 1479 A, and of C I at 1561 A and 1657 A, were observed in all four comets. Weak emission from several lines of the H2 Lyman band system, excited by solar Lyman-alpha and Lyman-beta pumped fluorescence, were detected in comet Lovejoy.

Comet C/2016 R2 (PanSTARRS) has a peculiar volatile composition, with CO being the dominant volatile as opposed to H\\(_2\\)O and one of the largest N\\(_2\\)/CO ratios ever observed in a comet. Using observations obtained with the \\textit{Spitzer Space Telescope}, NASA's Infrared Telescope Facility, the 3.5-meter ARC telescope at Apache Point Observatory, the Discovery Channel Telescope at Lowell Observatory, and the Arizona Radio Observatory 10-m Submillimeter Telescope we quantified the abundances of 12 different species in the coma of R2 PanSTARRS. We confirm the high abundances of CO and N\\(_2\\) and heavy depletions of H\\(_2\\)O, HCN, CH\\(_3\\)OH, and H\\(_2\\)CO compared to CO reported by previous studies. We provide the first measurements (or most sensitive measurements/constraints) on H\\(_2\\)O, CO\\(_2\\), CH\\(_4\\), C\\(_2\\)H\\(_6\\), OCS, C\\(_2\\)H\\(_2\\), and NH\\(_3\\), all of which are depleted relative to CO by at least one to two orders of magnitude compared to values commonly observed in comets. The observed species also show strong enhancements relative to H\\(_2\\)O, and even when compared to other species like CH\\(_4\\) or CH\\(_3\\)OH most species show deviations from typical comets by at least a factor of two to three. The only mixing ratios found to be close to typical are CH\\(_3\\)OH/CO\\(_2\\) and CH\\(_3\\)OH/CH\\(_4\\). While R2 PanSTARRS was located at a heliocentric distance of 2.8 AU at the time of our observations in January/February 2018, we argue that this alone cannot account for the peculiar observed composition of this comet and therefore must reflect its intrinsic composition. We discuss possible implications for this clear outlier in compositional studies of comets obtained to date, and encourage future dynamical and chemical modeling in order to better understand what the composition of R2 PanSTARRS tells us about the early Solar System.