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Abstract Kinetic deflection is a planetary defense technique delivering spacecraft momentum to a small body to deviate its course from Earth. The deflection efficiency depends on the impactor and target. Among them, the contribution of global curvature was poorly understood. The ejecta plume created by NASA’s Double Asteroid Redirection Test impact on its target asteroid, Dimorphos, exhibited an elliptical shape almost aligned along its north-south direction. Here, we identify that this elliptical ejecta plume resulted from the target’s curvature, reducing the momentum transfer to 44 ± 10% along the orbit track compared to an equivalent impact on a flat target. We also find lower kinetic deflection of impacts on smaller near-Earth objects due to higher curvature. A solution to mitigate low deflection efficiency is to apply multiple low-energy impactors rather than a single high-energy impactor. Rapid reconnaissance to acquire a target’s properties before deflection enables determining the proper locations and timing of impacts.

We report the detection of water vapor associated with main-belt comet 358P/PANSTARRS on UT 2024 January 8–9 using the NIRSPEC instrument on board JWST. We derive a water production rate of ${Q}_{{{\rm{H}}}_{2}{\rm{O}}}=(5.0\pm 0.2)\,\times {10}^{25}$ molecules s ^−1 , marking only the second direct detection of sublimation products of any kind from a main-belt comet, after 238P/Read. Similar to 238P, we find a remarkable absence of hypervolatile species, finding ${Q}_{{\mathrm{CO}}_{2}}\lt 7.6\times {10}^{22}$ molecules s ^−1 , corresponding to ${Q}_{{\mathrm{CO}}_{2}}/{Q}_{{{\rm{H}}}_{2}{\rm{O}}}\lt 0.2$ %. Upper limits on CH _3 OH and CO emission are also estimated. Photometry from ground-based observations shows that the dust coma brightened and faded slowly over ∼250 days in 2023–2024, consistent with photometric behavior observed in 2012–2013, but also indicates a ∼2.5× decline in the dust production rate between these two periods. Dynamical dust modeling shows that the coma’s morphology as imaged by JWST's NIRCam instrument on 2023 November 22 can be reproduced by asymmetric dust emission from a nucleus with a midrange obliquity ( ε ∼ 80°) with a steady-state mass-loss rate of ∼0.8 kg s ^−1 . Finally, we find similar Af ρ -to-gas ratios of ${\mathrm{log}}_{10}({Af}\rho /{Q}_{{{\rm{H}}}_{2}{\rm{O}}})=-24.8\pm 0.2$ for 358P and ${\mathrm{log}}_{10}({Af}\rho /{Q}_{{{\rm{H}}}_{2}{\rm{O}}})=-24.4\pm 0.2$ for 238P, suggesting that Af ρ could serve as an effective proxy for estimating water production rates in other active main-belt comets. The confirmation of water vapor outgassing in both main-belt comets observed by JWST to date reinforces the use of recurrent activity near perihelion as an indicator of sublimation-driven activity in active asteroids.

The NASA Double Asteroid Redirection Test spacecraft successfully impacted the Didymos–Dimorphos binary asteroid system on 2022 September 26 UTC. We provide an update to its preimpact mutual orbit and estimate the postimpact physical and orbital parameters, derived using ground-based photometric observations taken from 2022 July to 2023 February. We found that the total change of the orbital period was −33.240 ± 0.072 minutes (all uncertainties are 3 σ ). We obtained the eccentricity of the postimpact orbit to be 0.028 ± 0.016 and the apsidal precession rate was 7.3 ± 2.0 degrees day ^−1 from the impact to 2022 December 2. The data taken later in 2022 December to 2023 February suggest that the eccentricity dropped close to zero or the orbit became chaotic approximately 70 days after the impact. Most of the period change took place immediately after the impact, but in the few weeks following the impact it was followed by an additional change of $-{27}_{-58}^{+19}$ s or −19 ± 18 s (the two values depend on the approach we used to describe the evolution of the orbital period after the impact—an exponentially decreasing angular acceleration or the assumption of a constant orbital period, which changed abruptly some time after the impact, respectively). We estimate the preimpact Dimorphos–Didymos size ratio was 0.223 ± 0.012 and the postimpact is 0.202 ± 0.018, which indicate a marginally significant reduction of Dimorphos’ volume by (9 ± 9)% as the result of the impact.

NASA's Double Asteroid Redirection Test (DART) mission was the first to demonstrate asteroid deflection, and the mission's Level 1 requirements guided its planetary defense investigations. Here, we summarize DART's achievement of those requirements. On 2022 September 26, the DART spacecraft impacted Dimorphos, the secondary member of the Didymos near-Earth asteroid binary system, demonstrating an autonomously navigated kinetic impact into an asteroid with limited prior knowledge for planetary defense. Months of subsequent Earth-based observations showed that the binary orbital period was changed by –33.24 minutes, with two independent analysis methods each reporting a 1 σ uncertainty of 1.4 s. Dynamical models determined that the momentum enhancement factor, β , resulting from DART's kinetic impact test is between 2.4 and 4.9, depending on the mass of Dimorphos, which remains the largest source of uncertainty. Over five dozen telescopes across the globe and in space, along with the Light Italian CubeSat for Imaging of Asteroids, have contributed to DART's investigations. These combined investigations have addressed topics related to the ejecta, dynamics, impact event, and properties of both asteroids in the binary system. A year following DART's successful impact into Dimorphos, the mission has achieved its planetary defense requirements, although work to further understand DART's kinetic impact test and the Didymos system will continue. In particular, ESA's Hera mission is planned to perform extensive measurements in 2027 during its rendezvous with the Didymos–Dimorphos system, building on DART to advance our knowledge and continue the ongoing international collaboration for planetary defense.

With the successful impact of the NASA Double Asteroid Redirection Test (DART) spacecraft in the Didymos–Dimorphos binary asteroid system, we provide an initial analysis of the post-impact perturbed binary asteroid dynamics. To compare our simulation results with observations, we introduce a set of “observable elements” calculated using only the physical separation of the binary asteroid, rather than traditional Keplerian elements. Using numerical methods that treat the fully spin–orbit-coupled dynamics, we estimate the system’s mass and the impact-induced changes in orbital velocity, semimajor axis, and eccentricity. We find that the changes to the mutual orbit depend strongly on the separation distance between Didymos and Dimorphos at the time of impact. If Dimorphos enters a tumbling state after the impact, this may be observable through changes in the system’s eccentricity and orbit period. We also find that any DART-induced reshaping of Dimorphos would generally reduce the required change in orbital velocity to achieve the measured post-impact orbit period, and will be assessed by the ESA Hera mission in 2027.

We report observations of nine main-belt comets (MBCs) or candidate MBCs, most of which were obtained when the targets were apparently inactive. We find effective nucleus radii (assuming albedos of p _V = 0.05 ± 0.02) of r _n = (0.24 ± 0.05) km for 238P/Read, r _n = (0.9 ± 0.2) km for 313P/Gibbs, r _n = (0.6 ± 0.1) km for 324P/La Sagra, r _n = (1.0 ± 0.2) km for 426P/PANSTARRS, r _n = (0.5 ± 0.1) km for 427P/ATLAS, r _n < (0.3 ± 0.1) km for P/2016 J1-A (PANSTARRS), r _n < (0.17 ± 0.04) km for P/2016 J1-B (PANSTARRS), r _n ≤ (0.5 ± 0.2) km for P/2017 S9 (PANSTARRS), recently redesignated 455P/PANSTARRS, and r _n = (0.4 ± 0.1) km for P/2019 A3 (PANSTARRS). We identify evidence of activity in observations of 238P in 2021, and find similar inferred activity onset times and net initial mass-loss rates for 238P during perihelion approaches in 2010, 2016, and 2021. P/2016 J1-A and P/2016 J1-B are also found to be active in 2021 and 2022, making them collectively the tenth MBC confirmed to be recurrently active near perihelion and therefore likely to be exhibiting sublimation-driven activity. The nucleus of 313P is found to have colors of $g^{\prime} -r^{\prime} =0.52\pm 0.05$ and $r^{\prime} -i^{\prime} =0.22\pm 0.07$ , consistent with 313P being a Lixiaohua family member. We also report nondetections of P/2015 X6 (PANSTARRS), where we conclude that its current nucleus size is likely below our detection limits ( r _n ≲ 0.3 km). Lastly, we find that of 17 MBCs or candidate MBCs for which nucleus sizes (or inferred parent body sizes) have been estimated, >80% have r _n ≤ 1.0 km, pointing to an apparent physical preference toward small MBCs, where we suggest that Yarkovsky–O’Keefe–Radzievskii–Paddack spin-up may play a significant role in triggering and/or facilitating MBC activity.

AbstractWe have investigated the influence of lunarlike satellites on the infrared orbital light curves of Earth-analog extrasolar planets. Such light curves will be obtained by NASA''s Terrestrial Planet Finder (TPF) and ESA''s Darwin missions as a consequence of repeat observations to confirm the companion status of a putative planet and determine its orbit. We used an energy balance model to calculate disk-averaged infrared (bolometric) fluxes from planet-satellite systems over a full orbital period (one year). The satellites are assumed to lack an atmosphere, have a low thermal inertia like that of the Moon, and span a range of plausible radii. The planets are assumed to have thermal and orbital properties that mimic those of Earth, while their obliquities and orbital longitudes of inferior conjunction remain free parameters. Even if the gross thermal properties of the planet can be independently constrained (e.g.,via spectroscopy or visible-wavelength detection of specular glint from a surface ocean), only the largest (∼Mars-sized) lunarlike satellites can be detected by light curve data from a TPF-like instrument (i.e.,one that achieves a photometric signal-to-noise ratio of 10 to 20 at infrared wavelengths). Nondetection of a lunarlike satellite can obfuscate the interpretation of a given system''s infrared light curve so that it may resemble a single planet with high obliquity, different orbital longitude of vernal equinox relative to inferior conjunction, and in some cases drastically different thermal characteristics. If the thermal properties of the planet are not independently established, then the presence of a lunarlike satellite cannot be inferred from infrared data, which would thus demonstrate that photometric light curves alone can only be used for preliminary study, and the addition of spectroscopic data will be necessary. Key Words: Planetary systems—Planets and satellites: general—Astrobiology—Methods: data analysis. Astrobiology 9, 269–277. [ABSTRACT FROM AUTHOR]