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The NASA Double Asteroid Redirection Test (DART) mission performed a kinetic impact on asteroid Dimorphos, the satellite of the binary asteroid (65803) Didymos, at 23:14 UTC on 26 September 2022 as a planetary defence test1. DART was the first hypervelocity impact experiment on an asteroid at size and velocity scales relevant to planetary defence, intended to validate kinetic impact as a means of asteroid deflection. Here we report a determination of the momentum transferred to an asteroid by kinetic impact. On the basis of the change in the binary orbit period2, we find an instantaneous reduction in Dimorphos’s along-track orbital velocity component of 2.70 ± 0.10 mm s−1, indicating enhanced momentum transfer due to recoil from ejecta streams produced by the impact3,4. For a Dimorphos bulk density range of 1,500 to 3,300 kg m−3, we find that the expected value of the momentum enhancement factor, β, ranges between 2.2 and 4.9, depending on the mass of Dimorphos. If Dimorphos and Didymos are assumed to have equal densities of 2,400 kg m−3, β=3.61−0.25+0.19(1σ). These βvalues indicate that substantially more momentum was transferred to Dimorphos from the escaping impact ejecta than was incident with DART. Therefore, the DART kinetic impact was highly effective in deflecting the asteroid Dimorphos.

Although no known asteroid poses a threat to Earth for at least the next century, the catalogue of near-Earth asteroids is incomplete for objects whose impacts would produce regional devastation1,2. Several approaches have been proposed to potentially prevent an asteroid impact with Earth by deflecting or disrupting an asteroid1–3. A test of kinetic impact technology was identified as the highest-priority space mission related to asteroid mitigation1. NASA’s Double Asteroid Redirection Test (DART) mission is a full-scale test of kinetic impact technology. The mission’s target asteroid was Dimorphos, the secondary member of the S-type binary near-Earth asteroid (65803) Didymos. This binary asteroid system was chosen to enable ground-based telescopes to quantify the asteroid deflection caused by the impact of the DART spacecraft4. Although past missions have utilized impactors to investigate the properties of small bodies5,6, those earlier missions were not intended to deflect their targets and did not achieve measurable deflections. Here we report the DART spacecraft’s autonomous kinetic impact into Dimorphos and reconstruct the impact event, including the timeline leading to impact, the location and nature of the DART impact site, and the size and shape of Dimorphos. The successful impact of the DART spacecraft with Dimorphos and the resulting change in the orbit of Dimorphos7demonstrates that kinetic impactor technology is a viable technique to potentially defend Earth if necessary.

Some active asteroids have been proposed to be formed as a result of impact events1. Because active asteroids are generally discovered by chance only after their tails have fully formed, the process of how impact ejecta evolve into a tail has, to our knowledge, not been directly observed. The Double Asteroid Redirection Test (DART) mission of NASA2, in addition to having successfully changed the orbital period of Dimorphos3, demonstrated the activation process of an asteroid resulting from an impact under precisely known conditions. Here we report the observations of the DART impact ejecta with the Hubble Space Telescope from impact time T+ 15 min to T+ 18.5 days at spatial resolutions of around 2.1 km per pixel. Our observations reveal the complex evolution of the ejecta, which are first dominated by the gravitational interaction between the Didymos binary system and the ejected dust and subsequently by solar radiation pressure. The lowest-speed ejecta dispersed through a sustained tail that had a consistent morphology with previously observed asteroid tails thought to be produced by an impact4,5. The evolution of the ejecta after the controlled impact experiment of DART thus provides a framework for understanding the fundamental mechanisms that act on asteroids disrupted by a natural impact1,6.

The Lyman Alpha Mapping Project has detected five discrete low‐albedo anomalies in Lyman‐α (Ly‐α; 121.6 nm) nighttime reflectance maps. These anomalies reside on the nearside of the Moon within the southeastern Oceanus Procellarum and northwestern Mare Nubium, coincident with regions that have been observed to be photometrically anomalous at visible wavelengths. Some of the spectral properties of these regions within near ultraviolet (NUV) to visible (VIS) wavelengths are consistent with lunar swirls, and they have been reported as “probable swirls” in at least one of these studies. However, while these regions have low Ly‐α normal albedo values relative to nearby highlands regions and are consistent with Ly‐α albedo observations of other swirls, they do not spectrally redden at wavelengths >160 nm as other swirls have been shown to do and they are not associated with regions of higher magnetic intensity. Interestingly, while these anomalies are not easily discerned in NUV single‐band images, the anomalies can be discerned in NUV color ratio composites of the Lunar Reconnaissance Orbiter Camera Wide‐Angle Camera. At least one anomaly observed in the far ultraviolet is not easily observed in the NUV. Additional analyses of their thermal emission Christiansen Feature values, empirically corrected for maturity, suggest that regolith maturity is what differentiates them from their surroundings. While these anomalies may not be swirls, they may represent an “endmember” to swirls minus the influence magnetic materials, which have been hypothesized to provide solar wind shielding. If true, they may provide additional insight into the generation of lunar swirls. Global maps of the Moon derived by the Lyman Alpha Mapping Project are providing new and unique views of the surface through far ultraviolet (FUV) eyes. Enigmatic lunar surface features, previously only detected with Earth‐based telescopes collecting visible observations at multiple ratioed lighting geometries (which led to these features being labeled “photometric anomalies”), are readily discerned in nighttime observations at the FUV wavelength of 121.6 nm (i.e., Lyman‐alpha). Their dark albedo characteristics in this wavelength make it tempting to relate them to other enigmatic features called “lunar swirls” (also dark at this wavelength). However, these anomalies lack sinuous geomorphology and proximity to magnetic materials as is typical of lunar swirls. While they are not swirls, they may somehow be related. The lack of proximity to magnetic materials may play a part in differences observed between these two enigmatic lunar surface features. Additional analyses of Lyman Alpha Mapping Project FUV, Lunar Reconnaissance Orbiter Camera Wide‐Angle Camera near ultraviolet, Diviner thermal infrared, and Kaguya Multispectral Imager data products suggest that these anomalies have been less exposed to space weathering relative to their surroundings and are therefore categorized as “less mature.” The process that created these unusual surface features remains unknown. Nighttime Lyman‐a maps confirm five low‐albedo regions present on the lunar nearside, in low magnetic intensity areas unlike swirlsWhile most readily viewed in Lyman‐a, we note detection in NUV ratio, color composite, and TIR Christensen Feature data productsUV and TIR characteristics are consistent with a shallow/small‐scale change in maturity, porosity, or roughness; however, origin is unknown

This study investigates how individual large craters on Mercury (diameters of 25–200 km) can produce surface roughness over a range of baselines (the spatial horizontal scale) from 0.5 to 250 km. Surface roughness is a statistical measure of change in surface height over a baseline usually after topography has been detrended. We use root mean square deviation as our measure of surface roughness. Observations of large craters on Mercury at baselines of 0.5–10 km found higher surface roughness values at the central uplifts, rims, and exteriors of craters, while the crater floors exhibit the lowest roughness values. At baselines <10 km, the regions exterior to large craters with diameters >80 km have the highest surface roughness values. These regions, which include the ejecta and secondary fields, are the main contributors to the increased surface roughness observed in high‐crater density regions. For baselines larger than 10 km, the crater cavity itself is the main contributor to surface roughness. We used a suite of numerical models, utilizing the measured surface roughness obtained in the study, to model the cumulative effect of adding large craters to a surface. The results indicate that not all of the surface roughness on Mercury is due to fresh large craters but that impact craters likely contribute to the Hurst exponent from baselines of 0.5–1.5 km and the shape of the deviogram. The simulations show that the surface roughness varied around an asymptote at the baselines studied before the surface was covered in impact craters. Impact cratering is the main process by which many planetary bodies are roughened, where an increase in the number of craters is related to higher surface roughness. In this study, we use observations and artificial data to explore how individual complex craters on Mercury can change the surface topography and produce surface roughness. Observations of the surface roughness of complex craters on Mercury found surface roughness related to several different geologic features of the craters. We found that impact craters are the main source of surface roughness on Mercury. We modeled how impact crater density affects surface roughness and found that it is difficult to relate surface age to surface roughness. For horizontal scales less than 10 km, ejecta and secondary craters drive surface roughness around large cratersSurface roughness at scales of 0.5–250 km is not correlated with crater density (diameters of 20–120 km)