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 Dimorphos7 demonstrates that kinetic impactor technology is a viable technique to potentially defend Earth if necessary.

Magnetic nanoparticles (MNP) offer promise for local hyperthermia, thermoablative cancer therapy and microwave curing of polymers. Rosensweig's theory predicts that particle size dependence on RF magnetic heating of ferrofluids is chiefly determined by magnetic moment, magnetic anisotropy, and the viscosity of the fluid. Since relaxation times are thermally activated and material parameters can have strong T dependences, heating rates peak at a certain temperature. We extend the model to include the T dependence of the magnetization and anisotropy using mean field theory and literature reported T dependences of selected fluids considered for biomedical applications. We model materials with Curie temperatures near room temperature for which the magnetic properties are strongly T dependent to address the problem of self-regulated heating of ferrofluids.

Magnetic nanoparticles (MNPs) used as heat sources for cancer thermotherapy have received much recent attention. While the mechanism for power dissipation in MNPs in a rf field is well understood, a challenge in moving to clinical trials is an inadequate understanding of the power dissipation in MNP-impregnated systems and the discrepancy between the predicted and observed heating rates in the same. Here we use the Rosensweig [J. Magn. Magn. Mater. 252, 370 (2002)] model for heat generation in a single MNP, considering immediate heating of the MNPs, and the double spherical-shell heat transfer equations developed by Andrä et al. [J. Magn. Magn. Mater. 194, 197 (1999)] to model the heat distribution in and around a ferrofluid sample or a tumor impregnated with MNPs. We model the heat generated at the edge of a 2.15 cm spherical sample of FeCo/(Fe,Co)3O4 agglomerates containing 95 vol % MNPs with mean radius of 9 nm, dispersed at 1.5–1.6 vol % in bisphenol F. We match the model against experimental data for a similar system produced in our laboratory and find good agreement. Finite element models, extensible to more complex systems, have also been developed and checked against the analytical model and the data.

Metal-semiconductor eutectic liquids play a key role in both the fundamental understanding of atomic interactions and nanoscale synthesis and catalysis. At reduced sizes they exhibit properties distinct from the bulk. In this work we show an unusual effect that the formation of AuSi eutectic liquid layers is much easier for smaller thicknesses. The alloying reaction rate is enhanced by over 20 times when the thickness is reduced from 300 to 20 nm. The strong enhancement is attributed to a strain-induced increase in the chemical potential of the solid layer prior to the alloying reaction.

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Recent studies of transition metal alloys with compositions that place them near their limits of elastic stability (e.g. near the body-centered-cubic [BCC] to hexagonal-close-packed [HCP] transition) suggest interesting behavior for the dislocation cores. Specifically, the dislocation core size is predicted to diverge as the stability limit is approached. Here a simple analysis rooted in elasticity theory and the computation of ideal strength is used to analyze this divergence. This analysis indicates that dislocation core radii should diverge as the elastic limits of stability are approached in the BCC, HCP, and face-centered-cubic (FCC) structures. Moreover, external stresses and dislocationinduced stresses also increase the core radii. Density functional theory based total energy calculations are combined with anisotropic elasticity theory to compute numerical estimates of dislocation core radii.

We demonstrate a reversible phase transition in nanoparticles composed of a binary eutectic alloy, Ge–Au. The structure, 9 nm diameter nanoparticles embedded in silica, can be switched from bilobe to mixed using a 30 ns ultraviolet laser pulse. The structure can be switched back to bilobe by heating at 80 °C. The bilobe/mixed switching can be performed on the same sample at least ten times. Synchrotron X-ray diffraction studies reveal that the bilobe structure contains crystalline Ge and Au while the mixed structure consists of crystalline Ge and β Ge–Au.

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