Getting to Know Titan: Gravity-field measurements provide information on the interior structure of planets and their moons. Iess et al. (p. 457 ; published online 28 June) analyzed gravity data from six flybys of Saturn's moon, Titan, by the Cassini spacecraft between 2006 and 2011. The data suggest that Titan's interior is flexible on tidal time scales with the magnitude of the observed tidal deformations being consistent with the existence of a global subsurface water ocean.

Cassini's last look at Saturn's rings: During the final stages of the Cassini mission, the spacecraft flew between the planet and its rings, providing a new view on this spectacular system (see the Perspective by Ida). Setting the scene, Spilker reviews the numerous discoveries made using Cassini during the 13 years it spent orbiting Saturn. Iess et al. measured the gravitational pull on Cassini, separating the contributions from the planet and the rings. This allowed them to determine the interior structure of Saturn and the mass of its rings. Buratti et al. present observations of five small moons located in and around the rings. The moons each have distinctive shapes and compositions, owing to accretion of ring material. Tiscareno et al. observed the rings directly at close range, finding complex features sculpted by the gravitational interactions between moons and ring particles. Together, these results show that Saturn's rings are substantially younger than the planet itself and constrain models of their origin. Science , this issue p. 1046 , p. eaat2965 , p. eaat2349 , p. eaau1017 ; see also p. 1028

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 impact^3,4. 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, $${\beta =3.61}_{-0.25}^{+0.19}(1\sigma )$$β = 3.61 − 0.25 + 0.19 ( 1 σ ) . These β values 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.

Titan Through to the Core: Gravity measurements acquired from orbiting spacecraft can provide useful information about the interior of planets and their moons. Iess et al. (p. 1367 ; see the Perspective by Sohl ) used gravity data from four flybys of the Cassini spacecraft past Saturn's moon, Titan, to model the moon's gravity field and probe its deep interior structure. Their analysis implies that Titan is a partially differentiated body with a core consisting of a mix of ice and rock or hydrated silicates.

Constellations of satellites are being proposed in large numbers; most of them are expected to be in orbit within the next decade. They will provide communication to unserved and underserved communities, enable global monitoring of Earth and enhance space observation. Mostly enabled by technology miniaturization, satellite constellations require a coordinated effort to face the technological limits in spacecraft operations and space traffic. At the moment in fact, no cost-effective infrastructure is available to withstand coordinated flight of large fleets of satellites. In order for large constellations to be sustainable, there is the need to efficiently integrate and use them in the current space framework. This review paper provides an overview of the available experience in constellation operations and statistical trends about upcoming constellations at the moment of writing. It highlights also the tools most often proposed in the analyzed works to overcome constellation management issues, such as applications of machine learning/artificial intelligence and resource/infrastructure sharing. As such, it is intended to be a useful resource for both identifying emerging trends in satellite constellations, and enabling technologies still requiring substantial development efforts.

Saturn is orbited by dozens of moons, and the intricate dynamics of this complex system provide clues about its formation and evolution. Tidal friction within Saturn causes its moons to migrate outwards, driving them into orbital resonances that pump their eccentricities or inclinations, which in turn leads to tidal heating of the moons. However, in giant planets, the dissipative processes that determine the tidal migration timescale remain poorly understood. Standard theories suggest an orbital expansion rate inversely proportional to the power 11/2 in distance¹, implying negligible migration for outer moons such as Saturn’s largest moon, Titan. Here, we use two independent measurements obtained with the Cassini spacecraft to measure Titan’s orbital expansion rate. We find that Titan rapidly migrates away from Saturn on a timescale of roughly ten billion years, corresponding to a tidal quality factor of Saturn of Q ≃ 100, which is more than a hundred times smaller than most expectations. Our results for Titan and five other moons agree with the predictions of a resonance-locking tidal theory², sustained by excitation of inertial waves inside the planet. The associated tidal expansion is only weakly sensitive to orbital distance, motivating a revision of the evolutionary history of Saturn’s moon system. In particular, it suggests that Titan formed much closer to Saturn and has migrated outward to its current position.