Astronomical Units Research Articles

The structure of magnetic turbulence in the heliosheath region observed by Voyager 2 at 106 AU

It is currently believed that the turbulent fluctuations pervade the outermost heliosphere. Turbulence, magnetic reconnection, and their link may be responsible for magnetic energy conversion in these regions. The governing mechanisms of such anisotropic and compressible magnetic turbulence in the inner heliosheath (IHS) and in the local interstellar medium (LISM) still lack a thorough description. The present literature mainly concerns large scales which are not representative of the inertial-cascade dynamics of turbulence. Moreover, lack of broadband spectral analysis makes the IHS dynamics critically understudied. Our recent study [1] shows that 48 s magnetic-field data from the Voyager mission are appropriate for a spectral analysis over a frequency range of six decades, from 5 × 10−8 Hz to 10−2 Hz. Here, focusing on the Voyager 2 observation interval from 2013.824 to 2016.0, we describe the structure of turbulence in a sector zone of the IHS. A spectral break around 7 × 10−7 Hz (magnetic structures with size ℓ ≈ 1.3 Astronomical Units) separates the energy-injection regime from the inertial-cascade regime of turbulence. A second scale is observed around 6 × 10−5 Hz (ℓ ≈ 0.017 AU) and corresponds to a peak of compressibility and intermittency of fluctuations.

Two cold belts in the debris disk around the G-type star NZ Lupi

Context. Planetary systems hold the imprint of the formation and of the evolution of planets especially at young ages, and in particular at the stage when the gas has dissipated leaving mostly secondary dust grains. The dynamical perturbation of planets in the dust distribution can be revealed with high-contrast imaging in a variety of structures. Aims. SPHERE, the high-contrast imaging device installed at the VLT, was designed to search for young giant planets in long period, but is also able to resolve fine details of planetary systems at the scale of astronomical units in the scattered-light regime. As a young and nearby star, NZ Lup was observed in the course of the SPHERE survey. A debris disk had been formerly identified with HST/NICMOS. Methods. We observed this system in the near-infrared with the camera in narrow and broad band filters and with the integral field spectrograph. High contrasts are achieved by the mean of pupil tracking combined with angular differential imaging algorithms. Results. The high angular resolution provided by SPHERE allows us to reveal a new feature in the disk which is interpreted as a superimposition of two belts of planetesimals located at stellocentric distances of ~85 and ~115 au, and with a mutual inclination of about 5°. Despite the very high inclination of the disk with respect to the line of sight, we conclude that the presence of a gap, that is, a void in the dust distribution between the belts, is likely. Conclusions. We discuss the implication of the existence of two belts and their relative inclination with respect to the presence of planets.

Building protoplanetary disks from the molecular cloud: redefining the disk timeline

Context. Planetary formation models are necessary to understand the characteristics of the planets that are the most likely to survive. Their dynamics, their composition and even the probability of their survival depend on the environment in which they form. We therefore investigate the most favorable locations for planetary embryos to accumulate in the protoplanetary disk: the planet traps. Aims. We study the formation of the protoplanetary disk by the collapse of a primordial molecular cloud, and how its evolution leads to the selection of specific types of planets. Methods. We use a hydrodynamical code that accounts for the dynamics, thermodynamics, geometry and composition of the disk to numerically model its evolution as it is fed by the infalling cloud material. As the mass accretion rate of the disk onto the star determines its growth, we can calculate the stellar characteristics by interpolating its radius, luminosity and temperature over the stellar mass from pre-calculated stellar evolution models. The density and midplane temperature of the disk then allow us to model the interactions between the disk and potential planets and determine their migration. Results. At the end of the collapse phase, when the disk reaches its maximum mass, it pursues its viscous spreading, similarly to the evolution from a minimum mass solar nebula (MMSN). In addition, we establish a timeline equivalence between the MMSN and a “collapse-formed disk” that would be older by about 2 Myr. Conclusions. We can save various types of planets from a fatal type-I inward migration: in particular, planetary embryos can avoid falling on the star by becoming trapped at the heat transition barriers and at most sublimation lines (except the silicates one). One of the novelties concerns the possible trapping of putative giant planets around a few astronomical units from the star around the end of the infall. Moreover, trapped planets may still follow the traps outward during the collapse phase and inward after it. Finally, this protoplanetary disk formation model shows the early possibilities of trapping planetary embryos at disk stages that are anterior by a few million years to the initial state of the MMSN approximation.

Exploring the formation by core accretion and the luminosity evolution of directly imaged planets

Context. A low-mass companion to the two-solar mass star HIP 65426 has recently been detected by SPHERE at around 100 au from its host. Explaining the presence of super-Jovian planets at large separations, as revealed by direct imaging, is currently an open question. Aims. We want to derive statistical constraints on the mass and initial entropy of HIP 65426 b and to explore possible formation pathways of directly imaged objects within the core-accretion paradigm, focusing on HIP 65426 b. Methods. Constraints on the planet’s mass and post-formation entropy are derived from its age and luminosity combined with cooling models. For the first time, the results of population synthesis are also used to inform the results. Then a formation model that includes N-body dynamics with several embryos per disc is used to study possible formation histories and the properties of possible additional companions. Finally, the outcomes of two- and three-planet scattering in the post-disc phase are analysed, taking tides into account for small-pericentre orbits. Results. The mass of HIP 65426 b is found to be mp = 9.9−1.8+1.1 MJ using the hot population and mp = 10.9−2.0+1.4 MJ with the cold-nominal population. We find that core formation at small separations from the star followed by outward scattering and runaway accretion at a few hundred astronomical units succeeds in reproducing the mass and separation of HIP 65426 b. Alternatively, systems having two or more giant planets close enough to be on an unstable orbit at disc dispersal are likely to end up with one planet on a wide HIP 65426 b-like orbit with a relatively high eccentricity (≳ 0.5). Conclusions. If this scattering scenario explains its formation, HIP 65426 b is predicted to have a high eccentricity and to be accompanied by one or several roughly Jovian-mass planets at smaller semi-major axes, which also could have a high eccentricity. This could be tested by further direct-imaging as well as radial-velocity observations.

The Central 1000 au of a Pre-stellar Core Revealed with ALMA. I. 1.3 mm Continuum Observations

Abstract Stars like our Sun form in self-gravitating dense and cold structures within interstellar clouds that are referred to as pre-stellar cores. Although much is known about the physical structure of dense clouds just before and soon after the switch-on of a protostar, the central few thousand astronomical units (au) of pre-stellar cores are unexplored. It is within these central regions that stellar systems assemble and fragmentation may take place, with the consequent formation of binaries and multiple systems. We present Atacama Large Millimetre and submillimetre Array (ALMA) Band 6 observations (Atacama Compact Array and 12 m array) of the dust continuum emission of the 8 M ⊙ pre-stellar core L1544, with an angular resolution of 2″ × 1.″6 (linear resolution 270 au × 216 au). Within the primary beam, a compact region of 0.1 M ⊙, which we call a “kernel,” has been unveiled. The kernel is elongated, with a central flat zone with radius R ker ≃ 10″ (≃1400 au). The average number density within R ker is ≃1 × 106 cm−3, with possible local density enhancements. The region within R ker appears to have fragmented, but detailed analysis shows that similar substructure can be reproduced by synthetic interferometric observations of a smooth centrally concentrated dense core with a similar central flat zone. The presence of a smooth kernel within a dense core is in agreement with non-ideal magnetohydro-dynamical simulations of a contracting cloud core with a peak number density of 1 × 107 cm−3. Dense cores with lower central densities are completely filtered out when simulated 12 m array observations are carried out. These observations demonstrate that the kernel of dynamically evolved dense cores can be investigated at high angular resolution with ALMA.

A High-resolution Mid-infrared Survey of Water Emission from Protoplanetary Disks

Abstract We present the largest survey of spectrally resolved mid-infrared water emission to date, with spectra for 11 disks obtained with the Michelle and TEXES spectrographs on Gemini North. Water emission is detected in six of eight disks around classical T Tauri stars. Water emission is not detected in the transitional disks SR 24 N and SR 24 S, in spite of SR 24 S having pretransitional disk properties like DoAr 44, which does show water emission. With R ∼ 100,000, the TEXES water spectra have the highest spectral resolution possible at this time, and allow for detailed line shape analysis. We find that the mid-IR water emission lines are similar to the “narrow component” in CO rovibrational emission, consistent with disk radii of a few astronomical units. The emission lines are either single peaked, or consistent with a double peak. Single-peaked emission lines cannot be produced with a Keplerian disk model, and may suggest that water participates in the disk winds proposed to explain single-peaked CO emission lines. Double-peaked emission lines can be used to determine the radius at which the line emission luminosity drops off. For HL Tau, the lower limit on this measured dropoff radius is consistent with the 13 au dark ring. We also report variable line/continuum ratios from the disks around DR Tau and RW Aur, which we attribute to continuum changes and line flux changes, respectively. The reduction in RW Aur line flux corresponds with an observed dimming at visible wavelengths.

Unveiling a Few Astronomical Unit Scale Rotation Structure around the Protostar in B335

Abstract We report a kinematic structure in the innermost envelope of the low-mass Class 0 protostar IRAS 19347+0727 in the Bok globule B335 observed at the best angular resolution ever achieved for this source with ALMA. This is based on observations of complex organic molecule emission in the 1.2 mm band, which selectively traces a hot and dense area around the protostar. The distribution of the CH3OH and HCOOH emission is resolved, and a clear velocity gradient is observed. Moreover, the direction of the gradient is found to be different between these two molecules. These features are well explained by the model of an infalling and rotating gas, but not by the model of a Keplerian motion. The protostellar mass and the radius of the centrifugal barrier are determined to be 0.02–0.06 M ⊙ and <5 au, respectively. The different directions of the gradients observed for CH3OH and HCOOH are interpreted as the different sizes of their distributions. On the other hand, the SiO emission seems to trace a compact region at the closest vicinity of the protostar, which is a launching point of the outflow or a shocked region caused by the gas accretion onto the protostar. These results first reveal the transition zone from the infalling motion to the rotating motion in this representative isolated protostellar source, which has long been employed as a testbed for star formation studies.

Propulsive Force in Electric Solar Sails for Missions in the Heliosphere

An electric solar sail (E-sail) is a recent propulsion technology concept capable of reaching the edge of the Heliosphere. An E-sail consists of a set of bare tethers at high positive/negative bias capable of deflecting solar wind protons to produce thrust. The propulsive force calculation for a single tether was focused on outer planet missions, considering the characteristic plasma ambient at 1 astronomical unit. However, both ion and electron temperature and plasma density cover a wide range of values within the Heliosphere. Propulsive forces are determined here for the solar wind plasma within the entire Heliosphere. Results show that propulsive forces increase with ion temperature, whereas decrease with electron temperature. In addition, propulsive forces do increase with plasma density despite the sheath decreases.

Gravitational Weakening of Seismic Origin as a Driving Mechanism of Some Astronomical Anomalies

The anomalous increase of the astronomical unit, the mysterious secular increase of the lunar eccentricity and the flyby anomaly are important issues of modern astronomy. This study aims to determine the effect of earthquakes on the origin of these anomalies. Based on conventional physics, we found the existence of earthquake-induced gravitational weakening within the earth-moon-sun system and beyond. New equations of gravitation, including time dilation, are introduced that can explain the anomalous increase of the astronomical unit and the lunar orbit. A modified equation that includes the effect of massive quakes also explains the anomalous increase of the lunar eccentricity. Furthermore, the results of the present study can explain the flyby and Pioneer anomalies experienced by spacecraft during gravity assisted maneuvers. A modification of the 3rd Law of Kepler is also presented. Implications on the elliptic orbit of the earth, its reduced velocity and the occurrence of leap years are also discussed. Using the seismic-induced gravitational weakening model, probable trigger mechanisms of the faint young sun paradox and the Allais effect are also presented. An estimate of the age of the earth based on the observed values of the Hubble parameter and the USGS earthquake data for the past century is also presented. A new model of the seismic-driven expanding universe and a new equation to determine the expansion rate of galaxies and the universe is also proposed. The sudden earth’s retreat due to gravitational weakening and its implication on anomalous astronomical refraction and flight risk at night especially near the equatorial region, and its effect on the abrupt satellite orbital decay, spin and drift are also discussed. This study may also shed light on the occurrence of sinkholes and massive landslides. Finally, this study proposes a new equation that can explain the observed changes in the fine structure constant.

How planetary growth outperforms migration

Planetary migration is a major challenge for planet-formation theories. The speed of type-I migration is proportional to the mass of a protoplanet, while the final decade of growth of a pebble-accreting planetary core takes place at a rate that scales with the mass to the two-thirds power. This results in planetary growth tracks (i.e., the evolution of the mass of a protoplanet versus its distance from the star) that become increasingly horizontal (migration dominated) with the rising mass of the protoplanet. It has been shown recently that the migration torque on a protoplanet is reduced proportional to the relative height of the gas gap carved by the growing planet. Here we show from 1D simulations of planet–disc interaction that the mass at which a planet carves a 50% gap is approximately 2.3 times the pebble isolation mass. Our measurements of the pebble isolation mass from 1D simulations match published 3D results relatively well, except at very low viscosities (α < 10−3) where the 3D pebble isolation mass is significantly higher, possibly due to gap edge instabilities that are not captured in 1D. The pebble isolation mass demarks the transition from pebble accretion to gas accretion. Gas accretion to form gas-giant planets therefore takes place over a few astronomical units of migration after reaching first the pebble isolation mass and, shortly after, the 50% gap mass. Our results demonstrate how planetary growth can outperform migration both during core accretion and during gas accretion, even when the Stokes number of the pebbles is small, St ~ 0.01, and the pebble-to-gas flux ratio in the protoplanetary disc is in the nominal range of 0.01–0.02. We find that planetary growth is very rapid in the first million years of the protoplanetary disc and that the probability for forming gas-giant planets increases with the initial size of the protoplanetary disc and with decreasing turbulent diffusion.

A model for a dark matter core at the Galactic Centre

We consider a toy model for the supermassive compact object at the galactic center that does not require the presence of a black hole. We assume a matter distribution of weakly interacting particles with a density profile inferred from dark matter profiles in the outer regions. We show that rotation curves close to the center of the Milky Way galaxy can be explained within this model. We also show that the motion of test particles (stars) at distances of the order of 100 astronomical units can not be distinguished from the motion of corresponding particles in the Schwarzschild geometry. However, differences arise at shorter distances, suggesting that it could be possible to observationally test the validity of the model in the near future.

Evolution of a Magnetic Flux Rope toward Eruption

Abstract It is well accepted that a magnetic flux rope (MFR) is a critical component of many coronal mass ejections (CMEs), yet how it evolves toward eruption remains unclear. Here we investigate the continuous evolution of a pre-existing MFR, which is rooted in strong photospheric magnetic fields and electric currents. The evolution of the MFR is observed by the Solar Terrestrial Relations Observatory (STEREO) and the Solar Dynamics Observatory (SDO) from multiple viewpoints. From STEREO’s perspective, the MFR starts to rise slowly above the limb five hours before it erupts as a halo CME on 2012 June 14. In SDO observations, conjugate dimmings develop on the disk, simultaneously with the gradual expansion of the MFR, suggesting that the dimmings map the MFR’s feet. The evolution comprises a two-stage gradual expansion followed by another stage of rapid acceleration/eruption. Quantitative measurements indicate that magnetic twist of the MFR increases from 1.0 ± 0.5 to 2.0 ± 0.5 turns during the five-hour expansion, and further increases to about 4.0 turns per astronomical unit (au) when detected as a magnetic cloud at 1 au two days later. In addition, each stage is preceded by flare(s), implying reconnection is actively involved in the evolution and eruption of the MFR. The implications of these measurements on the CME initiation mechanisms are discussed.

Large-Scale Structure of Solar Wind beyond the Earth’s Orbit: Reconstruction Using the Data of Two-Site Measurements of Interplanetary Scintillations in the Decameter Radio Range

Solar wind is a set of flows with different parameters (the speed, the exponent of the spectrum of heterogeneities, the width, etc.). A bimodal character of the speed distribution of the solar wind was determined in spaceborne experiments. The measurements onboard the Ulysses spacecraft confirmed that the bimodal structure of solar wind continues to persist at relatively large distances from the Sun (to several astronomical units). However, there is one more possibility to determine the stream structure of solar wind. This is the method of interplanetary scintillations. The purpose of the paper is to reconstruct the stream structure of solar wind beyond the Earth’s orbit using the data on interplanetary scintillations obtained at two observational sites. The experiments were carried out at decameter wavelengths, since they are rather strongly scattered by the rarefied interplanetary plasma beyond the Earth’s orbit. The experimental data on interplanetary scintillations analyzed in this work were obtained in synchronous observations with the UTR-2 and URAN-2 radio telescopes. The parameters of solar wind and its stream structure were determined by comparison of the characteristics of interplanetary scintillations measured in the experiment (the dependences of the harmonic velocity of the cross-spectrum of scintillations and the power spectra) to those calculated with the models. To separate the interplanetary and ionospheric scintillations, the spectral, spatial, and frequency criteria were used. The results of this analysis show that solar wind beyond the Earth’s orbit consists of several streams that replace each other on the line of sight toward the radio source. These investigations prove the reliability and efficiency of the interplanetary scintillation method for reconstructing the stream structure of solar wind.

A warped disk around an infant protostar.

Recent exoplanet studies have revealed that the orbital planes of planets are not always aligned with one another or with the equatorial plane of the central star. The misalignment has been ascribed to gravitational scattering by giant planets and/or companion stars1-3 or to fly-bys in stellar cluster environments4. Alternatively, the misalignment could be natal: that is, such planets were born in a warped protostellar disk5,6. Warped disk structures have been reported in some transition disks and protoplanetary disks7,8, but not in the earlier stages of protostar evolution, although such a possibility is suggested by outflow morphology9,10. Here we report millimetre-wavelength dust continuum observations of the young embedded protostar IRAS04368+2557 in the protostellar core L1527 at a distance11 of 137 parsecs; the protostar's disk is almost edge-on12-16. The inner and outer parts of the disk have slightly different orbital planes, connected at 40 to 60 astronomical units from the star, but the disk has point symmetry with respect to the position of the protostar. We interpret it as a warped disk that is rotationally supported. Because there is no evidence for a companion source17,18, the warped structure must be due to either anisotropic accretion of gas with different rotational axes, or misalignment of the rotation axis of the disk with the magnetic field direction.

New Comparative Metric for Evaluating Spacecraft Radiation Shielding

Given the lack of human medical data available to calibrate and tie the sophisticated model calculations of various space radiation exposure metrics, such as “dose equivalent,” to the actual biological harm expected from long-term exposure to galactic cosmic ray (GCR) radiation in the region of cislunar space [nominally at 1 astronomical unit (AU) from the sun], a new spacecraft shielding evaluation metric is proposed that empirically links to the available medical data collected from International Space Station (ISS) operations, and thereby improves shielding material assessment. Adequate protection from radiation is one of the most important unknowns regarding the contemplated long-term spaceflights beyond the Earth’s protective zone afforded by its magnetosphere. This new radiation shielding evaluation metric relies on a comparison of the relative shielding effectiveness of a particular material or shielding design in the cislunar space radiation environment to the verified radiation exposure environment inside the ISS, where hundreds of astronauts have flown and spent considerable time, and for which there is voluminous medical data and continuing medical observation. Thus, the new metric is a measure of how well a proposed shielding protocol operating in the 1 AU cislunar radiation environment replicates the ISS radiation exposure levels. And, because it is calculated on a strict comparable basis, it mitigates the unavoidable errors introduced in the shielding evaluation process inherent in using different GCR models and radiation transport codes, as well as the various versions of radiation quality or weighting factors.

The Disk Substructures at High Angular Resolution Project (DSHARP). IX. A High-definition Study of the HD 163296 Planet-forming Disk

Abstract The Atacama Large Millimeter/submillimeter Array observations of protoplanetary disks acquired by the Disk Substructure at High Angular Resolution Project resolve the dust and gas emission on angular scales as small as 3 astronomical units, offering an unprecedented detailed view of the environment where planets form. In this Letter, we present and discuss observations of the HD 163296 protoplanetary disk that imaged the 1.25 mm dust continuum and 12CO J = 2–1 rotational line emission at a spatial resolution of 4 and 10 au, respectively. The continuum observations resolve and allow us to characterize the previously discovered dust rings at radii of 68 and 100. They also reveal new small-scale structures, such as a dark gap at 10 au, a bright ring at 15 au, a dust crescent at a radius of 55 au, and several fainter azimuthal asymmetries. The observations of the CO and dust emission provide information about the vertical structure of the disk and allow us to directly constrain the dust extinction optical depth at the dust rings. Furthermore, the observed asymmetries in the dust continuum emission corroborate the hypothesis that the complex structure of the HD 163296 disk is the result of the gravitational interaction with yet-unseen planets.

Probing the Inner Disk Emission of the Herbig Ae Stars HD 163296 and HD 190073

Abstract The physical processes occurring within the inner few astronomical units of protoplanetary disks surrounding Herbig Ae stars are crucial to setting the environment in which the outer planet-forming disk evolves and put critical constraints on the processes of accretion and planet migration. We present the most complete published sample of high angular resolution H- and K-band observations of the stars HD 163296 and HD 190073, including 30 previously unpublished nights of observations of the former and 45 nights of the latter with the CHARA long-baseline interferometer, in addition to archival VLTI data. We confirm previous observations suggesting that significant near-infrared emission originates within the putative dust evaporation front of HD 163296 and show that this is the case for HD 190073 as well. The H- and K-band sizes are the same within (3 ± 3)% for HD 163296 and within (6 ± 10)% for HD 190073. The radial surface brightness profiles for both disks are remarkably Gaussian-like with little or no sign of the sharp edge expected for a dust evaporation front. Coupled with spectral energy distribution analysis, our direct measurements of the stellar flux component at the H and K bands suggest that HD 190073 is much younger (<400 kyr) and more massive (∼5.6 M ⊙) than previously thought, mainly as a consequence of the new Gaia distance (891 pc).

The Disk Substructures at High Angular Resolution Project (DSHARP). I. Motivation, Sample, Calibration, and Overview

Abstract We introduce the Disk Substructures at High Angular Resolution Project (DSHARP), one of the initial Large Programs conducted with the Atacama Large Millimeter/submillimeter Array (ALMA). The primary goal of DSHARP is to find and characterize substructures in the spatial distributions of solid particles for a sample of 20 nearby protoplanetary disks, using very high resolution (∼0.″035, or 5 au, FWHM) observations of their 240 GHz (1.25 mm) continuum emission. These data provide a first homogeneous look at the small-scale features in disks that are directly relevant to the planet formation process, quantifying their prevalence, morphologies, spatial scales, spacings, symmetry, and amplitudes, for targets with a variety of disk and stellar host properties. We find that these substructures are ubiquitous in this sample of large, bright disks. They are most frequently manifested as concentric, narrow emission rings and depleted gaps, although large-scale spiral patterns and small arc-shaped azimuthal asymmetries are also present in some cases. These substructures are found at a wide range of disk radii (from a few astronomical units to more than 100 au), are usually compact (≲10 au), and show a wide range of amplitudes (brightness contrasts). Here we discuss the motivation for the project, describe the survey design and the sample properties, detail the observations and data calibration, highlight some basic results, and provide a general overview of the key conclusions that are presented in more detail in a series of accompanying articles. The DSHARP data—including visibilities, images, calibration scripts, and more—are released for community use at https://almascience.org/alma-data/lp/DSHARP.

Numerical study of dynamical friction with thermal effects – I. Comparison to linear theory

We investigate by means of numerical simulations with nested Cartesian meshes the force exerted on a massive and luminous perturber moving at constant speed through a homogeneous and opaque gas, taking into account thermal diffusion in the gas and the radiative feedback from the perturber. The force arising from the release of energy into the ambient medium by the perturber, or heating force, is directed along the direction of motion and induces an acceleration of the perturber. Its value is compared to analytic estimates in the low and high Mach number limits, and found to match those accurately. In addition, the drag exerted on a non-luminous perturber significantly departs from the adiabatic expression when thermal diffusion is taken into account. In the limit of a vanishing velocity, this drag tends to a finite value which we determine using linear perturbation theory and corroborate with numerical simulations. The drag on a non-luminous perturber in a non-adiabatic gas therefore behaves like dry or solid friction. We work out the luminosity threshold to get a net acceleration of the perturber and find it to be generally much smaller than the luminosity of accreting low-mass planetary embryos embedded in a gaseous protoplanetary disc at a few astronomical units. We also present in some detail our implementation of nested meshes, which runs in parallel over several \emph{Graphics Processing Units} (GPUs).

Relativistic Jets from Active Galactic Nuclei

The nuclei of most normal galaxies contain supermassive black holes, which can accrete gas through a disk and become active. These active galactic nuclei (AGNs) can form jets that are observed on scales from astronomical units to megaparsecs and from meter wavelengths to TeV energies. High-resolution radio imaging and multiwavelength/messenger campaigns are elucidating the conditions under which this happens. Evidence is presented that: ▪ Relativistic AGN jets are formed when the black hole spins and the the accretion disk is strongly magnetized, perhaps on account of gas accreting at high latitude beyond the black hole sphere of influence. ▪ AGN jets are collimated close to the black hole by magnetic stress associated with a disk wind. ▪ Higher-power jets can emerge from their galactic nuclei in a relativistic, supersonic, and proton-dominated state, and they terminate in strong, hot spot shocks; lower-power jets are degraded to buoyant plumes and bubbles. ▪ Jets may accelerate protons to EeV energies, which contribute to the cosmic ray spectrum and may initiate pair cascades that can efficiently radiate synchrotron γ-rays. ▪ Jets were far more common when the Universe was a few billion years old and black holes and massive galaxies were growing rapidly. ▪ Jets can have a major influence on their environments, stimulating and limiting the growth of galaxies. The observational prospects for securing our understanding of AGN jets are bright.