A rapid accumulation of observations and interpretation have followed in the wake of 1I 'Oumuamua's passage through the inner Solar System. We briefly outline the consequences that this first detection of an interstellar asteroid implies for the planet-forming process, and we assess the near-term prospects for detecting and observing (both remotely and in situ) future Solar System visitors of this type. Drawing on detailed heattransfer calculations that take both 'Oumuamua's unusual shape and its chaotic tumbling into account, we affirm that the lack of a detectable coma in deep images of the object very likely arises from the presence of a radiation-modified coating of high molecular weight material (rather than a refractory bulk composition). Assuming that 'Oumuamua is a typical representative of a larger population with a kinematic distribution similar to Population I stars in the local galactic neighborhood, we calculate expected arrival rates, impact parameters and velocities of similar objects and assess their prospects for detection using operational and forthcoming facilities. Using 'Oumuamua as a proof-of-concept, we assess the prospects for missions that intercept interstellar objects (ISOs) using conventional chemical propulsion. Using a "launch on detection" paradigm, we estimate wait times of order 10 years between favorable mission opportunities with the detection capabilities of the Large-Scale Synoptic Survey Telescope (LSST), a figure that will be refined as the population of interstellar asteroids becomes observationally better constrained. Subject headings: asteroids: individual (1I/2017 U1 ('Oumuamua)), galaxy: local interstellar matter

‘Oumuamua (I1 2017) was the first macroscopic (l ∼ 100 m) body observed to traverse the inner solar system on an unbound hyperbolic orbit. Its light curve displayed strong periodic variation, and it showed no hint of a coma or emission from molecular outgassing. Astrometric measurements indicate that ‘Oumuamua experienced nongravitational acceleration on its outbound trajectory, but energy balance arguments indicate this acceleration is inconsistent with a water ice sublimation-driven jet of the type exhibited by solar system comets. We show that all of ‘Oumaumua’s observed properties can be explained if it contained a significant fraction of molecular hydrogen (H2) ice. H2 sublimation at a rate proportional to the incident solar flux generates a surface-covering jet that reproduces the observed acceleration. Mass wasting from sublimation leads to monotonic increase in the body axis ratio, explaining ‘Oumuamua’s shape. Back-tracing ‘Oumuamua’s trajectory through the solar system permits calculation of its mass and aspect ratio prior to encountering the Sun. We show that H2-rich bodies plausibly form in the coldest dense cores of giant molecular clouds, where number densities are of order n ∼ 105, and temperatures approach the T = 3 K background. Post-formation exposure to galactic cosmic rays implies a τ ∼ 100 Myr age, explaining the kinematics of ‘Oumuamua’s inbound trajectory.

We show that the P ∼ 8 h photometric period and the astrometrically measured A ng ∼ 2.5 × 10 −4 cm s −2 non-gravitational acceleration (at r ∼ 1.4 AU) of the interstellar object 1I/2017 ('Oumuamua) can be explained by a nozzle-like venting of volatiles whose activity migrated to track the subsolar location on the object's surface. Adopting the assumption that 'Oumuamua was an elongated a × b × c ellipsoid, this model produces a pendulum-like rotation of the body and implies a long semiaxis a ∼ 5A ng P 2 /4π 2 ∼ 260 m. This scale agrees with the independent estimates of 'Oumuamua's size that stem from its measured brightness, assuming an albedo of p ∼ 0.1, appropriate to ices that have undergone long-duration exposure to the interstellar cosmic ray flux. Using ray-tracing, we generate light curves for ellipsoidal bodies that are subject to both physically consistent sub-solar torques and to the time-varying geometry of the Sun-Earth-'Oumuamua configuration. Our synthetic light curves display variations from chaotic tumbling and changing cross-sectional illumination that are consistent with the observations, while avoiding significant secular changes in the photometric periodicity. If our model is correct, 'Oumuamua experienced mass loss that wasted ∼ 10% of its total mass during the ∼ 100 d span of its encounter with the inner Solar System and had an icy composition with a very low [C/O] 0.003. Our interpretation of 'Oumuamua's behavior is consistent with the hypothesis that it was ejected from either the outer regions of a planetesimal disk after an encounter with an embedded M p ∼ M Nep planet or from an exo-Oort cloud.

The recently discovered population of interstellar objects presents us with the opportunity to characterize material from extrasolar planetary and stellar systems up close. The forthcoming Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will provide an unprecedented increase in sensitivity to these objects compared to the capabilities of currently operational observational facilities. We generate a synthetic population of ‘Oumuamua-like objects drawn from their galactic kinematics and identify the distribution of impact parameters, eccentricities, hyperbolic velocities, and sky locations of objects detectable with the LSST, assuming no cometary activity. This population is characterized by a clustering of trajectories in the direction of the solar apex and antiapex, centered at orbital inclinations of ∼90°. We identify the ecliptic or solar apex as the optimal sky location to search for future interstellar objects as a function of survey limiting magnitude. Moreover, we identify the trajectories of detectable objects that will be reachable for in situ rendezvous with a dedicated mission with the capabilities of the forthcoming Comet Interceptor or proposed Bridge concept. By scaling our fractional population statistics with the inferred spatial number density, we estimate that the LSST will detect of order ∼15 interstellar objects over the course of its ∼10 yr observational campaign. Furthermore, we find that there should be ∼1–3 and ∼0.0007–0.001 reachable targets for missions with propulsion capabilities comparable to Bridge and Comet Interceptor, respectively. These numbers are lower limits and will be readily updateable when the number density and size–frequency distribution of interstellar objects are better constrained.

Recently Squire & Hopkins (2018b) showed that charged dust grains moving through magnetized gas under the influence of any external force (e.g. radiation pressure, gravity) are subject to a spectrum of instabilities. Qualitatively distinct instability families are associated with different Alfvén or magnetosonic waves and drift or gyro motion. We present a suite of simulations exploring these instabilities, for grains in a homogeneous medium subject to an external acceleration. We vary parameters such as the ratio of Lorentz-to-drag forces on dust, plasma β, size scale, and acceleration. All regimes studied drive turbulent motions and dust-to-gas fluctuations in the saturated state, can rapidly amplify magnetic fields into equipartition with velocity fluctuations, and produce instabilities that persist indefinitely (despite random grain motions). Different parameters produce diverse morphologies and qualitatively different features in dust, but the saturated gas state can be broadly characterized as anisotropic magnetosonic or Alfvénic turbulence. Quasi-linear theory can qualitatively predict the gas turbulent properties. Turbulence grows from small to large scales, and larger-scale modes usually drive more vigorous gas turbulence, but dust velocity and density fluctuations are more complicated. In many regimes, dust forms structures (clumps, filaments, sheets) that reach extreme over-densities (up to 10 9 times mean), and exhibit substantial sub-structure even in nearly-incompressible gas. These can be even more prominent at lower dust-to-gas ratios. In other regimes, dust self-excites scattering via magnetic fluctuations that isotropize and amplify dust velocities, producing fast, diffusive dust motions.