Prospects for Backtracing 1I/`Oumuamua and Future Interstellar Objects

1I/`Oumuamua is the first of likely many small bodies of extrasolar origin to be found in the solar system. These interstellar objects (ISOs) are hypothesized to have formed in extrasolar planetary systems prior to being ejected into interstellar space and subsequently arriving at the solar system. This paper discusses necessary considerations for tracing ISOs back to their parent stars via trajectory analysis, and places approximate limits on doing so. Results indicate the capability to backtrace ISOs beyond the immediate solar neighborhood is presently constrained by the quality of stellar astrometry, a factor poised for significant improvement with upcoming Gaia data releases. Nonetheless, prospects for linking 1I or any other ISO to their respective parent star appear unfavorable on an individual basis due to gravitational scattering from random stellar encounters which limit traceability to the past few tens of millions of years. These results, however, do not preclude the possibility of occasional success, particularly after considering the potential for observational bias favoring the discovery of younger ISOs, together with the anticipated rise in the ISO discovery rate under forthcoming surveys.Comment: Accepted by ApJL; 7 pages, 3 figure

Relativistic Spacecraft Propelled by Directed Energy

Achieving relativistic flight to enable extrasolar exploration is one of the dreams of humanity and the long term goal of our NASA Starlight program. We derive a fully relativistic solution for the motion of a spacecraft propelled by radiation pressure from a directed energy system. Depending on the system parameters, low mass spacecraft can achieve relativistic speeds; thereby enabling interstellar exploration. The diffraction of the directed energy system plays an important role and limits the maximum speed of the spacecraft. We consider 'photon recycling' as a possible method to achieving higher speeds. We also discuss recent claims that our previous work on this topic is incorrect and show that these claims arise from an improper treatment of causality

Experimental Investigation of Wave Propagation Characteristics in Entangled Metallic Wire Materials by Acoustic Emission

In this paper, the response characteristics of wave propagation in entangled metallic wire materials (EMWMs) are investigated by acoustic emission. The frequency, amplitude of wave emission, and the pre-compression force of the specimen can be adjusted in the experimental setup. EMWM specimens fabricated from stainless steel wires and with different design parameters are tested in this work. The results show that waves of different amplitudes propagate in EMWMs with approximate linear characteristics and the fluctuation coefficient of wave passing ratios is calculated below 15%. The response spectrum of passing waves shows a distinct single-peak characteristic, with the peak response at approximately 14 kHz. The parameters of pre-compression force, porosity, wire diameter, helix diameter, specimen height, and the layered structure of specimens have no significant effect on the frequency characteristics but moderately affect the wave passing ratios. Notably, EMWMs exhibit a lower wave passing ratio (ranging from 0.01 to 0.18) compared to aluminum alloy and natural rubber. The characteristics of response spectrums can be successfully reproduced by the finite element simulation. This work demonstrates EMWMs’ potential as an acoustic frequency vibration isolation material, offering excellent performance and engineering design convenience

Orbital Deflection of Comets by Directed Energy

Cometary impacts pose a long-term hazard to life on Earth. Impact mitigation techniques have been studied extensively, but they tend to focus on asteroid diversion. Typical asteroid interdiction schemes involve spacecraft physically intercepting the target, a task feasible only for targets identified decades in advance and in a narrow range of orbits---criteria unlikely to be satisfied by a threatening comet. Comets, however, are naturally perturbed from purely gravitational trajectories through solar heating of their surfaces which activates sublimation-driven jets. Artificial heating of a comet, such as by a laser, may supplement natural heating by the Sun to purposefully manipulate its path and thereby avoid an impact. Deflection effectiveness depends on the comet's heating response, which varies dramatically depending on factors including nucleus size, orbit and dynamical history. These factors are incorporated into a numerical orbital model to assess the effectiveness and feasibility of using high-powered laser arrays in Earth orbit and on the ground for comet deflection. Simulation results suggest that a diffraction-limited 500 m orbital or terrestrial laser array operating at 10 GW for 1% of each day over 1 yr is sufficient to fully avert the impact of a typical 500 m diameter comet with primary nongravitational parameter A1 = 2 x 10^-8 au d^-2. Strategies to avoid comet fragmentation during deflection are also discussed.Comment: 13 pages, 12 figures; AJ, in pres

1I/2017 U1 (`Oumuamua) is Hot: Imaging, Spectroscopy and Search of Meteor Activity

1I/2017 U1 (`Oumuamua), a recently discovered asteroid in a hyperbolic orbit, is likely the first macroscopic object of extrasolar origin identified in the solar system. Here, we present imaging and spectroscopic observations of \textquoteleft Oumuamua using the Palomar Hale Telescope as well as a search of meteor activity potentially linked to this object using the Canadian Meteor Orbit Radar. We find that \textquoteleft Oumuamua exhibits a moderate spectral gradient of $10\%\pm6\%~(100~\mathrm{nm})^{-1}$, a value significantly lower than that of outer solar system bodies, indicative of a formation and/or previous residence in a warmer environment. Imaging observation and spectral line analysis show no evidence that \textquoteleft Oumuamua is presently active. Negative meteor observation is as expected, since ejection driven by sublimation of commonly-known cometary species such as CO requires an extreme ejection speed of $\sim40$ m s$^{-1}$ at $\sim100$ au in order to reach the Earth. No obvious candidate stars are proposed as the point of origin for \textquoteleft Oumuamua. Given a mean free path of $\sim10^9$ ly in the solar neighborhood, \textquoteleft Oumuamua has likely spent a very long time in the interstellar space before encountering the solar system.Comment: ApJL in pres

Hysteretic behaviour of uniaxially thermoformed auxetic foams under 3-point bending low-frequency vibration

The work describes experiments and models related to auxetic (negative Poisson’s ratio) foams subjected to low-frequency and variable amplitude 3-point bending loading. A custom 3-point bending vibration test rig is designed and used to perform the dynamic test of auxetic PU foam beams within low-frequency range (1–20 Hz) and 5 different displacement amplitudes. The auxetic foams tested in this work are manufactured using a simplified and relatively low-cost uniaxially thermoforming compression technique, which leads to the production of foams with transverse isotropic characteristics. Auxetic foam beam samples with two different cutting orientations and different thermoforming compression ratios rc (20–80%) are tested and compared, also with the use of theoretical Euler–Bernoulli-based and finite element models. The dynamic modulus of the foams increases with rc, ranging between 0.5 and 5 MPa, while the dynamic loss factor is marginally affected by the compression ratio, with overall values between 0.2 and 0.3. The auxetic PU foam has a noticeable amplitude-dependent stiffness and loss factors, while the dynamic modulus increases but slightly decreases with the frequency. The dynamic modulus is also 20–40% larger than the quasi-static one, while the dynamic and static loss factors are quite close. A modified Bouc–Wen model is also further developed to capture the amplitude and frequency-dependent properties of the conventional and auxetic foams with different volumetric compression ratios. The model shows a good agreement with the experimental results

Acoustic realization of projective mirror Chern insulators

Symmetry plays a key role in classifying topological phases. Recent theory shows that in the presence of gauge fields, the algebraic structure of crystalline symmetries needs to be projectively represented, which enables unprecedented topological band physics. Here, we report a concrete acoustic realization of mirror Chern insulators by exploiting the concept of projective symmetry. More specifically, we introduce a simple but universal recipe for constructing projective mirror symmetry, and conceive a minimal model for achieving the projective symmetry-enriched mirror Chern insulators. Based on our selective-excitation measurements, we demonstrate unambiguously the projective mirror eigenvalue-locked topological nature of the bulk states and associated chiral edge states. More importantly, we extract the non-abelian Berry curvature and identify the mirror Chern number directly, as conclusive experimental evidence for this exotic topological phase. All experimental results agree well with the theoretical predictions. Our findings will shine new light on the topological systems equipped with gauge fields.Comment: 5 figure

Long-period comet impact risk mitigation with Earth-based laser arrays

Long-period comets (LPCs) frequently transit the inner solar system, and like near-Earth asteroids (NEAs), pose a continued risk of impact with Earth. Unlike NEAs, LPCs follow nearly parabolic trajectories and approach from the distant outer solar system where they cannot be observed. An LPC on an Earth-impact trajectory is unlikely to be discovered more than a few years in advance of its arrival, even with significant advancements in sky survey detection capabilities, likely leaving insufficient time to develop and deliver an interception mission to deflect the comet. However, recent proposals have called for the development of one or more large ∼ 1 km laser arrays placed on or near Earth primarily as a means for photon propulsion of low-mass spacecraft at delta-v above what would be feasible by traditional chemical or ion propulsion methods. Such a laser array can also be directed to target and heat a threatening comet, sublimating its ices and activating jets of dust and vapor which alter the comet's trajectory in a manner similar to rocket propulsion. Simulations of directed energy comet deflection were previously developed from astrometric models of nongravitational orbital perturbations from solar heating, an analogous process that has been observed in numerous comets. These simulations are used together with the distribution of known LPC trajectories to evaluate the effect of an operational Earth-based laser array on the LPC impact risk