The Distribution of Earth-Impacting Interstellar Objects

TL;DR

This work addresses the distribution of Earth-impacting interstellar objects by constructing a probabilistic population synthesis that generates a vast ensemble of interstellar objects with MS-type kinematics. The authors predict Earth-impact probabilities and characterize the resulting radiants, velocities, and orbital elements, finding enhancements toward the solar apex and galactic plane, a velocity distribution skewed toward higher geocentric speeds, and a predominance of low-eccentricity hyperbolic orbits with perihelia near 1 au. The fastest impacts arise in spring, while the overall impact rate is higher in winter due to Solar gravitational focusing; impacts preferentially occur near the equator with a slight Northern-hemisphere bias. All distributions are provided as open-source data, independent of interstellar number density, albedo, and size-frequency assumptions, offering a practical tool for interpreting meteor observations and Rubin/LSST expectations for macroscopic interstellar bodies.

Abstract

In this paper we calculate the expected orbital elements, radiants, and velocities of Earth-impacting interstellar objects. We generate a synthetic population of $\sim10^{10}$ interstellar objects with M-star kinematics in order to obtain $\sim10^4$ Earth-impactors. The relative flux of impactors arriving from the direction of the solar apex and the galactic plane is enhanced by a factor of $\sim2$ relative to the mean. The fastest impactors also arrive from these directions, although Earth-impactors are generally slower than objects in the overall population. This is because the Earth-impacting subset contains a higher fraction of low-eccentricity hyperbolic objects which are more strongly affected by gravitational focusing. Earth-impacting interstellar objects are more likely to have retrograde orbits close to the ecliptic plane. A selection effect makes the distribution of inclination of Earth-impacting interstellar objects uniform/sinusoidal at low/high perihelion distances. In turn, low perihelion impactors have higher impact probability towards the ecliptic plane. The overall impactor population therefore exhibits an intermediate inclination distribution between uniform and sinusoidal. The highest velocity impacts are most likely to occur in the spring when the Earth is moving towards the solar apex. However, impacts in general are more likely to occur during the winter when the Earth is located in the direction of the antapex. Interstellar objects are more likely to impact the Earth at low latitudes close to the equator, with a slight preference for the Northern hemisphere due to the location of the apex. These distributions are independent of the assumed interstellar object number density, albedos, and size-frequency distribution and are publicly available.

Figures (11)

• Figure 1: The radiants towards the Earth of impacting interstellar objects. Interstellar objects tend to impact the Earth in the directions of the solar apex and the galactic plane. Radiants are shown in the geocentric frame.
• Figure 2: Interstellar objects impact the Earth with higher velocity when approaching from the solar apex and the galactic plane. Impactor velocity is calculated in the geocentric frame.
• Figure 3: The distribution of velocities of Earth-impacting interstellar objects are distinct from the overall interstellar object population. The addition of the Earth's velocity shifts the lognormal distribution to higher median values for the geocentric frame.
• Figure 4: Earth-impacting interstellar objects are significantly more likely to have perihelia close to the Earth and hyperbolic orbits near the low-eccentricity limit. The inclination distribution measured with respect to the ecliptic is uniform in angular space. In physical space, there is a larger fraction of impactor orbits in the ecliptic plane. This is because prograde and retrograde ecliptic interstellar objects are significantly more likely to cross the Earth-torus. The inset in the lower panel shows the low-eccentricity limit and has the same axes.
• Figure 5: The distributions of longitude of ascending node, argument of periapsis, and hyperbolic anomaly of the impactors are approximately symmetric. These distributions are similar to those in the overall interstellar populations Marceta2023a.