Meteoroids Scattered by the Earth

Near-Earth objects (NEOs) are typically fiercely monitored due to the inherent danger of their close encounters. Encounters with more massive objects at distances of a few lunar distances (LD) are relatively commonplace. However, fireball and meteor observation networks from around the world have witnessed ‘grazing’ events occur on several occasions [1, 2, 3, 4, 5]. Grazing events are characterized by their low impact angle and their possible re-entry into interplanetary space. These fireballs display how there are likely many smaller objects, that cannot be detected telescopically, that encounter the Earth all the time. Close encounters can quickly scatter meteoroids into drastically distinct orbits. This process is exemplified by the grazing fireball event detected by the Desert Fireball Network (DFN) in 2017 [5]. During this event, a ≥ 0.3 m object grazed the atmosphere coming from an Apollo-type orbit and exited with a JFC-like orbit. In order to characterize the population of objects in this small size range, we utilized the data collected by the Desert Fireball Network (DFN). The DFN is a continental-scale photographic fireball monitoring network covering over 2.5 million square kilometers of the Australian outback. The Earth’s close encounter flux in the 0.01-100 kg range was estimated using the impact flux observed by the DFN. To do this, several inherent biases had to be taken into account. Some of these biases include: limiting sensitivity of the fireball observatories, seasonal and diurnal variations in the flux, and gravitational focusing. These biases were all taken into consideration. The size-range analyzed in the DFN dataset was cutoff at small-sizes in order to remove the excess of fast, small meteoroids. Whereas, the diurnal and seasonal effects on the average flux of the DFN were considered negligible [6]. Most importantly, gravitational focusing must be corrected for or the flux of slower asteroidal material would be overestimated. The flux enhancement factor was accounted for using the global average enhancement determined by Opik [7], and scaled accordingly based on close encounter ¨ distance. In total, the close encounter population was modeled using 2.3 million test particles. The close encounter simulations, based on the DFN orbital dataset, demonstrated a significant population of close encounters at the centimeter/meter scale. Most of these bodies are negligibly affected during their close encounters; however, many experience considerable orbital changes (Fig. 1). Since the most likely objects to encounter the Earth are those with orbits more similar to the Earth, many close encounters come from asteroid-like (T_J > 3) objects. During the encounter, objects either gain or lose energy resulting in an inverse change to the objects T_J value. In total there appears to be a net gain of objects flung from asteroidal to JFC-like orbits. These encounters are considerably rare (about 0.16% of the total flux within 1.5 LD); however, considering the vast number of objects predicted to have close encounters at these small sizes, the size of this scattered population is not insignificant.

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