ASTEROIDAL TO COMETARY: A STORY OF A GRAZING FIREBALL Patrick M. Shober1, Trent Jansen-Sturgeon1, Eleanor K. Sansom1, Hadrien A.R. Devillepoix1, Martin C. Towner1, Phil A. Bland1, Martin Cupak1,Robert M. Howie1, and Benjamin A.D. Hartig1, 1Space Science & Technology Centre (SSTC), School ofEarth and Planetary Sciences, Curtin University, GPO Box U1987 Perth, Western Australia 6845, Australia(patrick.shober@postgrad.curtin.edu.au)

Introduction Grazing fireballs occur when a mete-oroid strikes the atmosphere at an extremely low-anglesuch that it has the opportunity to pass through the at-mosphere and re-enter interplanetary space. Objects thatexperience this often produce brilliant fireball displays.These events have been reported for hundreds of years.The first scientifically observed and triangulated grazingfireball was the ‘Great Daylight Fireball’ in 1972, whichlasted for over ∼ 100 sec, covered over 1500 km, andreached a minimum height of 58 km [1]. Since this spec-tacular event, there have been several other grazing fire-balls observed and described [2, 3, 4, 5].

Observations The Desert Fireball Network (DFN) isthe most expansive network of its kind in the world, withover 50 observatories in Western Australia and SouthAustralia [6, 7]. On July 7th, 2017, a 90 second graz-ing fireball was observed to travel over 1300 km throughthe atmosphere. The object was seen by ten DFN sta-tions, allowing for an accurate triangulation of the entireatmospheric trajectory (Fig. 1). Due to the large size ofthe DFN, nearly all (including beginning and end) of theatmospheric trajectory was able to be observed (2541 as-trometric datapoints). A summary of the observationsmade of event DN170707 01 and the fitted trajectoryare provided in Table 1. The number of observationsrefers to the number of 30-second exposures. Whereas,‘without timing’ denotes when observations of the fire-ball were collected; however, either the angular velocityof the meteoroid was too slow, or the fireball was toobright to distinguish the encoded de Bruijn sequence [8].

Methods When analyzing the 1972 daylight fireball,Ceplecha [1] recognized that the grazing trajectoryshould fit a hyperbola with added curvature due to at-mospheric drag. Thus, Ceplecha [1] fit osculating cir-cles to the trajectory of the 1972 grazing daylight fireballto account for this added curvature with reasonable ac-curacy. Within this study, we implemented a DynamicTrajectory Fit (DTF) triangulation method that fits theobservation rays directly to the equations of motion forfireballs [9]. This non-straight-line approach to the eventtriangulation represents the physical system more vera-ciously than previous studies. From the triangulated tra-jectory, several pre- and post-grazing orbital simulationswere initialized.

Results The observed meteoroid transited the atmo-sphere for over 90 seconds and reached a minimum

height of 58.5 km before returning to interplanetaryspace. The velocity of the meteoroid in the Earth-centered, Earth-fixed frame decreased from 15.7 to14.2 km s−1. However, in the Earth-centered inertialframe, the object had a net gain in angular momen-tum during the close encounter with the Earth. Thisslingshot-like trajectory flung the object from an Apollo-type to a JFC-like orbit. From the extensive numericalsimulations conducted, it was determined that the neworbit of the object is very similar to a natural JFC. Witha predicted dynamical lifetime of ∼ 200 kyrs, the objectwill have numerous close encounters with Jupiter overits lifetime. The first of these close-encounters will verylikely occur between January-March 2025, where the ob-ject has a very high probability of having an encounterwithin one Jupiter Hill radii. After this encounter, thefuture of the meteoroid becomes less clear. Although,the most likely final destination for the object is either tobe ejected from the Solar System or to be scattered intosome Centaur-like orbit based on our +500 kyrs simula-tions.

Conclusions The grazing fireball observed over Aus-tralia in 2017 was a spectacular event only matched bythe ‘Great Daylight Fireball’ of 1972 in penetration depthand duration. Lasting over 90 seconds, and reaching aminimum height of ∼ 58.5 km, the object was certainlydurable and ‘asteroid-like’ in composition. As a result ofthe encounter, this asteroidal object was inserted into aJFC-like orbit. Considering that telescopes cannot effec-tively detect close-encounters of objects like these (toofaint and too fast), there may be a non-negligible numberof objects being scattered onto orbits typically thoughtas dynamically distinct. Current work is being done toestimate this population of Earth-scattered objects.

References: [1] Z Ceplecha. In: Bull. Astron. Inst.Czechoslov. 30 (1979), pp. 349–356. [2] J Borovickaand Z Ceplecha. In: A&A 257 (1992), pp. 323–328.[3] P. Brown et al. In: Nature 367.6464 (1994), p. 624.[4] D. O. Revelle, R. W. Whitaker, and W. T. Armstrong.In: vol. 3116. 1997, pp. 156–167. [5] J.M. Madiedo et al.In: MNRAS 460.1 (2016), pp. 917–922. [6] PA Bland etal. In: Aust. J. of Earth Sci. 59.2 (2012), pp. 177–187.[7] R.M. Howie et al. In: ExA 43.3 (2017), pp. 237–266. [8] R.M. Howie et al. In: Meteorit. Planet. Sci52.8 (2017), pp. 1669–1682. [9] T. Jansen-Sturgeon etal. 2019.

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Table 1: Observations and triangulated trajectory for event DN170707 01.Entry Conditions Exit Conditions

Time (UTC) after 2017-07-07 12:33:45.900 12:35:16.050Height (km) 85.7 86.0Mass range (depending on density; kg) 14-92 9-62Latitude (deg) -28.69325 -28.41439Longitude (deg) 122.71606 136.33175Velocity (km s−1) 15.71± 0.13 14.24± 0.10Slope (deg) 4.6 deg 7.8 deg

Duration (sec) 90.15Minimum Height (km) 58.5Best Convergence Angle (deg) 45.9Number of Observations (with timing) 13Number of Observations (without timing) 7Number of Datapoints 2541

Figure 1: Triangulated trajectory for event DN170707 01, as seen over Western Australia and South Australia.

1237.pdf51st Lunar and Planetary Science Conference (2020)