People have reported witnessing brilliantly long-lasting and bright meteor processions for at least hundreds of years. The 1783 ‘Great Meteor’ was estimated to have traveled over 1600 km through the atmosphere over western Europe. The ‘Great Comet of 1860’, which was most likely an Earth-grazing fireball over the eastern United States, was accounted for in a painting by American landscape artist Frederic Church entitled ‘The Meteor of 1860’ and by American poet Walt Whitman in his poem ‘Year of Meteors’. Additionally, the ‘1913 Great Meteor Procession’ reported sightings across Canada, the north-eastern United States, Bermuda, and many ships in the Atlantic as far south as Brazil. The event was initially hypothesized to have been formed by a natural Earth satellite that had a grazing encounter with the atmosphere.
A grazing event is considered to be when a meteoroid impacts the atmosphere at an extremely low-angle relative to the horizon, and there are generally three possible outcomes. It can either escape back to interplanetary space after passing through the atmosphere, fully ablate, or slow down enough to fall to the Earth. The first scientifically observed and triangulated grazing event was not until 1972 over Canada and the northwestern United States. The 1972 fireball lasted about 100 seconds, covering over 1500 km, and reached a minimum height of 58 km. The mass of the object has been estimated at 100 000 to 1 000 000 kg with the most likely diameter of about 5 m, although the original analysis has since been shown to contain mistakes, and the values given should not be relied upon.
Since the 1972 fireball, there have been several atmospheric grazing events reported within scientific literature: In 1990, the first Earth-grazing fireball was observed by a photographic fireball network; it was detected by two Czech stations of the European Fireball Network, and was estimated to be 44 kg with the closest approach of 98 km. In October of 1992, a bright fireball endured for over 700 km over the eastern United States before dropping a meteorite in Peekskill, New York. In 1996 a fireball was observed to hit the western United States and only briefly escape for one orbit before allegedly impacting the Earth. On 29 March 2006, a roughly 40 sec grazing fireball was observed over Japan. The meteoroid traveled over 700 km through the atmosphere and reached a minimum height of 71.4 km. It appeared to come from a Jupiter Family Comet-like orbit and the spectra collected was consistent with a chondritic composition. On 7 August 2007, a grazing fireball was observed by the European Fireball Network originating from an Aten-type orbit. In June 2012, the first grazing meteoroid associated with a meteor shower in the scientific literature was recorded by 13 stations with a 98 km minimum altitude over Spain and Portugal and belonged to the daytime ζ-Perseid shower. In 2003, another grazing meteor, was detected over Ukraine before exiting back into interplanetary space. In December 2014, a 1200 km long grazing event occurred over Algeria, Spain, and Portugal and lasted approximately 60 seconds, reaching a minimum height of 75 km. On 31 March 2014, a 34 second fireball over Germany and Austria originating from an Apollo-type orbit was observed. The meteoroid was estimated to have an initial mass of about 200 kg, but no material is believed to have exited back into interplanetary space. Many meteorites may have survived to the ground, however, the uncertainty on the fall ellipse is very large due to the extremely shallow entry angle.
For some of these grazing meteoroids, the object was able to survive its passage through the atmosphere. The rock then re-entered interplanetary space on an altered orbit, sending material from one part of the inner Solar System to another. This is could be significant since various parts of the inner Solar System are thought to be dynamically and physically distinct from one another.
The classical view of the Solar System says that the Sun formed with a debris disk around it that was originally compositionally heterogeneous within bands of constant radial distance from the Sun. The ‘snow line’ denoted the boundary between the planetesimals in which water ice and other volatiles would be retained and the bodies which were unable to hold ice, thus remaining dry. This classically separated the small bodies within the Solar System into two main groups: comets and asteroids respectively.
Although, we have seen that this classical ideology does not usually fit our observations of the small bodies within the Solar System. The Solar System is complicated and dynamic. In the last 4.5 billion years, small bodies have been jumbled around and altered. The layout and distribution of the Solar System is much more complicated than the idealized stratified one we tend to imagine.
Within the inner Solar System there are short-period comets, main-belt objects, and near-Earth objects. Traditionally, the main-belt objects were considered asteroidal and inner Solar System in origin, and near-Earth objects primarily evolved from the main-belt object space after entering an orbital resonance. However, with the identifications of Main-Belt Comets and dry asteroidal material in the Kuiper Belt, we have realized that the material in the Solar System is more mixed than previously believed. Additionally, the starkly drawn lines between asteroidal and cometary material have since faded with the identification of active asteroids, extinct comets, and mixing between populations. In reality, the physical properties of small bodies in the Solar System most likely exist in a spectrum from primitive volatile-rich ('comet-like') to dry volatile-poor ('asteroid-like'). Planetary scientists are still trying to determine the most probable mechanism by which this mixing could have occurred, but several models such as the ‘Nice Model’ and the ‘Grand Tack’ have begun to elucidate some of these mysteries.
Jupiter family comets are a class of short-period comets, believed to have evolved from scattered disk and Kuiper belt orbits. Jupiter family comets are primitive and contain a large amount of hydrated minerals and volatile ices. They are also characterized by their orbits being strongly linked to the orbit of Jupiter, typically defined by their Tisserand’s parameter (a value calculated from the average distance at which a body orbits the Sun, and the inclination and eccentricity of its orbit) of between 2 and 3 with respect to Jupiter. Jupiter family comets usually have multiple low-velocity encounters with the gas-giant over their lifetime. These encounters with Jupiter make the orbits of Jupiter family comets more unpredictable compared to other small bodies, where the median dynamic lifetime of a Jupiter family comet is about 325 000 years. However, Jupiter family comets that display cometary features frequently encounter Jupiter at distances of no more than 0.1 AU (0.1 tines the distance at which the Earth orbits the Sun) making them highly unstable compared to a small subset of near-Earth 'asteroidal' Jupiter family comets which typically exist on more stable orbits comparatively. A tiny fraction of Jupiter family comets are also thought to decouple from Jupiter and become Encke-like comets through either non-gravitational perturbations or close planetary encounters.
Since 2003, the Desert Fireball Network has been operating observatories across south-western Australia to capture images of fireball events. The network has since grown from 4 observatories by 2007 to over 50 observatories in Western Australia and South Australia by 2015. No other fireball camera network in the world is this expansive. Furthermore, it has expanded this effort worldwide with the start of Global Fireball Observatory collaboration, with coverage area expected to increase to 2% of the Earth’s entire surface. This coverage area makes the Global Fireball Observatory particularly well suited to characterise grazing meteoroids and other more rare fireball events.
In a paper published on the arXiv database at Cornell University on 31 March 2020, Patrick Shober, Trent Jansen-Sturgeon, Ellie Sansom, Hadrien Devillepoix, Martin Towner, Phil Bland, Martin Cupák, Robert Howie, and Ben Hartig of the Space Science and Technology Centre at Curtin University, present a discussion of a 90 second extremely shallow fireball, which was observed to graze the atmosphere above Western Australia and South Australia, entering the atmosphere at a slope of roughly 4.6°, on 7 July 2017.
Ten Desert Fireball Network observatories made observations of the fireball as it traveled over 1300 km through the atmosphere. The luminous phase started at about 85 km and penetrated as deep as 58 km before ceasing to be visibly ablating at 86 km. This event is only equaled by the ‘Great Daylight Fireball of 1972’, which reached a similar depth and lasted about 9 seconds longer than our witnessed event. However, unlike the 1972 event, the Desert Fireball Network was able to photographically image a majority of the the atmospheric trajectory of the fireball (including the beginning and the end), with observations from many of the fireball observatories spread across Western Australia and South Australia. Thus, providing us with a substantial amount of data to accurately fit a trajectory to our observations (2541 astrometric datapoints). Unfortunately, due to the Desert Fireball Network’s viewing geometry at the beginning of the observed luminous trajectory, the initial observation convergence angle was only a few degrees. Therefore, the uncertainty associated with the initial velocity is higher than usual, however, still sufficient to determine what part of the Solar System the meteoroid originated.
At the meteoroid’s closest approach, a fragmentation event occurred in which a smaller piece of the primary object broke off. Desert Fireball Network observatories captured the fragmentation event on video, and an uncalibrated light curve was able to be extracted. There are no other instances of fragmentation observed during the trajectory. This fragmentation event was taken into account when triangulating the path of the primary and determining the mass of the meteoroid.
In the past, fireball and meteor observation networks estimated the trajectories they witnessed using a simplified straight-line-fit approach. These simplified straight-line fit techniques are sufficient enough to obtain meaningful results when the trajectory is shorter than 100 km. However, recent studies have shown that more satisfactory results can be obtained with the use of more rigorous methodologies. This is particularly true for a grazing fireball where the meteoroid is traveling hundreds to thousands of kilometers through the atmosphere. In previous grazing fireball studies, this non-linearity was accounted for in several different ways. It was first recognised that a grazing trajectory should fit a hyperbola when neglecting the atmosphere, but is otherwise slightly more curved due to the atmospheric drag experienced in 1979. Thus it was possible to fit osculating circles to the trajectory of the 1972 grazing daylight fireball to account for this added curvature with reasonable accuracy. A study of the 1990 Earth-grazing fireball utilised the fact that one of the observation stations was nearly directly below the fireball (passed nearly through zenith) and saw the entire trajectory. The authors of that study took their observations and performed a least-squares fit to an osculating circle at the point of pericenter, neglecting drag in this case based on fireball type. Similar methodologies using osculating circular trajectory fits have been utilised by other studies as well. A study of the 2003 grazing meteor over Ukraine triangulated the small, fast grazing, high-altitude meteor detected by video observatories in Ukraine by assuming minimal drag and fitting the observations to a hyperbolic orbit in the geocentric frame. Meanwhile, another study determined the atmospheric trajectory of a meteor over Spain belonging to the Daytime ζ-Perseid shower by using a segmented method-of-planes approach.
For standard Desert Fireball Network events, Shober et al. implement a modified straight-line least-squares method (drawing a straight line on a scatter plot so that the number of points above the line and below the line is about equal, and the line passes through as many points as possible) with an Extended Kalman Smoother (an algorithm that uses a series of measurements observed over time, containing statistical noise and other inaccuracies, and produces estimates of unknown variables that tend to be more accurate than those based on a single measurement alone) for velocity determination. Shober et al. then numerically determined the meteoroid’s orbit by including all relevant perturbations. Numerical methods are a slightly more accurate way to handle the orbit determination, especially for meteoroids that were slow or closely approached the Moon. For longer and/or shallower fireball events, where the meteoroid trajectory can have noticeable curvature, the straight-line least-squares method method cannot account for the non-linear motion. Within this study, Shober et al. implemented a Dynamic Trajectory Fit triangulation method that fits the observation rays directly to the equations of motion for fireballs. This non-straight-line approach to the event triangulation represents the physical system more veraciously. Consequently, the Dynamic Trajectory Fit method produces a much better fit to the observations compared to the straight-line least-squares method for both positions and velocities. Shober et al. then used this trajectory to numerically estimate the pre- and post-grazing orbits. Currently the Dynamic Trajectory Fit method does not provide adequate formal velocity errors, thus an Extended Kalman Smoother was utilised to determine the velocity uncertainties for the study.
During the Dynamic Trajectory Fit procedure, the meteoroid’s ballistic parameter and ablation coefficient are determined alongside its dynamic parameters, based directly on the line-of-sight observations. By assuming the meteoroid’s shape and density, a mass estimation can be deduced from the meteoroid’s fitted ballistic parameter.
After triangulating the grazing event, Shober et al. initialised several orbital integrations using the publicly available REBOUND code. They utilised the 15th order non-symplectic IAS15 integrator for their simulations of the event. This integrator is based upon the RADAU-15 integrator developed by Edgar Everhart in the 1980s. It improves upon its predecessor by minimizing the systematic error generated by the algorithm to well-below machine precision, implementing an adaptive time-step, and adding the ability to include in non-conservative forces easily while ensuring that the round-off errors are symmetric and at machine-precision.
From the trajectory determined by the Dynamic Trajectory Fit method, the pre- and post-atmospheric state vectors for the meteoroid can be used to initialize orbital simulations. These simulations contain N number of particles within the meteoroid state’s uncertainties produced by the triangulation. Currently, the Dynamic Trajectory Fit methodology does not provide formal uncertainties as model errors are not accounted for. Subsequently, for this event, we determined the velocity uncertainties using the Extended Kalman Smoother method in conjunction with the Dynamic Trajectory Fit trajectory fit. Additionally, Shober et al. assume a Gaussian distribution for the errors, although this may not be strictly true. However, the results from the integration should not deviate significantly due to this assumption. The particles’ positions are generated from the initial and final latitude, longitude, and height determined from the Dynamic Trajectory Fit triangulation. The speed of the particles and their right ascension and declination are given in the Earth-centered Earth-fixed frame and then converted to the Earth-centered inertial frame in order to generate the particles in the simulations.
Initial simulations were run within ±100 years of the grazing event in order to accurately characterize the short-term evolution of the meteoroid. The number of outputs recorded was increased so that any close encounters with Jupiter or the Earth would be well resolved. Afterward, a series of long-term integrations were done in a similar manner. The primary goal of these more extended integrations was to determine what were the lasting effects of the meteoroid’s grazing encounter with the Earth. Does it stay on a Jupiter Family Comet orbit as long as any typical Jupiter Family Comet, and where does it evolve to after? Each integration recorded the positions, velocities, and osculating orbital elements for the meteoroid particles for a total period of 500 000 years forward relative to the event epoch. Close encounters with other planets were also considered and inspected, particularly with Jupiter.
The Dynamic Trajectory Fit methods fit the pointwise observations much better than the straight-line least-squares method for an event that is thousands of kilometers in length. The pointwise heights are given by minimizing the angular distance between the lines-of-sight when at least two observations are made. If a center-difference is taken between all these points, a velocity scatter can be generated. The velocity scatter for event DN170707_01 is very large in some circumstances considering the low convergence angles especially for the beginning of the trajectory. A majority of the fireball’s trajectory was north of the Desert Fireball Network observatories. Thus reducing the accuracy of each measurement. However, since we gathered over 2500 datapoints from ten Desert Fireball Network observatories, a reasonably good trajectory was able to be extracted. There are also three distinct gaps in the observations of event DN170707_01 primarily due to the latency between the 30 second observation periods. These lapses in observations are typically only noticeable for the longest enduring fireballs observed by the Desert Fireball Network. The longest gap, towards the end of the trajectory, is compounded by the poor visibility for the Desert Fireball Network observatories in that area of the network due to the cloud coverage at the time.
During the Dynamic Trajectory Fit procedure, the ballistic parameter was determined throughout the trajectory based directly on the line-of-sight measurements, and hinges on the deceleration profile of the observed meteoroid. The meteoroid’s mass was estimated by assuming its shape and density. For instance, assuming a spheroid of chondritic density (3500 kg per cubic metre), the DN170707_01 meteoroid was estimated to have a 60 kg initial mass and a 40 kg outbound mass. A majority of the mass loss is predicted to have occurred during the fragmentation observed near the closest approach of the object. However, as minimal deceleration was observed during the luminous atmospheric encounter, this mass estimate would be more accurately viewed as a lower bound.
The loading ram pressure for the meteoroid at the time of fragmentation was also calculated. For event DN170707_01, Shober et al. determined the fragmentation height based on the time of fragmentation observed in the light-curve from video observations. They estimated the meteoroid to have fragmented at 58.49 km, just before the minimum height reached, with a velocity of 15.5 km per second. Shober et al. then used the NRLMSISE-00 global atmospheric model to determine the density of the atmosphere at the fragmentation height. The ram pressure experienced by the meteoroid just before fragmentation was calculated to be 0.084 megapascals. This very low-value is consistent with previosu studies, in which it was found that bulk strengths determined by initial fragmentation are consistently much lower than the strengths of recovered meteorites. Thus, this value likely reflects macro-scale fractures in the object and not the intrinsic material strength. For example, the Dingle Dell ordinary chondrite meteorite recovered by the Dynamic Trajectory Fit in 2016 also experienced similar low-pressure fragmentations (0.03-0.11 megapsacals) early in its brightflight, despite having a recovered bulk density of 3450 kg per metre cubed.
The meteoroid that skipped off the atmosphere over Western Australia and South Australia in July 2017 originally came from an orbit in the inner main-belt, between the 4:1 and the 3:1 mean-motion resonances with Jupiter. It most likely evolved into an Earth-crossing orbit after passing through either the 3:1 or the ν₆ complex, which are the two most significant entry routes into the near Earth object region. As a result of the grazing encounter with the Earth, the meteoroid was flung into an orbit with a higher energy. The geometry of the encounter enabled the meteoroid to gain angular momentum around the Sun. As a result, the semi-major axis and eccentricity both increased due to the increase in energy, and the object was inserted into a Jupiter Family Comet orbit. Hereon, the object’s future is strongly governed by its interactions with the gas-giant.
With an post-ecounter aphelion near Jupiter’s orbit, the meteoroid is likely to have multiple close-encounters with the planet in the future. Thus, the object is unpredictable on relatively short timescales compared to other small bodies in the Solar System. This is to be expected for an object on a Jupiter Family Comet-like orbit that originated from the trans-Neptunian region. The object will tend to decrease in eccentricity and slightly increase in semi-major axis over time. This will occur slowly for a majority of particles over about 10-100 thousand years, as Jupiter perturbs them. If the orbit of the meteoroid evolves into an orbit with a similar semi-major axis to Jupiter, the close encounters with the gas giant will begin to increase the eccentricity of the meteoroid again and throw the body towards the outer Solar System. The meteoroid is also nearly centered on the 2:1 mean motion resonance, however, this resonance is not as destabilizing as the other prominent resonances on such short timescales.
The first of these close-encounters will most likely occur between January and March, 2025 (roughly 7:52 years after encountering the Earth) in which the meteoroid will very likely come within 3 Jupiter Hill radii (astronomical body's Hill radius is the radius of the surrounding spherical region, the Hill sphere, within which smaller body's would tend to orbit the body) of the planet. A series of short-term highly resolved integrations were performed with 5000 test particles to analyse this first close encounter with Jupiter. The meteoroid is likely to get close to Jupiter (blue path), just 1.5 orbits after the observations of the fireball.
The well-constrained orbit prior to the close encounter with Jupiter rapidly spreads out in the orbital space. Following the likely meteoroid-Jupiter close encounter of 2025, the orbit of the meteoroid can only be treated statistically. The density plots show the evolution of the semi-major axis, eccentricity, and inclination of 10 000 test particles forward in time only 100 years. Most of the particles stay together, indicated by the darker portions of the plot. However, as seen by the multiple jumps in values over time, the meteoroid is likely to have a plethora of close encounters with Jupiter over its lifetime in a Jupiter Family Comet orbit, and every one of these encounters obscures the future of the object.
In order to determine the likelihood of future or previous close encounters with the Earth, two simulations with 5000 particles were integrated both backward and forward 20 years relative to the event. During these simulations, outputs were collected at a higher frequency in order to accurately characterize all possible close encounters. The probability that there was an encounter with the Earth within three and one Hill radii within 20 years prior to the grazing event was 2.4% and 0.7%, respectively. Additionally, the probability that a future close encounter with the Earth will occur within the proceeding 20 years after the grazing event is 1.4% and 0.5%, respectively. Therefore, the probability of having the opportunity to telescopically observe this object as it re-approaches the Earth is very slim. The most likely time for this to occur is in mid-July 2023, but there is still only a 1.1% chance that it will get within 3 Hill radii of the planet.
Further analysis using substantially longer integrations of test particles was performed in order to statistically characterise the meteoroid’s future. The longest of these simulations was a forward integration of 1000 test particles for 500 thousand years. Over the course of the 500 thousand years forward integration, most of the particles (60.1%) are eventually ejected from the Solar System, as expected. The vast majority of the particles that remain in the Solar System (heliocentric orbits) stay in Jupiter Family Comet orbits (as defined by the Tisserand’s parameter) for the entire integration.
There is an exponential decay in the number of particles in heliocentric and Jupiter Family Comet orbits. The average dynamical lifetime for the particles in Jupiter Family Comet orbits is approximately 200 thousand years, which is shorter than the roughly 325 thousand years dynamical lifetime estimate for Jupiter Family Comets. This is likely due to the initial post-grazing orbit, which has an aphelion very near the orbit of Jupiter. However, bodies in Jupiter Family Comet orbits that display cometary features are more likely to have multiple maximum 0.1 AU encounters with Jupiter, reducing the orbital stability compared to asteroidal interlopers within the population. Therefore, the Jupiter Family Comet-orbit dynamical lifetime for the meteoroid is indistinguishable from a Jupiter Family Comet from a more 'traditional' source region. The Jupiter Family Comet, asteroidal, and Long Period Comet categories are solely determined by the particles’ Tisserand’s parameter. Whereas, the Centaur and trans-Neptunian objects are defined as having orbits between Jupiter and Neptune, and beyond the orbit of Neptune, respectively. This classification does lend itself to including some Centaurs and trans-Neptunian objects when counting the number of Jupiter Family Comets.
A smaller fraction (31:6%) of the test particles evolve onto asteroidal or long-period cometary orbits as defined by their Tisserand’s parameter values. The majority of asteroidal particles are determined to be in the outer Solar System. These are particles that originated from Jupiter Family Comet space that were decoupled from Jupiter over time due to planetary perturbations and are now on Centaur or trans-Neptunian orbits. It has been estimated that there should be about 20 objects of kilometer-size from the main-belt being scattered by Jupiter every million years in today’s Solar System. The object discussed in this study differs in that it was gravitationally scattered by the Earth and then by Jupiter, resulting in the possible transfer of volatile-depleted inner Solar System material to the outer Solar System.
Within the current scientific literature, there have been in total ten grazing fireballs observed. However, in only six of these cases did the meteoroid survive the atmospheric passage and return to interplanetary space. These grazing events demonstrate the orbital changes experienced by meteoroids that come very close to Earth. In most of these occurrences, the objects experience a significant change to their orbits. Although, this does not necessarily change them enough to be orbitally reclassified. For the first photographically observed grazing fireball, in October of 1990, a 100 000-1 000 000 kg meteoroid in a higher inclination Apollo-type orbit with a Jupiter Family Comet-like Tisserand’s parameter was inserted into a lower-energy orbit with a Tisserand's parameter with respect to Jupiter of greater than 3. Thus, not only has a meteoroid with a more asteroid-like Tisserand's parameter become more cometary due to close encounters, but the reverse has also been observed. It has been shown that using the Tisserand’s parameter is a better metric to classify small Solar System bodies compared to the traditional arbitrary classification based on the orbital period. Nevertheless, as shown in Shober et al.'s study, small meter-sized objects occasionally experience close encounters with the Earth and have a sufficient orbital energy change to be reclassified even under this scheme.
If we consider an fireball event to be grazing simply when the initial slope of the trajectory is less than 5° and travelled over 100 km through the atmosphere, in the four years since the Desert Fireball Network has being fully operational, about 1.2% of the Desert Fireball Network dataset have been observed to be grazing events. Indicating that although somewhat uncommon, grazing events are not extremely rare. However, in most of the events detected, the meteoroid either does not survive the atmospheric passage or loses enough velocity to be incapable of re-entering interplanetary space.
Grazing fireballs indicate that meter-scale near Earth objects are occasionally inserted into categorically new orbits due to close encounters with the Earth, or indeed other planets. How effective this mechanism is for mixing material in the inner Solar System for small objects is still to be determined. Current work is being done to produce an artificial dataset of close encounters undetected by telescopes based upon the entire orbital dataset of the Desert Fireball Network. This analysis will be extremely valuable to conclusively determine how significant this process is for small objects in the inner Solar System. If it is non-negligible, what populations in the near-Earth space may be more or less contaminated by genetically unrelated material, how significant are the orbit alterations, and what may this imply about where meteorites come from?
On 7 July 2017, the Desert Fireball Network observed a more than 1300 km long grazing fireball by ten of its high-resolution digital fireball observatories. The meteoroid transited the atmosphere for over 90 seconds and reached a minimum height of 58.5 km before returning to interplanetary space. This fireball is only matched by the notorious ‘Great Daylight Fireball of 1972’, which penetrated to a very similar depth in the atmosphere but lasted 9 seconds longer. As a result of the grazing encounter with the Earth, the meteoroid observed by the Desert Fireball Network underwent a natural slingshot maneuver in which it was transferred from an asteroidal Apollo-type orbit to a Jupiter Family Comet orbit. Additionally, numerical integration of the object forward 500 kyrs indicated that it will most likely stay in a Jupiter Family Comet orbit for about 200 thousand years, indistinguishable from any other Jupiter Family Comet. Considering there are likely many small objects that go telescopically undetected that have close encounters with the Earth, there may be a non-negligible amount of meter-sized objects in modified orbits within the inner Solar System.
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