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Comets, asteroids and meteors may be considered as the debris of the solar system.  They are members of the group of objects known as Small Solar System Bodies, which are any solar system body too small to meet the definition of either a planet or dwarf planet.  The CAM Section is involved in the observation of these bodies, which are important to our understanding of the formation and evolution of our solar system.

Definition of terms

The following definitions are from those accepted by the IAU.

An asteroid, also known as a minor planet, is an irregularly shaped rocky body orbiting the Sun that does not meet the definition of either a planet or dwarf planet.

A comet is a small body orbiting the Sun, and comprising a substantial fraction of its composition as volatile matter, which can sublimate under the effect of solar radiation to form a coma.

A meteoroid is a solid natural object of a size roughly between 30 µm and 1m moving in, or coming from, interplanetary space.

A meteor is the light and associated physical phenomena (heat, shock, ionization), which result from the high speed entry of a solid object from space into a gaseous atmosphere.  A meteor brighter than visual magnitude –4 is termed a bolide or a fireball.

A meteorite – is any natural solid object that survived the meteor phase in a gaseous atmosphere without being completely vaporized.

A meteor shower is a group of meteors produced by meteoroids of the same meteoroid stream, and may be formed by either a comet or asteroid.

What observations can be made?

The Section is involved with the following observations:

Comets – for observations to be of scientific value the observer should concentrate on:

• Estimates of the total visual magnitude of the comet, preferably made over the entire apparition to allow construction of a light curve
• Estimates of the diameter of the coma
• Estimates of the degree of condensation of the comet
• Estimates of the length and position angle of the tail
• Detailed visual descriptions, sketches and photographs of the comet

A guide to observing comets can be found here: Observing Comets

Asteroids – determination of size and shape by timing occultations of stars by asteroids, light curve studies to determine rotation rates.

Meteors – visual counts in order to determine time of maxima and generate activity profiles of showers, recording of shower meteor magnitudes to determine population index, and plotting of meteors to determine radiant position and structure.  In addition the Section participates in global video monitoring networks, in order to detect new meteor streams and confirm showers listed by the IAU Meteor Data Centre (MDC).

A guide to observing meteors can be found here: Observing Meteors

Fireballs – any meteor of magnitude -4 or brighter should be reported

A form for reporting fireballs can be found here: Report a Sighting

A diary of events and observing details for specific events is published regularly in CAMnotes.

The latest issue can be found here:

Observers that would like to contribute to Section activities or requiring more information can contact the Director at CAMassa.saao.ac.za

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22 December 2022

Tim Cooper is co-author of a new paper titled ‘An observing campaign to search for meteoroids of Bennu at Earth’ in the latest publication of the journal Icarus and describes our results of three years of video monitoring to detect meteors from asteroid 101955 Bennu.

Magda Streicher and Tim also conducted a visual campaign during 2022, and results will be published in MNASSA at a later date.

Abstract

An observing campaign was conducted in the Southern Hemisphere using low-light video camera triangulation to measure the trajectories and orbits of meteoroids with a possible origin at asteroid Bennu. New CAMS (Camera for Allsky Meteor Surveillance) video camera networks were established in Australia, Chile, and Namibia, and networks in New Zealand and South Africa were expanded. During observing periods in September 2019, 2020, and 2021, we measured 7672, 4936, and 5890 orbits, respectively. Based on the non-detection of predicted meteoroid trail encounters, Bennu’s meteoroid production rate was not >1.5 kg/s during 1500–1800 CE. Indeed, the current production rate is many orders of magnitude lower. Bennu may have an associated annual meteoroid stream of much older ejecta at a particle flux density of ≤1.3 × 10−6 km−2 h−1, based on seven Bennu-like orbits detected during the first three years of observations.

The paper can be viewed at: https://doi.org/10.1016/j.icarus.2022.115403

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23 June 2022
Bright fireball seen from the Western Cape on Saturday 18 June 2022 by Tim Cooper

Several hundred tonnes of solid particles enter Earth’s atmosphere every day. Most of these are small particles left behind by comets as they round the Sun. Particles travelling through space are called meteoroids. Particles entering the atmosphere at high speed, typically 20-70 km/sec, become visible as meteors (or popularly as ‘shooting stars’) and burn up before they reach the ground. Very bright meteors are called fireballs, and those that are seen to explode are called bolides. If the object survives its passage through the atmosphere and reaches the ground it is called a meteorite. These objects are much larger than ordinary meteors and are due to entry of fragments of asteroids into the atmosphere.

During the early morning of Saturday 18 June, a small fragment of asteroid entered the atmosphere resulting in a very bright bolide, which was widely seen over the Western Cape’s Garden Route region. This is the fourth very bright fireball seen over the Western Cape in recent years.

Time and duration

Most reports gave the time as 05h47 SAST. One observer confirmed this time from a GPS, so it can be considered as correct. Some videos were posted online which showed time stamps with a different time, but these time stamps are taken from PCs which were uncalibrated and therefore unreliable. Many gave the duration of the passage of the fireball as three to four seconds, but again the attention of most people was only drawn to the presence of the fireball by the very bright flash, at which point the object disrupted. There were probably few sightings of the object when it began its fiery path in the upper atmosphere at about 120 km altitude.

Brightness and colour

The brightness of very bright fireballs is always subjective, as there are few bright objects with which to compare. Most comparisons are made against the sun, or full Moon, neither of which was visible at the time, thought the 82% illuminated Moon was high in the north at the time. Estimates of the brightness were between 50-100 times the brightness of the Moon. Various colours were reported and included nearly the entire spectrum from red, orange, yellow, green and blue, and bright white. There were many reports of bright green, which is typical of bright meteors, becoming red as the object descended towards the horizon.

Fragmentation, persistent train and sounds heard

Most agreed that the object exploded during its path, splitting into numerous fragments. One observer said there were four or five large fragments, and another said there were more than 50 smaller fragments. There were no reports of any smoke train left behind after the fireball disappeared. Only two people said they heard sounds, one like a crack, the other three distinct explosions.

Path of the fireball

From various reliable eye witness accounts we reconstructed the path of the fireball as shown by the yellow arrow. The fireball was first seen just south and perhaps west of Mossel Bay. The yellow arrow may have started further to the west, but was only really noticed around the time of the bright flash, by which time it was south of the coast. The fireball burned out and entered ‘dark flight’ at which stage it had been slowed down to the extent that it no longer caused visible light, around 120 km to the south, and perhaps slightly east of Plettenberg Bay.

Conclusion

A small fragment of asteroid entered the atmosphere on Saturday 18 June at 05h47 SAST, resulting in a bright fireball visible to the south of the Garden Route. The object disrupted in a bright flash, leaving several fragments which continued and burned out to the south of Plettenberg Bay.

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August 2021
Bolide over the Western Cape

Several persons reported hearing sounds at around 1am on the morning of August 9.  Locations included Riversdale, Mossel Bay, Oudtshoorn, Boggoms Bay, George, Hartenbos, Klein Brak River, Wilderness and De Rust, spanning a horizontal distance of around 115 km.  The most westerly location where sounds were reported was Riversdale, where it was described as a rumble, like distant thunder.  The most easterly was from several observers in the environs of George, where the sounds were mostly described as rumbling like thunder.  From Oudtshoorn the sound was also likened to distant thunder.  Others said it sounded like an explosion, houses shook and windows rattled.

Only three visual sightings were received of the bolide.  The best description was received from Ted Nutting, who was outdoors at the time and saw the passage of a bright green fireball in a clear sky, duration about 4 seconds moving west to east, and disintegrated into four or five fragments with a bright flash before disappearing.  Sounds were heard about two minutes later, as up to four distinct ‘bangs’ and tailing off afterwards like the sound of a jet flying over.  Triangulation of the start and end points gives a tentative path from west to east, seen towards the north from George, but to the south from near De Rust, where the start and end azimuths were 230° and 180° respectively, traveling parallel to a rooftop, at altitude 60-70°.  Frankie Dos Santos saw the bolide through a window facing azimuth 330° from Hoekwil, near Wilderness.  He described the fireball as ‘very bright, much bigger than a normal shooting star’, moving left to right (towards north east) at an altitude of about 45°, and very fast with a duration of about a second.  The sky was overcast with thin patchy clouds and the appearance was like seeing car headlights through fog.  The bolide possibly began ablation near overhead and just south of Oudtshoorn, and disrupted in the vicinity above Kammanassie Nature Reserve.  Fragments may have fallen as meteorites near to the Kammanassie Mountain range.

In order to determine the fall location of possible meteorites, sufficient video footage is required which shows both the passage and disruption of the bolide, or shadows cast by the flash, which are in the anti-direction of the disruption.  Unfortunately, despite requests on various forums to check security cameras, only one clip was received, from Godwin Pangel in George.  The clip shows a brief very bright flash, with duration less than 1 second, and was followed 2m56s later by a booming sound.  The sky is seen to be mainly overcast at the time of the bright flash.  The video footage was calibrated against internet time to give the time of the flash as August 9, 01:00:30 SAST.  The time lag of the sound gives a distance to the flash of approximately 60km.

The event was not detected by NASA fireball detectors, neither were there any reports to the ASSA, AMS nor IMO reporting forums.  I conclude that a bright bolide passed roughly west to east over the Western Cape, disrupting with a bright flash, and with accompanying sounds.  Insufficient video evidence could be obtained that could have enabled determination of a strewn field site for meteorites.

Several misconceptions arose in the social media in connection with the event.  Firstly the statement on some social media pages that the event was a ‘skyquake’.  There is no accepted scientific definition of what constitutes a ‘skyquake’; rather it appears to be a loose term originating on the internet describing rumbling sounds from unexplained sources, including possibly the sonic boom from meteors.  Since the bolide was observed visually, the suggestion that it was a ‘skyquake’ is clearly of no consequence.

Secondly that the event might be linked to the Perseid meteor shower, which normally peaks during the night of August 11/12, and showed an unexpected surge in activity this year on August 14 at 10am SAST, the outburst lasting about two hours.  The August 9 event was not related to the normal activity or the outburst of the Perseids.  The Perseid meteor stream is known debris from comet 109P/Swift-Tuttle, and particles from comets are too small to produce effects like that witnessed over the Western Cape on August 9.  The radiant of the meteors in the constellation of Perseus only rises early morning and is highest before dawn, and even then the radiant does not rise above the horizon at any time as seen from the George/Mossel Bay area.  Finally, the radiant is located in the north-east in the early morning and cannot explain any meteor observed on a trajectory from west to east.

Thirdly the misconception that what was seen was a meteorite.  Particles travelling through space are termed meteoroids.  If they enter the atmosphere then the resultant streak of light is termed a meteor.  Bright meteors are termed fireballs, and if seen to explode they are called bolides.  If the object survives its passage through the atmosphere, reaches the ground and is recovered then it is termed a meteorite.  Since no videos were obtained to triangulate the path and hence no fragments could be found on the ground, the event on August 9 is classified as a bolide.

Possible path of the August 9 bolide.  Yellow pins are locations where sounds were heard, red pins are locations of the three observers who actually saw the bolide, green pin is the location of the one video which caught the bright flash.

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April 2021
The Botswana super-bolide and meteorites from asteroid 2018 LA

Meteorite MP-19, found by Tim Cooper in Central Kalahari Game Reserve on October 12, 2018. Image courtesy Dr Peter Jenniskens.

Almost three years since the entry of asteroid 2018 LA into the atmosphere over Botswana on June 2, 2018, the scientific results generated by sixty-six authors, collectively known as the ‘2018 LA Consortium’, and after an in-depth analysis of the recovered meteorites from 2018 LA, are now out in an article in Meteoritics and Planetary Science.

Discovered just a few hours earlier, asteroid 2018 LA entered earth’s atmosphere at 16h44 UT, and resulted in a bolide which reached magnitude -23 during its disruption at an altitude of 27.8 km. The resultant explosion deposited meteorites over a strewn field located in the northern part of the Central Kalahari Game Reserve (CKGR) in Botswana. Several videos were secured which captured the visible passage of the bolide, including the bright explosion, which enabled a precise determination of the location at which the disruption occurred. The screen grabs at right show the passage as seen from a commercial property in Gaborone just prior to the disruption of the meteor.

Left: Screen grabs of the bolide from a security camera in Gaborone. Images reproduced with kind permission of Beverly Lombard.

ASSA’s Tim Cooper calibrated videos of the bolide to help determine the location of the strewn field and also calibrated the footage which enabled photometry and subsequently the construction of the light curve of the meteor. An initial search during June 18-23, 2018 found one meteorite (MP-01), now referred to as Motopi Pan. Following revised astrometry, a new search was mounted with a team comprising members from the Botswana Geophysics Institute, Okavango Research Institute, Department of National Museum and Monuments (Botswana), the Department of Wildlife & National Parks, the Astronomical Society of Southern Africa (ASSA) and under the guidance of meteor astronomer Dr Peter Jenniskens from the SETI Institute. The search during October 9-12, 2018 found an additional 22 fragments of asteroid 2018 LA, all collectively known as Motopi Pan, and including fragment MP-19 (image top left) found by Tim Cooper on October 12, 2018. The discovery of these fragments now enabled a complete characterisation of the meteorites from asteroid 2018 LA, and determination of its origin in the solar system.

Left: Rubria Crater on asteroid 4 Vesta, probable source of the meteorites from 2018 LA. Image credit NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Following extensive analysis using multiple techniques, these show the Motopi Pan meteorite to be a HED polymict breccia derived from howardite, eucrite, and diogenite lithologies.

The findings and pre-atmospheric orbit are consistent with an origin for Motopi Pan at asteroid 4 Vesta, possibly from the 10 km crater Rubria (left image) during an impact ~22 Ma ago.

The full scientific results and findings can be found in the Meteoritics and Planetary Science article. The full story of the appearance of the bolide, eye-witness accounts, analysis of video footage to determine the strewn field location, measurement of the brightness of the meteor, and the October 2018 search for meteorites from asteroid 2018 LA will appear in the June issue of MNASSA.

Members of the October 2018 search team which found an additional 22 fragments of the Motopi Pan meteorite. Standing left to right Tim Cooper (ASSA), Oliver Moses (ORI), Mohutsiwe Gabadirwe (BGI), Thebe Kemosedile (ORI), Sarah Tsenene (DWNP), Kabelo Dikole (BGI), Mosarwa Babutsi (Botswana National Museum, Gaborone), kneeling Kagiso Kgetse (DWNP) and Peter Jenniskens (SETI Institute). ASSA = Astronomical Society of Southern Africa, BGI = Botswana Geoscience Institute, ORI = Okavango Research Institute of the University of Botswana at Maun, DWNP = Department of Wildlife and National Parks. Photo by team member Odirile Sempho.