Meteorites and Meteor Showers

By David Bryant

Every year in mid-August astronomers all over the world eagerly anticipate the Perseid Meteor Shower: some years the sky can be full of ‘shooting stars’, on others hardly a handful are seen.
This year was pretty good: I personally observed over sixty meteors in around two hours and even managed to photograph one as it passed close to the familiar ‘W’ shaped constellation, Cassiopeia:

As many of you will know (either from your own reading or my previous articles) meteors are the tracks made by small stony or nickel-iron grains glowing and burning up due to frictional heating as they enter the Earth’s atmosphere. The vast majority of these are smaller than an apple and do not survive their fiery passage to reach the Earth’s surface.
Just occasionally a larger object will make it to ground level: it is then called a meteorite.
The most abundant types of meteorite are:
• Stony meteorites. These are left over from the formation of the Solar System or come from the surface of other planets.
A couple of hundred meteorites (known as achondrites) are known to have been blasted from the surface of the Moon or Mars by meteorite or cometary impacts: about the same number originated in the same way on asteroids such as 4-Vesta.

•Stony irons. Pallasites and mesosiderites are thought to derive from the core-mantle layer of shattered planets, or from collisions between stony and nickel-iron bodies.
• Iron meteorites These may have either condensed directly within the Solar Nebula or be the cores of disrupted, differentiated planetissimals.
Some meteor showers, however, are associated with the return of debris from periodic comets to our region of the Solar System.Beyond the furthest reaches of the Solar System, at a distance between 5,000 to 100,000 AU, (1 Astronomical Unit = the distance from the Earth to the Sun: around 150 million kilometres) lies an encompassing shell of trillions of cometary nuclei: the Oort Cloud. Since their formation from the solar nebula or capture by the Sun, these objects have collected a regolith of carbonaceous material and dust particles.

For reasons not yet fully understood (but possibly associated with the gravitational influence of passing dwarf stars) the orbits of these bodies can be perturbed, causing them to tumble inwards towards the Sun. Should this occur, by the time an Oort Cloud object reached the inner Solar System it would be travelling at a velocity of one hundred thousand kilometres an hour and will have acquired an outer shell of rocky material.

Encountering the electromagnetic radiation and streams of particles from the Sun, the icy nucleus may develop a coma and tail, becoming visible from the Earth as a typical comet.
In common with the great majority of these objects, it may swing around the Sun and pass harmlessly back into deep space. Possibly, however, it will be captured and enter an elliptical orbit, becoming a periodic comet that makes regular returns to the inner Solar System. Eventually it will lose most of its mass through sublimation, leaving a cloud of stony debris that might give rise to a new meteor shower, should its orbit cross that of the Earth.
The recent investigation of Comet 67P/Churyumov–Gerasimenko by the twin probes Rosetta / Philae demonstrated what many meteoricists had long suspected: although comets are primarily composed of water-ice, their surface is a regolith composed of whatever debris they have passed through during their adventures in space: it is this material that is responsible for periodic meteor showers like the Perseids in August (associated with Comet Swift-Tuttle), the Orionids in October (associated with Comet Halley) and the Leonids in November (associated with Comet 55P/Tempel-Tuttle)
Whenever a ‘good’ meteor shower (like those above) is due, the event is highlighted in the press and on TV. In the past there have been some incredible ‘meteor blizzards, like that in the old print below. Sadly, most years, non-astronomers are disappointed by the reality: even the one or two a minute I saw this August could not really be described as a spectacle.

Perhaps it is the publicity given to some showers or perhaps it is the antics of a couple of my American colleagues on TV making people more meteorite-aware, but after every Perseid Shower for the past four or five years I have received numbers of e-mails from people who think they have found one!
Here’s a recent example:

‘Hi! Is this a meteorite? I was watching out for meteors the other night when suddenly I noticed a black rock on the lawn: I’m certain it wasn’t there before!
I just went through the check list on your site: it is strongly magnetic, has a fusion crust, thumbprints, and there seems to be a widdsmanstatten pattern inside .I also clipped a little of edge and it is bright. I have added some pictures for you to look at. If it is a meteorite, I would be happy to offer it to you at the right price.’
Sad to say, I have no recollection of a single meteorite having been conclusively proven to have fallen during a periodic meteor shower. I should imagine the reasons are quite straight-forward:
• The size of particle that the Earth encounters when it passes through a field of cometary debris is quite small: probably the size of a grain of rice – far too small to reach the ground.

• The majority of periodic comets have visited the inner Solar System sufficiently often that most of the Earth-crossing debris has already been ‘hoovered up’.
• The trail of debris left by a comet is spread very thinly along its entire orbit around the Sun: the tiny region of the orbit of Halley’s Comet that the Earth crosses each year must statistically contain very few large chunks.
• The chances of anyone being fortunate enough to see one of these hit the ground and locate it are about the same as winning the lottery every week for a year!
In conclusion, the vast majority of meteoric material that arrives on Earth (about 300 tonnes a day!) is not of cometary origin: any that is will generally be far too small to reach the surface or be discovered.
If you’re interested in finding out more about comets, meteorites and their occasional impacts on our planet, my third book ‘Danger from the Skies’ is now available on Amazon or from my sales page:

Ice from Above!

By David Bryant

A recent TV documentary (of the somewhat over-dramatic variety!) sought to explain several recent – and very damaging - falls of ice. Arriving at high speed from cloudless skies, these have battered roofs, cars and aircraft. The conclusion of the program was that these were examples of ‘mega-hailstones’, poorly-understood phenomena, more usually called megacryometeors by the Scientific community.

Around 50 of these have been recorded so far this century, varying in mass from 0.5 kg to real giants such as a Brazilian example of over 50 kg: a specimen with a mass of 200kg was reportedly seen falling in Scotland in the nineteenth century!

Meteorologists on the documentary spent much of the program establishing a mechanism by which huge chunks of ice could form in the upper atmosphere, other than in the conventional nursery of the convection currents of a cumulo-nimbus cloud. As most people will be aware, the powerful updraughts inside such clouds (which are typically associated with thunderstorms) allow the formation of hailstones. These may gyrate inside the cloud, accumulating mass until they are too heavy to remain aloft: hailstones the size of golf balls are not that uncommon.
However, they generally display a layered cumulate structure similar to that of an onion, while megacryometeors do not.

At the time of writing, no generally accepted mechanism for generating and supporting such large masses in the upper atmosphere has been forthcoming, although some of the theories put forward seem credible at first glance.

That chunks of ice occasionally fall from aircraft is undeniable and may be placed in two separate categories. The first (of
which I have personal experience) is generated by the dumping of liquid waste from on-board lavatories. Some years ago a
local radio station invited me to interview a lady who had been struck on the arm while hanging out her laundry. On arrival,
I asked to see the object that had hit her: to my amazement, she opened her freezer and took out a polythene bag, inside which I saw a bluish lump of ice similar to that in the photo:


The lady was less than thrilled when I told her she’d been storing lavatory waste from an aircraft among her fish fingers and
frozen chips! It had an obvious smell of disinfectant, so I’m at a loss to know how the lady for one minute thought it was a
Of course, icing on the fuselage and wings of aircraft still poses a threat to aviation safety, and flying through Supercooled Large Droplet (SLD) conditions can generate chunks of ice: these could fall to the ground and cause damage, but would not, I feel, be confused with genuine extraterrestrial ice meteors that should show signs of flowlines and ablation.
So then: is it possible that ice meteors could reach the Earth from space and pass through the atmosphere to the ground?
An online search will quickly discover a good number of learned publications that appear to answer this question with a resounding ‘no’. The majority maintain that a vast initial mass of tens of thousands of tonnes would be required for a football-sized chunk to reach ground level. But is this necessarily the case? The assumption is that frictional ablation would melt away most of the object’s mass, but this ignores certain factors:
1) Objects entering the atmosphere from deep space may hit the Earth head-on, at a combined velocity of 220,000kph or more. But equally, they may ‘creep up’ on the Earth from behind, with a closing velocity of just a few thousand kph, reducing frictional heating by a huge amount
2) Our putative ice meteor would be at a temperature just a few degrees above absolute zero (-273 degrees C) Whilst the outer layers would indeed become extremely hot, they would slough off like the heat shield of a re-entering Apollo spacecraft, taking heat energy with them.

Moreover, like the tiles on the five Shuttle Orbiters, ice is a pretty poor conductor of heat, so that the interior could be  expected to survive better than, say, a piece of rock or iron.
Assuming, then, that some of the ice that falls from the sky may indeed originate in space, two questions immediately occur:
Where would ice meteorites originate? The Solar System is full of water-ice: billions of tons make up most of the mass of each of the trillions of comets in the Oort Cloud and Kuyper Belt.
Additionally, the Asteroid Main Belt must hold thousands of captured objects from these remote regions: these, we know, are
occasionally deflected into the inner Solar System.
I have written elsewhere that it is my belief that the majority of large craters on the inner planets and their satellites are the result of cometary (rather than asteroidal) impacts:
certainly, recent research has shown that to be the case with the Gilf-el-Kebir in Egypt:

 How could we prove an extraterrestrial origin?
The relative concentrations of two isotopes of oxygen (17O, and 18O ) is used to assign an origin to planetary and asteroidal
meteorites. Cometary water should display relative oxygen isotope concentrations different to that of terrestrial water.
At the time of writing, just a few megacryometeors have been tested: these have all had terrestrial ROICs. But the samples tested were just a tiny fraction of the numbers that fall on the Earth: it is unscientific to discount the possibility of ice
meteorites on such a small sample. Should a sample be found to have an exotic ROIC, it could then be examined for evidence of presolar grains, interplanetary dust and regalith fragments it had picked up during its wanderings in space.
In conclusion, I suggest there is a high probability that chunks of cometary ice do reach the Earth’s surface from time to time:
I have written this article in the hope that not all astronomers will continue to dismiss a possible extraterrestrial origin for these.

David Bryant

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