Asteroids, Comets and Meteors
In the beginning a massive swirling cloud of dust and gas circled the young Sun. The dust particles collided with each other and formed into larger bits of rock. This process continued until they reached the size of boulders. Eventually this process of accretion formed the planets of our solar system.
Billions of small space rocks never evolved. Amazingly, many of these mysterious worlds have been altered very little in the 4.6 billion years since they first formed. Their relatively pristine state makes the comets, asteroids and some meteors wonderful storytellers with much to share about what conditions were like in the early solar system. They can reveal secrets about our origins, chronicling the processes and events that led to the birth of our world. They might offer clues about where the water and raw materials that made life possible on Earth came from.
Current Known Count:
Raw Materials For Life?
Comets and asteroids probably delivered some of the water and other ingredients that allowed the complex chemistry of life to begin on Earth. The amino acid glycine was discovered in the comet dust returned to Earth by the Stardust mission. Glycine is used by living organisms to make proteins. The discovery supports the theory that some of life's ingredients formed in space and were delivered to Earth long ago by meteorite and comet impacts.
Protection From Possible Impacts:
Impacts are capable of ending life as well as advancing it. These cosmic collisions are as natural as rain, although they happened a lot more often when the solar system was young. Scientists believe stray objects or fragments from earlier collisions slammed into Earth in the past, playing a major role in the evolution of our planet.
An asteroid is a celestial body - composed of rock, metal or a mixture of both - that is orbiting the Sun. Most of them are in the asteroid belt between Mars and Jupiter. Even though there are millions of asteroids with sizes up to more than 500 km (like Pallas and Vesta) they are of no danger to the planet Earth. The biggest body in the asteroid belt - Ceres - is officially not called an asteroid anymore but a dwarf planet. If you try to envision the asteroid belt don't get fooled by some science fiction films: traveling around in the asteroid belt with your spacecraft does not require constant steering in order to avoid crashes with asteroids. The scale of the solar system is so immense that even inside the asteroid belt the average distance between two asteroids is above one million km - or three times the distance between Earth and the Moon.
Some asteroids have very elliptical trajectories, crossing the orbits of the inner planets Mars, Earth or Venus. The cause of these elliptical trajectories could be collisions within the asteroid belt or the gravitational influence of the massive planet Jupiter changing the orbits of some asteroids gradually over time (see orbital resonance). All asteroids with orbits so eccentric that they cross Earth's orbit are called 'Apollo asteroids', 'Amors' approach the Earth but do not cross Earth's orbit. Apollo asteroids are doomed to sooner or later collide with one of the inner planets, usually within a few million years of their orbit becoming so eccentric. The largest Apollo asteroid - 1866 Sisyphus - has a diameter of about 9 km, similar to the asteroid that caused the Chicxulub event, the giant meteorite impact that caused the extinction of the dinosaurs. Anyhow, Sisyphus and none of the other big Apollo asteroids will collide with Earth in the next millennia; which does not mean that smaller bodies can cause local damage. Many of you will remember the Chelyabinsk event which took place on the 15th of February 2013. Fortunately no people were killed during this event and today you can even buy a Chelyabinsk meteorite.
Every asteroid or meteoroid that survives its passage through Earth's atmosphere (and this is the rare exception) can be advanced to be called a meteorite. Meteorites are made of rock (stony meteorites), metal (iron meteorites) or a mixture of these two materials (stony-iron meteorites or pallasites). Pallasites form beautiful olivine crystals that are embedded into a metal matrix. Scientists are eager to study meteorites since they are the very first material that was formed in our early solar system, almost 4.6 billion years ago.
Generally speaking, meteoroids are all the smaller objects in orbit around the Sun. Most of them originate from comets that lose gas and dust when they approach the Sun. Other meteoroids are basically small asteroids. There is no exact diameter that distinguishes an asteroid from a meteoroid. Wikipedia states 10 meters; other trustworthy sites call anything smaller than 1 km a meteoroid. Anyhow, the vast majority of all meteoroids are just a few millimeters and less in size. The smallest and by far the most numerous ones have sizes of small dust particles and are called micrometeoroids; they do not leave any visible trace behind when they enter the Earth's atmosphere.
The ones about the size of a pebble leave behind a flash of light when they completely vaporize. Most people call this flash a "shooting star" or a "falling star", but more accurately spoken this is a meteor. A meteor is the light that you can see when a small meteoroid enters the Earth’s atmosphere. This normally happens with speeds between 11 and 73 km/s and at altitudes of about 75-120 km. Under a clear sky an observer can see 5 to 10 meteors per hour, especially after midnight when the Earth has rotated so far that the observer's part of the sky is positioned in the direction of the Earth's motion around the Sun. During so called meteor showers the rate of observable meteors per hour can increase significantly. Meteor showers are caused when the Earth crosses higher than usual concentrations of particles that are themselves in an eccentric orbit around the Sun. Since the orbit of these particles is fixed, we encounter this stream every year at the same time - just its density cannot be foreseen. This sometimes leads to sparse meteor showers and sometimes very intense meteor showers with more than 1000 meteors per hour, also called meteor outbursts or meteor storms. The meteors we see can be debris from a comet (> 90% of all meteors we see) or an asteroid. The most famous meteor showers are the Perseids in mid-August (caused by Comet 109P/Swift-Tuttle) and the Leonids (mid-November). The meteors during these meteor showers almost all emerge from the same section of the sky; indeed the meteor showers are named for the constellations from which the meteors appear to originate.
But what causes the light path of the meteor that we can see in the sky? Smaller meteoroids will be heated by adiabatic compression until the point when they completely disintegrate. However, the light emission we observe is mainly caused by interactions between evaporated and detached components of the fast moving meteoroid and air molecules. Both the meteoroid atoms and the air ionize during this encounter. When the free electrons recombine with the ionized atoms in the tail of the meteoroid they emit the light that we can observe. The light track can have a length of up to several tens of kilometers and an initial diameter of a few meters. The color of the meteor is an indicator of the material of the meteoroid; e.g., a yellow color is caused by iron, a blue-green color by copper and a red color by silicate material.
A meteor that is larger and brighter than normal is called a fireball; brighter than the brightest planet in our night sky (Venus). If these fireballs also break apart or explode during their atmospheric flight - sometimes accompanied by considerable audible sounds - they are called a bolide.
Comets are asteroid-like objects which are composed of ice, dust and rocky particles; that's why they are also called 'dirty snowballs'. The sizes of their nuclei vary between a few hundred meters to tens of kilometers in diameter; their visible tails can extend to above 150 million km in length. They originate from outside Neptune's orbit and - like many asteroids and meteoroids - are unmodified remnants of the formation of our solar system about 4.568 billion years ago. When comets approach the Sun the solar radiation and solar winds cause particles to sublimate and detach from the comet, forming a tail of particles which often makes them visible in the night sky even to the naked eye. We say 'sublimate' (a direct phase transition from the solid to the gas phase) since with zero pressure in space, water will not exist in the liquid phase. Anyhow, below its surface there can also be reservoirs of liquid water which can vaporize and feed jets of water vapor.
Comets orbit as around the Sun on elliptical orbits until all of their volatile material has evaporated away. The orbital periods vary between a few years (like comet Encke) and tens of millions of years. While we can observe Halley's Comet every 75 years we need to wait 106 000 more years until we see Comet Panstarrs (C/2011 L4), our guest in 2013, the next time.
Short-period comets mainly originate from the Kuiper belt, a region in the solar system with many millions of icy bodies extending from about 30 AU (about the orbit of Neptune) to 50 AU. If some of these icy bodies get too close to Neptune during their orbit they may be deflected and enter a new, eccentric orbit which will make them become short-period comets. Long-period comets normally originate from the Oort cloud, a region between 2000 AU and 50000 AU (or about one light year) away from the Sun. The Oort cloud consists of trillions of icy objects with diameters above 1 km. With these huge numbers we can be sure that there will be no shortage of comets visiting the inner part of the solar system in the future. But what causes these icy objects in the Oort cloud to leave their stable orbit and approach the inner part of the solar system? Without any "push" they would certainly continue orbiting in the Oort cloud forever. But gravitational perturbations of nearby passing stars and the galactic tide can cause these comets to change their trajectory around the Sun and approach the inner parts of the solar system. The star Gliese 710 will approach within a distance of just 1 light year from the Sun in about 1.4 million years, scratching the Oort cloud and causing many objects to change their trajectories around the Sun.
Comets from the Kuiper belt tend to orbit the Sun within the plane of the solar system because the Kuiper belt itself is aligned with the plane of the solar system. Comets from the Oort cloud can arrive from all different directions since the Oort cloud has a spherical shape. A comet's tail is caused by gas and dust particles that are sublimated and/or vaporized by sunlight and then blown away by the solar wind. The tail always streams out in the direction opposite to the Sun, but it does not arise until the comet enters the inner parts of the solar system (somewhere between Mars and Jupiter), so that the sunlight can sufficiently heat up the comet.
|Asteroid||A celestial body bigger than 10 m orbiting the Sun, mainly between Mars and Jupiter|
|Meteroid||Similar to an asteroid, but significantly smaller. Mostly debris of comets, sometimes debris of asteroids.|
|Meteor||A bright tail of light caused by a meteoroid during its atmospheric flight, also called a shooting star or falling star.|
|Fireball||A very bright meteor (brighter than the planet Venus).|
|Bolide||A fireball that explodes during its atmospheric flight, often with visible fragmentation.|
|Meteorite||The part of a meteoroid or asteroid that survives the passage through our atmosphere and reaches the Earth's surface.|
|Comet||A smaller celestial body mainly composed of ice and dust. If a comet approaches the Sun it can generate a tail of gas and/or dust.|
Most bodies in the solar system with a visible solid surface exhibit craters. On Earth we see very few because geological processes such as weathering and erosion soon destroy the obvious evidence. On bodies with no atmosphere, such as Mercury or the Moon, craters are everywhere. Without going into detail, there is strong evidence of a period of intense cratering in the solar system that ended about 3.9 billion years ago. Since that time cratering appears to have continued at a much slower and fairly uniform rate. The cause of the craters is impacts by comets and asteroids. Most asteroids follow simple circular orbits between the planets Mars and Jupiter, but all of these asteroids are perturbed, occasionally by each other and more regularly and dramatically by Jupiter. As a result some find themselves in orbits that cross that of Mars or even Earth. Comets on the other hand follow highly elongated orbits that often come close to Earth or other major bodies to begin with. These orbits are greatly affected if they come anywhere near Jupiter. Over the eons every moon and planet finds itself in the wrong place in its orbit at the wrong time and suffers the insult of a major impact.
The Earth's atmosphere protects us from the multitude of small debris, the size of grains of sand or pebbles, thousands of which pelt our planet every day. The meteors in our night sky are visible evidence of this small debris burning up high in the atmosphere. In fact, up to a diameter of about 10-meters (33 feet), most stony meteoroids are destroyed in the atmosphere in thermal explosions. Obviously some fragments do reach the ground, because we have stony meteorites in our museums. Such falls are known to cause property damage from time to time. On October 9, 1992, a fire ball was seen streaking across the sky all the way from Kentucky to New York. A 27-pound stony meteorite (chondrite) from the fireball fell in Peekskill, New York, punching a hole in the rear end of an automobile parked in a driveway and coming to rest in a shallow depression beneath it. Falls into a Connecticut dining room and an Alabama bedroom are well documented incursions in this century. A 10-meter body typically has the kinetic energy of about five nuclear warheads of the size dropped on Hiroshima, however, and the shock wave it creates can do considerable damage even if nothing but comparatively small fragments survive to reach the ground. Many fragments of a 10-meter iron meteoroid will reach the ground. The only well-studied example of such a fall in recent times took place in the Sikhote-Alin Mountains of eastern Siberia on February 12, 1947. About 150 US tons of fragments reached the ground, the largest intact fragment weighing 3,839 pounds. The fragments covered an area of about 1 x 2 kilometers (0.6 x 1.2 miles), within which there were 102 craters greater than 1 meter in diameter, the largest of them 26.5 meters (87 feet), and about 100 more smaller craters. If this small iron meteoroid had landed in a city, it obviously would have created quite a stir. The effect of the larger pieces would be comparable to having a car suddenly drop in at supersonic speeds! Such an event occurs about once per decade somewhere on Earth, but most of them are never recorded, occurring at sea or in some remote region such as Antarctica. It is a fact that there is no record in modern times of any person being killed by a meteorite. It is the falls larger than 10 meters that start to become really worrisome. The 1908 Tunguska event was a stony meteorite in the 100-meter class. The famous meteor crater in northern Arizona, some 1219 meters (4,000 feet) in diameter and 183 meters (600 feet) deep, was created 50,000 years ago by a nickel-iron meteorite perhaps 60 meters in diameter. It probably survived nearly intact until impact, at which time it was pulverized and largely vaporized as its 6-7 x 1016 joules* of kinetic energy were rapidly dissipated in an explosion equivalent to some 15 million tons of TNT! Falls of this class occur once or twice every 1000 years.
There are now over 100 ring-like structures on Earth recognized as definite impact craters.
There are now more than 150 known asteroids that come nearer to the Sun than the outermost point of Earth's orbit. These range in diameter from a few meters up to about 8 kilometers. A working group chaired by Dr. David Morrison, NASA Ames Research Center, estimates that there are some 2,100 such asteroids larger than 1 kilometer and perhaps 320,000 larger than 100 meters, the size that caused the Tunguska event and the Arizona Meteor Crater. An impact by one of these larger meteors in the wrong place would be a catastrophe, but it would not threaten civilization. However, the working group concluded that an impact by an asteroid larger than 1-2 kilometers could degrade the global climate, leading to widespread crop failure and loss of life. Such global environmental catastrophes, which place the entire population of the Earth at risk, are estimated to take place several times per million years on average. A still larger impact by an object larger than about 5 kilometers is damaging enough to cause mass extinctions. In addition there are many comets in the 1-10 kilometer class, 15 of them in short-period orbits that pass inside the Earth's orbit, and an unknown number of long-period comets. Virtually any short-period comet among the 100 or so not currently coming near the Earth could become dangerous after a close passage by Jupiter.
|Impact Diameter||Kinetic Energy At||Airburst Altitude||Average Frequency (Years)||Recorded Fireballs (CNEOS) (1988-2018)|
|4 m (13 ft)||3 kt||0.75 kt||42.5 km (139,000 ft)||1.3||54|
|7 m (23 ft)||16 Kt||5 Kt||36.3 km (119,000 ft)||4.6||15|
|10 m (33 ft)||47 kt||19 Kt||31.9 km (105,000 ft)||10||2|
|15 m (49 ft)||159 kt||82 kt||26.4 km (87,000 ft)||27||1|
|20 m (66 ft)||376 kt||230 kt||22.4 km (73,000 ft)||60||1|
|30 m (98 ft)||1.3 Mt||930 kt||16.5 km (54,000 ft)||185||0|
|50 m (160 ft)||5.9 Mt||5.2 Mt||8.7 km (29,000 ft)||764||0|
|70 m (230 ft)||16 Mt||15.2 Mt||3.6 km (12,000 ft)||1900||0|
|85 m (279 ft)||29 Mt||28 Mt||0.58 km (1,900 ft)||3300||0|
|Based on density of 2600 kg/m3, speed of 17 km/s, and an impact angle of 45°|
|Impact Diameter||Kinetic Energy At||Crater Diameter||Frequency (Years)|
|100 m (330 ft)||47 Mt||3.4 Mt||1.2 km (0.75 mi)||5200|
|130 m (430 ft)||103 Mt||31.4 Mt||2 km (1.2 mi)||11,000|
|150 m (490 ft)||159 Mt||71.5 Mt||2.4 km (1.5 mi)||16,000|
|200 m (660 ft)||376 Mt||261 Mt||3 km (1.9 mi)||36,000|
|250 m (820 ft)||734 Mt||598 Mt||3.8 km (2.4 mi)||59,000|
|300 m (980 ft)||1270 Mt||1110 Mt||4.6 km (2.9 mi)||73,000|
|400 m (1,300 ft)||3010 Mt||2800 Mt||6 km (3.7 mi)||100,000|
|700 m (2,300 ft)||16100 Mt||15700 Mt||10 km (6.2 mi)||190,000|
|1,000 m (3,300 ft)||47000 Mt||46300 Mt||13.6 km (8.5 mi)||440,000|
|Based on ρ = 2600 kg/m3; v = 17 km/s; and an angle of 45°|
Based on crater formation rates determined from the Earth's closest celestial partner, the Moon, astrogeologists have determined that during the last 600 million years, the Earth has been struck by 60 objects of a diameter of 5 km (3 mi) or more. The smallest of these impacts would leave a crater almost 100 km (60 mi) across. Only three confirmed craters from that time period with that size or greater have been found: Chicxulub, Popigai, and Manicouagan, and all three have been suspected of being linked to extinction events though only Chicxulub, the largest of the three, has been consistently considered. The impact that caused Mistastin crater generated temperatures exceeding 2,370 °C, the highest known to have occurred on the surface of the Earth.
Besides direct effect of asteroid impacts on a planet's surface topography, global climate and life, recent studies have shown that several consecutive impacts might have an effect on the dynamo mechanism at a planet's core responsible for maintaining the magnetic field of the planet, and might eventually shut down the planet's magnetic field
An impact event is commonly seen as a scenario that would bring about the end of civilization. In 2000, Discover Magazine published a list of 20 possible sudden doomsday scenarios with an impact event listed as the most likely to occur
More Earth Impacts:
1. The leading theory of the Moon's origin is the giant impact theory, which postulates that Earth was once hit by a planetoid the size of Mars; such a theory is able to explain the size and composition of the Moon, something not done by other theories of lunar formation.
2. Evidence of a massive impact in South Africa near a geological formation known as the Barberton Greenstone Belt was uncovered by scientists in April 2014. They estimated the impact occurred about 3.26 billion years ago and that the impactor was approximately 37–58 kilometers (23–36 miles) wide. The crater from this event, if it still exists, has not yet been found.
3. Two 10-kilometre sized asteroids are now believed to have struck Australia between 360 and 300 million years ago at the Western Warburton and East Warburton Basins creating a 400-kilometer impact zone. According to evidence found in 2015 it is the largest ever recorded. A third, possible impact was also identified in 2015 to the north, on the upper Diamantina River, also believed to have been caused by an asteroid 10 km across about 300 million years ago, but further studies are needed to establish that this crustal anomaly was indeed the result of an impact event.
20th Century Imapcts:
One of the best-known recorded impacts in modern times was the Tunguska event, which occurred in Siberia, Russia, in 1908. This incident involved an explosion that was probably caused by the airburst of an asteroid or comet 5 to 10 km (3.1 to 6.2 mi) above the Earth's surface, felling an estimated 80 million trees over 2,150 km2 (830 sq mi)
In February 1947, another large bolide impacted the Earth in the Sikhote-Alin Mountains, Primorye, Soviet Union. It was during daytime hours and was witnessed by many people, which allowed V. G. Fesenkov, then chairman of the meteorite committee of the USSR Academy of Science, to estimate the meteoroid's orbit before it encountered the Earth. Sikhote-Alin is a massive fall with the overall size of the meteoroid estimated at approximately 90,000 kg (200,000 lb). A more recent estimate by Tsvetkov (and others) puts the mass at around 100,000 kg (220,000 lb). It was an iron meteorite belonging to the chemical group IIAB and with a coarse octahedrite structure. More than 70 tonnes (metric tons) of material survived the collision.
21st Century Hits:
In the late 20th and early 21st century scientists put in place measures to detect Near Earth objects, and predict the dates and times of asteroids impacting Earth, along with the locations at which they will impact. The International Astronomical Union Minor Planet Center (MPC) is the global clearing house for information on asteroid orbits. NASA's Sentry System continually scans the MPC catalog of known asteroids, analyzing their orbits for any possible future impacts. Currently none are predicted (the single highest probability impact currently listed is ~7 m asteroid 2010 RF12, which is due to pass earth in September 2095 with only a 5% predicted chance of impacting)
Currently prediction is mainly based on cataloging asteroids years before they are due to impact. This works well for larger asteroids (> 1 km across) as they are easily seen from a long distance. Over 95% of them are already known and their orbits have been measured, so any future impacts can be predicted long before they are on their final approach to Earth. Smaller objects are too faint to observe except when they come very close and so most cannot be observed before their final approach. Current mechanisms for detecting asteroids on final approach rely on wide-field ground based telescopes, such as the ATLAS system. However, current telescopes only cover part of the Earth and even more importantly cannot detect asteroids on the day-side of the planet, which is why so few of the smaller asteroids that commonly impact Earth are detected during the few hours that they would be visible. So far only four impact events have been successfully predicted in advance, all from innocuous 2-5m diameter asteroids and detected a few hours in advance.
In April 2018, the B612 Foundation reported "It’s 100 per cent certain we’ll be hit [by a devastating asteroid], but we’re not 100 per cent certain when." Also in 2018, physicist Stephen Hawking, in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation to launch a mission to intercept an asteroid. The preferred method is to deflect rather than disrupt an asteroid.
So, what are the chances of Planet Earth being hit by an Asteroid, Comet or Meteorite?
Throughout history and before, this planet shows signs of being struck by meteors and asteroids; some very large ones. Look at the K-T extinction event. Evidence has been found regarding this happening, yet people still do not believe it. There are craters all over the world, both on land and underwater, showing some type of a strike. Look at the Meteor Crater in Arizona. It is there. No one can say it did not happen. I don not believe the U.S. Government or the state of Arizona made the crater to make someones vacation better.
Based on my research and what I've written about in this article I truly believe that some type of impactor [asteroid, meteor or comet] will hit this planet again. I really don't know when this will happen, but predictions from scientists state sometime around 2095. Not sure where they get that date. It will happen, may not be that year, but it will. I hope everyone is prepared.