Friday, February 22, 2013

Asteroids, Meteor and Meteorite - (English)

It 's been a long time since the last post but the recent celestial phenomena involving our planet and the many questions addressed by colleagues inspired this post dedicated to space closer to us and, sepcifically, to those celestial objects that may come into contact with the Earth. I'm not an expert of course and the informations given are only a small part of the argument, I tried to gather the information that I found most interesting and that I hope will be useful to clarify some concepts and unravel in the great quantity of ​​information that has been spread, not always correct. (Clicking on the pictures you can see them in their original size). Versione italiana qui.

Let's start with some definitions even if it is not an easy thing. From my layman reckon that the classification into three categories, "Planets", "Dwarf Planets" and "Small Solar System Bodies" for the celestial bodies orbiting the Sun, established by International Astronomical Union (IAU) in 2006 has made significant doubts and increased confusion.

Planet: celestial body that: is in orbit around the Sun; has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; has cleared the neighbourhood around its orbit. The Planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Dwarf Planet: celestila body that: is in orbit around the Sun; has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; has not cleared the neighbourhood around its orbit; is not a satellite. The Dwart Planets are: Ceres, Pluto, Haumea, Makemake and Eris.

Small Solar System Bodies: all other objects, except satellite, orbiting the Sun and that do not fall in the previous two categories.

We start with the identification of the main areas of the solar system in which they are distributed the Small Solar System Bodies.

Main Belt: it's between the orbits of Mars and Jupiter at a distance of between 1.7 AU and 4.0 AU from the Sun and with orbital periods ranging between 3 and 6 years. It's the area where there are a majority of asteroids, although recently findings suggest even the presence of some comets.

Kuiper Belt: extends beyond the orbit of Neptune, on the ecliptic plan, between 30 AU and 50 AU. Contains bodies of considerable size: asteroids and comets. The objects inside are called KBO (Kuiper Belt Object). Have been so far identified over 800 items, among them 3 Dwarf Planets: Pluto, Haumea and Makemake.

Scattered Disc: the beginning coincides with the outermost zone of the Kuiper Belt but with orbits far above and below the ecliptic plan. Extends beyond 100 AU.

Oort Cloud: it's a region that is hypothesized may extend, in spherical form, from about 50 AU up to 1 LY and that is estimated to contain approximately 10,000 billion objects. Should contain mainly comets.
Ceres and Vesta (Credit: NASA)
Asteroid (or very small planet or planetoids or minor planet): a celestial body of non planetary dimensions, with size and shape variables, inactive, rocky. The first to be discovered was Ceres, which was at the time the largest known with a diameter of about 950 km. To give proper information is worth to specify that, in reality, Ceres in 2006 was elevated to the rank of Dwarf Planet.

Most orbit in the Main Belt, a group, the Trojans, shares its orbit with Jupiter and another group, the Centaurs, orbiting between Jupiter and Neptune.

Others have been identified, in areas previously desribed, beyond the orbit of Neptune

The Nucleus of Hartley 2 Comet (Credit: JPL)
Comet celestial body relatively small, composed of ice and dust and sometimes active, able to develop, on evaporation, an atmosphere of dust and gas (coma) and sometimes also one or more tails. Based on the orbital parameters are divided into short-period comets, long-term and non-periodic, with parabolic or hyperbolic orbits that take them to leave the Solar System. It is believed that the short-period comets come from the Kuiper Belt and the long-term from the Oort Cloud.

Meteoroid: fragments of Asteroids and Comets.

Meteor (or shooting stars): light phenomenon that occurs when a Meteoroid vaporizes entering the Earth's atmosphere.

Meteorite: Meteoroid that survives the passage into the atmosphere, without vaporize completely, hitting the ground. 

NEO (Near Earth Object): celestial bodies that pass near the Earth and can intersect the orbit.

Radar Image of Teoutatis (Credit: NASA, JPL)

NEA (Near Earth Asteroids): asteroids whose orbit is closest to the Earth, less than 0.3 AU. The largest known is 1036 Ganymede with a radius of about 31 km. 9.621 have been identified, since today, on an estimated total of about 300,000.

PHA (Potentially Hazardous Asteroids): potentially hazardous Asteroids, their orbit is within the 0.05 AU from the Earth and whose absolute magnitude is less than 22 mag. The largest known today is Teoutatis 4170. Since today have been identified 1382 PHA.

Palermo Scale: is a logarithmic scale, used by professionals, whose equation allows you to combine the probability of impact with the kinetic energy released. Values ​​less than -2 indicate events without consequences, those between -2 and 0 indicate situations that need to be monitored. Values ​​above zero indicate an event out of the ordinary and it surely deserves attention. The first object to passing the value 0 was NTZ 2002, an asteroid about the size of 2 km that could cross the Earth on 1 February 2019.

The Torino Scale (Credit: Richard. P. Binzel MIT)
Torino Scale: is a scale used to communicate with the public and has, therefore, a more immediate and simplified formulation. The values ​​range between 0 and 10, the higher the value the higher the risk. 0 - no risk, 1 - normal risk, 2-4 - deserves attention by astronomers, 5-7 - threatening, 8-10 - certain collision. The name comes from the city of Turin, where he played in 1999 a conference on NEOs during which was revised the original version of the scale.

The space around us is far from empty. The Solar System is populated by billions of bodies larger or smaller, as you can guess from the data provided in the definitions, which are the remains of a planetary nebula from which it originated.
Due to their number, the encounter with these objects can not be definitely excluded and there are many evidences of such impacts. The simplest is below, or should I say above, the eyes of all: the Moon. The craters, of which is covered, are due to collisions with celestial bodies in more or less recent times and the Moon formation, according to recent theories, could be due to the impact of a Mars-sized body with the Earth.
There is vast evidence of the impacts within the Solar System, as showes in the images of the following examples: Phobos, one of the moons of Mars; Deimos, one of the satellites of Saturn that looks like the "Death Star" from Star Wars; Jupiter struck by fragments of Comet Shoemaker-Levy between 16 and 22 July 1994.

Phobos captured by Viking 1 (Credit: NASA)
Deimos (Credit: NASA)
Impact on Jupiter caused by Shoemaker-Levy comet (Credit: NASA)
The Barringer Crater (Credit: NASA)
On the Moon, the absence of an atmosphere and an active geological history have allowed their preservation.On Earth impacts are less obvious and more difficult to detect as they are subject to severe erosion and only the availability of new types and systems of increasingly sophisticated research have led to the discovery. The most famous, known and easily identifiable is the Barringer Crater, known also as Meteor Crater, 1.200 m wide and 170 m deep found in Arizona. The formation dates back to about 50,000 years ago and was caused by a nickel-iron asteroid about the size of 50 m.The Earth, like the other bodies in the Solar System, is constantly bombarded by material from space. It is estimated that every day about 100 tons of dust and small particles hit the atmosphere. The phenomenon is, therefore, not uncommon.

When a meteoroid enters the atmosphere its speed, already high, is accelerated by the attraction gravitation. It is not possible to give a unique reference parameter in view of the numerous factors to be taken into account such as the direction and angle of arrival, the shape, etc., but to give an order of magnitude average speeds oscillate between 10 Km/s and 70 km/s (36,000 Km/h and 252,000 Km/h).At the entrance into the atmosphere the air in front of the object is strongly compressed (ram pressure) and, like all gases, heats up to temperatures above 1600° C able to vaporize and burn the rocks in contact (ablation). Due to the fact that the meteoroid not burn directly, but the atmosphere that surrounds it, in the case in which hit the earth's surface the meteorite would not necessarily be hot or molten, because it would be consumed only in the outer layers, and this would allow to surviv to extraterrestrial bodies, some theories suggest that life in fact has been brought on the Planet by Asteroids and Comets (Panspermia).The strong pressures involved, the very high temperatures and the chemical and physical changes which are to occur in the meteoroid can also lead to its explosion, with the propagation of shock waves of considerable intensity. Bright light is created by the ionization of the air that makes the celestial body considerably larger than the reality.The event occurred in Russia 15 February 2013 was therefore not anything extraordinary. It's an event that occurs on average once every 100 years.

The estimates of NASA on the object before entering into the atmosphere report a size between 15 m and 17 m and a mass between 7,000 t and 10,000 t. The entrance into the atmosphere occurred at an estimated speed of 18 km/s (64,000 km/h) and 32.5 seconds have elapsed from the entrance to the disintegration, which occurred at an altitude of about 20 km.The damage was caused by the shock wave that reached supersonic speeds, creating the so-called "Sonic Bang", similar to that produced by plane when pass the sound barrier, and hit people and things. There weren't , therefore, hit by fragments of the meteor.The numerous references to a "meteor shower" are derived from the fragmentation of the meteoroid and the term has been misused. We refer to a meteor shower when there is a periodic phenomena such as meteor showers, like the Perseids also known as the "Tears of St. Lawrence" in August, during which time you can attend to hundreds of trails per hour.
Without wishing to make a list of such events is sufficient, as an example, remember what happened June 30, 1908 in Tunguska (Siberia) where the shock wave created by the explosion of a fragment of an asteroid or comet nucleus, which took place an estimated share of around 10 km, devastated an area of ​​over 2,000 km/sq breaking down, it is estimated, up to 80 million trees. There was at least one person killed and people, even at a distance of 60 kilometers, were hurled to the ground.

10 August 1972 a meteoroid pass throug the atmosphere (Credit: James M. Baker)
Another example of this happened 10 August 1972 when a meteor crossed the Earth's atmosphere at an altitude of about 57 km coming into contact with the atmosphere over Utah (USA) and came out over Alberta (Canada). It was estimated that the size was between 3 and 14 meters and entered into the atmosphere with a grazing angle, which allowed him to "survive" the passage.

The first object to being identified before it hit the Earth was 2008 TC3 that was discovered on 6 October 2008 to approximately 20 hours before impact. The body, estimated to be about 4 m and a mass of 80 tonnes, has exploded at an altitude of about 10 km above the Nubian Desert in Sudan, as expected. About 600 fragments were recovered in the desert.

If you are intrigued on the internet there are numerous examples of impact craters discovered on our planet. The oldest was discovered in Greenland and dates back to approximately 3 billion years ago. That the effect of which was the extinction of the dinosaurs is Chicxulub in the Yucatan, dropped 65 million years ago.

A great starting point is the Earth Observatory della NASA site.

Finally, an Italian example. The first crater, or craters, impact discovered in Italy is located in Abruzzo, precisely in the plane of the Prati del Sirente. At this link you can deepen the discovery
Prati del Sirente (Abruzzo)
I hope to have provided useful information and not to have added more confusion.


Reference Link:
* NASA Asteroids and Comets Impact Hazards
* NASA JPL NEO Program
* IAU Near Earth Object

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