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Astronomy
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Oct 30, 2023
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Module 2.1- The Origin of the Solar System
Nebular Hypothesis The Sun and the bodies that orbit around it are thought to have begun as a nebula
(an immense cloud of gas and dust in space; also called a “molecular cloud”).
The nebula that became our solar system began as a large irregularly shaped mass of gas and dust
in space. Within the nebula the pressure of the gases act outwards to cause it to expand while gravitational forces (forces that pull bodies towards each other) act to cause the nebula to collapse onto itself.
The force of gravity prevailed over gas pressure and the nebula collapsed and began to spin.
As the diameter of the nebula was reduced, the rate of spin increased.
Due to the interaction of the pressure and gravitational forces, as the nebula was spinning it became flatter and formed a broad disk as the nebula continued to collapse.
As the density of the centre of the disk increased along with its temperature the core of the nebula became the protosun.
As the protosun continued to increase in density and its temperature reached about 10 million degrees, nuclear fusion began within the protosun. Nuclear fusion is a reaction that involves the formation of new chemical elements by combining atomic nuclei. For example, through nuclear
fusion Hydrogen nuclei and subatomic particles can combine to form Helium. The onset of nuclear fusion further increased the temperature of the protosun and at this stage it effectively
became a star, in the case of our solar system one we call Sol, or the sun. With further increase in temperature other elements with larger and larger nuclei (up to Iron on the periodic table) were
produced by nuclear fusion. Within the cloud swirling eddies developed drawing matter towards their centres to form the protoplanets.
As the protosun became even hotter gases were driven off the inner region of the Solar System. The protoplanets became solid planets and continued their orbit, governed by the initial spin of the nebula.
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The Nebular Hypothesis is attractive because it explains many features of the Solar System. For example, the orbits of the planets lie in a plane with the sun at its centre. This plane is called the "orbital" plane or "ecliptic" plane and it is also the plane of the early disk-shaped nebula.
The Nebular Hypothesis also explains why the planets mostly rotate in the same direction and their axes of rotation are nearly perpendicular to the orbital plane. This direction of rotation was inherited from the direction of spin of the eddies in the spinning nebula that formed the protoplanets.
In our solar system Venus and Uranus do not rotate in the same direction as the other planets.
Venus’s rotational axis is at right angles to the plane of the planets (the ecliptic plane) but it rotates in the opposite direction compared to the other planets.
Uranus rotates about an axis that is almost parallel to the plane of the planets.
Modern thinking is that the rotations of both planets were affected by major collisions with other bodies very early in their history.
Module 2.2- The Planets of the Solar System
The image below shows the 8 planets and their relative sizes. The four small planets that are closest to the Sun are the "inner planets" and the four large planets that lie further from the Sun are the "outer planets".
The Inner Planets
Earth-like, "rocky" planets: metallic cores, dominated by silicon and oxygen compounds.
Name
Diameter (km)
Diameter as a % of Earth
diameter
Distance from Sun
(AU)
Mercury
4,880
38%
0.37
Venus
12,103
95%
0.72
Earth
12,742
100%
1.00
Mars
6,779
53%
1.52
*
Astronomical Units (AU)
: 1 unit is the distance from Earth to the Sun, 149,597,900 km.
The Outer Planets
“Gas Giants” with thick atmospheres that become denser and hotter towards their rocky cores.
Name
Diameter (km)
Diameter as a % of Earth
diameter
Distance from Sun
(AU)
Jupiter
139,822
1,097%
5.2
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Saturn
116,464
914%
9.5
Uranus
50,724
398%
19.2
Neptune
49, 244
386%
30.1
Formation of the Planets
The formation of the planets is a captivating process. The universe consists of countless galaxies,
each containing numerous stars. Around some of these stars, there are planets, much like those orbiting our own star, the Sun. The Nebular Hypothesis is the most widely accepted explanation for how the Sun and planets in the solar system may have formed. When our solar system was first created, it was believed that all that existed was a cold, spinning cloud of gas called the Solar Nebula. This nebula resulted from an uneven distribution of gases throughout the universe. As gravitational pull began to condense the gas toward the center, the rotation speed increased, causing the cloud to flatten and create an accretion disk. Matter continued to collect as the growing force of gravity drew it toward the center. Eventually, the gas warmed from increasing pressure, leading to a ball of hot gas forming in the center of the accretion disk, creating a protostar, also known as the Sun. When enough gas gathered in the center of the protostar, the pressure generated enough heat to fuse atoms, forming a star. Outside the star, matter formed into
clumps of gas, dust, and rock, creating protoplanets. These protoplanets grew by trapping material in their gravitational fields. Since they all formed from the same cloud of gas and dust, they travel around the Sun in the same direction and in the same plane. The Nebular Hypothesis also explains the arrangement of the planets. Heat and solar winds from the Sun swept lighter gases farther out into the developing solar system, placing rocky terrestrial planets (Mercury, Venus, Earth, and Mars) closer to the Sun, while gas giants (Jupiter, Saturn, Uranus, and Neptune) formed in the cooler outer region. The solar system continued to evolve, with large asteroids slamming into planets and planets differentiating into layers as they cooled. While the Nebular Hypothesis cannot be directly tested, it serves as a useful description for how a solar system forms, explaining the consistent direction and plane of planetary orbits and the arrangement of rocky and gas giant planets. The next time you gaze at the stars twinkling in the night sky, imagine new planets forming through these very steps. Module 2.3 - Small Solar System Bodies
Small solar system bodies are most commonly made up of solid material from the nebula that either did not accumulate into planets or did form larger objects but were shattered into many fragments during collisions near the end of planet formation. Note that while asteroids (small
solar system objects that are largely made up of rock and/or metals) and comets (small solar system objects that are made up of ice, gases, dust and rocky material) are mentioned in this section but greater details of these objects are provided in Module 2.6.
Asteroids
In comparison to the planets asteroids are mostly relatively small solar system bodies that are largely made of rocky and/or metallic materials and are found throughout the solar system. The region with the largest concentration of asteroids is found just beyond the orbit of Mars out to about half way to Jupiter’s orbit and is known as the Asteroid Belt (white dots in the image below). These asteroids range in size up to about 800 km in diameter, the largest object being the object called "Ceres". Ceres was thought to be a planet when it was first discovered in 1801 but it was soon recognized to be just one of many objects in this region of space. We'll see more about Ceres in the more detailed section on Asteroids in the next section of this module.
Just beyond the Asteroid Belt are three other clusters of asteroids that are also shown in the figure above.
The Hilda Asteroids (orange dots in the above figure) form three clusters that extend from the outer limit of the Asteroid Belt to the edge of Jupiter's orbit.
Two other clusters of asteroids (both shown as green dots) are the Trojan and Greek Asteroids that orbit behind and ahead of Jupiter, respectively, along the planet's orbital path. Note that the Trojan and Greek asteroid clusters appear to be extensions of two of the Hilda asteroid clusters.
The Hilda, Trojan and Greek Asteroids are in orbits that are controlled by the immense gravity of
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Jupiter as it orbits the Sun.
There are millions of asteroids in the solar system. Many are believed to consist of material that didn't come together to form planets during the period of planet formation in the nebula. Some asteroids are thought to be fragments of planets or planetoids that were broken up due to collisions during the late stage of planet formation.
Note that there are many asteroids in orbit between Mars and the sun; many of these are termed “Near Earth Objects” and include asteroids that pose the greatest risk of collision with Earth. We'll focus on Near Earth Objects later in this Module.
Trans-Neptunian Small Solar System Bodies
Asteroids make up the majority of small solar system bodies that orbit between Neptune and the Sun but there are many, many more small bodies in orbits about the Sun that are beyond the orbit
of Neptune. These objects are called “trans-Neptunian Small Solar System Bodies” or "trans-
Neptunian objects".
Trans-Neptunian objects contain large amounts of water-ice and other volatile compounds and in the past this composition distinguished them from asteroids. However, large volumes of water are now known to make up many asteroids so this distinction is no longer true. Trans-Neptunian objects differ from asteroids by being located beyond Neptune.
The objects are made up of material that was left-over from the nebula.
The Trans-Neptunian bodies occur in two distinct regions: the Kuiper Belt
and the Oort Cloud
and both of these regions are shown in the following image.
Both are regions of space around our solar system that, combined, contain up to trillions of small, icy bodies that become comets
when their orbits are disturbed by the gravity of other objects in space and they fall towards the sun in the centre of our solar system.
Kuiper Belt
: a disk-shaped region past the orbit of Neptune, 30 to 100 AU from the Sun.
Oort Cloud:
a huge spherical “cloud” of many billions of icy bodies, surrounding the outer limits of the Solar System and extending approximately 3 light years (about 30 trillion kilometers) from the Sun.
Interstellar Solar System Objects
Interstellar objects are the newest class of small bodies that periodically turn up in our solar system. These are objects that normally pass through the solar system, having arrived here from
"interstellar space" (the space between star systems), so that they are not native to our own solar system. They are normally only "small solar system bodies" temporarily
because they are just "passing through" solar system; the exception would be if they crashed into a solar system object
during their passage through our solar system or if they become gravitational bound to the sun or
another planet. An important difference between solar system bodies and interstellar objects is that the latter are not "gravitationally bound" to any star (including the Sun). They are thought to
include objects that are compositionally similar to comets or asteroids but the majority of interstellar objects are thought to be icy bodies (like comets in our solar system). Such objects are distinguished from similar objects from within our solar system by their trajectory; an interstellar object typically follows a strongly hyperbolic trajectory (eccentricity greater than 1) that shows it arriving from outside of the solar system and then continuing on along a path that takes it out of the solar system after circling the Sun. The figure below compares the trajectory of
a comet from within our solar system to the trajectory of the first interstellar object to be found in
our solar system (described in detail, below). The comet has a trajectory that is elliptical (grey path) in shape whereas the interstellar object has a trajectory that is hyperbolic (green path) in shape.
This first interstellar object was formally recognized as being from beyond our solar system in early November 2017 (its discovery date is reported as Oct. 19, 2017). It is named "'Oumuamua" which in Hawaiian means "first messenger from afar” and NASA reports that it is pronounced “oh MOO-uh MOO-uh”. It's official designation number is "1I/2017 U1('Oumuamua)". The first part of the name (1I) means that it is the first ("1") Interstellar ("I") object to be recognized. It was identified as an object from outside of our solar system due to its speed and its orbital path. It was traveling faster than any object that is just falling at speeds determined solely by the Sun's gravitational force. Such speeds could have developed if the object had passed nearby one of our largest planets, the gas giants, with strong gravity that could
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have caused the object to accelerate. However, an analysis of the objects trajectory indicated that
it had come nowhere near one of our giant planets so it must have arrived at high speed from outside of our solar system. Its orbit has the highest eccentricity (1.20) ever observed for an object in our Solar System and that trajectory indicated that the object originated from beyond the solar system and that, after circling the sun, it would return the object back to interstellar space.
‘Oumuamua is a relatively small object with a length of approximately 400 metres; it is thought to be “cigar-shaped” with intermediate and minor axes lengths close to 40 metres. This shape was thought to be surprising by some and, coincidentally, it is similar to the shape that is sometimes recommended for interstellar space ships. This led some to suggest that it was actually an alien spacecraft but continued observation as it moved away from the Sun provided no supporting evidence of this suggestion. When it was first observed ‘Oumuamua was thought to have been a comet but its designation was changed to an asteroid when it did not develop a tail (see Module 2.6) as comets typically do
as they passed near the sun. However, subsequent analysis suggested that the object is covered by a surface coating formed by very long-term bombardment by cosmic rays. It was proposed that the material beneath the surface of the object may well be icy in nature but because the volatile components are protected by the coating from the effects of solar radiation as it circles the sun it does not develop a tail. While no comet tail was seen as Oumuamua moved away from
the sun its velocity increased in a manner that suggested that it was gaining speed due to vaporization of ice, suggesting that it was, in fact made up largely of ice. That raised the question
of what kind of ice might not produce a prominent tail like those associated with most comets. As of the spring of 2021 it is widely believed that Oumuamua is made of nitrogen ice like that which covers much of the surface of the dwarf planet Pluto . This means that Oumuamua may be a chunk of material from a body that was similar to Pluto, at the farthest reaches of some distant solar system, before it began its long journey that brought to our solar system in 2017.
A second interstellar object, named 2I/Borisov, was identified in 2019 and was soon confirmed to
have originated from beyond our solar system but, unlike Oumuamua, Borisov it had a well-
developed tail as it passed closest to the sun so that it was clearly an interstellar comet
. Borisov is rather spherical in form with a diameter of about 975m. The following image is an artists conception if this newest interstellar object.
To date (April 2023) 1I/Oumuamua and 2I/Borisov are the only officially confirmed interstellar objects in our solar system although there are a couple of "candidate" interstellar meteors that may well have originated from outside of the solar system. It has been estimated that there should be hundreds of interstellar objects as large as Oumuamua that have been gravitationally captured within our solar system. These objects may eventually be distinguished from objects that are "native" to our solar system by detailed analysis of their composition and their orbital path. Presumably as these interstellar objects that are permanently in our solar system are identified they will eventually outnumber the ones like Oumuamua and Borisov that are with us
only briefly, never to return again.
Before leaving the topic it is important to mention that there is another class of interstellar objects that have not yet turned up on our solar system, and hopefully one never does travel close
to us, and these are larger objects called "rogue planets". Rogue planets are "interstellar" objects that are much larger than "small objects" in our solar system because they are are comparable in size to planets or even small stars. However, they are similar to other "interstellar" objects in that they are not gravitationally bound by any star so that they wander about the universe at speeds that are estimated at being up to 13000km/s (almost 5% of the speed of light). Rogue planets attain such high speeds if they are accelerated by the gravitation of large black holes. The closest rogue planet to our solar system is named WISE 0855-0714, it is about 7.1 light years from Earth and it is 3 to 10 times the size of Jupiter!
Module 2.4 - The Problems With Pluto
Problems with Pluto led to turmoil in the astronomical community in 2006 when its status as a planet came under attack. The outcome of the debate that ensued changed our understanding of the planets forever.
Discovered on February 18, 1930, Pluto was considered to be the 9th planet of our solar system until late in the summer of 2006. On September 13, 2006 Pluto was reclassified as a dwarf planet along with a growing number of similar objects in our solar system. The problems with Pluto were that it differed in many ways from the other planets and these differences strongly suggested that Pluto did not form in the same manner as the other planets within the evolving nebula. For that reason many people believed that it was not a true planet.
The problems with Pluto!
1.
It rotates in the opposite direction to most planets which inherited their direction of rotation from the nebula.
2.
It is NOT a gas giant like the other "outer" planets; Pluto is like the inner "rocky" planets,
largely lacking a gaseous atmosphere.
3.
Pluto has a strongly eccentric orbit (meaning that it is strongly elliptical rather than circular like most planets). This orbit is not consistent with most of the other planets much more circular orbits that were established by the rotating nebula.
4.
Its orbital path is tilted at 17° to the plane of the orbits of most of the other planets; again,
this orbital plane differs from the orbital planes of the other planets where determined by the plane of the rotating nebula.
The figure below shows the orbits of the planets and Pluto, illustrating points 3 and 4, above.
Pluto's orbit about the Sun takes 248 Earth years and it follows a strongly elliptical path so that its distance from the Sun varies considerably. The average distance from Pluto to the Sun is 5.9 billion km but it varies from a maximum distance of 7.38 billion km to a minimum of 4.44
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billion km. The distance from Neptune to the Sun varies from 4.45 billion km to 4.55 billion km.
So, for parts of it's orbit Pluto is closer to the Sun than Neptune; this was most recently the case from January 1979 until February 1999.
The problems with Pluto had been known for many, many years; it was too different from the other planets to have formed in the same manner as the other planets so it likely didn't belong in the list of true "planets". On the other hand, astronomers didn't really have a good definition of what a planet really is so there was little basis for making a decision regarding Pluto's status. Besides, for almost 80 years school children had learned of the "9" planets, not the "8" planets, so that Pluto was firmly fixed in public knowledge that would be difficult to change.
However, the problems with Pluto became an important concern when a new object was discovered and formally named 2003 UB313. This object has a diameter of 2,326km (compared
to Pluto's 2368 km diameter) and, like Pluto, it has a highly eccentric orbit about the Sun (see the
image below) and an orbital plane that is at a high angle to the orbital plane of the original 8 planets. In 2005 the discoverers of 2003 UB313 argued that because Pluto was considered to be a planet then 2003 UB313 should also be classified as a planet. There was resistance to growing the list to 10 planets because everyone was accustomed to having 9 planets. There was a concern
that if another planet was added that the list would become endless. The fact was that many astronomers knew that Pluto (and 2003 UB313) were both likely "Kuiper Belt Objects" and if all
Kuiper Belt Objects as large as Pluto were included as planets the list would become very, very long as our ability to see deeper into the fringes of our solar system improves over time.
The matter of 2003 UB313 was the main event at the 2006 International Astronomical Union (IAU) General Assembly which was held in Prague in August of that year. To support the argument for assignment of 2003 UB313 as a planet a detailed proposal was prepared that defined objects that orbit the sun in a detail that had not previously been undertaken with any real degree of success.
The following definition was proposed prior to the 2006 meeting and, if adopted, would mean that 2003 UB313 would become officially recognized as a planet:
Planets: orbit a star (the sun), have enough mass to produce gravity strong enough to make them spherical, not a star or moon of another planet.
If this definition was approved by the General Assembly not only would 2003 UB 313 be added to the list of planets but Pluto would remain a planet. In addition, the object Charon (sometimes thought of as a moon of Pluto) along with Ceres (the largest known asteroid) would be added to the list of planets bringing the total to 12 planets in our solar system. Furthermore any objects that are discovered in future that satisfy the definition would also become planets.
The diagram below shows how this definition of "planets" would change our vision of the solar system.
The outcome of the 2006 IAU General Assembly
The proposed definition of "planet" was rejected by the majority of participants and the following three definitions were approved:
1. “Planet” is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has “cleared the neighbourhood*” around its orbit. Note: "Cleared the neighbourhood means that the planet has used its gravity to “absorb” or control other bodies in the vicinity of its orbit (i.e., its neighborhood).
2. A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. 3. “Small Solar-System Bodies” are defined as all objects that are not planets, dwarf planets or satellites of planets or dwarf planets.
Source: www.universetoday.com/525/plutos-out-of-the-planet-club
Because Pluto's orbit crosses the path of many Kuiper Belt objects it does not satisfy the criteria of having "cleared its neighbourhood" so it does not meet the definition of "planet" as approved by the General Assembly; for that reason Pluto was dropped from the list of planets. For the same reason 2003 UB313 and Ceres did not meet the new definition and were excluded from the list of planets. However, all three objects do fit the definition of Dwarf Planet so they do have some elevated status above small solar system objects. As for Charon, it remains a moon of Pluto.
The following figure illustrates the planets and dwarf planets as they were defined at the 2006 IAU General Assembly.
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Planetary update:
●
The object formerly known as 2003 UB313 was officially named “
Eris
”, after the Greek god of discord.
●
Its moon will be named Dysomia
(the daughter of Eris).
●
The dwarf planet Pluto has a new official name: 134340 Pluto
How many dwarf planets are there in the Solar System?
In 2006 there were three dwarf planets that had been formally assigned to this category of object.
According to Mike Brown (Professor of Planetary Astronomy at the California Institute of Technology and the discoverer of 2003 UB313), as of January 6, 2023, there are:
10 objects which are nearly certainly dwarf planets, 27 objects which are highly likely to be dwarf planets, 68 objects which are likely to be dwarf planets,
130 objects which are probably dwarf planets, and 741 objects which are possibly dwarf planets.
These numbers were the most up-to-date values available on March 25, 2023. You can check for
more recent values at Prof. Brown's web site at http://www.gps.caltech.edu/~mbrown/dps.html#table
Big News about a former planet now dwarf planet - Pluto
Prior to 2015 the best images of Pluto came from Hubble Space Telescope that was launched in April of 1990. Before Hubble became operational the best images that we had were merely several pixels of light in otherwise dark space due to the immense distance to Pluto (average distance: 5.91 billion kilometres). The following shows a Hubble image of Pluto:
In 2006 NASA launched an interplanetary space probe called "New Horizons" that would fly close to Pluto and provide an opportunity for images with a resolution that were previously impossible. On July 15, 2015, New Horizons made its closest approach to Pluto at distance of 12,500 km. The day before the closest approach we could see Pluto as shown in the following photograph:
Module 2.5 - The Formation of the Earth and the Moon
Accumulation of material within the nebula to form the Earth and other planets was a three-stage
process that was completed by about 4.56 Billion years ago.
Stage 1 ●
Dust-size particles come into close contact and "stick" together, gradually forming larger objects.
●
Turbulence in the nebula is thought to play an important role in bringing small objects into close proximity.
●
The objects grow until they have sufficient mass so that their gravitational attraction of the larger objects pulls smaller objects into them and they grow into “Planetesimals” that exceed 1 km in diameter.
Stage 2
●
Planetesimals underwent relatively rapid accretion as they came together to form larger objects called "planetary embryos".
●
This stage saw the formation of tens to about 100 objects ranging in size from that of our moon to the size of Mars; these large planetary embryos are often called "Protoplanets".
●
This period of planet formation is thought to have taken only a million years.
Stage 3:
●
Protoplanets from Stage 2 are attracted to each other due to their gravity, resulting in massive collisions and forming larger objects that ultimately became the planets of the
solar system.
●
This stage is believed to have taken 50 to 100 million years. Approximately half of the heat within the Earth today was produced by the energy released during collisions of the protoplanets during this stage of Earth's formation.
●
During the later part of Stage 3 the Moon came into existence.
The formation of the moon
The Lunar orbit suggests that it was captured from debris ejected from the Earth
during a stage 3 impact.
Towards the end of Stage 3 the Earth had an ocean of molten rock; melting due to the energy released from frequent large collisions.
It is believed that the formation of the moon began when one or possibly two very large objects (up to Mars in size) had oblique collisions with Earth and ejected a ring of molten debris into orbit around the planet. Once in orbit the molten debris accreted (came together) in a manner similar to the accretion of the planets.
It is estimated that the Moon was pretty much complete after a decade following
the causal collisions with Earth. Right after its formation the moon was likely a molten mass but it cooled to form a solid crust when impact frequency and magnitude diminished.
Since its formation the moon has slowly been moving farther and farther from Earth. Just following completion of Lunar construction it was in an orbit about the Earth that was about 15 time closer than the current orbit. A billion years later the orbit was 4 times closer than today; it has been moving away from the Earth ever since it formed.
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Module 2.6 - Comets, Meteorites and Asteroids
The following diagram shows the rate of change in the number of space objects that have collided with Earth since it formed. During the third stage of planetary formation (see Module 2.5) the rate at which objects collided with Earth was a billion times the current rate of impact. The rate of impact decreased rapidly as fewer and fewer objects were left in the vicinity of Earth, having accreted to construct the planet. By 4.3 billion years ago
the impact rate
was reduced to about 20 times the current rate
. From 3.8 to 4.1 billion years ago
there was a period when the impact rate rose to almost 1000 times the
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current rate
when it is believed that a disturbance in the asteroid belt caused an
increase in the number of small bodies entering the inner portion of the solar system. This period is shown in the diagram below as the "late heavy bombardment". By 3.5 billion years
ago the impact rate had fallen to current levels and has remained more or less constant since that time.
According to NASA about 100 tonnes of material from space comes into Earths'
atmosphere every day, mostly as very fine particles. On average, only one body
with a diameter of 1 metre will reach Earth each year.
Comets
A comet is a mixture of ices, both water and frozen gases (carbon dioxide, methane, ammonia) and dust and are often called “dirty snowballs”. Comets are
made up of material that was not incorporated into planets when the solar system was formed (most comets are about 4.6 billion years old). Most comets have elliptical orbits about the Sun as shown in the following illustration. Some
comets orbit within the planetary region of the solar system (as in the example below) whereas others have an orbital path that takes them very far out into the Oort Cloud, taking millions of years for a complete orbit about the Sun.
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Most comets began as trans-Neptunian objects
(objects from beyond the orbit of Neptune but their orbits have been disturbed in such a way as to produce new
orbital paths that circles the sun.
Anatomy of a Comet
The next figure shows the important components of a comet.
For most of its orbit about the sun a comet is just a solid mass made up mostly of ice and frozen gases along with dust and other solids. This solid mass makes up the nucleus
of the comet and it is very hard to see against that blackness of space.
However, once the dark solid mass of a comet approaches the Sun two changes happen:
1. The nucleus heats up and the water ice and solid (frozen) gases melt and change to a gaseous state. As a result of melting, dust and other solid debris that had been frozen into the nucleus is released. The liberated gas and solid particles lag behind the moving comet to form two tails (see below for details) that extend away from the nucleus.
2. Ultraviolet light from the sun causes the gas and dust to fluoresce or glow so that the entire comet, including its tails, becomes prominently visible. As the comet comes closer to the sun it becomes more brilliant and its tail becomes longer as it crosses the night sky.
It is during this period when the comet appears as in the photograph below.
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Coma
: this is the brightest part of the comet and is a glowing dense cloud of water, carbon dioxide and other neutral gases derived from the nucleus.
Dust tail:
composed of smoke-sized dust particles driven off the nucleus and come behind after the comet has passed.
Gas tail
: these are charged gas ions that are released from the nucleus.
The tails do not lie along the path that the comet has followed (like the exhaust contrail does behind a jet flying high over head) because the material making up
the tails is pushed by solar wind. Solar wind is made up of charged particles that follow straight-line paths that radiate outward from the sun. Gas ions originating from the comet are more readily moved by solar winds than is the dust. Thus, the solar wind can cause separation of the gas ions from dust particles. The gas ions are readily pushed along a line that parallels the solar wind (the line passing through the centres of the comet and the sun). The dust tail is also pushed towards that line but much less effectively so that the dust tail
is at an angle to the line paralleling the solar wind. This is illustrated in the figure below. Note that when the path of the comet is at a small angle to the direction of solar wind there is not enough separation of the gases and dust to
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cause the development of two tails. Two tails are best developed when the line from the sun to the comet (and, therefore the line followed by solar wind) is at a right angle to the path of the comet. In the figure below you can see that as the comet's path moves around the sun the angular difference between the two tails changes.
Because a comet loses volatiles (water and gases) with every pass by the sun, after several hundreds of passes by the Sun only a rocky object remains.
Where do comets come from?
Answer: the Kuiper Belt and the Oort Cloud (see Module 2.3).
Both are regions of space around our Solar system that, combined, contain up to
trillions of small, icy bodies that become comets when their orbits are disturbed by the gravity of other objects in space and they fall towards the sun in the centre of our solar system.
Gravitational interaction with the outer planets can also disturb the orbit of icy bodies, sending them on their elliptical orbits around the sun where we see them
as comets.
Exploration of comets
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Spacecraft from Earth have visited Comets over the last decade giving us a close look at these visitors from the outer regions of our solar system. The section linked below provides information on the 2014-15 European Space Agency's Rosetta Mission which resulted in the first "soft" landing on a comet and the NASA's 2005 Deep Impact Mission which crashed a probe into a comet.
Rosetta Mission, 2014
On 12 November 2014, ESA's Rosetta mission
soft-landed its Philae probe on comet 67P/Churyumov-Gerasimenko (abbreviated as 67P/C-G), the first time in
history that such an extraordinary feat has been achieved.
Deep Impact, 2005
On July 4, 2005, JPL’s spacecraft Deep Impact’s
mission was complete when its
impactor collided with Comet Temple 1.
The purpose of the mission was to observe the formation of a crater and to analyse the debris that was ejected to determine the comet’s composition.
The impact crater was made by a 370 kg mass that was launched from Deep Impact.
Asteroids
Asteroids are relatively small (metres to less than 800 kilometres), dense objects
that orbit the Sun. Most are not massive enough to develop a spherical shape so
that many of them are irregular in shape. They are largely made up of inner solar system material that was not formed into planets or fragments of planets produced by collisions early in the history of the solar system.
Asteroids come in a variety of forms, examples of which are given below.
Asteroid Gaspra Like most asteroids, Gaspra has a heavily cratered surface due to billions of
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years of exposure to smaller objects that have been drawn into it by its gravity.
Asteroid Ida
Ida is large enough to have its own satellite in orbit around it. Ida's Satellite (Dactyl) is 1.5km in diameter
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Asteroid 25143 Itokawa
This is a relatively small asteroid 690 by 300 metres that circles the sun in an orbit that crosses Earth's orbit. It is particularly notable because it appears to be an aggregate of broken up rock material with little observable cratering on its surface.
Asteroid Eros
Eros is a large asteroid (33x13x13km) with an orbit approximately 17,800,000km from the Sun.
Eros rotates with a strong wobble due to a giant gouge that may be a scar from a
past collision?
Ceres - more than just an asteroid!
While Ceres is member of the asteroid belt, and the largest (946 km in diameter)
of the asteroids, it is also a "dwarf planet" as we currently define dwarf planets.
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When I first started teaching about asteroids the best photos of Ceres that could be found were blurry and pixelated blotches of light. When the Hubble Space Telescope was put into a low orbit around Earth it increased our ability to see deeper into space than ever before and produced images like that below of the largest of the asteroids.
NASA's Dawn mission visited Ceres in the Asteroid Belt over 2015-16, came to
within 375 km of the surface of the dwarf planet/asteroid. The Classification of Asteroids
Asteroids are classified on the basis of the light that is reflected from the surface
of the object. One such measure is the proportion of light reaching a surface that is reflected back from that surface; this measure of reflectivity of an object is called its "albedo" and it varies from 0 (no light is reflected which would appear as a very black object) to 1.0 (all of the light is reflected which would appear as a bright object).
The three most common types of asteroid are:
C-type: Extremely dark asteroids with albedo of 0.03 (only 3% of light reaching
the surface is reflected). Over 75% of all asteroids are this type and they are thought to be "stony" bodies with a composition comparable to the composition of solids in the nebula.
S-type: about 17% of all asteroids of this type which is relatively bright (albedos
range from 0.10 to 0.22) and they are thought to be made of the metals nickel-
iron mixed with rocks that are similar to those making up the Earth's lithosphere.
M-type: making up the majority of asteroids that are not C- or S-type, these asteroids have albedos in the range 0.10 to 0.18 (overlapping with S-type) and they are believe to be almost pure nickel-iron.
Absolute magnitude and size of asteroids
The Absolute Magnitude
(normally denoted by the capital letter
H
) of an asteroid is essentially a measure of how bright it is and brightness will vary with
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the asteroid's size and its albedo. For constant albedo the brightness increases with the size of the asteroid; for constant size the brightness increases with the albedo of the asteroid. due to the manner in which absolute magnitude is calculated the smaller the magnitude the larger the size of asteroid.
The absolute magnitude of an asteroid may be used to estimate its size. However, because the exact albedo of an asteroid may not be known, the size that is determined from absolute magnitude will span a range that reflects the natural range in variation in the albedo of asteroids.
For example, an asteroid with magnitude of 3.0 ranges in size from 670km to 1490km. An asteroid with a magnitude of 20 will range in size from 270m to 590m in size.
Asteroids: How big is big?
As we will see later in this module, the mass of an asteriod, which is governed by its overall size and the density of the material that it is made of, is an important variable in determining how much damage would result if it collided with the Earth.
Meteoroids, meteors, meteorites and bolides
This section describes the relatively small objects that are found in space and which commonly make it through the Earth's atmosphere and crash onto its surface.
It is estimated that about 100 tonnes of material from space enters the Earth's atmosphere each day...much of this burns up in the atmosphere.
Here are some definitions to begin with:
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Meteoroid
: a piece of stone or metal that travels in space (smaller than an asteroid, from dust size to a metre in diameter)
Meteor:
a meteoroid that falls towards the Earth, heating up due to friction and glowing as it crosses the sky.
Meteorite:
a meteor that lands on the Earth’s surface.
Bolide:
a large, particularly bright meteor that often explodes (sometimes called
a "fireball").
Classification of Meteorites
Because meteorites actually fall to Earth they have been classified according to their composition. The figure below shows a simple classification of meteorites.
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Module 2.7 - The Risk of Small Solar System Bodies
Today there is considerable concern about space objects colliding with Earth, despite the paucity of recorded strikes directly on humans:
●
Ancient records from China indicate that people have been killed by falling meteorites; no such deaths are well documented from modern times.
●
A meteorite killed a dog when it fell in Egypt in 1911.
●
Elizabeth Hodges, of Sylacauga, Alabama, was given a terrible bruise on the side by a falling meteorite in 1954.
●
A young boy was struck on the head by a meteorite that had been slowed down by the leaves of a banana plant in Uganda in 1992.
●
On February 15, 2013, the Chelyabinsk bolide exploded over Russia and its shock wave resulted in 1491 injuries, largely from glass and debris but also included temporary blindness due to the brightness of the fireball. There were no reported injuries directly attributable to meteorite fragments hitting people.
●
In 2009 Gerrit Blank (see below) claimed to have been hit in the hand by a glowing pea sized meteorite. His claim is widely believed to have been a hoax!
Woman nearly struck by meteorite in British Columbia on October, 2021!
Over the night of Oct. 4, 2021, Ruth Hamilton of Golden BC was woken by her barking dog when a soft-ball sized meteorite passed through the metal roof of her house and then crashed through her bedroom ceiling, landing on the pillow beside her head! See Ms. Hamilton's photo of the situation below. The event was a particularly close call that may well have been fatal if the meteorite had been just centimetres to the right as seen in the photo. Ms. Hamilton contacted Dr. Phil McCausland at the University of Western Ontario (and an Adjunct Professor at Brock) who confirmed that the object that nearly hit her was a meteorite made up of solid material that came together about 4.5 Billion years ago as part of an asteroid that orbited within the Asteroid Belt. That asteroid remained within the Asteroid Belt for a few billion years until it was ejected from its orbital path due to a catastrophic collision with another small space object about 470 million years ago. That collision produced many fragments, at least one of which was ejected from the Belt and sent on a new orbital path that,
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after millions of years, led to its collision with the Earth. Upon reaching the Earth's atmosphere that object exploded in to smaller fragments, one of which ended it's journey on Ms. Hamilton's pillow! So why the concern today?
Three events over the 20th Century heightened concern that objects in space may pose a serious risk to humans and all other life on Earth. These events were:
The Cretaceous-Paleogene (K-Pg) Extinction Event
(Formerly known as the "Cretaceous-Tertiary (K-T) Extinction Event)
Recognition that a major impact led to the extinction of the dinosaurs and much other life led to serious consideration that a similar event could happen at any time!
If you recall, at the end Module 1, we learned about Luis and Walter Alvarez, who found the first evidence that the mass extinction that wiped out the dinosaurs and many other groups of organism was likely the result of the collision of a large space object with Earth 66 million years ago. This was the first good proof that an object or objects from space could have devastating effects on the Earth.
The Tunguska Event
On June 30, 1908, at 7:30am a 15 megaton blast was felt over a large area of Siberia (the Hiroshima nuclear explosion was about 0.02 megatons; 750 X Hiroshima).
●
The blast was an airburst (explosion) of a 60 m diameter asteroid.
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●
The explosion was heard in London England.
●
Over 60,000 trees were flattened over an area of 2000km
2
Comet Shoemaker-Levy 9 (SL-9)
In 1992, Comet Shoemaker-Levy 9 (SL-9) passed near to Jupiter when it broke up into at least 21 separate fragments, up to 2 km in diameter. The pieces dispersed over several million kilometres along its orbit as shown in the image below.
Between 16 July 1994 and 22 July 1994 the fragments impacted the upper atmosphere of Jupiter. This was the first collision of two solar system bodies ever witnessed (and it was watched worldwide on television).
The first fragment struck Jupiter with energy equal to about 225,000 megatons of TNT creating a plume which rose about 1000 km above the planet.
A later fragment struck with an estimated energy equal to 6,000,000 megatons of TNT (about 600 times the estimated arsenal of the world). The fireball rose about 3000 km above the surface of the planet.
These three events illustrated that:
●
Such impacts were possible and not just the stuff of SciFi. We could see it happen (with the aid of space telescopes).
●
Major impacts can have a devastating effect on all life on Earth.
●
Even minor impacts on Earth (Tunguska) that have taken place in recent record could kill millions and cause billions of dollars in damage.
●
Governments and insurance companies developed concern regarding the risks and costs of such events.
These events caused scientists and politicians to pause and think about what is known about the risk of space objects to the Earth. The answer was repeatedly the same: very, very little
.
In light of these three 20th Century events researchers have focused on several questions:
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●
What has been the frequency of impacts with Earth?
●
How many objects are close enough to Earth to pose a risk?
●
What happens when an object of a given size arrives at Earth?
●
How do we assign a level of “risk” to space objects?
●
What does the geologic record tell us about major impacts (the past is the key to the present)?
Frequency of Impacts Today
The average time between impacts can be resolved for smaller regions of Earth to evaluate human risk:
For example, in the diagram above it can be seen that a Tunguska-class impact (60m diameter object) can be expected to take place somewhere on Earth once every 300 years. However, of greater interest, in terms of the outcome of such an event, is how frequently such an event can be expected over populated areas of the Earth. The probability must be weighted by the proportion of the area of the Earth that is populated by humans. For example, ●
Average interval between impacts for the entire Earth (100% of the Earth's surface)= 300 years.
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●
Average interval between impacts for populated areas (which make up about 10% of Earth surface)= 3,000 years
●
Average interval between impacts for world urban areas (which make up 0.3% of Earth surface)= 100,000 years ●
Average interval between impacts for U.S. urban areas (which make up 0.03% of Earth surface) = 1,000,000 years The Search for Near Earth Objects (NEO): How many objects are out there
that pose a risk to Earth?
This is a question that hadn't really been considered seriously until the latter part
of the 20th century. While we watched Jupiter being bombarded by huge cometary fragments we had no real idea of whether or not there were many comparable objects in a position to collide with our home planet.
By 1998 sufficient concern had been generated that NASA created the Near Earth Object Observations Program
in an effort to, as quickly as possible, identify objects that pose a risk to Earth.
Near Earth Objects*
include both Near Earth Asteroids (NEAs) and
Near Earth Comets (NECs) as defined below:
NEA
s are asteroids that are in orbits in which they come to within 1.3 AU of the
Sun;
NEC
s are comets in orbits in which they come to within 1.3 AU of the Sun AND have orbital periods (time for a complete orbit) of less than 200 years.
Potentially Hazardous Asteroids (
PHAs
) are asteroids that come to within 0.05 AU of the Earth and have a minimum diameter of 110m.
*Definitions are derived from information at http://neo.jpl.nasa.gov/neo/groups.html
on April 21, 2015. Recall that AU is the
abbreviation for "Astronomical Unit" and 1AU is the distance from the Earth to the Sun.
In its first year of operation (1998) the Near Earth Object Observations Program's budget was 10.5 million dollars and it's mandate was to discover 90%
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of all NEOs that were 1km in size or larger over the next 10 years. The data that
is needed to fulfil this mandate comes from large field telescopes that swept the skies taking digital photographs. Photos from the same region of space but at different times are digitally overlain to identify objects that displayed a particular type of motion.
It has been estimated that there are approximately 1000 NEAs that exceed 1 km in size. As of April 21, 2015, 871 NEAs that are 1km in size or greater have been discovered; 5 asteroids greater than 1km have been discovered so far in 2015 so it is believed that the initial target will be reached well before the end of
2015.
As they close in on meeting their initial mandate the focus of the search is now on discovering 90% of all NEOs that are larger than 140 m; to help achieve this goal the Program's budget was increased to $40 million in 2014.
The following figure shows how well the program has done in discovering NEAs based on observations spanning the period from January 1980 to May 6, 2021. The blue region of the diagram represents all NEAs discovered over that period, the orange region represents all NEAs discovered that are larger than 140m in size and the red region represents all NEAs that are larger than 1km in size. Note in particular the sharp increase in the number of newly discovered NEAs at the beginning of 1998 when the NEO Observations Program began. Note also that the number of discoveries of large NEAs has been increasing very
slowly over the last few years because most of them have already been discovered. In contrast, many more of the smaller NEAs are being discovered every year because there remains a large number yet to be found.
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As of June 15, 2023 Near Earth Object discoveries includes 121 Near Earth Comets and 32,154 Near Earth Asteroids, totalling 32,275 Near Earth Objects.
Of the 32,154 NEAs, 852 are 1km in diameter or larger and 10,481 are 140 metres in diameter or larger.
Of the 32,154 NEAs 2,340 are Potentially Hazardous Asteroids and 151 of them
are 1km in diameter or larger.
When the program began in 1998 there were 500 NEOs, 447 of these were NEAs and 200 of the NEAs were 1km or greater in size.
The Near Earth Object Observations Program has certainly gone a long way to answering the question of "How many objects are out there that pose a risk to Earth?"
.....
but there are still many left to discover.
An additional value of the findings of the NEO Program is that all newly discovered asteroids are reported directly to the Near-Earth Object Human Space Flight Accessible Targets Study (NHATS) that began in September
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2010. The NEO Program shares its data with NHATS and this data will inform planning for possible operations such as commercial mineral and metal recovery
in the future. What was the topic of science fiction just a couple of decades ago will likely become a reality in your lifetime.
What Happens When an Object Impacts With Earth?
The outcomes of an object from space impacting on the Earth's surface depends on a number of factors. The size and speed of the object are most important because combined they determine how much energy is released on impact.
●
Asteroids approach the Earth at speeds of 15 to 25 km/sec (54,000-90,000
km/hr).
●
Comets can approach the Earth at speeds up to 70 km/sec (252,000 km/hr).
Depending on the mass (volume X density) the atmosphere may slow the object down to about 200 km/hr.
The energy released upon impact is the Kinetic Energy
(denoted by "
E
") of the object.
Kinetic Energy = E = ½ mV
2
●
Where m
is the mass of the object and V
is its velocity.
●
As the mass of the object increases so does the Kinetic Energy
●
Double the mass leads to a doubling of the Kinetic Energy
●
The Kinetic Energy increases with the square of the velocity; when velocity increases by a factor of two (i.e., it doubles) the Kinetic Energy increases by a factor of 4 (four times the value).
The available Kinetic Energy determines the outcomes of the impact.
It is crucial to understand that all large impact craters are formed by the explosive release of kinetic energy that takes place on impact. The explosive release of kinetic kinetic energy produces a great deal of heat and compressional
shock waves waves that travel symmetrically outward in all directions.
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Specific effects at impact can include: Heat wave
●
Much of the kinetic energy may be converted to heat that radiates outward
from the impact or atmospheric explosion.
●
Radiant heat is also derived from the temperature of the object due to friction as it passed through the atmosphere, by compression of air, from the "fireball" if the object explodes to release energy as light and heat.
●
Can incinerate the area close to the event and start fires all around the site.
Pressure wave (atmospheric shock wave)
●
Shock wave front due to compression of the air from the explosion in the air on upon impact followed by winds that can exceed 500km/hr.
●
The shock wave can knock down buildings and trees all around the site.
●
Hurricane-like winds cause further devastation but, on the bright side, may blow out fires.
●
The following video shows a compilation of videos showing the shock wave generated by a 17m diameter bolide that exploded over Chelyabinsk, Russia, in February 2013. The pressure wave produced by the Chelyabinsk bolide is reported to have caused injury to almost 1500 people due to flying glass and other debris but no injuries were due to a direct hit by fragments of the object.
Crater Formation
●
Impact of an object on the Earth surface results in an explosion that displaces crustal material into the atmosphere leaving a large crater on the
surface.
●
In general, the impact crater is 20 to 30 times the diameter of the impacting object, depending on the nature of the material making up the surface that is impacted.
●
The impact crater and its immediate surroundings are the site of complete devastation.
●
We will examine crater formation in some detail a little later in this module.
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The following photo shows Wolf Creek crater in Australia; this crater formed about 300,000 years ago. The crater is 875 metres across and its rim rises 25 metres above the surrounding plains and the crater floor is 50 metres below the top of the rim.
Rain of small rocks and dust
●
Material ejected from a crater (both asteroid and Earth material) or produced as an object explodes in the air can travel for thousands of kilometres.
●
Large debris falls relatively close to the impact whereas dust is carried in the atmosphere for years.
●
Secondary damage to anything remaining in the region around the impact or explosion.
●
Hot material that is ejected can cause fire for considerable distance around
the impact site.
●
Increased atmospheric dust due to an impact can cause a reduction in sunlight reaching the Earth's surface and result in significant global cooling. Tsunamis
●
When an asteroid impacts on a large water body (e.g., the ocean) a wave is generated that travels very quickly over the water surface, steepening and flowing onshore along coasts. Wave speeds have been recorded at almost 800 kilometres per hour (generated by earthquakes, not asteroids).
●
At the shoreline waves can reach over 100 metres in height and wash out buildings for kilometres away from the shore.
●
Large objects impacting on the ocean surface can generate tsunamis of much greater magnitude than has been seen in historic times.
Earthquake
●
Much of the energy released on impact will produce shock waves through
the Earth, thus generating earthquakes.
●
Earthquakes produced by terrestrial processes can produce events up to approximately 9.5 on the Richter Scale (a logarithmic scale that is proportional to the amount of energy released in the crust due, most commonly, the breaking of rock in the crust).
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●
A large impacting object, like that that caused the mass extinction 66 million years ago could generate a magnitude 13 earthquake.
●
Surface shock waves can devastate the landscape (including buildings) hundreds of kilometres from the impact site.
A footnote on human-made junk in space.
There are currently millions of bits of debris orbiting the Earth that was produced by human activity. This material is often called "space debris", "space
junk", "space litter", etc. and some of it poses a direct risk to orbiting satellites that cross the path of large pieces of space junk.
In 2013 it was reported that there were over 170,000,000 such objects smaller than 1cm were in orbit above the Earth. Wikipedia reports that there are about 670,000 pieces of debris that ranges from 1 to 10 cm in size and about 29,000 are larger than 10cm.
Space debris consists of
:
●
frozen bits of nuclear reactor coolant that are leaking from old satellites
●
jettisoned spacecraft parts
●
nuts and bolts
●
solar cells
●
abandoned satellites
●
paint chips
●
nuclear reactor cores
●
spent rocket stages
●
solid fuel fragments
●
Most of the debris in space is small but it's travelling extremely fast. Below altitudes of 2,000 km, the average relative impact speed is 36,000km/h.
●
A 1mm metal chip could do as much damage as a .22-caliber long rifle bullet. Bits this size don't generally pose a large threat to spacecraft, but can erode more sensitive surfaces and disrupt missions.
●
A pea-sized ball moving this fast is as dangerous as a 400-lb safe travelling at 100 km/h. Objects this large may penetrate a spacecraft; this could be fatal.
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●
A metal sphere the size of a tennis ball is as lethal as 25 sticks of dynamite. An object like this will penetrate and seriously damage a spacecraft.
Large pieces of human space junk can pose a significant risk when they return to Earth at densely populated locations. In the April of 2018 the world waited nervously for a collision with Earth by an entire space station the size of a school bus that had been put into orbit by China. The space station, Tiangong-1,
launched in September of 2011, finally returned to Earth on April 2, 2018, after a long period of orbital decay that made it difficult to provide much forewarning
of precisely what part of the Earth would be at risk. Fortunately, re-entry was over the Southern Pacific Ocean where it reportedly mostly burned up during its
descent. Quantifying the Risk of Impacts
Grollmann, (original source was http://www.gcr.com/sharedfile/pdf/Topics11Grollmann-en.pdf but this is NO LONGER AVAILABLE) described the types of damages as evaluated by the insurance industry. Note that in the following descriptions the period between impacts is, in some cases, smaller than the scientific data support.
Grollmann divided space objects up by size into four classes of asteroids and described the nature and magnitude of the various outcomes of impacts for each class.
Type I Asteroid: ranging from 0-30 m in diameter; 10,000 – 50,000/year.
●
Normally explodes before impact into dust and small fragments.
●
On March 27, 2003, such an explosion took place and the fragments (the size of tennis balls) crashed into several houses in Park Forest, Illinois.
●
Fragments cause damage but no risk of heat wave, earthquakes, etc., but shock waves can be locally
significant (e.g., the Chelyabinsk bolide)
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Like the Earth, the moon is also bombarded by space debris and a NASA program monitor's lunar impact events constantly. On March 17, 2013, the most
spectacular such event took place when a 30 to 40 cm diameter meteoroid weighing about 40 kg and travelling at 25 km/sec collided with the lunar surface
releasing energy equivalent to 5 tons of TNT. Unlike Earth these relatively small meteoroids impact the lunar surface rather than exploding in the atmosphere because there is no atmosphere on the moon. The video, below, shows and discusses the March 17, 2013 lunar impact.
Type II Asteroid: 50 m diameter; every 250 yrs.
●
FExplodes in the air.
●
Over land the heat wave starts fires within several kilometres below the explosion.
●
Heat and pressure waves cause extensive damage within 25 or 30 kilometres of explosion.
●
Diminishing damage from pressure wave and winds to almost 100 kilometres.
●
Damage can exceed that of a major earthquake.
●
10-15 metre tsunamis can cause extensive damage to large coastal cities (e.g., Vancouver, San Francisco, Tokyo if the Pacific receives the impact).
Type III Asteroid:
1 km diameter; every 100,000 yrs.
●
Objects of this size impact the surface; a 1 km object would create a 20 to 30 km diameter crater.
●
Very heavy damage for 500 km around the impact site due to heat and pressure wave.
●
A major earthquake would add to extensive damage.
●
Forest fires rage across the entire continent due to extensive heat wave and falling hot debris.
●
Local climatic change would have an effect on fauna and flora for decades
to come.
●
An impact at sea would send masses of water upwards to 10 km. Tsunamis would make landfall as waves hundreds of metres high. Los Angeles, Tokyo, Hong Kong, Miami…..destroyed except for concrete-
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reinforced ruins.
Type IV Asteroid: 10 km diameter; every 50 million yrs.
●
Impact crater: 300 km in diameter.
●
Entire continent destroyed.
●
Falling masses of molten rock would start forest fires world-wide.
●
Magnitude 12 or greater earthquake would just add to the devastation.
●
Auxiliary damage as nuclear power plants are destroyed.
●
Global cooling of climate due to dust in the atmosphere would set back both plants and animals.
●
Global food supply jeopardized.
How do we assign a level of "risk" to space objects?
We can define "risk" as being the likelihood of some negative outcome arising from an event or action.
In general the level of risk depends on the frequency of the event causing risk and it's scale (i.e., the extent of damage and the number of people affected).
Car accidents occur with a high frequency but they are of very small scale, normally affecting only a few people per accident. However, car accidents have
a relatively high frequency so that everyone has a fairly high risk of being affected by a car accident.
Airplane crashes occur very infrequently but their scale is moderate, possibly affecting up to hundreds of people. The risk of an airplane crash is considered to be low because of the very low frequency.
Asteroid impacts have a very low frequency but potentially have a very large-
scale effects (may affect billions of people) but overall the risk is low to the very
low frequency.
However, for the average person the risk of death due to an asteroid impact
is only twice the risk of dying from a "snake, bee or venomous bite or sting" according to an article in Livescience in 2005 entitled "The Odds of Dying".
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The Torino Scale
The Torino Scale is a measure of the risk posed by a given asteroid or comet expressed as a value from 0 to 10 with each value defined as shown in the following table.
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Module 2.8 - Impact Crater Formation and Craters on Earth
The Formation of Impact Craters
Impact craters form when an object explodes on impact with Earth's surface, releasing its kinetic energy and displacing crustal material, ejecting it into the atmosphere leaving a large crater on the surface (1 km diameter asteroid produces a 20 to 30 km diameter crater). Crater formation takes place in three stages as described below:
1. Contact/compression Stage.
This first stage in crater formation is very brief, a fraction of a second in duration, beginning with the instant of contact of the object with the Earth's surface. At the point of impact the pressure and temperature increase suddenly
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due to the explosive release of kinetic energy. Rock in immediate area of the impact is vaporized and surrounding rock melts due to the high temperatures. In
subsequent stages, rocks in the area immediately adjacent to the impact are displaced upwards and other material is ejected from the crater. "Spalled" material, derived from the impacting object, subsequently accumulates over the impact site. The crustal rocks beneath the impact site are compressed, briefly, to
form a short-lived "transient" crater (see stage 3).
2. Excavation Stage
This is the stage when material is ejected upwards, away from the impact site. Rock beneath the impact continues to be compressed into a short-lived “transient” crater. The Chicxulub Crater would have taken less than 2 minutes to complete this stage.
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3. Modification Stage.
For craters larger than 4km diameter a peak rises up in the centre as rock that was compressed and pushed downwards in Stage 2 rebounds and rises upward. This "central uplift" reaches 10% of the crater diameter and this rebounding event takes only a few minutes
. At the end of this stage the crater walls slump into the crater. Initially large slumps occur but these that become smaller and less frequent over time as the initially steep walls become stable.
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Anatomy of a Crater
The following figure shows the characteristics of simple
craters (without a central uplift) and complex
craters that have a central uplift and are floored with solidified molten rock. Note that both craters display a complex array of fractures beneath the crater. Identification of these fractions is an important indicator that a crater is formed by an impact rather than by other processes.
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Some notable examples of craters on Earth:
crater at Brent, Ontario. Rim diameter: 3.8km Age: 400 million years.
Meteor Crater, Arizona, Rim diameter: 1.2 km Age:49,000 years.
Haviland crater, Kansas, < 1000 years old, 15 metres diameter
Macha Crater (see arrow), Russia, 7000 years old, 300 m in diameter.
New Quebec Crater, 1.4 million years old, 3.4 km in diameter.
Module 2.9 - Close Approaches of NEOs
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So we humans have found a lot of debris in space, much of it left over from the nebula, that is close by to Earth and we've been able to determine where the debris will be for hundreds of years into the future so we know the risk that those objects pose. The question now is just how much warning will we have before a significant object collides with Earth.
Here’s an answer from David Morrison of NASA that he gave a few years ago:
“With so many of even the larger NEOs remaining undiscovered, the most likely warning today would be zero -- the first indication of a collision would be
the flash of light and the shaking of the ground as it hit.
In contrast, if the current surveys actually discover a NEO on a collision course, we would expect many decades of warning. Any NEO that is going to hit the Earth will swing near our planet many times before it hits, and it should be discovered by comprehensive sky searches like Spaceguard. In almost all cases, we will either have a long lead time or none at all.”
What space object has posed the greatest risk?
2002 NT7?
●
A 2 km diameter asteroid with an impact velocity of 28 km/s
●
When first discovered it was thought to be in an orbit that would have it collide with Earth on January 31, 2019
●
2002 NT7 was the first space object to be assigned a high risk of impact when it was first discovered.
●
With further observations the projected orbit became more accurate and showed that it would miss the Earth on the collision date by a large margin. As a result, it was removed from the list of threatening objects. Another close call: 2002 MN June 14, 2002
●
On June 14, 2002, asteroid 2002 MN passed within the moon’s orbit of
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the Earth (within 120,000 km of Earth and about 1/3 the distance from the
Earth to the moon).
●
The asteroid is the size of a football field (50 – 120 m in diameter) and is traveling at 37,000 km/hr.
Which Potentially Hazardous Asteroid will come closest?
99942 Apophis ●
When first discovered it was thought that this 325 metre diameter asteroid
could very possibly impact with Earth on April 13, 2029.
●
As more data became available it was found that it will come very, very close to Earth; the current estimate is that it will come to within 31,000 km of Earth (close enough to collide with some of our satellites that are in
orbit about the Earth.)
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It will come so close in 2029 that its interaction with Earth's gravity may alter its orbit so that it may come even closer on Friday, April 13, 2036.
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By 2016 enough new data had been collected on Apophis' orbit to rule out
the likelihood of an impact with Earth in 2036 but, at that time, the orbital
projections indicated a serious threat of impact in 2068. So Apophis continues to be under constant scrutiny because as new data becomes available the projections of its orbital path in future become more and more accurate.
Collision Avoidance Strategies
Several approaches have been suggested:
1. Land astronauts (including Bruce Willis) on the object, drill into it and plant nuclear bombs.
●
The explosions should break the object up into smaller, harmless pieces
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(or, a gazillion chunks of asteroid will destroy the Earth rather than a single very large thud).
2. Detonate nuclear explosives at selected locations in space near the object.
●
The blasts will “nudge” the object away from the blasts, sending it off the course for collision with Earth.
3. Attach "solar sails" to the object and let solar energy push it out if its collision course.
●
Solar sails would use solar winds to push the object into a new, safer orbit.
4. Attach rockets to the object to send it off on a new orbit that poses no risk to the Earth.
5. Kinetic impactors. Crash an object into the asteroid so that it is pushed just enough out of its original orbit and out of the collision course.
Sept. 27 2022 Update
: On Sept. 26, NASA made its first attempt at altering an
asteroid's orbit by impacting it with a spacecraft (Approach 5, above) is the DART (Double Asteroid Redirection Test) Mission
that has targeted a small asteroid named Dimorphos that is in orbit around a much larger asteroid named Didymos. Dimorphos currently makes a complete orbit around Didymos about once every 11 hours and 55 minutes and this period of time is referred to as its “orbital period”. NASA hoped that impacting Dimorphos will increase its orbital speed enough to reduce that orbital period by at least 73 seconds. Some scientists are predicting that Dimorphos may speed up enough to reduce the orbital period by as much as almost 10 minutes. Land and space telescopes will be watching Dimorphos closely to measure any changes in its orbital period in order to determine whether or not the experiment was successful. If it was successful the technique may be used in the future to alter the path of asteroids that are projected to strike Earth.
Module 2.10 - The Nemesis Hypothesis
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he current interest in space objects that pose a risk to Earth aims at finding those
particular objects for which a reaction by humans might save us from a devastating impact. Today we evaluate risk on the assumption that the number of dangerous objects in space around us does not vary significantly with time. Thus, the historical impact rate that we see will not be significantly more or less in the future.
However, over long periods of time there may well be variation in risk that significantly changes the probability of large impacts. The Nemesis Hypothesis arose when paleontologists realized that extinction events appear to take place with a regular periodicity of about 26 million years. This, in turn, led them to suggest that the frequency of impacts with Earth may vary with this same periodicity. This suggestion raises the obvious question of "why would impact rates vary with a 26 million years periodicity"? The Nemesis Hypothesis was developed as a possible explanation for such variation in impact rates over long periods of time. It was developed by colleagues of Louis Alvarez and is described in some detail in the following video.
Summary:
Impacts and mass extinctions
Mass extinction involves the loss of many groups of organisms over a relatively short period of time. Many mass extinctions have taken place over geologic time; arrows in the figure, below, point to the 5 major
extinctions of the past 600 million years.
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Modern thinking is that the demise of the dinosaurs was due to the effects of a large number of significant impacts over the span of several hundred thousands of years.
The vast amount of dust and debris that was sent up into the atmosphere due to the impacts is thought to have caused a prolonged period of cold climate. Recent literature suggests that smoke and ash from global forest fires that followed the impact may have contributed significantly to cooling the Earth. Dinosaurs and many other groups of organisms could not adapt to the cold temperatures and became extinct. By closely examining data on the extinction rates on Earth over the past 275 million years paleontologists Dave Raup and Jack Sepkowski found that major extinction events actually take place quite regularly about every 26 million years (see the following figure in which green dots represent peaks in extinction
rate that are believed have resulted from impact events). Raup and Sepkowski went further, suggesting that because three of the mass extinctions were likely due to impacts that they all might be caused by impact events and this would suggest that every 26 million years there was an increase in the probability of impacts with the Earth (i.e., clusters of impacts every 26 million years).
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Why their is a 26 million year periodicity to the extinctions?
It has been suggested that the regular periodicity may be due to the existence of a companion star to the Sun that has been named "Nemesis”. Nemesis is postulated to be a dwarf star; 1/3 the size of the sun and 1/1000 as bright. Nemesis would circle the Sun in an orbit that was 2.8 light years across and it would take 26 million years for one complete passage around the sun.
The suggestion is that Nemesis' orbit would bring it close to the Oort Cloud (a vast area with billions of frozen objects that become comets when they orbit about the sun) once every 26 million years and at that time the dwarf star's gravity would disturb the objects, sending them towards the sun. This would result in a “comet shower” reaching the inner solar system and lasting for a few million years and over that time the chances of collisions with Earth is increased.
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In this way the number of impacts with Earth would greatly increase every 26 million years, or so, as Nemesis makes its way around it's orbital path.
The Nemesis Hypothesis has yet to be substantiated…..
The Nemesis Star has not been discovered.
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