Since its origins, the Earth has been frequently bombarded by asteroids and comets, which have
often caused large mass extinctions, such as that of dinosaurs, so these celestial bodies
can rightfully be considered the most dangerous for our planet; the impacts of large asteroids
or comets on Earth is also at the center of the plot of many films of the fantasy genre,
such as Armageddon, Deep Impact or the more recent Greenland.
Let's see their most salient features.
Asteroids are all those small rocky bodies present above all in the Main Belt of our
Solar System between the orbits of the planets Mars and Jupiter, therefore at a distance
from the Sun between 2 and 3 astronomical units (remember that an astronomical unit
is the average distance Earth - Sun).
In addition to the Main Belt, there are also other populations of asteroids, such as the
Trojans, which orbit at the Lagrangian points L4 and L5 of Jupiter.
Lagrangian points, studied by the Italian mathematician Giuseppe Luigi Lagrange, from
whom they take their name, are points of gravitational equilibrium corresponding to stable configurations
in which three bodies can remain in equilibrium for an indefinite time, in the absence of
external perturbations.
In the case of Jupiter, the two Lagrangian points L4 and L5, in which there are the Trojan
asteroids, together with the Sun and the planet form an equilateral triangle, so they are
on the same orbit as Jupiter, at an angular distance of 60 degrees from the planet.
There is still an outer asteroid belt, known as the Edgeworth - Kuiper Belt, at a distance
between 30 and 50 astronomical units from the Sun, therefore beyond the orbit of Neptune.
On average, asteroids travel through orbits that are slightly inclined on the ecliptic
plane (the plane of the Earth's orbit).
To date, the asteroids with precisely known orbits amount to more than 100,000, while
an even greater number of other objects await to be observed more extensively in order to
derive a more reliable value of the orbital parameters.
The bodies with a diameter greater than 200 km are only 26, the others are smaller, up
to those with a few meters in diameter.
Comets are small icy bodies that travel through very elongated and inclined elliptical orbits
on the ecliptic plane; when they arrive near the Sun, solar heat and radiation heat the
small icy nuclei, so the pressure of the gases trapped inside them increases; these gases
then escape, breaking the surface crust of the cometary nucleus and producing jets of
dust and gases that the comet leaves behind, thus forming the two tails: the ion tail,
produced by the interaction of these particles with the solar wind, more bluish, and the
dust tail, more yellowish.
After perihelion, the comet moves away from the Sun and the activity of its nucleus decreases
until it ceases, due to the weakening of the intensity of solar radiation.
When the Earth, traveling its orbit around the Sun, passes through the deposits of dust
left by comets, these particles enter the Earth's atmosphere and burn by friction with
the Earth's atmosphere, producing meteors, improperly called "shooting stars", which
many of you will surely have observed during the hot summer nights: among the most famous
meteor showers the Perseids of August, the Leonids of November and the Geminids of December.
According to their orbital period, comets are divided into short-period comets, which
take less than 200 years to travel their orbit around the Sun, such as Halley's comet, with
an orbital period of 76 years, and long-period comets, such as the spectacular C / 2020 F3
Neowise, with an orbital period of 6,688 years, which takes more than 200 years to make a
revolution around the Sun.
Short-term comets are believed to come from the Edgeworth - Kuiper Belt, while those of
long period from the Oort Cloud, the outermost part of the Solar System extended up to 100,000
astronomical units and probably containing about 200 billion icy cometary nuclei.
Comets and asteroids are scientifically very interesting, because they provide us with
valuable information on the origin and evolution of the Solar System: it is believed that they
are the residues derived from the formation of planets.
According to the most accredited hypothesis, the Solar System was formed about 4.5 billion
years ago due to the gravitational contraction of a nebula of gas and dust: in this period
it was a rather turbulent place, because the same planets were formed by aggregation of
matter as a result of violent impacts, which, as a side effect, also produced innumerable
rocky debris that have been preserved until today in the form of asteroids and comets.
But it is true that all asteroids and comets represent a danger to our planet?
Actually no, of these only NEOs represent a potential danger for the Earth: NEO is in
fact an acronym that means "Near Earth Objects", and indicates that population of bodies that
orbit around the Sun on trajectories that are potentially capable of impact with the
Earth.
Given that asteroids, comets and therefore also NEOs are celestial bodies with a negligible
mass compared to those of the Sun or planets, it is very likely that, passing very close
to a planet, their orbit will be perturbed by the gravitational attraction of this one,
so it becomes very difficult to predict its future orbital evolution.
It is believed that there are about twenty NEOs with diameters greater than 5 km (such
as the one that probably caused the extinction of the dinosaurs 65 million years ago), another
thousand with diameters between 1 and 5 km and even 150,000 larger than 100 m.
The risk of impact with a NEO larger than one kilometer can be considered quite exceptional
in the context of risks deriving from natural disasters for two reasons:
1) The consequences of the impact are far greater than those of any other natural or
man-made disaster (including a nuclear war); 2) The probability of such an event occurring
within a human life is extremely low.
To try to quantify the risk of impact of a NEO on Earth, in 1999, during the IMPACT international
conference on the NEO problem held in Turin, Italy, the Turin Scale was launched, divided
into 11 steps, from grade 0 with zero probability of collision, to grade 10 representing the
maximum collision probability on a global scale: in principle, grade 0 has no consequence.
Grade 1 requires continuous control of the object.
Grades 2 - 4 require special attention and possible intervention studies.
Grades 5 - 7 are to be considered alarming and require preparation of interventions.
Grades 8 - 10 represent safe collisions and require action.
Let's now see each step in detail:
Grade 0 - The probability of a collision is zero, or so low that it is considered zero.
This class also applies to small objects, such as meteors and fireballs that burn in
the atmosphere and to rare meteorite falls that rarely cause damage.
Grade 1 - The object makes a close passage with the Earth, but does not represent an
extraordinary level of danger.
The probability of a collision is extremely low.
The news does not deserve to be brought to the attention of public opinion.
New observations will most likely lead to a Grade 0 reassignment.
Grade 2 - The object makes a passage close to the Earth but not entirely unusual.
While it deserves the attention of astronomers, there is no need to bring the news to the
attention of the public, as the possibility of a collision is very unlikely.
New observations will likely lead to a Grade 0 reassignment.
Grade 3 - The object makes a close encounter that deserves the attention of astronomers.
The calculations give a probability of 1%, or higher, of an impact capable of causing
only local destruction.
Grade 4 - Like 3, but the destruction would be on a regional scale.
Grade 5 - Close encounter of an object that poses a serious but still uncertain threat
of regional devastation.
This requires a high level of attention by astronomers to establish with certainty whether
the collision will take place.
If the fly-by is scheduled for a period of less than a decade, government plans to mitigate
the threat must be guaranteed.
Grade 6 - Like 5, but, if the fly-by is scheduled for less than three decades, government threat
mitigation plans must be guaranteed.
Grade 7 - A very close encounter with a large object, which can occur within this century,
poses an unprecedented but still uncertain threat of catastrophe on a global scale.
For such a case to occur this century, emergency plans will have to be put in place on an international
scale, especially to determine urgently and with certainty whether the collision will
take place.
Grade 8 - The collision is sure but the destruction would be on a local scale.
These events occur on average between 1 and 50 times per thousand years.
Grade 9 - The collision is sure but the destruction would be on a regional scale.
These events occur on average between once every 10,000 years and once every 100,000
years.
Grade 10 - The collision is sure but with global scale destruction that can threaten
the future of mankind.
These events occur on average no more than once every 100,000 years.
Finally, let's see what could be the possible defenses against the risk of impact with a
NEO with a diameter greater than 100 m.
Both NASA and other space agencies have proposed essentially two types of approaches: the deviation
of the NEO from its trajectory or its destruction.
To achieve these objectives, several techniques have been considered, let's see some of them.
Nuclear techniques: based on the use of thermonuclear charges of significant power to deflect or
destroy a NEO on a collision course with the Earth.
Their great advantage is the enormous energy released (about 1 million times higher than
that of traditional explosives): it has been calculated that, in order to disturb the speed
of NEO of 1 cm / s with dimensions of 100 m, 1 km and 10 km, it would be necessary,
respectively, explosive charges with energies of 0.1 - 1 kiloton, 0.1 - 1 megaton and 0.1
- 1 gigaton (for comparison the energy released by the nuclear devices of Hiroshima and Nagasaki
is around 20 kiloton).
The disadvantage of nuclear techniques, in addition to the danger of radiation, consists
in the fact that the results are difficult to control: there is always the risk that
the asteroid will fragment into several blocks still directed towards the Earth and at this
point the problem would be multiplied rather than be solved.
Kinetic energy transfer: the basic idea is to make the NEO on a collision course with
the Earth to impact in a certain direction with a bullet of considerable mass.
The problem is that this mass would be too high: to give sufficient energy to a NEO with
a diameter between 1 and 5 km, in such a way as to impart the necessary thrust to deflect
it, it would be necessary to have a projectile of the order of 1500 tons.
This is an excessively high mass to be launched into space, whatever spacecraft is used (the
Space Shuttle was capable of carrying about 20 tons).
Mass drivers and space tugs: anchored to the surface of the NEO to be deflected, they operate
ion motors (for tugs) or based on the extraction of material from the asteroid itself (mass
drivers) capable of exert a force of low intensity but prolonged over time on the asteroid.
In this way, acting in the long term, the period of revolution of the asteroid can be
varied just enough to avoid collision with the Earth.
Solar Sails: they use solar radiation pressure to impart small but constant thrust to deflect
dangerous NEOs.
Since the pressure of solar radiation is very low, it is clear that in order to have the
necessary thrust to deflect an object of dimensions between 1 and 10 km in a period of a few years,
it would be necessary to have sails with diameters of the order of a hundred kilometres.
Solar collectors: this technique can be compared to that of Archimedes' "burning mirrors".
In this case, solar sails with a diameter between 1 and 10 km and suitably treated with
reflective materials could be used as collectors of the sunlight which would then be directed
on the surface of the asteroid or comet whose material, subjected to the very high temperatures
of concentrated radiation, would vaporize generating a jet of matter and a consequent
thrust.