bodies in the solar system,
solid surface, show extensive
evidence of cratering, caused by impact with NEOs.
Terrestrial impact craters larger than about 10 km
|Crater Name||Location||Diameter, km|
[Neukum,G., 'Impact Record in the Solar System', in "Near Earth Objects",Usually NEOs are classified on the basis of their orbits. NEOs can be
1997, Annals of the NY Academy of Sciences, vol. 822, p.318]
Terrestrial planets and asteroids Gaspra and Ida show striking similarities
in their crater size distribution characteristics [Neukum,G., 'Impact Record
in the Solar System', in "Near Earth Objects", 1997, Annals of the NY Academy
of Sciences, vol. 822, p.318].
Asteroids are a special kind of small planets orbiting the sun.
'Most asteroids are made of rock; but some are composed of metal, mostly
iron and nickel' [ Asteroids, Comets and NASA ].
The majority of asteroids lie on the main asteroid belt, a zone located within
the orbits of Mars and Jupiter.
The orbits of Asteroids are elliptical, more or less eccentric, some times ap-
proaching the cometary type. Orbits of some Asteroids are known to cross
that of the Earth.
Orbits of others do not currently cross the Earth's one, but orbital perturbation
could occur by many causes, making impacts possible in the future.
The asteroids that have the potential to impact our planet are usually called
Earth-Crossing Asteroid (ECA).
It can be estimated that the population of ECAs can be approximated by the
N = k * D b
where N is the expected number of ECAs of Diameter D [Km], the constant
K ~ 1,950 and the exponent b ~ -2.22.
From the above equation it follows that the number N of ECAs as a function
of diameter D is expected to be:
Nevertheless, the fraction ECAs discovered as a function of the size
regarded to be the following:
|> 14||~ 100|
|6 - 3||~ 35|
|4 - 2||~ 15|
|2 - 1||~7|
Threat of Asteroid impact.
Usually asteroids do not survive the passage through the Earth's atmosphere;
they burn up on contact with the atmosphere during their descent and they
leave the luminous trail known as meteor.
Large asteroids may crash into Earth, however, and create craters, such as
the Arizona's Crater near Flagstaff. [Asteroids and Comets Impact Hazard ]
Among the immediate effects of impact with asteroid, one can
crater [~ 20 times the diameter of the NEO]
ejection of debris into sub orbital trajectories
The darkening of the Sun by the enormous and extensive clouds of dust,
will cause extreme cooling [threatening survival probability of animals]
and inability of plants to do photosynthesis, severely threatening the
ecosphere on the global scale [Rampino, M.R., Haggerty, B.M. & Pagano,
T.C., 1997, in "Near Earth Objects", Annals of the NY Academy of Sciences,
vol. 822, p.403].
firestorms and smoke clouds caused
by the reentry in the atmosphere of ejected matter;
destruction of the ozone layer
triggering of volcanism and seismic activity
significant environmental disaster
massive loss of plant and animals
The NEO that devastated 2000 square kilometers of Siberian forest in 1908
[Tunguska] was about 60-meters in diameter; it exploded with a force of
12.5 megatons, ~ 100 times the Hiroshima atomic bomb. [ Threat from asteroids ]
The NEO impacted the earth ~65MY ago, causing the disappearance of
Dinosaurs, was 10km diameter , like the Martian moon Phobos [thought to
be a captured Main Belt asteroid] [Sagan, C., 1994, 'The pale blue dot',
Random House, NY].
'The most satisfactory explanation of the origin of our own Moon..is that it
was formed almost 4.5 billion years ago, when a world the size of Mars
struck the Earth' [Sagan, C., 1994, 'The pale blue dot', Random House, NY].
The expected relationship between the Asteroid diameter and the
of impact with Earth, is usually regarded to be the following:
|10||1.5 x 108||< 5|
|100||3.1 x 105||10-3|
Recently recorded sub-critical impacts of bodies with diameters of 10-50m
are the following:
|1930||Brazil||Tunguska like airburst|
|1937||Estonia||8.5 m crater from NEO of ~50 ton|
|1947||Sikhote-Alin||1-14m craters, by 100 iron NEOs|
|1965||Revelstoke||High Air burst. No damage on ground|
|1966||Kincardine||High Air burst. No damage on ground|
|1969||College, Alaska||High Air burst. No damage on ground|
|1972||Grand Teton|| NEO tracked 1500 Mi through
atmosphere before bouncing off
|1990||Sterlitamak||5 m crater|
|1992||Peekskill||Bolide witnessed across eastern USA|
|1994||Micronesia||High Air burst. No damage on ground|
|1997||Texas||High Air burst. No damage on ground|
|1997||Greenland||Medium Air burst. No damage on ground|
" Potentially Hazardous Asteroids (PHAs) are currently defined based
parameters that measure the asteroid's potential to make threatening close
approaches to the Earth. Specifically, all asteroids with an Earth Minimum
Orbit Intersection Distance (MOID) of 0.05 AU or less and an absolute
V-magnitude (H) of 22.0 or less are considered PHAs. In other words,
asteroids that can't get any closer to the Earth (i.e. MOID) than 0.05 AU
(roughly 7,480,000 km or 4,650,000 mi) or are smaller than about 150 m
(500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%) are not
considered PHAs. The current list of PHAs is obtained from the Minor
Planet Center on a daily basis.
There are currently 272 known PHAs.
This ``potential'' to make close Earth approaches does not mean a PHA
will impact the Earth. It only means there is a possibility for such a threat"
[from : Potentially Hazardous Asteroids ]
destroyed nearly all living organisms on the Earth surface.
Lousiana State University, Off. Univ. Relations
PHAs approaching Earth to ~ 0.2
AU during 2001
'Meteoroids are defined as being solid particles in space larger than 100 µm
in size..and smaller than about 10 meters..' [Steel,D.,1997, in 'Near Earth
Objects. Annals of New York Academy of Sciences, vol. 822, p.31].
'During the period from about 1960 to the early 1980's a number of large
bolides (meteor-fireballs) entered the atmosphere which were sufficiently
large to generate blast waves during their drag interaction with the air...
detected by microbarographs which were operated by the US Air Force..':
Meteoroid Airwaves Events
Taken by US Air Force [1960-1974]
|Date||Source Location|| Source
Active comets can also cross the Earth's orbit with the potential for collision
[Earth Crossing Comets, ECC]. It can be inferred [Everhart,1967] that 10 to
20% of all short-period comets can be regarded as ECC. It is usually estimated
that the population of short-period ECC is likely to comprise ~20-40 objects
larger than 1 km diameter, ~ 100 - 150 larger than 0.5 km diameter, and ~
2000 - 4000 larger than 0.1 km diameter.
The number of ECCs [NECC/year] are assumed to be obtained from the
Short period comets usually originate in the Edgeworth-Kuiper Belt, 35 to
1000 AU [1 Astronomical Unit, AU, is the average Earth<>Sun distance
~ 150 million km].
The main constituent of comet nuclei is ice, mixed with dust and hydrocarbons.
Long Period Comets [LPC] originate in a cloud of debris surrounding the Sun
at distances between 20,000 and 100,000 AU [~ 2 light years].
Gravitational perturbations from nearby stars and/or shock wave from massive
explosion of a supernova, or other perturbations, are thought to cause LPC to
unexpectedly approach the Sun.
Each year, a significant number [5 to 10] LPC approach the Sun.
Unlike short period comets, whose orbits are relatively close to the ecliptic, long
period comets have orbits that are randomly orientated on the celestial sphere.
Their orbital periods are longer than 200 years, and they will often only return
after thousands or even millions of years (or not at all).
Comets start to brighten markedly when they reach a heliocentric distance of 3
AU, at which point their water ice begins to sublimate, but discovery at that
juncture would give only some months of warning of an impact.
SURVEILLANCE PROGRAMS"During 1993/94, several significant events occurred that raised public and
professional awareness of the threat coming from possible impact of NEO
- The much publicized impact of Comet Shoemaker-Levy 9 on Jupiter
in the summer of 1994;
- The "near-miss" of Asteroid 1994 XM1 in December 1994;
- The publication of a position paper, entitled "Response to the Potential
Threat of a Near-Earth-Object Impact" by the AIAA in January 1995.
The US Congress requested that NASA consider the threat with more urgency,
and directed them to work with the US Air Force (who already operate a net-
work of telescopes used to track man made satellites). The NEO Survey
Science Working Group was established." [ Surveillance Programs]
Afterwards, some parallel Programs were established elsewhere.
The major discovery programs active at the beginning of 1999 [these projects
find about 15-20 new NEOs every month].
[NASA Surveillance Programs , Surveillance Programs ]
According to studies carried out in both the United States and Russia on
methods to avoid potential NEO impacts, once a NEO's threat has been
identified, two possible courses of action seem to be available :
- Destruction, probably with high yield nuclear weapons [although incomplete
disruption of the object would subject the Earth to multiple impacts from
pieces of the original body].
- Deflection / Acceleration / Deceleration: the orbit of a potential impactor
could be modified by detonation of nuclear weapons close enough to the
NEO, as required to avoid impact.
[ Surveillance Programs]
'Who could deny that man could somehow also make
the heavens, could he only obtain the instruments?'
[M. Ficino, ~1474]