A Nature's threat from the sky

All bodies in the solar system, having visible solid surface, show extensive
evidence of cratering, caused by impact with NEOs.

                         On the Earth surface we see very few obvious craters because geological
                         processes such as volcanism and erosion soon destroy the evidence.

Terrestrial impact craters larger than about 10 km

Crater Name Location Diameter, km
Chicxulub Mexico          180
Chesapeake Bay USA            90
Morokweng S. Africa            70
Charlevoix Canada            54
Montagnais Canada            45
Miolnir Norway            40
Clearwater (west) Canada            36
Clearwater (east) Canada            26
Kamensk Russia            25
Ries Germany            24
Gosses Bluff Australia            22
Suavjavi Russia           16
Bosumtwi Ghana           10.5
                     [Neukum,G., 'Impact Record in the Solar System', in "Near Earth Objects",
                           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].
                            Usually NEOs are classified on the basis of their orbits. NEOs can be
                            categorized as Amors, Apollos, or Atens, according to whether their orbits
                            lie outside that the Earth [Amors], cross that of the Earth with period greater
                            than 1 year [Apollos], or cross that of the Earth with period less than 1 year
                            [Athens]. Cometary objects are classed as short period if their periods are
                            less than 20 years, intermediate period if their periods are between 20 and
                            200 years, and as long period [or "new"] if their periods are greater than
                            200 years [NEOs from NASA].

                         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:

   Diameter, Km 
   Number of 
           0.1      320,000 
           0.25         42,200
           0.5          9,040
           1          1,950
           2             420
         10               12

                            Nevertheless,  the fraction ECAs discovered as a function of the size are
                            regarded to be the following:

  Diameter, Km   % ECAs 
         > 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 include:
                                    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 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 probability
                            of impact with Earth, is usually regarded to be the following:

 Diameter,m   Estimated 
 Frequency, year
         10   1.5 x 108        < 5
       100   3.1 x 105           10-3
    1,000     2,000            10-5

                             Recently recorded sub-critical impacts of bodies with diameters of 10-50m
                             are the following:

  Date       Location               Impact Details
   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
                                        [Table from  Recent Subcritical Impacts ,  with minor modifications]

                       " Potentially Hazardous Asteroids (PHAs) are currently defined based on
                        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 ]

According to recent studies, the oldest impact event occurred 3470 ± 2 million years ago.  The asteroid was between 12 and 30 miles in diameter, and smashed into the earth with the energy close to 1 billion atomic bombs, creating a  tsunami more than half a mile high that raced around the world at about 500 miles / hour .  The heat generated by the impact evaporated the upper 30-300 feet of water of the oceans, and 
destroyed nearly all living organisms on the Earth surface.
 Lousiana State University, Off. Univ. Relations
[G.R. Byerly, D.R. Lowe, J.L. Wooden & X. Xie, 2002, Science 297, 1325-1327]

PHAs approaching Earth to ~ 0.2 AU during 2001

Earth-Approach DATE
Oct 31, 2002
Nov 01, 2002
Jan 17, 2003
May 20, 2003
Jun 27, 2003
Jul 19, 2003
Aug 16, 2003
Oct  21, 2003
Nov 28, 2003
Jan 15, 2004
Feb 02, 2004
                                                     Torino Impact Scale of the above Asteroids = 0

                                   '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..':

Some Basic Meteoroid Airwaves Events
Taken by US Air Force [1960-1974]

  Date  Source Location  Source Energy,
11/02/60        9N, 43E          10
09/26/62       30N, 35E          20
09/27/62       32N, 60E          30
08/03/63       51S, 24E      1,100
11/30/64     18N, 123W           10
06/12/66     51N, 164E             8
01/08/71      30N, 40E             6
04/14/72       13S, 78E           14
*  1 kt TNT = 4.185x1012  Joules
                        [Revelle, D.O., 'Historical Detection of Atmospheric Impacts by Large
                            Bolids using Acoustic-Gravity Waves', in "Near Earth Objects",
                            Annals of the NY Academy of Sciences, vol. 822, p.318].
                    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
                    following relationship:
                                  NECC/year= 180*D-1.97

                  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
                with Earth:
                    - 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 ProgramsSurveillance 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]