[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].

                         ASTEROIDS
                         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
                            relationship:
                            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 are
                            regarded to be the following:
 

                            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:
                                    blast
                                    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
                                tsunami
                                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 probability
                            of impact with Earth, is usually regarded to be the following:
 

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

                       " 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 ]

                               Meteoroids
                                   '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..':

          COMETS
                    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 [N_ECC/year] are assumed to be obtained from the
                    following relationship:
                                  N_ECC/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).
                  [http://impact.arc.nasa.gov/introduction/faq-hq.html]

                 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 Programs ,  Surveillance Programs ]

                    Countermeasures

                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]

---