ASTEROID TYPES & GROUPS
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    potentially hazardous asteroid

    large asteroid colliding with the Earth
    A near-Earth asteroid (NEA) whose orbit can bring it so close to Earth that there is a risk (typically very small) of a future collision. Past asteroid and comet impacts have had dramatic effects both on a local scale (as in the case Tunguska event) and globally (for example, the mass extinction at the Cretaceous-Tertiary boundary).

    The growing realization that Earth will inevitably be hit hard again at some point, with potentially catastrophic results for human and other life, has led to the setting up of several programs to detect and monitor NEAs (or, more generally, NEOs – near-Earth objects – which also include short-period comets) and near-Earth threatening objects. Among these projects are the Spacewatch Project, LINEAR, the Spaceguard Foundation, and the Jet Propulsion Laboratory's NEO Program.

    Among recent close encounters was one on June 14, 2002, when an asteroid the size of a football pitch, catalogued as 2002MN, passed Earth at a distance of around 120,000 km – less than a third the distance to the Moon – traveling at over 10 km/s (23,000 mph). This was bettered by 1994XM1, which came to within 105,000 km of Earth in December 1994.


    The threat from Apophis

    potentially hazardous asteroid approaching Earth
    The current biggest threat of a collision with Earth is posed by the asteroid Aphophis. This 400-meter-wide space rock will come closer to Earth in 2029 than the orbits of many communications satellites – but it will not hit the planet. The concern centers on the small chance that its orbit could be altered enough during the flyby to put the rock on a collision path for its return in 2036. In 2006, the Planetary Society announced a $50,000 prize to be given to the designers of the mission that would allow the asteroid's orbit to be tracked with the most precision. The competition has support from the US and European space agencies. It was won in February 2008 by a team led by Atlanta-based SpaceWorks Engineering. The Foresight spacecraft, if it goes ahead, will launch in about 2012, rendezvous with Apophis, determine its center of mass, and take regular measurements of its position relative to the asteroid. These measurements will be used to reduce the uncertainties in Apophis' orbit.


    TNT equivalent of asteroid impacts

    Even a relatively small asteroid, with a diameter of 100 m, could have a devastating effect on the Earth and its life because of its high incoming velocity. The kinetic energy (energy of motion) of an object is given by the formula

    KE = ½mv2
    where m is the mass of the object and v its velocity. If the velocity is high, then velocity squared is large and this makes the kinetic energy large if the mass is significant. The table below shows the equivalent in megatons of TNT of the impact of asteroids of various masses and velocities, assuming a typival rock-like density for an asteroid of 2.5 kg/m3. For comparison, the yields of the Hiroshima and Nagasaki atomic bomb explosions were approximately 15,000 tons and 21,000 tons, respectively. In other words, the smallest impact considered in the table (that of a 100-meter-wide asteroid traveling at 15 km/s) would be roughly 100 times more powerful than the devastating Hiroshima-Nagasaki attacks.


    Diameter (meters) Mass (million tons) Velocity (km/s)

    15

    20

    25

    30
    100 1.3 35 62 97 140
    200 10 280 500 780 1,100
    300 35 950 1,700 2,600 3,800
    400 84 2,200 4,000 6,200 9,000
    600 280 7,600 13,000 21,000 30,000
    800 670 18,000 32,000 50,000 72,000
    1,000 1,300 35,000 62,000 97,000 140,000
    2,000 10,000 280,000 500,000 780,000 1,100,000
    3,000 35,000 950,000 1,700,000 2,600,000 3,800,000
    4,000 84,000 2,200,000 4,000,000 6,200,000 9,000,000
    5,000 160,000 4,400,000 7,800,000 12,000,000 18,000,000
    6,000 280,000 7,600,000 13,000,000 21,000,000 30,000,000
    7,000 450,000 12,000,000 21,000,000 33,000,000 48,000,000
    8,000 670,000 18,000,000 32,000,000 50,000,000 72,000,000
    9,000 950,000 26,000,000 45,000,000 71,000,000 100,000,000
    10,000 1,100,000 35,000,000 62,000,000 97,000,000 140,000,000



    Related entries

       • Torino scale
       • cosmic collisions, biological effects


    Related category

       • ASTEROIDS AND OTHER MINOR PLANETS: TYPES AND GROUPS


    Archived news
    Asteroid makes close Earth pass (Jan 30, 2008)
    Near-Earth asteroids buzz US government (May 24, 2005)



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