Definitions:
Albedo: the ratio of reflected sunlight to incident sunlight; it measures the reflectivity and the intrinsic brightness of an object, together with its distance from the Sun and the observer.
Magnitude (relative and absolute): the measurable brightness of any celestial object depends on many parameters such as the object's size and the distance from the observer (a candle very near you is much brighter than a very far -and very bright- star!). For this reason, the brightness of any celestial body, measured directly as you can see it, is also called
relative magnitude. Since all the objects in the solar system are moving (and changing), the relative magnitude of an object changes in time. It is therefore necessary to define an
absolute magnitude for every class of objects (which can be asteroid, stars etc). An absolute magnitude is a measure of brightness that is independent of distance. In case of objects of the Solar System, absolute magnutide is a measure of the brightness an object would have if it were 1 AU from the Earth and the Sun and a phase angle (the angle Sun-object-Earth) of 0 deg.
Main Belt Asteroids:asteroids orbiting in the region of interplanetary space between the orbits of Mars and Jupiter.
NEAs are divided into four classes:
- Amor: objects having orbits with perihelion distance between 1 AU and 1.3 AU.
- Apollo: objects having orbits with semimajor axis greater than 1 AU and perihelion distance less than 1 AU . As a consequence their orbits can cross the Earth orbital path.
- Aten: objects having orbits with semimajor axis less than 1 AU and aphelion distance greater than 1 AU. As a consequence their period of revolution is less than one year and their orbits can cross the Earth orbital path.
- IEO: Inner Earth Objects have orbits entirely inside that of the Earth. As a consequence they are extremely difficult to discover because they remain always close to the Sun. IEOs are also called Atira, from the name of the first discovered object of this kind.
- Due to planetary perturbations which modify the orbit of a NEA, an object can change its class in time.
NECs are comets which have a perihelion distance of less than 1.3 AU and an orbital period of less than 200 years.
PHAs(Potentially Hazardous Asteroids) are objects which can in principle come closer than 0.05 AU (7.5 million km) to Earth.
Torino scale: the Torino scale is a classification (similar to the Ritcher scale for earthquakes) to quantify the hazard posed by a NEO which takes into account its kinetic energy and its impact probability. It ranges from 0 to 10 to indicate an increasing chance of collision. In its graphic form colors are also used to give information on the level of danger of the event. More info at
http://spaceguard.iaps.inaf.it/tumblingstone/dictionary/torino.htm
Palermo scale: in the Palermo scale the risk of a possible impact is compared to the 'background risk' posed by similar (or larger) NEOs between now and the time of impact. The Palermo scale has a "0" value set on the background risk and increasing/decreasing values following a powers-of-ten (logarithmic) law. Should an object have PS=2 then the corresponding event is one hundred times more likely to happen than a background similar event in the same period of time. Until now only slightly positive values of the Palermo scale have been recorded. For a detailed treatment see the original paper
Quantifying the risk posed by potential Earth impacts by Steven R. Chesley (JPL), Paul W. Chodas (JPL), Andrea Milani (Univ. Pisa), Giovanni B. Valsecchi (IASF-CNR) and Donald K. Yeomans (JPL),
Icarus 159, 423-432 (2002),
http://neo.jpl.nasa.gov/risk/doc/palermo.pdf
Size. Unless close range observations are available (e.g. carried out by an exploration mission or a planetary radar during a close approach) the size of an object is derived from its brightness (measured by the absolute magnitude H) and assuming an average reflectivity of the surface. As a consequence the figures reported in the tables retain a high level of approximation. Magnitude-size conversion tables can be found at the Minor Planet Center (
Conversion of Absolute Magnitude to Diameter).
In the NEOCC database if no better information on the size of a NEO is available the diameter of an object (D, in km) is derived from the absolute magnitude (H) assuming an albedo (p) of 0.1 and using the following equation:
D = 1329 * p-1/2 * 10-0.2H
ESA - European Space Agency
Because even after getting destroyed in multiple pieces they will not come down.
