Definitions & Assumptions


The ratio of reflected sunlight to incident sunlight. Given an albedo and the distance of the asteroid from the Sun and the observer, the size of an object can be estimated from its brightness.


Asteroid taxonomy

Asteroid taxonomies are classification systems that allow to group asteroids into classes on the basis of their reflectance spectra (i.e. the reflectivity in different wavelengths). Each class is thought to correspond to one (or more) physical compositions of the surface layer of the object. Various taxonomical systems have been developed over the years, mostly based on the different used wavelength ranges for the observations. Although not identical, they share most of the basic classes. They are all characterised by a broad division in two large groups, called C and S complexes, thought to correspond to carbonaceous and siliceous asteroids, respectively.


Lunar Distance (LD)

A common reference unit to measure distances in the Earth neighbourhood. It corresponds to the average distance between the Earth and the Moon, approximately 384 400 km. 


Magnitude (apparent and absolute)

The measurable brightness of any celestial object depends on many parameters such as the object size and the distance from the observer (a candle close you is much brighter than a very far - and very bright - star!). For this reason, the brightness of any celestial body as it appears to the observer is called apparent magnitude. When magnitudes are presented on this webpage, they are usually defined (unless otherwise noted) as "visual magnitudes" (symbol V), corresponding to wavelengths centred around the peak sensitivity of the human eye (~550 nm). Since all the objects in the Solar System are moving, the apparent magnitude of an object changes in time. It is therefore useful to define an absolute magnitude, a measure of brightness that is independent of distance. For a Solar System object, the absolute magnitude is a measure of the brightness an object would have if it were at 1 au from both the observer and the Sun, and at a phase angle (the angle Sun-object-Earth) of 0 degrees. The commonly used symbol for the absolute magnitude is H, and when not specified it is also referred to the visual band.


Maximum brightness at close approach

This quantity, which is used in the 'Close Approches' table, gives an estimate of the observability of an object during a close encounter. It allows individual observers to quickly ascertain whether an observation is feasible.
In the context of this website, the maximum brightness (or minimum apparent magnitude) is defined for a location that is at the sub-asteroid location on Earth, usually corresponding to a geometry from where the asteroid is close to its ideal observability. However, if the approach is very close, there could be places on the Earth surface with better phase angles, and therefore with higher brightness.

The computation is executed as follows:

1.     The geocentric ephemeris around the close approach date is computed.

2.     The magnitude is computed for a point on the Earth surface at the sub-asteroid position.

3.     If the asteroid is in the night sky as seen from the sub-asteroid position, then this magnitude is considered to be the maximum brightness.


Near-Earth objects (NEOs)

Asteroid or comets that may come close to the Earth. Generally, they are defined as asteroids (near-Earth asteroids, NEAs) or comets (near-Earth comets, NECs) with a perihelion distance of 1.3 au or less. For NECs, the additional requirement of having a period shorter than 200 years is imposed.

NEOs are divided into four groups, named after the first object in the respective groups:

Amor: objects having orbits with perihelion distance between 1.017 au and 1.3 au.

Apollo: objects having orbits with semimajor axis greater than 1 au and perihelion distance less than 1.017 au. Their orbital period is larger than 1 year and their orbits can cross the Earth orbital path.

Aten: objects having orbits with semimajor axis less than 1 au and aphelion distance greater than 0.983 au. Their orbital period is smaller than 1 year and their orbits can cross the Earth orbital path.

- Atira: also called Inner Earth orbit Object (IEOs) or Apohele. They 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.

Due to planetary perturbations which modify the orbit of an NEO, an object can change its group over time.


Palermo Scale (PS)

This is a numerical scale (similar to the Richter scale for earthquakes) used to quantify the impact risk associated with a given NEO. The formal definition is 


PS = log10 (P / (fΔT))


where P is the impact probability (from 0 to 1), fb is the background impact frequency (number per year) and ΔT is the time until impact (in years). The background impact frequency can be estimated as fb= 0.03 E-0.8, with E the energy released by the impact expressed in megatons (Mt) of TNT equivalent. The Palermo scale has been defined with the goal of being more quantitative than the Torino 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 defines the value of zero for an impact risk equal to the background risk and gives increasing/decreasing values following a logarithmic (powers-of-ten) law. Should an object have PS=2, then the corresponding event is one hundred times more likely to happen than a similar or more powerful event occurring randomly (from the background) during 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),


Potentially Hazardous Asteroids (PHAs)

Asteroids that can in principle come closer than 0.05 au (7.5 million km) to Earth and have an absolute magnitude of 22 or brighter. This corresponds to a size of roughly 140 m.



The approximate diameter of an object, assuming that the object is a sphere. A direct measurement of size and shape of an asteroid is often difficult. It can be estimated using radar systems, observing the asteroid occulting a star, with thermal infrared observations or by sending spacecraft missions to the asteroid. For most objects, the size is derived from its brightness (measured by the absolute magnitude) and assuming an average reflectivity (albedo) of the surface. The diameter of an object (D, in m) is derived from the absolute magnitude (H) and albedo (p) using the following equation:


D = 1.329 * 106 * p-1/2  * 10-0.2H


See, for example, Harris, A.W., Lagerros, J.S.V. 2002, in Asteroids III, eds. W.F. Bottke, A. Cellino, P. Paolicchi, and R. Binzel (Tucson: Univ. Arizona Press), 205. The albedo is often unknown and has to be assumed, resulting in large uncertainties in the size determination. A magnitude-size conversion table can be found at the Minor Planet Center. In the database of the NEO Coordination Centre, sizes computed with an assumed albedo range (0.05 to 0.25 as default) are marked with the star symbol (*).


Threatening objects

Within ESA's Planetary Defence Office, objects that have a probability greater than 0 to impact our planet are called threatening objects, independently of their sizes. They are all included in our Risk List.


Torino Scale (TS)

The Torino scale is a classification (similar to the Mercalli scale for earthquakes) to quantify the hazard posed by an NEO, taking into account its kinetic energy and its impact probability.  It ranges from 0 to 10 to indicate an increasing collision threat. In its graphical representation, colours are also associated to each level, to provide a more direct visualisation of the danger level of the event. More info at


TNT equivalent

A customary measure of energy, often used to estimate the energy released by an impact. One kiloton of TNT equivalent (1 kt) corresponds to the energy released by the explosion of 1000 tons of TNT. In SI units, 1 kt TNT equivalent corresponds to 4.184 * 1012 J.