NEO Characterisation

The inner Solar System is populated, in addition to the Sun itself and the terrestrial planets, by a swarm of small bodies that are collectively called NEOs (Near-Earth objects). The population of NEOs is composed by those asteroids and comets that can, from time to time, approach the Earth; conventionally, the condition separating them from generic comets and asteroids is given by perihelion distance (glossary) q less than 1.3 AU.

Comets are generally thought to be made of a mixture of ices (frozen gases including water vapour, methane, ammonia, carbon dioxide and hydrogen cyanide) and dust that wasn't incorporated into the planets when the Solar System was formed about 4.5 billion years ago. The sublimation (glossary) of ices when such bodies come in the proximity of the Sun are responsible for the peculiar cometary activity (coma, dust and ion tails) and make them visible sometimes by naked eye.

Typical orbits for inner solar system asteroids
Typical orbits for inner solar system asteroids

NEOs, can be characterised in many ways. Two of the more common classification schemes are by;

  • Orbital elements
  • Taxonomic class

Orbital Properties

The primary source of near-Earth objects (NEOs) is the main asteroid belt between the orbits of Mars and Jupiter. Since the epoch of planetary formation collisions between objects in the main belt have given rise to a very broad distribution of sizes. The largest asteroid in the main belt is (1) Ceres with a diameter of about 950 km. Ceres is probably an example of a primordial body that has remained largely unaltered since the time of planet formation 4.5 billion years ago. However, most bodies in the main belt are thought to be collisional fragments with sizes ranging from hundreds of kilometres down to pebble-sized objects and dust grains.

The strong gravitational fields of Jupiter and Saturn can perturb the orbits of small asteroids in the main belt and increase their eccentricity until they cross the orbits of one or more of the inner planets. Asteroids that can come close to the Earth’s orbit are called near-Earth asteroids (NEAs) and are traditionally classed by astronomers in three groups according to their orbital parameters,named after the archetypal members of each group (See Table below).

Orbital Properties

NEO

Description

Near-Earth Comets (NECs)

q<1.3 AU, short period comets with P<200 years

Near-Earth Asteroids (NEAs)

q<1.3 AU

Atens

Earth-crossing NEAs with semi-major axes smaller than Earth's, a<1.0 AU, Q>0.983 AU.

Apollos

Earth-crossing NEAs with semi-major axes larger than Earth's, a>1.0 AU, q<1.017 AU

Amors

Earth-approaching NEAs with orbits interior to Mars' but exterior to Earth's. a>1.0 AU, 1.017

Potentially Hazardous Asteriods (PHAs)

NEAs whose Minimum Orbit Intersection Distance (MOID) (glossary) with the Earth is 0.05 AU or less and whose absolute magnitude (H) (glossary) is 22.0 or brighter. MOID<=0.05 AU, H<=22.0

The Apollo and Aten asteroids are on so-called “Earth-crossing orbits” which do not necessarily actually intersect the Earth’s orbit at the present time but have the potential to enter the Earth’s capture cross-section as a result of gravitational interactions with the Earth and other planets.

Inner-Earth Objects (IEOs) are a largely undiscovered population of objects that have orbits with aphelia less than 0.983 AU, i.e. that lie entirely within the orbit of the Earth. As in the case of Amors, members of this group could become hazardous to the Earth in the future as their orbits evolve. IEOs are very difficult to detect from the ground and very little is known about their number and sizes. Confirmed detections of only a few IEOs have been made to date.

Physical properties

From Earth-based observations, we know that there are many different types of “animal” in the NEO “zoo”; stony, carbonaceous, cometary nuclei, rubble piles, binary systems, monolithic slabs (it is entirely possible that there are rare species awaiting discovery).

The physical properties of NEOs are fundamental to the assessment of impact risk in a number of ways.

A 100-m iron-nickel asteroid will almost certainly act differently to a 100-m carbonaceous body. Hence at some point we will need to obtain precise physical information for a range of sizes and compositional types. Space missions provide the “ground-truth” data which are required to fully understand how these parameters may be best constrained from the ground. If a high-probability impactor is discovered, Earth-based measurements will provide the initial information upon which a mitigation strategy can be based.

Spectroscopy is the main source of information on the mineralogy of asteroid surfaces. The taxonomic type of an asteroid can be used as a rough indicator of the reflectivity or albedo of its surface. Broadly speaking, carbonaceous asteroids such as D, P and C types have dark surfaces with albedos typically in the range 0.03 – 0.1. Most asteroids with spectra dominated by silicates or metals have intermediate albedos, although certain minerals, e.g. enstatite, can give rise to remarkably high albedos of 0.5 or above. Although only a small fraction of NEAs, as far as taxonomic classes are concerned, have been unambiguously classified, in general the mineralogy of NEAs appears to reflect that of main-belt asteroids so it is not surprising that a broad range of albedos is observed amongst NEAs. In particular, a few of them (M-type, or metallic type) seems to be of high metallic content, which is important for their possible exploitation, while C-type (or carbonaceous type) bodies would represent a sample of pristine material characterizing the outer asteroid main belt.

The range of spectrally identified species is listed in the Table below. The information is from the NEOMAP final report (link).

Physical Properties

Taxonomic Type

Typical Albedo (glossary)

Probable mineral composition

D,P

0.03-0.06

Carbon, organics, silicates

C, B, F, G

0.03-0.10

Carbon, organics, hydrated silicates

M

0.1-0.2

Metals, enstatite (glossary)

S

0.1-0.3

Silicates, metals

Q

0.2-0.5

Silicates, metals

V

0.2-0.5

Silicates (pyroxene, feldspar)

E

0.3-0.6

Enstatite + other iron-poor silicates

X

0.03-0.6

Unknown

The taxonomic distribution of Near Earth Asteroids

For the whole main belt the population is 75% of type C, 15% of type S, and 10% of other types [Zellner B. (1979) Asteroid taxonomy and the distribution of the compositional types.]. The S class is the dominant class in the inner asteroid belt (region between 2 AU and 2.5 AU from the Sun), while asteroids evolving in the outer asteroid belt (region between 2.5 AU and 4 AU) are mostly of C-type.

With the use of Earth-based telescopes in recent years we have learned;

  • that NEOs with diameters > 200m are probably either highly fractured or rubble piles;
  • new thermal models have been applied to NEOs to determine diameters and albedos more accurately;
  • radar observations have demonstrated that binary objects are common within the NEO population.

However, a single mission to a NEO, the NEAR–Shoemaker spacecraft, substantially increased our knowledge of the bulk properties of silicate NEOs and revealed for the first time the metre-scale surface structure and properties. At the same time, the mission underlined the fact that measurements by in-situ spacecraft can be orders of magnitude more precise than those obtained from the ground.

Finally, another (more subtle) goal is the knowledge of properties of the population such as albedo and surface composition. Our current understanding of the overall risk from NEOs is based on an extrapolation from the detected population. To perform this extrapolation successfully we must understand the surface reflectance properties of NEOs such as albedo and colour within the population as a whole, so that the observational biases can be properly accounted for.

Last update: 8 May 2012

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