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What are Small Bodies?

PDS-SBN deals with small bodies. What are they?

There are a number of links about small bodies research, images, and other resources. You will find them on our consolidated Related Links page.


Comets are icy planetesimals formed in the outer solar system. They are studied as an aid to understanding the starting composition of the solar system. Throughout history, comets have been viewed with wonder by many different civilizations. At times they were feared and considered to be omens, usually bad. Scholars tried to explain the existence of comets through both scientific and religious reasoning. Today, science has succeeded in answering many of the questions posed by the ancients, but it has also produced many more of its own.

Comets are thought to be the remnants of the material which gave birth to the solar system. As a large cloud of primordial gas contracted it eventually formed into the Sun and the planets around it. However, some of this material did not get incorporated into these objects and it is this leftover material which is thought to be what formed the comets.

Comets are small, irregularly shaped bodies composed mainly of ice and dust. Most comets have highly elliptical orbits which take them very close to the Sun and back out into deep space, far beyond the orbit of Pluto. These comets will take hundreds of thousands or even millions of years to complete one orbit. Other comets will never pass far beyond Pluto's orbit and have orbital periods less than 200 years. These comets with short orbital periods are called periodic comets.

The main body of the comet is called the nucleus and is composed of dust particles trapped in a mixture of ices of water, carbon monoxide, carbon dioxide, methane, and ammonia. The nucleus is typically only a few kilometers in diameter and is the only part of the comet that exists when it is far from the Sun. As the comet approaches the inner solar system, the solar heat generates a halo of evaporated gas and dust which sublimes from the nucleus and spreads out around it, forming the coma. The coma will grow larger in size as the comet gets closer to the Sun. An invisible hydrogen envelope also forms engulfing the nucleus in a huge cloud of neutral hydrogen stretching out through millions of kilometers of space.

There are two types of tails associated with visible comets. The first is composed of microscopic dust particles which usually form a broad and gently curved tail. The second, straighter and thinner, is composed of ionized gas .The radiation pressure and solar wind from the Sun cause the dust and ions to be blown away from the comet's nucleus and coma producing the visible dust and ion tails. Consequently, the tails of comets are always directed away from the Sun.


main belt asteroids
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Main belt Asteroids Positions projected onto the ecliptic plane for 4 October 2000 of the planets Mercury through Jupiter and 7722 numbered asteroids with accurately known orbits.
© 1998 Institute for Remote Exploration

Asteroids are small rocky bodies which orbit the Sun. Also known as minor planets, they were formed from the leftover material of our solar system which did not get incorporated into the Sun or planets. The largest asteroid, 1 Ceres, was discovered in 1801 by the Sicilian astronomer Giuseppe Piazzi. He had been studying the area between Mars and Jupiter where it was believed, at the time, an as yet undiscovered planet would be located. At first, it was thought 1 Ceres might be this planet but its size was too small for this to be the case.

There are over 269,000 asteroids that have been numbered and more are being found every month. When an asteroid is discovered it is given a temporary name consisting of six characters. The first four characters are numbers identifying the year of the discovery, the next character is a letter indicating the half-month of the year in which the discovery occurred ('I' is omitted and 'Z' is unused), and the last character is a second letter indicating sequentially the asteroids discovered in that half-month ('I' is omitted). For example, the asteroid 1995 CR would be the eighteenth asteroid discovered in the first half of February of 1995. If there are more than twenty-five asteroid discoveries in a half-month then the last character will start over with 'A' but with a numerical subscript added on. Should the asteroid be studied long enough for its orbit to be determined it will be given a permanent name and number. The discoverer submits a name to the International Astronomical Union: Minor Planet Center, Commission 20.

Near-Earth Asteroids (NEAs) reside in the area near Earth, around 1.0 AU, and have perihelia less than or equal to 1.3 AU. Three sub-groups of NEAs have been defined. The Aten group contains the innermost asteroids, which have a semi-major axis less than 1.0 AU and a perihelion distance greater than 0.983 AU. The Apollo group asteroids have a semi-major axis greater than 1.0 AU and perihelion distance less than 1.017 AU. The Amor group asteroids have semi-major axis greater than 1.0 AU and perihelion distance between 1.017 and 1.3 AU. The Aten and Apollo asteroids cross the orbit of the Earth while the Amor asteroids do not reach Earth's orbit but do cross that of Mars.

Kirkwood Gap
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Kirkwood Gap This figure shows the frequency of asteroids vs. the semi-major axis of asteroids.

Most Main Belt asteroids are located between the orbits of Mars and Jupiter (1.8 to 4.0 AU) in a region known as the asteroid belt. Within the belt are gaps where there are few or no asteroids at all. These gaps are known as Kirkwood gaps and are the result of Jupiter's large gravitational pull on objects in these areas. Objects which might have started in the Kirkwood gaps would have had an orbital period that was a simple fraction of Jupiter's period. Such an asteroid would then have been accelerated by Jupiter's regular pull and, in time, thrown into an eccentric orbit in which it could collide with another object in the solar system.

The Trojan asteroids are two groups of asteroids located along the orbital path of Jupiter, 60 degrees ahead and behind the planet. These positions are called Lagrange points after the French theorist Joseph Louis Lagrange. Lagrange suggested that when objects in the same plane form an equilateral triangle with the Sun they can share the same orbit without catching up to or colliding with each other.

Centaurs are asteroids in the outer solar system which have orbits lying between those of Saturn and Uranus. These objects may, in fact, be more like comets or Kuiper Belt objects than asteroids. However, since the Centaurs are in orbit beyond Jupiter they do not often get a chance to get close enough to the Sun to warm up enough to show cometary activity, so they are still considered to be asteroids.

Asteroids are classified into types according to their albedo and colors or spectra, with the most common types being C, S, and M. C-type asteroids are very dark, with a low albedo of 0.03-0.06, and their spectra are flat, making their color grayish. S-type asteroids are moderately bright, having an albedo of 0.10-0.22. Their spectra have moderate to strong absorption bands and their colors are greenish to reddish. It is believed they are composed of metal-rich silicates. M-type asteroids are moderately bright, have an albedo of 0.10-0.18, with spectra which have few if any absorption bands and their colors are reddish. They are believed to be composed predominantly of metallic species, iron and nickel. There are also a dozen or so other rarer types of asteroids included in the classification scheme with their own letter designations associated with clustering of brightness and color or spectral properties.


a measure of how efficiently a surface or dust particle reflects light; sometimes defined as the ratio of the amount of light reflected by an object and the amount of incident light; a body with the albedo of 0 reflects no light, with the albedo of 1 reflects 100% of the incident light.
aphelion ('Q'):
the point in the orbit of a planet, comet, asteroid, etc. that is furthest from the Sun.
astronomical unit (AU):
a unit of length in astronomy; it is equal to the average distance from the Earth to the Sun; 1 AU = 149,597,870 kilometers (92,955,806 miles).
a measure of the asteroid/comet brightness variations with the wavelength; often color is determined as the difference of brightness in two wavelengths or two filters; if an object is equally bright as seen through different filters, it is referred to as gray; if the body is brighter at longer wavelength or red filters, it is referred to as being red.
eccentricity ('e'):
a measure of how far from a circular shape an ellipse is; numerically, the eccentricity e = 1 - ( perihelion / semi-major axis). The eccentricity e = 0 for a circle and e = nearly 1 for very long, skinny ellipses.
a close curve resembling a flattened circle; the sum of the distances from a point on the ellipse to two fixed points (foci) is always constant.
an atom or group of atoms (molecule) that carries a positive or negative electric charge as a result of having lost or gained one or more electrons.
to be converted into ions by gaining or losing electrons.
kirkwood gaps:
major axis:
the axis passing through the foci of an ellipse; the largest diameter distance between opposite points of an ellipse.
perihelion (perihelia, pl) ('q'):
the point in the orbit of a planet, comet, asteroid, etc. that is closest to the Sun.
period ('P'):
time for an object to complete one orbit.
semi-major axis ('a'):
half the length of the major axis of an ellipse; the semi-major axis of an orbiting body is equal to the average distance from the body to the Sun.
'aJupiter' = semi-major axis of Jupiter
a chemical compound that contains silicon, oxygen, and usually one or more other common elements (often metals); silicates are the main rock and mineral forming compounds.
solar wind:
a tenuous flow of gas and energetically charged particles, mostly protons and electrons -- plasma -- which stream from the Sun.
spectrum (spectra, pl):
electromagnetic radiation arranged in order of wavelength; a rainbow is a natural spectrum of visible light from the Sun; spectra are often punctuated with emission or absorption lines, which can be examined to reveal the composition and motion of the radiating source.
sublime (or sublimate):
the turning of a solid directly into a gas without going through the intermediate liquid phase, e.g., the vapor of 'dry ice' (the sublimation of frozen carbon dioxide).
The Tisserand parameter (or Tisserand invariant) is a combination of an object's orbital elements in such a way that the result remains essentially constant, even if the object's orbit is perturbed by planetary perturbations. The value of Tisserand's criterion is also used to classify different types of comet's and asteroids into dynamical groups.
'TJupiter' = Tisserand Invariant with respect to Jupiter.