# A Kuiper Belt Glossary

This page lists acronyms and definitions of terms used in Kuiper belt research. If there are some terms and/or definitions you would like added, send them to: ekonews@boulder.swri.edu.

Some definitions have been taken (often verbatim) from the following sources:
This astronomical glossary provided by the International Comet Quarterly (ICQ).
Encyclopedia of the Solar System, 1999 (Academic Press)

## Acronyms

AU
Astronomical Unit
EKO
Edgeworth-Kuiper Object
KBO
Kuiper Belt Object (see Edgeworth-Kuiper Object)
TNO
Trans-neptunian Object (includes Edgeworth-Kuiper Objects and Scattered Disk Objects)
SDO
Scattered Disk Object

## Definitions

Aphelion
The point in an orbit that is farthest from the Sun.
Albedo
The fraction of the incident light that is reflected by a surface. A surface with albedo of zero reflects no light, and with albedo of 1 reflects 100% of the incident light
Arc Minute (arcmin)
A unit of angular measure equal to 1/60th of a degree. An arc minute also uses the notation of a single quote, e.g., 5 arcmin = 5´
Arc Second (arcsec)
A unit of angular measure equal to 1/3600th of a degree. An arc second also uses the notation of a double quote, e.g., 5 arcsec = 5´´
Astronimical Unit (AU)
The average distance between the Earth and the Sun. 1 AU is about 150 million kilometers (or, more precisely, 1.4959787066×10¹³ cm), or about 93 million miles.
Centaur
An ``outer planet crosser''. A minor body whose heliocentric orbit is between Jupiter and Neptune and typically crosses the orbits of one of the other outer giant planets (Saturn, Uranus, Neptune). The orbits of the Centaurs are dynamically unstable due to interactions with the giant planets, so they must be transition objects from a larger reservoir of small bodies to potentially active inner solar system objects. The Kuiper belt is believed to be this source reservoir.
Cubewano
A ``main Kuiper belt'' object, beyond about 41 AU and not controlled by resonances with the outer planets (see MPEC 1997-P12, Marsden, B.).
Degree (deg)
A unit of angular measurement. There are 360 degrees in a full circle. The view of the sky with unobstructed horizons is 180 (e.g., from north to south or east to west). There are 60 arc minutes in a degree and 3600 arc seconds. Adegreee also uses the notation of a raised circle, e.g., 5 degrees = 5°
Eccentricity (e)
The measure of how elliptical or circular is an orbit. The eccentricity is equal to (1-b²/a²)1/2, where a and b are the major and minor axes of the elliptical orbit. Circular orbits have e=0, elliptical orbits have 0<e<1, radial and parabolic orbits have e=1, and hyperbolic orbits have e>1.
Ecliptic
The plane of the Earth's orbit around the Sun. Most objects in the solar system are in orbits very close to the ecliptic plane. Conversely, the ecliptic is the path of the sun against the sky with respect to the stars.
Ecliptic comet
An object which has escaped the Kuiper belt or the scattered disk and which is being scattered inward to an orbit crossing one or more of the planets. The designation of such objects as comets assumes an icy composition because of their likely formation far from the Sun in the colder regions of the solar nebula. Ecliptic comets include Centaur objects and Jupiter-family comets.
Elongation
The angular separation between the Sun and a solar system object as viewed by an observer (usually on the Earth).
Edgeworth-Kuiper Object (EKO)
(also known as: Kuiper Belt Object, Trans-neptunian Object) A minor body that resides in the Kuiper belt. Their sizes can range up to a few hundred kilometers.
Inclination
The angle between the plane of an orbit and some reference plane, usually the ecliptic.
Jupiter-family comet
An active comet in a low to moderate inclination orbit with a semi-major axis less than that of Jupiter's orbit. Most Jupiter-family comets are in orbits which cross or can closely approach Jupiter's orbit. The Jupiter-family comets are a subset of the ecliptic comets.
Kuiper Belt
Where one can find Kuiper belt objects. To be more specific, the collection of minor bodies with heliocentric orbits beyond Neptune, reaching out to possibly hundreds of astronomical units.
Kuiper Belt Object (KBO)
(see Edgeworth-Kuiper Object)
Lightcurve
The variation in brightness due to an object's rotation. A lightcurve can be due to shape effects (seeing different cross-sectional areas of an object as it rotates), albedo changes on the surface (seeing different bright and dark patches as the object rotates), or some combination of the two.
Major Axis (a)
The axis passing through the focii of an ellipse. The largest diameter distance between opposite points of an ellipse.
Mean Motion Resonance
The dynamical situation where the ratio of the orbital periods of two orbiting objects can be expressed as the ratio of two small integers (e.g., Plutinos are EKOs that are in a 3:2 resonance with Neptune, meaning they orbit two times in the time Neptune completes three orbits). Mean motion resonances can lead to strong changes in the orbit of one or both of the bodies or can actually enhance orbital stability, depending on the precise nature of the resonance.
Minor Axis (b)
The axis passing through the center of an ellipse, perpendicular to the major axis. The shortest diameter distance between opposite points of an ellipse.
Perihelion
The point in an orbit that is closest to the Sun.
Phase Angle
The angle from the sun to an object to an observer. I.e., an object's phase angle is the angular separation (elongation) between the Sun and the Earth as would be observed from that object.
Phase Function
The change in the brightness of an object as a function of the phase angle. In general, an object gets brighter as the phase angle decreases. The function is usually fairly smooth except for at small (approaching zero degres) phase angles where there may be a ``spike'' of increased brightness. The phase function is usually described as the change in magnitude (brightness) per degree of phase angle.
Planetesimal
A small body formed in the early solar system by accretion of dust and ice (if present) near the central plane of the solar nebula.
Plutino
An EKO whose motion is controlled by the 3:2 resonance with Neptune (see ``The Plutinos'', Jewitt, D. & Luu, J. 1996, in Completing the Inventory of the Solar System, ASP Conf. Proc., 107, 255). Also see this discussion.
Pluto
A planet? An EKO?
Precession
The slow, smooth increase or decrease of an angle. When used to describe the changing orientation or an orbit, one can imagine an ellipse (the orbit) spinning around a central point. When used to describe a spinning body, it is the circular gyration of the body's rotation axis.
Resonance
(see mean motion resonance or secular resonance).
Rotational Lightcurve
(see Lightcurve)
Scattered Disk
The minor bodies in high eccentricity orbits in the ecliptic plate beyond Neptune. Objects in the scattered disk may be escapees from the Kuiper belt and/or may be scattered Uranus-Neptune planetesimals.
Scattered Disk Object (SDO)
An object that resides in the scattered disk.
Secular Resonance
The dynamical situation where there is commensurability among the frequencies associated wit the rates of precession of the argument of perihelion or of the regression of the nodal line of two bodies. Secular resonances can lead to very large changes in one or more of the bodies.
Semi-Major Axis
One half of the major axis of an elliptical orbit. The semi-major axis is commonly thought of as the average distance between an object and the body it is in orbit about.
Semi-Minor Axis
One half of the minor axis of an elliptical orbit.
Trans-neptunian Object (TNO)
This class of objects includes Edgeworth-Kuiper Objects and Scattered Disk Objects.

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