Glossary main content.



The collection of material onto a formed or forming star, planet, or moon. Asteroid-rocky or metallic body smaller than a planet that orbits a star.

asteroid belt

The region of the Solar System where most of the asteroids orbit. The asteroid belt lies between the orbits of Mars and Jupiter.

astronomical unit (AU)

The average distance between the Earth and the Sun (149.6 million km), used as a convenient "yardstick" for distances in the Solar System.


The scientific study of the Universe.astrophysicsThe branch of astronomy that deals with the physics of astronomical objects and phenomena.


A gaseous envelope surrounding a star, planet, or satellite, and bound to it by gravity. avian Relating to or characteristic of birds.


Having a sensitive dependence on a system's initial conditions. In a chaotic system, the initial conditions are not known with complete precision, so it is difficult to predict an outcome. Weather is an example.

charge-coupled device (CCD)

A device that uses a light-sensitive silicon chip to detect and quantify photons of light. The light signal is collected into pixels, or picture elements.


A body of rock and ice that orbits the Sun. When a comet nears the inner Solar System, some ice evaporates and forms an extended and diffuse atmosphere. This is blown away from the Sun by the solar wind and radiation pressure, forming a prominent tail of gas and dust.


A depression on a planet, moon, or meteorite created by the impact of an external body or by volcanic activity.daguerreotypeAn early photographic process where an image is made directly onto a light-sensitive silver-coated metallic plate, without using a negative.


The capacity of a physical system to do work. Energy can be converted among its various forms (motion, light, mass, etc.) but the total amount of energy remains constant.exposureThe presentation of photographic film or plates to light.


The force of attraction acting between any two masses.

impact probability

The statistical likelihood that an asteroid or comet will hit Earth.

infrared (IR)

Invisible electromagnetic radiation (light) with wavelengths longer than red light and shorter than microwaves. Infrared light occupies the spectral band extending from 0.75 to about 200 micrometers.


A hard, brittle metallic element rare in terrestrial rocks but common in some asteroids.jet streamA high-speed air current in the stratosphere or upper troposphere.

Kepler's Laws of Planetary Motion

Three laws discovered by Johannes Kepler: (1) A planet follows an elliptical orbit with the Sun at one focus. (2) The line from a planet to the Sun sweeps out equal areas in equal times, so the planet moves faster the closer it is to the Sun. (3) The square of a planetàs orbital period is proportional to the cube of its average distance from the Sun, so distant planets have longer periods than close ones. These laws apply to all bodies in gravitational orbits, not just planets.

kilometer (km)

A unit of length equal to 1,000 meters, or 0.62 miles.

margin of error

The uncertainty of a measured quantity. The more precise the instrument or technique used for the measurement, the smaller the margin of error.


A unit of energy used to describe nuclear warheads. The same amount energy as 1 million tons of TNT.


A bright streak of light produced by a small fragment of rock or metal that burns up as it enters the atmosphere.


A fragment of rock and/or metal that has landed on Earth from interplanetary space. Most meteorites come from asteroids, but a few are from other planets or their satellites.


A body in space that is smaller than an asteroid and at least as large as a speck of dust.


A naturally-occurring, homogeneous inorganic element or compound having a definite chemical composition and orderly internal structure, crystal form, and characteristic chemical and physical properties.

minor planet

An asteroid.

near-Earth object

Asteroids or comets that pass within 1.3 astronomical units of the Sun.


Relating to or using light in visible wavelengths.


The path of one celestial body moving around another under the force of gravity.

orbit solution

A likely orbit path calculated for an asteroid or comet by measuring its position on the sky over time.

orbital period

The time interval for a body to complete one orbit around another.


A slight disturbance in the orbit of one celestial body around another caused by the gravitational attraction of a third body.


An astronomical body with enough mass for its gravity to make it spherical but not enough to generate nuclear energy. Planets have nonintersecting orbits around stars. Some might drift freely in space.


An upwelling of matter moving away from a source of heat.


The acronym for "radio detection and ranging" . A technique in which radio signals are transmitted and their reflections off objects such as asteroids are received and studied.

radio waves

Low energy electromagnetic radiation, with long wavelengths and low frequencies.refractor telescopeA telescope that uses lenses to bring light to a focus.


A body that orbits around a larger body.


Particles ranging in size from smaller than sand to huge boulders that are carried along and then deposited by natural agents such as wind, water, and ice. Organisms and chemical precipitation can also produce sediment.

spectrum (pl. spectra)

The range of electromagnetic radiation (light) expressed in terms of frequency or wavelength. A rainbow displays the spectrum of visible light.spheruleA hard bead of minerals that condenses out of airborne materials from meteorite collisions.


A self-luminous body held together by its own gravity and with a central temperature and pressure sufficient to generate nuclear energy.


The layer of the atmosphere above the troposphere. The stratosphere extends from approximately 10 to 50 km above Earth's surface.


Relating to the internal processes that alter the outer layers of a terrestrial planet or moon.telescopeAn instrument designed to gather and focus electromagnetic radiation (light) to study celestial objects and events.


The path of a body in motion through space.troposphereThe lowest layer of Earthàs atmosphere in contact with Earthàs surface. Most weather occurs within the troposphere. The troposphere extends from Earthàs surface to elevations of approximately 10 to 15 km.


A very large ocean wave that can be produced by an underwater earthquake, landslide, volcanic eruption, or the rare impact of a comet or asteroid in the ocean.


The distance between successive wave crests, or troughs. Light of different wavelengths has different properties, such as color.