glossary

asteroid: A small rocky or metallic body that orbits a star. In our Solar System, most are found within the main asteroid belt, which falls roughly between the orbits of Mars and Jupiter.

asteroid impact mitigation: Plans for reducing the consequences of an asteroid impact on Earth, or preventing such an impact from occurring at all.

Aurora Borealis: An aurora that occurs in northern regions of the Earth (also called northern lights). The southern lights are called an Aurora Australis. An aurora is a glowing atmospheric phenomenon that occurs when charged particles collide with the atmosphere of the Earth near the poles. They are especially bright after solar storms.

comet: Made of ice and dust, these frozen pieces of left-over planets move in elliptical orbits around the Sun. As they approach it and begin to melt, comets may release glowing
tails of dust and gas millions of miles long. In our Solar System, most are found outside the orbit of Neptune, in regions called the Kuyper Belt and the Oort Cloud.

electromagnetic spectrum: The entire range of electromagnetic radiation (light) that objects emit, reflect, or transmit. In order of increasing wavelength (decreasing
frequency and energy), the spectrum ranges from gamma rays through X-rays, ultraviolet light, visible light, infrared radiation, and microwaves to radio waves.

energy: The power behind all phenomena, energy flows from place to place and form to form. Heat energy is the disorderly motion of molecules; chemical energy is the arrangement of atoms; mechanical energy is moving bodies or elastically distorted shapes; and gravitational energy is the separation of mutually attracting masses.

galaxy: A massive, gravitationally-bound assembly of stars, interstellar clouds, and dark matter.

gravity: The force of attraction acting between any two masses.

isotope: Every element comes in multiple varieties, called isotopes. Each isotope has the same number of protons (positively charged particles), but a different number of neutrons
(electrically neutral particles).

K-T impact: The collision of an asteroid with Earth that happened 65 million years ago, contributing to a mass extinction of species (including the non-avian dinosaurs). "K-T" is the geologists term for the boundary between the Cretaceous and Tertiary periods in geological history.

magnetosphere: The volume of space around a star or a planet in which the global magnetic field influences the motion of charged particles.

meteor: If a piece of a comet or asteroid falls to Earth, as it heats up and passes through our atmosphere it is called a meteor. If it survives to land on Earth, it's called a meteorite.

meteor shower: A large number of meteors that appear together and that seem to come from the same area in the sky, and probably come from the same source.

moon: A natural satellite orbiting around a planet.

nuclear fusion: Energy is released when light atoms, like hydrogen and helium, combine into heavier ones. This is the reaction that powers the Sun. Nuclear fission occurs when
heavier atoms like uranium and plutonium split into lighter ones, which also releases energy. Nuclear reactions are far more powerful than chemical ones.

planet: An astronomical body with enough mass for its gravity to make it spherical but not enough to generate nuclear energy. Planets have non-intersecting orbits around stars or drift freely in space.

proton: A positively-charged subatomic particle. Every atomic nucleus contains one or more protons.

solar wind: A stream of high-speed, ionized particles ejected primarily from the Sun.

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

variable star: A star that varies noticeably in brightness.