Discovery Guides Areas


Planetary Defense: Preventing a World of Trouble
(Released November 2005)

  by Salvatore A. Vittorio  


Key Citations

Web Sites




Albedos: Fractions of the total light incident on reflecting surfaces, especially celestial bodies, which are reflected back in all directions.

Amor: A family of near-Earth asteroids that have average orbital diameters in between the orbits of Earth and Mars and perihelia slightly outside Earth's orbit (1.017-1.3 astronomical units). Amor asteroids often cross the orbit of Mars, but they do not cross the orbit of Earth.

Aphelia: The points on planetary orbits that are farthest from the Sun

Apollo: A family of near-Earth asteroids that have average orbital diameters greater than that of the Earth and perihelia less than Earth's aphelion.

Apogee: That point in an orbit at which the moon or an artificial satellite is most distant from the Earth.

Asteroid: One of the many small celestial bodies revolving around the Sun, most of the orbits being between those of Mars and Jupiter. Also known as minor planet or planetoid.

Asteroid belt: The region between 2.1 and 3.5 astronomical units (AU) from the sun where most of the asteroids are found. Asteroids are small planetary bodies revolving around the sun; most of the orbits are between the planets Mars and Jupiter.

Atens: A family of near-Earth asteroids that have average orbital diameters closer than one astronomical unit (1 AU), the distance from the Earth to the Sun and aphelia of greater than Earth's perihelion, placing them usually inside the orbit of Earth.

Biosphere: The life zone of the Earth, including the lower part of the atmosphere, the hydrosphere, soil, and the lithosphere (the rigid outer crust of rock) to a depth of about 1.2 miles (2 kilometers).

Comet: A nebulous celestial body having a fuzzy head surrounding a bright nucleus; comets are one of three major types of bodies moving in closed orbits about the sun, the others being the planets and asteroids (minor planets); in comparison with the planets, comets are characterized by their more eccentric orbits and greater range of inclinations to the ecliptic plane.

Delta-v: In general physics, delta-v is simply the change in velocity. In astrodynamics (the study of the application of celestial mechanics to the creation of artificial satellite orbits), delta-v is a scalar measure for the amount of "effort" needed to carry out an orbital maneuver, i.e., to change from one orbit to another. A delta-v is typically provided by the thrust of a rocket engine. The time-rate of delta-v is the magnitude of the acceleration, i.e., the thrust per kilogram (kg) total current mass, produced by the engines. The actual acceleration vector is found by adding the gravity vector to the vector representing the thrust per kg.

Eccentric orbits: Orbits of celestial bodies that deviate markedly from a circle.

K-T impact: The Cretaceous-Tertiary (K-T or KT) extinction event, also known as the KT boundary, which was a period of massive extinction of species, about 65.5 million years ago. It corresponds to the end of the Cretaceous Period and the beginning of the Tertiary Period in Earth's geological history.

Geostationary transfer orbit (GTO): An intermediate orbit between a low Earth orbit (LEO) and a geosynchronous orbit. A geosynchronous orbit is a geocentric orbit that has the same orbital period as the sidereal (stellar referenced) rotation period of the Earth. A geostationary transfer orbit is used to move a satellite from LEO into a geostationary orbit.

Gravity assist: Also known as swingby or flyby; an interplanetary space vehicle maneuver whereby the vehicle closely approaches a target planet without impacting the planet or going into an orbit around it. The vehicle uses the energy obtained from the planet's gravitational field to change the speed or shape of the spacecraft's orbit.

Low Earth orbit (LEO): A circular orbit around Earth between the atmosphere and the Van Allen radiation belt, with a low angle of inclination. These boundaries are not firmly defined, but are typically around 350-1400 km above the Earth's surface, with inclination angles less than 60 degrees from the equator. This is generally below intermediate circular orbit (ICO) and far below geostationary orbit. Orbits lower than this are not stable, and will decay rapidly because of atmospheric drag. Orbits higher than this are subject to early electronic failure because of intense radiation and charge accumulation.

Libration point: Any one of five points in the orbital plane of two massive particles (such as planets or moons) in circular orbits around a common center of gravity (such as the Sun), where a third particle of negligible mass (such as a spacecraft) can remain in equilibrium.

Meteoroid: Any solid object moving in interplanetary space that is smaller than a planet or asteroid but larger than a molecule.

Momentum: For a single nonrelativistic particle, the product of the mass and the velocity of a particle.

Near-Earth asteroids (NEAs): A subset of the NEOs, are asteroids whose orbit intersects Earth's orbit and which may therefore pose a collision danger, as well as being most easily accessible for spacecraft from Earth.

Near-Earth objects (NEOs) : Celestial bodies whose orbits are nudged into Earth's neighborhood by the gravitational attraction of other planets.

Newton: The unit of force in the meter-kilogram-second system, equal to the force which will impart an acceleration of 1 meter per second squared to the International Prototype Kilogram mass. Symbolized as N.

Perihelion: That orbital point nearest the Sun when the Sun is the center of gravitational attraction.

Parking orbits: Temporary Earth orbits during which space vehicles are checked out and their trajectories carefully measured to determine the amount and time of increase in velocity required to send them into a final orbit or into space in the desired direction.

Semi-major axis: Either of the equal line segments into which the major axis of an ellipse is divided by the center of symmetry.

Specific impulse: A performance parameter of a rocket propellant, expressed in seconds, equal to the thrust in pounds divided by the weight flow rate in pounds per second. Also known as specific thrust.

Torino Scale: An important tool for categorizing the Earth impact hazard associated with newly discovered NEOs, equivalent to the "Richter Scale" but for NEOs. This scale was created by Professor Richard P. Binzel at the Massachusetts Institute of Technology and revised at an international conference on NEOs held in Torino, Italy, in June 1999. The Torino scale utilizes numbers that range from 0 to 10, where 0 indicates an object that has a zero or negligibly small chance of collision with the Earth, or that is too small to penetrate the Earth's atmosphere intact in the event that a collision does occur. A 10 indicates that a collision is certain, and the impacting object is so large that it is capable of precipitating a global disaster. An object is assigned a value based on its collision probability and its kinetic energy (proportional to its mass times the square of its encounter velocity).

Tsunami: A long-period sea wave produced by a seaquake or volcanic eruption; it may travel for thousands of miles. Also known as seismic sea wave.

Trans-Neptunian object (TNO): is any object in the solar system which orbits the Sun at a greater distance on average than the planet Neptune. The Kuiper belt, Scattered disk and Oort cloud are names for three divisions of this volume of space. The planet Pluto and its moon Charon are trans-Neptunian objects, and if Pluto had been discovered today, it might not have been called a planet.

Volatiles: Matter which is readily passed off by evaporation.