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Asteroid Terms: Explained

Asteroid Terms: Explained

There are interesting asteroid characters in our solar system, including an asteroid that has its own moon and even one that is shaped like a dog bone! Our OSIRIS-REx mission launches at 7:05 p.m. EDT today and will travel to asteroid Bennu.


Scientists chose Bennu as the target of the OSIRIS-REx mission because of its composition, size and proximity to Earth. Bennu is a rare B-type asteroid (primitive and carbon-rich), which is expected to have organic compounds and water-bearing minerals like clays.

Our OSIRIS-REx mission will travel to Bennu and bring a small sample back to Earth for study.


When talking about asteroids, there are some terms scientists use that might not be in your typical vocabulary…but we’ll help with that!

Here are a few terms you should know:

Orbital Eccentricity: This number describes the shape of an asteroid’s orbit by how elliptical it is. For asteroids in orbit around the sun, eccentricity is a number between 0 and 1, with 0 being a perfectly circular orbit and 0.99 being a highly elliptical orbit.

Inclination: The angle, in degrees, of how tilted an asteroid’s orbit is compared to another plane of reference, usually the plane of the Earth’s orbit around the sun.

Orbital Period: The number of days it takes for an asteroid to revolve once around the sun. For example, the Earth’s orbital period is 365 days.

Perihelion Distance: The distance between an asteroid and the sun when the asteroid is closest to the sun.

Aphelion Distance: The distance between the asteroid and the sun when the asteroid is farthest away from the sun.

Astronomical unit: A distance unit commonly used to describe orbits of objects around the sun. The distance from the Earth to the sun is one astronomical unit, or 1 AU, equivalent to about 93 million miles or 150 million kilometers.

Diameter: A measure of the size of an asteroid. It is the length of a line from a point on the surface, through the center of the asteroid, extending out to the opposite surface. Irregularly shaped asteroids may have different diameters depending on which direction they are measured.

Rotation Period: The time it takes for an asteroid to complete one revolution around its axis of rotation. For example, the rotation period of the Earth is approximately 24 hours, or 1 day.

Spectral Type: The classification of an asteroid, based on a measurement of the light reflected by the asteroid. 

Asteroid Terms: Explained

Watch live launch coverage of OSIRIS-REx to asteroid Bennu starting at 5:30 p.m, on NASA TV: 

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