The
Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered (
2062 Aten2062 Aten is an asteroid that was discovered at the Palomar Mountain Observatory by Eleanor F. Helin, who is now the principal scientist for the NEAT project. It is named after Aten, the Egyptian god of the sun....
, discovered January 7, 1976 by
Eleanor F. HelinEleanor Francis "Glo" Helin was an American astronomer. She was principal investigator of the Near Earth Asteroid Tracking program of NASA's Jet Propulsion Laboratory. Some sources give her name as Eleanor Kay Helin...
). They are defined by having
semi-major axesIn geometry, the semi-major axis is used to describe the dimensions of ellipses and hyperbolae.- Ellipse :The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
of less than one
astronomical unitAn astronomical unit is a unit of length roughly equal to the mean distance between the Earth and the Sun. It is approximately 150 million kilometres ....
(the distance from the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
to the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
). Note that, because asteroids' orbits can be highly eccentric, an Aten orbit need not be entirely contained within Earth's orbit; in fact, nearly all known Aten asteroids have their aphelion greater than one
AUAn astronomical unit is a unit of length roughly equal to the mean distance between the Earth and the Sun. It is approximately 150 million kilometres ....
. Those that have their aphelion entirely within the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's orbit are known as
Apohele asteroidApohele asteroids are a subclass of Aten asteroids. They have not only their perihelion at less than one AU , but also their aphelion; that is, their entire orbit is within Earth's....
s. As of March 2008 there are only five known Apoheles ((163693) Atira, , , and ) and four suspected Apoheles .
The smallest semi-major axis is that of , at 0.642 AU (its eccentricity of 0.688 takes it from a perihelion of 0.200 AU, well within
MercuryFor the liquid metallic element, see Mercury .Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three...
's orbit, to an aphelion of 1.084 AU), although seems to have an even smaller one (0.635 AU; eccentricity 0.532 ranging from 0.297 to 0.973 AU —enough to cross
VenusVenus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6...
' orbit but not Mercury's).
For a brief time near the end of 2004, the asteroid
99942 Apophis99942 Apophis is a near-Earth asteroid that caused a brief period of concern in December 2004 because initial observations indicated a small probability that it would strike the Earth in 2029...
(then known only by its provisional designation ) appeared to pose a threat of causing an Earth
impact eventAn impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Impact events have been a plot and background element in science fiction since knowledge of real impacts became established in the scientific mainstream.-Sizes and...
in 2029, but earlier observations were found that eliminated that possibility, although a very small possibility remains for 2036.
See also
Aten asteroids (category)
External links