![Asteroid 2004 FH](https://upload.wikimedia.org/wikipedia/commons/0/0a/Asteroid_2004_FH.gif)
An Earth-crosser is a
near-Earth asteroid whose
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
crosses that of
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
as observed from the
ecliptic pole
An orbital pole is either point at the ends of an imaginary line segment that runs through the center of an orbit (of a revolving body like a planet, moon or satellite) and is perpendicular to the orbital plane. Projected onto the celestial sphe ...
of Earth's orbit. The known numbered Earth-crossers are listed here. Those Earth-crossers whose
semi-major axes are smaller than Earth's are
Aten asteroid
The Aten asteroids are a dynamical group of asteroids whose orbits bring them into proximity with Earth. By definition, Atens are Earth-crossing asteroids . The group is named after 2062 Aten, the first of its kind, discovered on 7 January ...
s; the remaining ones are
Apollo asteroid
The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, discovered by German astronomer Karl Reinmuth in the 1930s. They are Earth-crossing asteroids that have an orbital semi-major axis greater than that of the Earth ...
s. ''(See also the
Amor asteroid
The Amor asteroids are a group of near-Earth asteroids named after the archetype object 1221 Amor . The orbital perihelion of these objects is close to, but greater than, the orbital aphelion of Earth (i.e., the objects do not cross Earth's orb ...
s.)''
An asteroid with an Earth-crossing orbit is not necessarily in danger of colliding with Earth. The orbit of an Earth-crossing asteroid may not even intersect with that of Earth. This apparent contradiction arises because many asteroids have highly inclined orbits, so although they may have a
perihelion
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ellip ...
less than that of Earth, their paths can never cross. An asteroid for which there is some possibility of a collision with Earth at a future date and which is above a certain size is classified as a
potentially hazardous asteroid
A potentially hazardous object (PHO) is a near-Earth object – either an asteroid or a comet – with an orbit that can make close approaches to the Earth and is large enough to cause significant regional damage in the event of impact. They are ...
(PHA). Specifically, an asteroid is a PHA if its Earth
minimum orbital intersection distance
Minimum orbit intersection distance (MOID) is a measure used in astronomy to assess potential close approaches and collision risks between astronomical objects. It is defined as the distance between the closest points of the osculating orbits o ...
(MOID) is <0.05
AU and its
absolute magnitude
Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...
is 22 or brighter. The concept of PHA is intended to replace the now abandoned strict definition of ECA (Earth-crossing asteroid) which existed for a few years. Determining if an asteroid was an ECA required calculation of its orbits millennia into the future, including planetary gravitational
perturbations, to assess whether a collision with Earth was possible and this has proved to be impractical.
Having a small MOID is not a guarantee of a collision. On the other hand, small gravitational perturbations of the asteroid around its orbit from planets that it passes can significantly alter its path. For instance,
99942 Apophis
99942 Apophis is a near-Earth asteroid and potentially hazardous asteroid with a diameter of that caused a brief period of concern in December 2004 when initial observations indicated a probability up to 2.7% that it would hit Earth on April&nb ...
will approach Earth so closely in 2029 that it will get under the orbit of the Earth's geostationary satellites.
[Dr. Tony Phillips, "Friday the 13th, 2029", ''Science@NASA'']
retrieved online
May 3, 2009.
Of the Earth-crossing asteroids,
3753 Cruithne is notable for having an orbit that has the same period as Earth's.
List
''Notes:'' † outer-grazer
See also
*
Earth-grazing fireball
An Earth-grazing fireball (or Earth grazer) is a fireball, a very bright meteor that enters Earth’s atmosphere and leaves again. Some fragments may impact Earth as meteorites, if the meteor starts to break up or explodes in mid-air. These phenom ...
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List of asteroid close approaches to Earth
This is a list of examples where an asteroid or meteoroid travels close to the Earth. Some are regarded as potentially hazardous objects if they are estimated to be large enough to cause regional devastation.
Near-Earth object detection techno ...
*
Asteroid impact avoidance
Asteroid impact avoidance comprises the methods by which near-Earth objects (NEO) on a potential collision course with Earth could be diverted away, preventing destructive impact events. An impact by a sufficiently large asteroid or other NEOs ...
*
List of Mercury-crossing minor planets
A Mercury crosser is an asteroid whose orbit crosses that of Mercury. The Mercury crossers proper have aphelia outside Mercury's (0.4667 AU) and perihelia inside Mercury's (0.3075 AU), whereas those listed here as outer grazers have perihelia wit ...
*
*
List of Mars-crossing minor planets
A Mars-crossing asteroid (MCA, also Mars-crosser, MC) is an asteroid whose orbit crosses that of Mars. Some Mars-crossers numbered below 100000 are listed here. They include the two numbered Mars trojans 5261 Eureka and .
Many databases, for ...
*
List of Jupiter-crossing minor planets
A Jupiter-crosser is a minor planet whose orbit crosses that of Jupiter. Jupiter trojans can be inner grazers (105), outer grazers (52), co-orbitals (183), and crossers (537). Discounting them, there is one numbered, 7 non-numbered, and 19 cometar ...
*
List of Saturn-crossing minor planets
A Saturn-crosser is a minor planet whose orbit crosses that of Saturn. The known numbered Saturn-crossers (as of 2005) are listed below. There is only one inner-grazer (944 Hidalgo) and no outer-grazers or co-orbitals known; most if not all of the ...
*
List of Uranus-crossing minor planets
A Uranus-crosser is a minor planet whose orbit crosses that of Uranus. The numbered Uranus-crossers (as of 2005) are listed below. Most, if not all, are centaurs.
Notes: † inner-grazer; ‡ outer-grazer
* 2060 Chiron †
* 5145 Pholus
* 5335 ...
*
List of Neptune-crossing minor planets
A Neptune-crosser is a minor planet whose orbit crosses that of Neptune. The dwarf planet Pluto is the most massive example of this class of object. The known numbered Neptune-crossers (as of 2005) are:
''Notes'': ‡ outer-grazer
† 134340 P ...
References
External links
Very Close Approaches (<0.01 AU) of PHAs to Earth 1900–2200Upcoming Close Approaches (<0.10 AU) of Near-Earth Objects to Earth-Sormano Astronomical Observatory
-Sormano Astronomical Observatory
-Sormano Astronomical Observatory
-Minor Planet Center
{{DEFAULTSORT:List Of Earth-Crossing Minor Planets
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Earth-crossing
Earth-crossing
Planetary defense