An Earth-crosser is a near-Earth asteroid whose orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
crosses that of Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
as observed from the ecliptic pole
The ecliptic pole is the point on the celestial sphere where the sphere meets the imaginary line perpendicular to the ecliptic plane, the path the Earth travels on its orbit around the Sun.There are two ecliptic poles:...
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 group of near-Earth asteroids, named after the first of the group to be discovered . They are defined by having semi-major axes of less than one astronomical unit...
s; the remaining ones are Apollo asteroid
The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, the first asteroid of this group to be discovered by Karl Wilhelm Reinmuth...
s. See also the Amor asteroid
The Amor asteroids are a group of near-Earth asteroids named after the asteroid 1221 Amor. They approach the orbit of the Earth from beyond, but do not cross it. Most Amors do cross the orbit of Mars...
An asteroid with an Earth-crossing orbit is not necessarily in danger of colliding with the Earth. The orbit of an Earth-crossing asteroid may not even intersect with the orbit of the Earth. This apparent contradiction arises because many asteroids have highly inclined orbits, so although they may have a perihelion less than that of the Earth, their paths can never cross. An asteroid for which there is some possibility of a collision with the Earth at a future date and which is above a certain size is classified as a Potentially Hazardous Asteroid (PHA). Specifically, an asteroid is a PHA if its Earth Minimum Orbital Intersection Distance (MOID) is <0.05 AU
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
and its absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
is 22 or less. 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 peturbations, 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 peturbations 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 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. Additional observations provided improved predictions that eliminated the possibility of an impact on...
will approach the Earth so closely in 2029 that it will get under the orbit of the Earth's geostationary satellites. The Earth will change the trajectory of Apophis and the result may be an even closer approach in the future, possibly 2036. It is not currently possible to accurately predict the path of Apophis subsequent to the 2029 encounter because its present orbit is not yet known to a sufficiently high precision — very small differences prior to the planetary encounter can produce large differences in orbit after the encounter.
Of the Earth-crossing asteroids, 3753 Cruithne
3753 Cruithne is an asteroid in orbit around the Sun in approximate 1:1 orbital resonance with the Earth. It is a periodic inclusion planetoid orbiting the Sun in an apparent horseshoe orbit. It has been incorrectly called "Earth's second moon", but it is only a quasi-satellite. Cruithne never...
is notable for having an orbit that has the same period as Earth's.
ListNotes: † outer-grazer
- Earth-grazing fireballEarth-grazing fireballAn Earth-grazing fireball is a small solar system body that enters the Earth's atmosphere and leaves again. If it starts to break up in the atmosphere it can become an Earth-grazing meteor procession, and some fragments may impact the Earth...
- Asteroid deflection strategiesAsteroid deflection strategiesAsteroid mitigation strategies are "planetary defense" methods by which near-Earth objects could be diverted, preventing potentially catastrophic impact events. A sufficiently large impact would cause massive tsunamis or an impact winter, or both...
- List of Mercury-crossing minor planets
- List of Venus-crossing minor planets
- List of Mars-crossing minor planets
- List of Jupiter-crossing minor planets
- List of Saturn-crossing minor planets
- List of Uranus-crossing minor planets
- List of Neptune-crossing minor planets
- Very Close Approaches (<0.01 A.U.) of PHAs to Earth 1800–2200
- Upcoming Close Approaches (<0.10 A.U.) of Near-Earth Objects to Earth
- Table of next close approaches to the Earth