About 5,000 orbits have been calculated, but 100,000 asteroids may exist.
Orbits are elliptical but nearly circular and near the plane of the ecliptic.
Their orbits are very similar to the planets' orbits.
The Kirkwood gaps are found in the orbits of belt asteroids and are formed by Jupiter's strong gravitational influence.
Orbits of other Asteroids
Asteroids: Orbital Characteristics
Found in the same orbit with Jupiter, but are 60o ahead and behind the planet.
They are stable positions in Jupiter's orbit where the gravity of the Sun and Jupiter cancel.
Such positions called Lagrange points.
Orbits cross Earth's orbit.
About 50 known Apollo asteroids, but may be as many as 1,000.
All potential "Earth-colliders".
Eros is an example. It is about 30 kilometers across.
Asteroids: Orbital Characteristics
Other asteroid groups
A few stray asteroids have been found that lie completely outside of the asteroid belt.
Chiron is the most famous example.
Its orbit carries it between Saturn and Uranus.
Chiron may actually be a dormant comet that has lost most of its volatiles.
When it is closest to the Sun, a very diffuse atmosphere forms around it.
If Chiron is a comet, it is the largest one known with a diameter of about 180 kilometers.
Chiron is unusual
has a detectable coma, indicating that it is a cometary body,
over 50,000 times the characteristic volume of a comet, a size more commensurate with a large asteroid
its curious orbit is unstable on time scales of a million years, indicating that it hasn't been in its present orbit long.
Mt. Wilson Observatory - California 02 April 1995 Charles Morris, TIE telescope
Chiron was the first of four bodies discovered so far with similar orbits and properties.
These bodies have been designated Centaurs, after the race of half-man/half-horse beings from Greek mythology, in recognition of their dual comet/asteroid nature.
It is believed that the Centaurs may be objects which have escaped from the Kuiper belt.
Distribution of asteroid semi-major axes shows some prominent gaps caused by resonances with Jupiter's orbital motion. These are known as the Kirkwood Gaps .
Asteroid in resonance with Jupiter receives a strong gravitational tug from planet each time they are close together. If asteroid's period is in a well-defined ratio with that of Jupiter’s, the effects reinforce each other.
Near-Earth and Trojan Asteroids
Although most asteroids orbit in the main belt, 2-3 A.U. from the Sun, an additional class of asteroids orbit at the distance of Jupiter and are called the Trojan asteroids.
Trojan asteroids are locked into a 1:1 orbital resonance with Jupiter.
Several hundred such asteroids are now known; it is estimated that there may be a thousand or more altogether.
Trojan Asteriods: Lagrange Points
There are exactly five places in the solar system where a small body can orbit the Sun in synchrony with Jupiter. These places are known as the Lagrange points of Jupiter's orbit.
All five Lagrange points revolve around the Sun at the same rate as Jupiter.
Near Earth Asteroids (NEAs)
Atens: semi-major axes <1.0 AU; aphelion distance >0.983 AU
Apollos:semi-major axes >1.0 AU; perihelion distance <1.017 AU
Amors: perihelion distances between 1.017 and 1.3 AU and only cross Mars’ orbit
NEAs are asteroids that closely approach the Earth.
Most are small, diameter < 1 km
Certain groups of asteroids have elliptical orbits that cross the orbit of Earth and other inner planets.
About 300 asteroids are known to cross Earth's orbit.
However, about 1500 unknown NEO’s are estimated to exist.
The good news is that none of the known asteroids will strike the Earth.
The bad news is that we have discovered only a fraction of the total number of Earth-crossing asteroids, so there are many for which we do not know the orbit.
Animation of Aten asteriod
Near Earth Asteroid Rendezvous
NEAR spacecraft launched in 1996.
fly by Mathilde, C-type in main belt
gravity measurements yield density of 1.6 g/cm3
match orbit with Eros, a C-type asteroid and largest of Earth-approaching asteroids
determine surface composition and density
Mathilde and Rhode Island
Close to Earth:Binary Asteroids
Binary asteroids -- two rocky objects orbiting about one another appear to be common in Earth-crossing orbits. If one is ever found headed our way, it could be double trouble.
composed of plasma; laced with rays and streamers caused by interactions with the solar wind.
Tails of bright comets can be 150 million kilometers (1 AU) in length, making them the "largest" objects in the Solar System.
Many comets have two tails:
gas tail (or ion tail) composed of ions blown out of the comet away from the Sun by the solar wind, and
dust tail composed of dust particles liberated from the nucleus as the ices are vaporized.
Comets interact gravitationally with the Sun and other objects.
Their motion is also influenced to some degree by gases jetting out of them, so their orbits are not completely determined by gravity.
Most comets’ orbits appear to be elliptical, or in some cases parabolic.
The most common comets are called short-period comets that have only mildly elliptical orbits that carry them out to a region lying from Jupiter to beyond the orbit of Neptune. These are normally seen only with telescopes.
Comets visible to the naked eye are rare and are thought to come from a great spherical cloud of cometary material surrounding the Solar System called the Oort Cloud.
English astronomer Edmund Halley used Newton's new theory of gravitation to determine the orbits of comets from their recorded positions in the sky as a function of time.
He found that the bright comets of 1531, 1607, and 1682 had the same orbits, and concluded that these were different appearances of the same comet.
He used his calculations to predict the return of this comet in 1758.
If one traces back in the historical records for recordings of bright comets and their positions in the sky, it can be concluded that Comet Halley has been observed periodically as far back as 240 B.C.
Halley in 1910
Halley in 1986
Comet Shoemaker-Levy 9
In July of 1994, fragments of Comet Shoemaker-Levy 9 impacted the planet Jupiter. The points of impact could be observed by the Galileo spacecraft.