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A quasi-satellite is an object in a specific type of
co-orbital configuration In astronomy, a co-orbital configuration is a configuration of two or more astronomical objects (such as asteroids, moons, or planets) orbiting at the same, or very similar, distance from their primary, i.e. they are in a 1:1 mean-motion resonance ...
(1:1
orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
) with a
planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
(or
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...
) where the object stays close to that planet over many orbital periods. A quasi-satellite's orbit around the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
takes the same time as the planet's, but has a different eccentricity (usually greater), as shown in the diagram. When viewed from the perspective of the planet by an observer facing the sun, the quasi-satellite will appear to travel in an oblong retrograde loop around the planet. . In contrast to ''true'' satellites, quasi-satellite orbits lie outside the planet's
Hill sphere The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sph ...
, and are unstable. Over time they tend to evolve to other types of resonant motion, where they no longer remain in the planet's neighborhood, then possibly later move back to a quasi-satellite orbit, etc. Other types of orbit in a 1:1 resonance with the planet include horseshoe orbits and tadpole orbits around the Lagrangian points, but objects in these orbits do not stay near the planet's longitude over many revolutions about the star. Objects in horseshoe orbits are known to sometimes periodically transfer to a relatively short-lived quasi-satellite orbit, and are sometimes confused with them. An example of such an object is . A quasi-satellite is similar to an object in a
distant retrograde orbit A distant retrograde orbit (DRO), as most commonly conceived, is a spacecraft orbit around a moon>M2>>M3. So DRO is a general three-body problem solution. It's just that most practical near-term uses for the concept at three-body problems in our S ...
, in a different context. The latter term is usually used for a space probe or artificial satellite in a retrograde orbit around a moon, and the period may be much shorter than that of the moon, whereas the term "quasi-satellite" usually refers to an object like an asteroid whose period is similar to that of the planet of which it is considered to be a quasi-satellite. But in both cases, the object (asteroid, space probe) viewed in a reference frame that rotates with the two main objects (once a year for Sun-Earth, once a month for Earth-Moon) appears to move retrograde compared to that rotation, thus lengthening its sidereal period. So a quasi-satellite (with low inclination) tends to stay in certain constellations rather than going through the whole zodiac. Quasi-satellites with high eccentricity can get quite far from their planet, more than an
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
for quasi-satellites of Earth such as . The word "geosynchronous" is sometimes used to describe quasi-satellites of the Earth, because their motion around the Sun is synchronized with Earth's. However, this usage is unconventional and confusing. Conventionally,
geosynchronous satellite A geosynchronous satellite is a satellite in geosynchronous orbit, with an orbital period the same as the Earth's rotation period. Such a satellite returns to the same position in the sky after each sidereal day, and over the course of a day tra ...
s revolve in the prograde sense around the Earth, with orbital periods that are synchronized to the Earth's rotation.


Examples


Venus

Venus has one known quasi-satellite, . This asteroid is also a Mercury- and
Earth-crosser An Earth-crosser is a near-Earth asteroid whose orbit crosses that of Earth as observed from the ecliptic pole of Earth's orbit. The known numbered Earth-crossers are listed here. Those Earth-crossers whose semi-major axes are smaller than E ...
; it seems to have been a "companion" to Venus for approximately the last 7000 years only, and is destined to be ejected from this orbital arrangement about 500 years from now.


Earth

As of 2016, Earth had five known quasi-satellites: * * * * * On the longer term, asteroids can transfer between quasi-satellite orbits and horseshoe orbits, which circulate around Lagrangian points L4 and L5. By 2016, orbital calculations showed that all five of Earth's then known quasi-satellites repeatedly transfer between horseshoe and quasi-satellite orbits. 3753 Cruithne, , and are minor planets in horseshoe orbits that might evolve into a quasi-satellite orbit. The time spent in the quasi-satellite phase differs from asteroid to asteroid. Quasi-satellite is predicted to be stable in this orbital state for several hundred years, in contrast to which was a quasi-satellite from 1996 to 2006 but then departed Earth's vicinity on a horseshoe orbit.
469219 Kamoʻoalewa 469219 Kamoʻoalewa (), provisionally designated , is a very small asteroid, fast rotator and near-Earth object of the Apollo group, approximately in diameter. At present it is a quasi-satellite of Earth, and currently the smallest, closest, ...
() is thought to be one of the most stable quasi-satellites found yet of Earth. It stays between 38 and 100  lunar distances from the Earth.


Ceres

The dwarf-planet asteroid 1 Ceres is believed to have a quasi-satellite, the as-yet-unnamed .


Neptune

is a temporary quasi-satellite of Neptune. The object has been a quasi-satellite of Neptune for about 12,500 years and it will remain in that dynamical state for another 12,500 years.


Other planets

Based on simulations, it is believed that
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus ( Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of ...
and Neptune could potentially hold quasi-satellites for the age of the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
(about 4.5 billion years), but a quasi-satellite's orbit would remain stable for only 10 million years near
Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
and 100,000 years near Saturn. Jupiter and Saturn are known to have quasi-satellites. , a co-orbital to Jupiter, intermittently becomes a quasi satellite of the planet, and will next become one between 2380 and 2480.


Artificial quasi-satellites

In early 1989, the Soviet ''
Phobos 2 ''Phobos 2'' was the last space probe designed by the Soviet Union. It was designed to explore the moons of Mars, Phobos and Deimos. It was launched on 12 July 1988, and entered orbit on 29 January 1989. ''Phobos 2'' operated nominally throug ...
'' spacecraft was injected into a quasi-satellite orbit around the Martian moon Phobos, with a mean orbital radius of about from Phobos. According to computations, it could have then stayed trapped in the vicinity of Phobos for many months. The spacecraft was lost due to a malfunction of the on-board control system.


Accidental quasi-satellites

Some objects are known to be accidental quasi-satellites, which means that they are not forced into the configuration by the gravitational influence of the body of which they are quasi-satellites. The
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...
s
Ceres Ceres most commonly refers to: * Ceres (dwarf planet), the largest asteroid * Ceres (mythology), the Roman goddess of agriculture Ceres may also refer to: Places Brazil * Ceres, Goiás, Brazil * Ceres Microregion, in north-central Goiás ...
and
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...
are known to have accidental quasi-satellites. In the case of Pluto, the known accidental quasi-satellite,
15810 Arawn 15810 Arawn, provisional designation , is a trans-Neptunian object (TNO) from the inner regions of the Kuiper belt, approximately in diameter. It belongs to the plutinos, the largest class of resonant TNOs. It was named after Arawn, the ruler of ...
, is, like Pluto, a
plutino In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the l ...
, and is forced into this configuration by the gravitational influence of Neptune. This dynamical behavior is recurrent, Arawn becomes a quasi-satellite of Pluto every 2.4 Myr and remains in that configuration for nearly 350,000 years.


See also

*
Artificial satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisoto ...
*
Natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
*
Temporary satellite A temporary satellite is an object which has been captured by the gravitational field of a planet and thus has become the planet's natural satellite, but, unlike irregular moons of the larger outer planets of the Solar System, will eventually either ...
*
Satellite system (astronomy) A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object (incl. sub-brown dwarfs and rogue planets) or minor planet, or its barycenter. Generally speaking, it is a set of natural satellites (moons), alt ...


References


External links


Quasi-satellite Information PageAstronomy.com: A new "moon" for EarthDiscovery of the first quasi-satellite of Venus
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University of Turku sv, Åbo universitet , latin_name = Universitas Aboensis , image_name = University of Turku.svg , motto = ''Vapaan kansan lahja vapaalle tieteelle'' , established = 1920 , type ...
news release (August 17, 2004) {{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System Moons