Quasi-satellite
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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 resonanc ...
(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 relation ...
) with a
planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...
(or
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
) 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 centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
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 is a common model for the calculation of a Sphere of influence (astrodynamics), gravitational sphere of influence. It is the most commonly used model to calculate the spatial extent of gravitational influence of an astronomical ...
, 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 point In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium (mechanics), equilibrium for small-mass objects under the gravity, gravitational influence of two massive orbit, orbiting b ...
s, 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, 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 AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...
for quasi-satellites of
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
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 satellites 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, 524522 Zoozve. This asteroid is also a Mercury- and Earth-crosser; it seems to have been a "companion" to Venus for approximately the last 7,000 years only, and is destined to be ejected from this orbital arrangement about 500 years from now.


Earth

As of 2023, Earth had seven known quasi-satellites: * 164207 Cardea * * * * * * On the longer term, asteroids can transfer between quasi-satellite orbits and horseshoe orbits, which circulate around
Lagrangian point In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium (mechanics), equilibrium for small-mass objects under the gravity, gravitational influence of two massive orbit, orbiting b ...
s 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 () 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 Ceres ( minor-planet designation: 1 Ceres) is a dwarf planet in the middle main asteroid belt between the orbits of Mars and Jupiter. It was the first known asteroid, discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical O ...
is believed to have a quasi-satellite, the as-yet-unnamed .


Neptune

is a temporary quasi-satellite of
Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...
. 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. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a Supercritical fluid, supercritical phase of matter, which astronomy calls "ice" or Volatile ( ...
and
Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...
could potentially hold quasi-satellites for up to the age of the
Solar System The Solar SystemCapitalization 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 "Sola ...
(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 List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...
and 100,000 years near
Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...
. 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'' 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. In 2005, aerospace engineer Thomas Gangale proposed a quasi-satellite orbit for spacecraft to provide communications relays between Earth and crews on Mars when Mars is in solar conjunction and direct communication is blocked by the Sun for several weeks.


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 around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
s Ceres and
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...
are known to have accidental quasi-satellites. In the case of Pluto, the known accidental quasi-satellite, 15810 Arawn, 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 la ...
, and is forced into this configuration by the gravitational influence of Neptune. This dynamical behavior is recurrent where Arawn becomes a quasi-satellite of Pluto every 2.4 Myr and remains in that configuration for nearly 350,000 years.


See also

* * * *


References


External links


Quasi-satellite Information PageAstronomy.com: A new "moon" for EarthDiscovery of the first quasi-satellite of Venus
nbsp;– University of Turku news release (August 17, 2004) {{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System Moons