(65803) Didymos I Dimorphos (

provisional designation Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been cal ...

S/2003 (65803) 1) is a

minor-planet moon A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. , there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are importan ...

of the near-Earth asteroid 65803 Didymos, with which it forms a

binary system A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that th ...

. It has a diameter of and has been characterised as a low-density

rubble pile In astronomy, a rubble pile is a celestial body that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the ...

. Discovered in 2003 at the

Ondřejov Observatory The Ondřejov Observatory (; cs, Observatoř Ondřejov) is the principal observatory of the Astronomical Institute () of the Academy of Sciences of the Czech Republic. It is located near the village of Ondřejov, southeast of Prague, Czech ...

, Dimorphos was the target of the

Double Asteroid Redirection Test Double Asteroid Redirection Test (DART) is a NASA space mission aimed at testing a method of planetary defense against near-Earth objects (NEOs). It was designed to assess how much a spacecraft impact deflects an asteroid through its transfe ...

(DART), a

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...

space mission that deliberately collided a spacecraft with the moon in September 2022 to alter its orbit around Didymos. The

ESA , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (120 ...

mission '' Hera'' is planned to arrive at the Didymos system in 2026 to further study the effects of the collision on the moon.

Nomenclature

The Working Group for Small Bodies Nomenclature of the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

(IAU) gave the satellite its official name on 23 June 2020. The name ''Dimorphos'' is derived from a Greek word () meaning 'having two forms'. The justification for the new name reads: "As the target of the DART and ''Hera'' space missions, it will become the first celestial body in cosmic history whose form was substantially changed as a result of human intervention (the DART impact)". Prior to the IAU naming, the nickname ''Didymoon'' was used in official communications.

Characteristics

The primary body of the binary system, Didymos, orbits the Sun at a distance of 1.0 to 2.3 AU once every 770 days (2 years and 1 month). The pathway of the orbit has an

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...

of 0.38 and an

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...

of 3° with respect to the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...

. On 4 October 2022 Didymos made an Earth approach of . Dimorphos moves in a nearly equatorial, nearly circular orbit around Didymos, with an

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

of 11.9 hours. Its orbit period is synchronous with its rotation, so that the same side of Dimorphos always faces Didymos. Dimorphos's orbit is retrograde relative to the

plane, in conformity with Didymos's retrograde rotation. The final few minutes of pictures from the DART mission revealed an egg-shaped body covered with boulders, suggesting it has a

structure. Dimorphos is around in diameter, compared to Didymos at . Dimorphos does not have a confirmed mass, but it is estimated to be about (5.5 million tons), or about the same mass and size as the

Great Pyramid of Giza The Great Pyramid of Giza is the biggest Egyptian pyramid and the tomb of Fourth Dynasty pharaoh Khufu. Built in the early 26th century BC during a period of around 27 years, the pyramid is the oldest of the Seven Wonders of the Ancient Worl ...

, when assuming a density of similar to Didymos. It is currently the smallest celestial object given a formal name by the IAU.

Observation

The primary asteroid Didymos was discovered in 1996 by Joe Montani of the Spacewatch Project at the University of Arizona. The satellite Dimorphos was discovered on 20 November 2003, in photometric observations by

Petr Pravec Petr Pravec (born September 17, 1967) is a Czech astronomer and a discoverer of minor planets, born in Třinec, Czech Republic. Pravec is a prolific discoverer of binary asteroids, expert in photometric observations and rotational lightcurves ...

and colleagues at the

in the Czech Republic. Dimorphos was detected through periodic dips in Didymos's brightness due to mutual eclipses and occultations. With his collaborators, he confirmed from the Arecibo radar delay-Doppler images that Didymos is a binary system.

Exploration

On 24 November 2021,

and the

Applied Physics Laboratory The Johns Hopkins University Applied Physics Laboratory (Applied Physics Laboratory, or APL) is a not-for-profit university-affiliated research center (UARC) in Howard County, Maryland. It is affiliated with Johns Hopkins University and emplo ...

launched an impactor spacecraft towards Dimorphos as part of their

(DART). DART was the first experiment conducted in space to test asteroid deflection as a method of defending Earth from

potentially hazardous asteroids 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 ar ...

. Following a ten-month journey to the Didymos system, the impactor collided with Dimorphos on 26 September 2022 at a speed of around . The collision successfully decreased Dimorphos's

around Didymos by minutes. Fifteen days prior to its collision, the impactor released

LICIACube Light Italian CubeSat for Imaging of Asteroids (LICIACube, ) is a six-unit CubeSat of the Italian Space Agency (ASI). LICIACube is a part of the Double Asteroid Redirection Test (DART) mission and carries out observational analysis of the Didy ...

, a 6U

CubeSat A CubeSat is a class of miniaturized satellite based around a form factor consisting of cubes. CubeSats have a mass of no more than per unit, and often use commercial off-the-shelf (COTS) components for their electronics and structure. CubeSats ...

operated by the

Italian Space Agency The Italian Space Agency ( it, Agenzia Spaziale Italiana; ASI) is a government agency established in 1988 to fund, regulate and coordinate space exploration activities in Italy. The agency cooperates with numerous national and international enti ...

that photographed the impact and the resulting dust plume as it performed a close flyby of the Didymos system. Spacecraft and observatories such as Hubble, James Webb, ''

Lucy Lucy is an English feminine given name derived from the Latin masculine given name Lucius with the meaning ''as of light'' (''born at dawn or daylight'', maybe also ''shiny'', or ''of light complexion''). Alternative spellings are Luci, Luce, Lu ...

'', SAAO and

ATLAS An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...

also captured the dust plume trailing the Didymos system in the days following the impact. As part of its Hera mission,

currently plans to launch three spacecraft to the Didymos system in 2024 to further study the aftermath of the impact.

Footnotes

References

External links

* {{Solar System moons (compact) Asteroid satellites Named minor planets Radar-imaged asteroids Active asteroids Astronomical objects discovered in 2003 Minor planets visited by spacecraft