IAU approves name of target of first NASA and ESA planetary defence missions

Asteroid Didymos’ moon receives its new name just over a year before deflection mission launch


Credit: NASA/Johns Hopkins APL/Steve Gribben

The International Astronomical Union has just approved an official name for a tiny asteroid satellite set to become the first-ever target of an asteroid deflection mission. The satellite is the smaller of two bodies in the near-Earth asteroid system Didymos, and will now be distinguished from its primary object by the name Dimorphos.

In July 2021, just over a year from now, NASA will launch the Double Asteroid Redirection Test (DART) mission. This first full-scale demonstration of asteroid deflection technology will target the smaller body in the binary asteroid system known as Didymos, and will be followed in 2024 by the ESA space probe Hera. In recognition of the asteroid moon’s significance in these pioneering missions, it has been given an official name: Dimorphos.

Together, the DART and Hera missions, and the international research collaboration known as the Asteroid Impact and Deflection Assessment (AIDA), will demonstrate deflection technology that could be used to protect Earth from hazardous asteroids by shunting them off a collision course. Didymos does not pose a risk of striking Earth, and is the most easily reachable asteroid system of its size, with a primary body about 780 metres in diameter. The smaller body is about 160 metres in diameter — roughly the size of the Great Pyramid in Egypt.

First spotted in 1996 by Joe Montani of the Spacewatch Project at the University of Arizona, the asteroid system’s binary nature was not discovered until 2003, when Petr Pravec of the Ondřejov Observatory in the Czech Republic found the characteristic signs of a binary system in his observations. It was at this point that it received its official name. As the original discoverer, Montani suggested the system be named Didymos, meaning twin in Greek, to reflect its composition, a name which the IAU quickly approved.

While the larger body and the system as a whole have gone by the name Didymos since then, the smaller body has been referred to by many names, including the provisional designation S/2003 (65803) 1 and the nicknames Didymos B and Didymoon, but was never given its own official name. When it was identified as an ideal target for AIDA, the DART and Hera teams decided to seek a permanent designation and an official name by which it could be distinguished from its larger companion.

The body that officially approves the names of minor planets and their satellites is the IAU Working Group Small Body Nomenclature (WGSBN), under the IAU Division F Planetary Systems and Astrobiology [1]. The WGSBN received the proposal to name the satellite Dimorphos, with the following citation: “Dimorphos, Greek for ‘having two forms’, is the smaller member of the (65803) Didymos system. 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).” The WGSBN accepted the proposal, and the object obtained its final designation as (65803) Didymos I = Dimorphos.

The name of the moon was suggested by Kleomenis Tsiganis, a planetary scientist at the Aristotle University of Thessaloniki and a member of both the DART and Hera teams. He explains that the name Dimorphos “has been chosen in anticipation of its changes. It will be known to us in two very different forms, the one seen by DART before the impact, and the other seen by Hera a few years later.

At present there are 546 077 numbered minor planets, of which 22 129 have official names [2]. The catalogue of minor planets and comets is maintained by the Minor Planet Center (MPC) [3], a service of the IAU. Most of the minor planets belong to the Main Asteroid Belt, in the region between the orbits of Mars and Jupiter. There are 22 735 classified as Near Earth Asteroids (NEA), many of which have orbits that approach the Earth. Didymos belongs to the NEA group. In the minor planet catalogue there are also many examples from the outer regions of the Solar System, objects known as Centaurs and Trans-Neptunian Objects (TNOs). Many minor planets have companions; 390 binaries and 15 triple systems have so far been discovered [4]. The binary comprising Didymos and Dimorphos has become the 26th system with approved names for its components. Dimorphos, with a size of 160 metres in diameter, is also one of the smallest objects to get a permanent name.

After its launch next year, DART is scheduled to reach Dimorphos in 2022, deliberately colliding with it and creating a kinetic impact intended to alter the satellite’s trajectory. The ESA space probe Hera will be launched two years later and is scheduled to arrive at Dimorphos in 2027, where it will perform a close-up survey to assess the effects of the DART impact on the satellite’s form and orbit.

The DART impact will be recorded by the LICIACube CubeSat, provided by the Italian Space Agency, which will be carried to Didymos by DART and deployed a few days before the collision. Longer-term effects will be studied by telescopes here on Earth and in space. The Hera mission will also deploy two CubeSats, including the Juventas CubeSat, which will use a low-frequency radar to scan the interior structure of Dimorphos, the first such scan to be performed by a spacecraft. The results of Hera’s detailed investigation will be compared with the observations recorded by DART before the collision, providing important insights into the effects of the impact.

G. Tancredi, president of IAU Div. F and external member of the DART and Hera investigation teams, remarks that: “The IAU has been closely following the development of the research on the Near-Earth Objects and their threat for life on Earth. The IAU Working Group Near Earth Objects (WGNEO) was formed in the early 1990s to coordinate the NEO studies and to provide timely advice on any objects that threaten collision with the Earth.” [5]


[1] The IAU Division F Planetary Systems and Astrobiology deals with our Solar System, extrasolar planetary systems, and bioastronomy. It has 2370 members distributed all around the world. The President for the Division in the period 2018-2021 is Gonzalo Tancredi. The Division F WG Small Bodies Nomenclature (WGSBN) has responsibility for the naming and designation of small bodies (except satellites of major planets) in the Solar System. It covers the naming of minor planets including Near Earth Asteroids and Trans-Neptunian Objects, comets, dwarf planets and satellites of minor planets. The WGSBN chair in the period 2018-2021 is Jana Tichá and the Vice-Chair is Keith Noll. The Division F WG Near Earth Objects is chaired by Patrick Michel who is one of the leading scientists of the ESA’s Hera mission.

[2] Data taken from the MPC Orbit (MPCORB) Database.

[3] The Minor Planet Center (MPC) operates at the Smithsonian Astrophysical Observatory, under the auspices of Division F of the International Astronomical Union (IAU). The MPC’s operating funds come from a NASA Near-Earth Object Observations programme grant.

[4] Data from the webpage: Asteroids with Satellites, by Wm. Robert Johnston.

[5] The IAU Symposium 374: Astronomical Hazards for Life on Earth will be held during the next IAU General Assembly in Busan in 2021 and will be an opportunity to address this relevant topic for the future of humanity.

More information

The IAU is the international astronomical organisation that brings together more than 14 000 professional astronomers from more than 100 countries worldwide. Its mission is to promote and safeguard astronomy in all its aspects, including research, communication, education and development, through international cooperation. The IAU also serves as the internationally recognised authority for assigning designations to celestial bodies and the surface features on them. Founded in 1919, the IAU is the world’s largest professional body for astronomers.


  • MPEC from MPC
  • NASA press release
  • ESA Hera

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