65803 Didymos

Discovery

Didymos was discovered on 11 April 1996 by the University of Arizona Steward Observatory's, and Lunar and Planetary Laboratory's, Spacewatch survey using its 0.9-meter telescope at Kitt Peak National Observatory in Arizona, United States. The binary nature of the asteroid was discovered by others; suspicions of binarity first arose in Goldstone delay-Doppler echoes, and these were confirmed with an optical lightcurve analysis, along with Arecibo radar imaging on 23 November 2003.

Orbital characteristics

Didymos orbits the Sun at a distance of 1.0–2.3 AU once every 770 days (2 years and 1 month). Its orbit has an eccentricity of 0.38 and an inclination of 3° with respect to the ecliptic. The minimum distance between the orbit of Earth and the orbit of Didymos is currently 0.04 AU, but will change as the asteroid is perturbed. In November 2003 it passed 7.18 million km from Earth; it will not come that near again until November 2123, with a distance of 5.86 million km. Didymos also occasionally passes very close to Mars: it will fly by Mars at a distance of 4.68 million km in July 2144. The Earth approach of October 2184, at a distance of 8.6 million km, is listed with an uncertainty region of roughly ±.

Didymos spends one third of its time orbiting far enough from the Sun to cross the inner parts of the main asteroid belt, where collisions with other asteroids are more probable. This means that about every 73–84 thousand years, an object impacts Didymos with the energy of the DART mission satellite. Over its median NEA lifetime of 8 to 10 million years, Didymos probably has been hit several tens of times.

Physical characteristics

In the SMASS classification, Didymos was classified as an Xk-type asteroid, which transitions from the X-type to the rare K-type asteroids. Subsequent visible and near-infrared spectroscopy showed it to be silicate in nature, which also qualifies it as a stony S-type asteroid. It rotates rapidly, with a period of 2.26 hours and a low brightness variation of 0.08 magnitude (), which indicates that the body has a nearly spheroidal shape. Radar observations confirmed this spheroidal shape, showing it to be oblate due to its rapid rotation.

Satellite

Main article: Dimorphos

Didymos is a binary asteroid with a satellite in its orbit. The minor-planet moon, named Dimorphos, moves in a mostly circular retrograde orbit with an orbital period of 11.9 hours. It measures approximately 160 m in diameter compared to 780 m for its primary (a mean diameter-ratio of 0.22). It was previously known by its provisional designation and had been informally called "Didymoon" or "Didymos B".

Naming

This minor planet was named "Didymos", Greek for "twin", due to its binary nature. The name was suggested by the discoverer, University of Arizona Lunar and Planetary Laboratory astronomer Joseph L. "Joe" Montani, who made the naming proposal to the International Astronomical Union after the binary nature of the object was detected. The approved naming citation was published on 13 July 2004 (M.P.C. 52326).

The proper name for the satellite Didymos B comes from the word "Dimorphos", Greek for "having two forms". The meaning of the name represents how the form of Dimorphos's orbit will change after the collision with NASA's Double Asteroid Redirection Test (DART) spacecraft, though in fact the change will be only a very slight change in its orbital parameters. Appropriately, Dimorphos serves dual roles as both a test target and as a part of a blueprint for a modality for future planetary protection. The name of the moon was suggested by planetary scientist Kleomenis Tsiganis at the Aristotle University of Thessaloniki, Greece.

Two boulders (saxa) have been given names of traditional drums.

Exploration

In the early 2010s, Didymos moon, Dimorphos was to be the principal target of proposed robotic mission by the ESA and NASA, called the Asteroid Impact & Deflection Assessment (AIDA) mission. The ESA dropped out, and the mission did not proceed.

NASA redefined mission requirements and decided to proceed with a 2020s mission to visit Didymos with an impactor, which had been considered as a part of the earlier AIDA mission, named the Double Asteroid Redirection Test or DART. The NASA mission was intended to test whether a spacecraft impact could successfully deflect an asteroid on a collision course with Earth. The DART spacecraft was launched on 24 November 2021, and impacted Dimorphos on September 26, 2022. It was accompanied by the Italian Space Agency's (ASI) six-unit LICIACube flyby Cubesat that was released 15 days before impact to observe the asteroid and DART's impact.

DART was the first spacecraft to intentionally target and successfully visit an asteroid known to have a minor-planet moon (The binary asteroid was targeted by the PROCYON mission before it failed, 243 Ida was visited by the Galileo spacecraft but its moon was unknown until then, Pluto was considered a planet until a few months after the launch of New Horizons, and 3548 Eurybates's and 15094 Polymele's moons were not discovered until months before and after Lucy's launch, respectively). Didymos is the most easily reachable asteroid of its size from Earth, requiring a delta-v of only for a spacecraft to rendezvous, compared to to reach the Moon.

After two weeks of analysis, NASA announced that the collision shortened Dimorphos's orbital period around Didymos by 32 minutes, far more than the minimum requirement of 73 seconds and the success benchmark of 10 minutes. The measurement has an uncertainty of ±2 minutes.

Another mission to Didymos was approved in November 2019 and launched in October 2024, with the arrival at Didymos being expected in 2026. ESA's Hera mission is planning to survey the dynamical effects of the DART impact and measure the characteristics of the crater made by DART.

Notes

References

|archive-url=https://web.archive.org/web/20241220045232/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%2765803%27&START_TIME=%272184-Oct-14%2010:15%27&STOP_TIME=%272184-Oct-15%27&STEP_SIZE=%271%20day%27&QUANTITIES=%2720,39%27 |archive-date=2024-12-20 |url-status=live

References

1. {{OED. didymous
2. (2024-05-03). "Recent collisional history of (65803) Didymos". Nature Communications.
3. (2015). "The binary near-Earth Asteroid (175706) 1996 FG3 — an observational constraint on its orbital evolution". Icarus.
4. "[http://phys.org/news/2015-06-telescopes-focus-esa-asteroid-mission.html Telescopes focus on target of ESA's asteroid mission] {{Webarchive. link. (18 August 2015 " at phys.org (30 June 2015))
5. "Nomenclature Search Results".
6. (26 September 2022). "Brace for impact: DART successfully slams into asteroid". [[NASASpaceFlight.com]].
7. (2022-09-07). "DART Sets Sights on Asteroid Target".
8. Potter, Sean. (2021-11-23). "NASA, SpaceX Launch DART: First Test Mission to Defend Planet Earth".
9. Greshko, Michael. (2021-11-23). "This NASA spacecraft will smash into an asteroid—to practice saving Earth".
10. (2010). "Delta-v for spacecraft rendezvous with all known near-Earth asteroids".
11. "NASA says DART's asteroid impact was a huge success".
12. (11 October 2022). "NASA Confirms DART Mission Impact Changed Asteroid's Motion in Space".
13. "Planetary defence mission Hera heading for deflected asteroid".
14. [https://web.archive.org/web/20230406002045/http://iaaweb.org/iaa/Scientific%20Activity/conf/pdc2019/IAA-PDC-19-04-09ab.pdf Numerical modelling of the DART impact and the importance of the Hera mission.] Sabina D. Raducan, Thomas M. Davison, Gareth S. Collins. PDC 2019. Washington, D.C., USA.
15. "JPL Small-Body Database Browser: 65803 Didymos (1996 GT)". [[Jet Propulsion Laboratory]].
16. "65803 Didymos (1996 GT)". Minor Planet Center.
17. "MPC/MPO/MPS Archive". Minor Planet Center.
18. (February 2015). "AIDA: Asteroid Impact and Deflection Assessment mission under study at ESA and NASA". Observatoire de la Côte d'Azur.
19. "Asteroid Impact & Deflection Assessment". ESA.
20. (20 September 2014). "(65803) Didymos". johnstonsarchive.net.
21. "LCDB Data for (65803) Didymos". Asteroid Lightcurve Database (LCDB).
22. (November 2003). "(65803) 1996 GT". IAU Circular.
23. (March 2006). "Photometric survey of binary near-Earth asteroids". Icarus.
24. (March 2004). "35th Lunar and Planetary Science Conference". .
25. (September 2012). "Absolute magnitudes of asteroids and a revision of asteroid albedo estimates from WISE thermal observations". Icarus.
26. (September 2020). "Radar observations and a physical model of binary near-Earth asteroid 65803 Didymos, target of the DART mission". Icarus.
27. (July 2022). "NASA's Double Asteroid Redirection Test (DART): Mutual Orbital Period Change Due to Reshaping in the Near-Earth Binary Asteroid System (65803) Didymos". The Planetary Science Journal.
28. (July 2022). "Preimpact Mutual Orbit of the DART Target Binary Asteroid (65803) Didymos Derived from Observations of Mutual Events in 2003–2021". The Planetary Science Journal.
29. (January 1998). "Coordinate transformations". European Southern Observatory.
30. Talbert, Tricia. (23 June 2020). "NASA's First Planetary Defense Mission Target Gets a New Name". NASA.
31. (23 June 2020). "MPEC 2020-M83 : (65803) Didymos I = Dimorphos". International Astronomical Union.
32. (23 June 2020). "IAU Approves Name of Target of First NASA and ESA Planetary Defence Missions". International Astronomical Union.