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Dimorphos

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Dimorphos
High-resolution view of Dimorphos, created by combining the final 10 full-frame images obtained byDART's Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO). Dimorphos is oriented so that its north pole is toward the top of the image. Taken seconds before impact on September 26, 2022.
Discovery[1]
Discovered byPetr Pravecet al.[a]
Discovery siteOndřejov Observatory
Discovery date20 November 2003
Designations
Designation
Didymos I
Pronunciation/daɪˈmɔːrfəs/dy-MOR-fəs
Named after
Greek word for "having two forms"[2]
S/2003 (65803) 1
Didymos B
"Didymoon"
Orbital characteristics[3]: 28 [4]
Epoch26 September 2022 23:14:24.183UTC
(JD2459849.4683355;impact time)[5]: 5 [3]: 28 
1.206±0.035 km(pre-impact)[3]: 28 
1.144±0.070 km(post-impact)[6]: 5 
Eccentricity≈0(pre-impact)[7]: 15 
0.021±0.014[8]or0.0247±0.0002[9]: 16 (post-impact)
11.921473±0.000044hr
(11h 55m 17.3s ± 0.2s; pre-impact)[3]: 28 
11.3676±0.0014 hr
(11h 22m 03.4s ± 5.0s; post-impact)[8]
0.177 m/s(pre-impact)[b]
Inclination169.3°±1.0°with respect toecliptic[c]
Satellite of65803 Didymos
Physical characteristics[5]: 9 
Dimensions177 × 174 × 116m (±2 × 4 × 2m)
151±5 m(volume equivalent)
7.58×104m2[11]
Volume(1.81±0.18)×106m3
Mass(1.33±0.30)×109kg(if density is0.6–0.7 g/cm3)[12]
≈4.3×109kg(if same density as Didymos)[5]: 9 
0.6–0.7 g/cm3[12]
2.4±0.9 g/cm3(if same as Didymos)[7]: 29 
≈11.9 hr(synchronous;pre-impact)[13]
chaotic(post-impact)[14]
Albedo0.15±0.02[5]: 6 
Spectral type
S[15]
21.4±0.2[1][d]

Dimorphos(formal designation(65803) Didymos I;provisional designationS/2003 (65803) 1) is anatural satelliteormoonof thenear-Earth asteroid65803 Didymos,with which it forms abinary system.The moon was discovered on 20 November 2003 byPetr Pravecin collaboration with other astronomers worldwide. Dimorphos has a diameter of 177 meters (581 ft) across its longest extent and it was the target of theDouble Asteroid Redirection Test(DART), aNASAspace mission that deliberately collided aspacecraftwith the moon on 26 September 2022 to alter its orbit around Didymos. Before the impact by DART, Dimorphos had a shape of anoblate spheroidwith a surface covered inbouldersbut virtually nocraters.[16]The moon is thought to have formed when Didymos shed its mass due to its rapid rotation, which formed an orbiting ring of debris that conglomerated into a low-densityrubble pilethat became Dimorphos today.[17][18][19]

The DART impact reduced Dimorphos'sorbital periodaround Didymos by 33 minutes and ejected over 1 million kilograms (2.2×10^6lb) of debris into space, producing a dust plume that temporarily brightened the Didymos system and developed a 10,000-kilometer (6,200 mi)-longdust tailthat persisted for several months.[20][21][22]The DART impact is predicted to have caused global resurfacing and deformation of Dimorphos's shape, leaving animpact craterseveral tens of meters in diameter.[23][15][24]Post-impact observations of brightness fluctuations within the Didymos system suggest that the impact may have either significantly deformed Dimorphos into an ellipsoidal shape or may have sent it into achaoticallytumblingrotation.[8][25]If Dimorphos was in a tumbling rotation state, the moon will be subjected to irregulartidal forcesby Didymos before it will eventually return to atidally lockedstate within several decades.[14][26][27]TheESAmissionHerais planned to arrive at the Didymos system in 2026 to further study the effects of DART's impact on Dimorphos.

Discovery

[edit]
Radar images of Didymos and Dimorphos taken by theArecibo Observatoryin 2003

The primary asteroid Didymos was discovered in 1996 by Joe Montani of theSpacewatch Projectat the University of Arizona.[1]The satellite Dimorphos was discovered on 20 November 2003, inphotometricobservations byPetr Pravecand colleagues at theOndřejov Observatoryin the Czech Republic. Dimorphos was detected through periodic dips in Didymos's brightness due to mutualeclipsesandoccultations.With his collaborators, he confirmed from theArecibo radar delay-Dopplerimages that Didymos is a binary system.[28][13]

Etymology

[edit]

TheWorking Group for Small Bodies Nomenclatureof theInternational Astronomical Union(IAU) gave the satellite its official name on 23 June 2020.[29]The nameDimorphosis derived from a Greek word (Δίμορφος) meaning 'having two forms'.[30][31][e]The justification for the new name reads: "As the target of the DART andHeraspace 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) ".[2]The name was suggested by Kleomenis Tsiganis, aplanetary scientistat theAristotle University of Thessalonikiand a member of both theDARTandHerateams.[2]Prior to the IAU naming, the nicknameDidymoonwas used in official communications.[32]

Exploration

[edit]

On 24 November 2021,NASAand theApplied Physics Laboratorylaunched an impactor spacecraft towards Dimorphos as part of theirDouble Asteroid Redirection Test(DART).[33][34]DART was the first experiment conducted in space to testasteroid deflectionas a method of defending Earth frompotentially hazardous asteroids.[35]Following a ten-month journey to the Didymos system, the impactor collided with Dimorphos on 26 September 2022 at a speed of around 24,000 kilometers per hour (15,000 miles per hour).[35][36]The collision successfully decreased Dimorphos'sorbital periodaround Didymos by32±2minutes.[37][29][38][39]Fifteen days prior to its collision, the impactor releasedLICIACube,a 6UCubeSatoperated by theItalian Space Agencythat photographed the impact and the resulting dust plume as it performed a close flyby of the Didymos system.[33][40][41][42]Spacecraft and observatories such asHubble,James Webb,Lucy,SAAOandATLASalso captured the dust plume trailing the Didymos system in the days following the impact.[43][44][45][22]As part of itsHera mission,ESAcurrently plans to launch three spacecraft to the Didymos system in 2024 to further study the aftermath of the impact.[38][46][47]

A trail of dust streams from Dimorphos in thisHubble Space Telescopephoto taken about three months after the collision. The asteroid is surrounded by blue dots, which are boulders ranging from 1 to 6.7 metres across that were ejected by the impact.

The DART impact on the center of Dimorphos decreased the orbital period, previously 11.92 hours, by 33±1 minutes. This large change indicates the recoil from material excavated from the asteroid and ejected into space by the impact (known as ejecta) contributed significant momentum change to the asteroid, beyond that of the DART spacecraft itself. Researchers found the impact caused an instantaneous slowing in Dimorphos' speed along its orbit of about 2.7 millimeters per second — again indicating the recoil from ejecta played a major role in amplifying the momentum change directly imparted to the asteroid by the spacecraft. That momentum change was amplified by a factor of 2.2 to 4.9 (depending on the mass of Dimorphos), indicating the momentum change transferred because of ejecta production significantly exceeded the momentum change from the DART spacecraft alone.[48]While the orbital change was small, the change is in the velocity and over the course of years will accumulate to a large change in position.[49]For a hypothetical Earth-threatening body, even such a tiny change could be sufficient to mitigate or prevent an impact, if applied early enough. As thediameter of Earthis around 13,000 kilometers, a hypothetical asteroid impact could be avoided with as little of a shift as half of that (6,500 kilometers). A2 cm/svelocity change accumulates to that distance in approximately 10 years.

Dart Impact seen by LICIACube

By smashing into the asteroid DART made Dimorphos anactive asteroid.Scientists had proposed that some active asteroids are the result of impact events, but no one had ever observed the activation of an asteroid. The DART mission activated Dimorphos under precisely known and carefully observed impact conditions, enabling the detailed study of the formation of an active asteroid for the first time.[48][50]Observations show that Dimorphos lost approximately 1 million kilograms after the collision.[51]Impact produced a dust plume that temporarily brightened the Didymos system and developed a 10,000-kilometer (6,200 mi)-longdust tailthat persisted for several months.[20][21][22]The DART impact is predicted to have caused global resurfacing and deformation of Dimorphos's shape, leaving animpact craterseveral tens of meters in diameter.[23][15][24]The impact has likely sent Dimorphos into achaoticallytumblingrotation that will subject the moon to irregulartidal forcesby Didymos before it will eventually return to atidally lockedstate within several decades.[14][26][27]Additionally, the impact changed Dimorphos shape from a roughly symmetrical "oblate spheroid" to "a flat-topped oval", or "triaxial ellipsoid".[52][53][54]

Size and shape

[edit]

Dimorphos is approximately 170 meters (560 ft) in diameter, compared to Didymos at 780 meters (2,560 ft). Dimorphos does not have a confirmed mass, but it is estimated to be about5×109kg(5.5 million tons), or about the same mass and size as theGreat Pyramid of Giza,when assuming a density of2.17 g/cm3similar to Didymos.[55]It is one of the smallest celestial objects given a formal name by the IAU, after367943 Duendeand469219 Kamoʻoalewa.[2]

The final few minutes of pictures from the DART mission revealed an egg-shaped body covered with boulders, suggesting it has arubble pilestructure.[56][57]

Surface

[edit]

Five boulders (saxa) and six craters have been given names of traditional drums from several cultures. They are approximately 10 meters across or smaller:[58]

Named features
Name Pronunciation Feature Named after Date approved[58]
Atabaque Saxum UK:/ˌætəˈbæki/AT-ə-BAK-ee
US:/ˌɑːtəˈbɑːki/AH-tə-BAH-kee		 boulder atabaque(Brazil) 25 Jan 2023
Bodhran Saxum /ˈbɔːrɑːn/BOR-ahn boulder bodhrán(Ireland) 25 Jan 2023
Caccavella Saxum /ˌkækəˈvɛlə/KAK-ə-VEL-ə boulder caccavella
a.k.a. putipù (Italy)		 25 Jan 2023
Dhol Saxum /ˈdɔːl/DAWL boulder dhol(India) 25 Jan 2023
Pūniu Saxum /ˈpuːni.uː/POO-nee-oo boulder pūniua.k.a kilu (Hawaii) 25 Jan 2023
Bala Crater /ˈbælə/BAL-ə crater balafon(Guinea, Senegal, Mali) 14 Nov 2023
Bongo Crater /ˈbɒŋɡoʊ/BONG-goh crater bongo(Cuba) 14 Nov 2023
Marimba Crater /məˈrɪmbə/mə-RIM-bə crater marimba(Central America) 14 Nov 2023
Msondo Crater /ɛmˈsɒndoʊ/em-SON-doh crater msondo(Tanzania) 14 Nov 2023
Naqqara Crater /næˈkɑːrə/na-KAR-ə crater naqqara(naker) (Mid East and India) 14 Nov 2023
Tamboril Crater /ˌtæmbəˈrɪl/TAM-bər-IL crater tamboril(Uruguay,Candombe) 14 Nov 2023
Left:Surface of Dimorphos, captured by DART two seconds before impact.Right:Composite map of Dimorphos with named features.

Orbit and rotation

[edit]
Animation of DART around Didymos - Impact on Dimorphos
DART·Didymos·Dimorphos

The primary body of the binary system, Didymos, orbits the Sun at a distance of 1.0 to 2.3AUonce every 770 days (2 years and 1 month). The pathway of the orbit has aneccentricityof 0.38 and aninclinationof 3° with respect to theecliptic.On 4 October 2022 Didymos made an Earth approach of 10.6 million km (6.6 million mi).[59]Dimorphos moves in a nearly equatorial, nearly circular orbit around Didymos, with anorbital periodof 11.9 hours. Its orbit period is synchronous with its rotation, so that the same side of Dimorphos always faces Didymos. Dimorphos's orbit isretrograderelative to theeclipticplane, in conformity with Didymos's retrograde rotation.[60]

Dimorphos's rotation is being slowed down by theYORP effect,with an estimated rotation perioddoubling timeof 86,000 years. However, because it is in orbit around Didymos,tidal forceskeep the moon locked insynchronous rotation.[61]

See also

[edit]
  • 354P/LINEAR– a main-belt asteroid that was naturally impacted by another asteroid sometime before 2010
  • P/2016 G1 (PanSTARRS)– another main-belt asteroid that was impacted by an asteroid in 2016

Footnotes

[edit]
  1. ^ Astronomers involved in the discovery of Dimorphos includeP. Pravec,L.A.M. Benner, M.C. Nolan, P. Kusnirak, D. Pray, J.D. Giorgini, R.F. Jurgens,S.J. Ostro,J.-L. Margot, C. Magri, A. Grauer, and S. Larson. The discovery used lightcurve and radar observations from theJet Propulsion Laboratory,Pasadena,CA;National Astronomy and Ionosphere Center / Arecibo Observatory,Arecibo,PR;andOndrejov Observatory,Ondřejov,CZ.[1]
  2. ^For acircular orbitwith negligible eccentricity, as in the case of Dimorphos, the meanorbital speedcan be approximated by the timeTit takes to complete one revolution around its orbitalcircumference,with the radius being itssemi-major axisa:.
  3. ^Thomas et al. (2023) give Dimorphos's orbit pole orientation in terms ofecliptic coordinates,whereλisecliptic longitudeandβisecliptic latitude.[3]: 28 βis the angular offset from theecliptic plane,whereasinclinationiwith respect to the ecliptic is the angular offset of theorbital polefrom theecliptic north pole,atβ= +90°;iwith respect to the ecliptic would be thecomplementofβ.[10]Therefore, givenβ= –79.3°,i= 90° – (–79.3°) = 169.3°from the ecliptic.
  4. ^Absolute magnitude of Dimorphos calculated from the addition of its magnitude difference to Didymos's absolute magnitude: 18.07 + 3.29 ≈ 21.4.[1]
  5. ^The nameDimorphoswas suggested byplanetary scientistKleomenis Tsiganis at theAristotle University of Thessaloniki.Tsiganis explained that the name "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 byHeraa few years later. "[2]

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[edit]
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