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Dimorphos
High-resolution view of Dimorphos, created by combining the final 10 full-frame images obtained by DART'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 Pravec et 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]
Epoch 26 September 2022 23:14:24.183 UTC
(JD 2459849.4683355; 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] or 0.0247±0.0002[9]: 16  (post-impact)
11.921473±0.000044 hr
(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 to ecliptic[c]
Satellite of65803 Didymos
Physical characteristics[5]: 9 
Dimensions177 × 174 × 116 m (± 2 × 4 × 2 m)
151±5 m (volume equivalent)
7.58×104 m2[11]
Volume(1.81±0.18)×106 m3
Mass(1.33±0.30)×109 kg (if density is 0.6–0.7 g/cm3)[12]
≈ 4.3×109 kg (if same density as Didymos)[5]: 9 
Mean density
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 designation S/2003 (65803) 1) is a natural satellite or moon of the near-Earth asteroid 65803 Didymos, with which it forms a binary system. The moon was discovered on 20 November 2003 by Petr Pravec in collaboration with other astronomers worldwide. Dimorphos has a diameter of 177 meters (581 ft) across its longest extent.

Dimorphos is the smallest asteroid to be photographed and visited by a spacecraft. It was the target of the Double Asteroid Redirection Test (DART), a NASA space mission that deliberately collided a spacecraft with the moon on 26 September 2022 to alter its orbit around Didymos. Before the impact by DART, Dimorphos had a shape of an oblate spheroid with a surface covered in boulders but virtually no craters.[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-density rubble pile that became Dimorphos today.[17][18][19]

The DART impact reduced Dimorphos's orbital period around Didymos by 33 minutes and ejected over 1 million kilograms (2.2×10^6 lb) of debris into space, producing a dust plume that temporarily brightened the Didymos system and developed a 10,000-kilometer (6,200 mi)-long dust tail that persisted for several months.[20][21][22] The DART impact is predicted to have caused global resurfacing and deformation of Dimorphos's shape, leaving an impact crater several 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 a chaotically tumbling rotation.[8][25] If Dimorphos was in a tumbling rotation state, the moon will be subjected to irregular tidal forces by Didymos before it will eventually return to a tidally locked state within several decades.[14][26][27] The ESA mission Hera is 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 the Arecibo Observatory in 2003

The primary asteroid Didymos was discovered in 1996 by Joe Montani of the Spacewatch Project at the University of Arizona.[1] The satellite Dimorphos was discovered on 20 November 2003, in photometric observations by Petr Pravec and colleagues at the Ondřejov Observatory 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.[28][13]

Naming

[edit]

The Working Group for Small Bodies Nomenclature of the International Astronomical Union (IAU) gave the satellite its official name on 23 June 2020.[29] The name Dimorphos is 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 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)".[2] The name was suggested by Kleomenis Tsiganis, a planetary scientist at the Aristotle University of Thessaloniki and a member of both the DART and Hera teams.[2] Prior to the IAU naming, the nickname Didymoon was used in official communications.[32]

Exploration

[edit]

On 24 November 2021, NASA and the Applied Physics Laboratory launched an impactor spacecraft towards Dimorphos as part of their Double Asteroid Redirection Test (DART).[33][34] DART was the first experiment conducted in space to test asteroid deflection as a method of defending Earth from potentially 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's orbital period around Didymos by 32±2 minutes.[37][29][38][39] Fifteen days prior to its collision, the impactor released LICIACube, an Italian Space Agency CubeSat that 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 as Hubble, James Webb, Lucy, SAAO and ATLAS also captured the dust plume trailing the Didymos system in the days following the impact.[43][44][45][22] As part of its Hera mission, ESA launched three spacecraft to the Didymos system in 2024 to reach this asteroid system in December 2026 to further study the aftermath of the impact.[38][46][47]

A trail of dust streams from Dimorphos in this Hubble Space Telescope photo 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's 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 the diameter of Earth is around 13,000 kilometers, a hypothetical asteroid impact could be avoided with as little of a shift as half of that (6,500 kilometers). A 2 cm/s velocity change accumulates to that distance in approximately 10 years.

Dart Impact seen by LICIACube

By smashing into the asteroid DART made Dimorphos an active 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)-long dust tail that persisted for several months.[20][21][22] The DART impact is predicted to have caused global resurfacing and deformation of Dimorphos's shape, leaving an impact crater several tens of meters in diameter.[23][15][24] The impact has likely sent Dimorphos into a chaotically tumbling rotation that will subject the moon to irregular tidal forces by Didymos before it will eventually return to a tidally locked state 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]

DART spacecraft's final images of Dimorphos, (from 11.5 seconds before impact).

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 about 5×109 kg (5.5 million tons), or about the same mass and size as the Great Pyramid of Giza, when assuming a density of 2.17 g/cm3 similar to Didymos.[55] It is one of the smallest celestial objects given a formal name by the IAU, after 367943 Duende and 469219 Kamoʻoalewa.[2]

The final few minutes of pictures from the DART mission revealed an egg-shaped body covered with boulders, suggesting it has a rubble pile structure.[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 pūniu a.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 2.3 AU once every 770 days (2 years and 1 month). The pathway of the orbit has an eccentricity of 0.38 and an inclination of 3° with respect to the ecliptic. 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 an orbital period 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 ecliptic plane, in conformity with Didymos's retrograde rotation.[60]

Dimorphos's rotation is being slowed down by the YORP effect, with an estimated rotation period doubling time of 86,000 years. However, because it is in orbit around Didymos, tidal forces keep the moon locked in synchronous 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]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Dimorphos

View on Grokipedia
from Grokipedia
Dimorphos is a small moonlet, approximately 160 meters (525 feet) in , that orbits the larger near-Earth Didymos every 11 hours and 23 minutes, forming a system approximately 1.2 kilometers (0.75 miles) across. Discovered through radar observations at the on November 23, 2003, Dimorphos was selected as the target for 's (DART) mission due to its accessibility and the opportunity to measure changes in its orbit without posing any risk to . The DART spacecraft, launched in November 2021, intentionally collided with Dimorphos on September 26, 2022, at a speed of about 6.6 kilometers per second (4.1 miles per second), marking the first demonstration of the kinetic impact technique for planetary defense. This impact shortened Dimorphos's around Didymos by 33 minutes and 15 seconds—from its pre-impact duration of 11 hours and 55 minutes to approximately 11 hours and 22 minutes (as of March 2024)—exceeding the mission's success criteria by more than 25 times and confirming the method's effectiveness in altering an asteroid's trajectory. The collision ejected over a million kilograms of material into space, creating a comet-like tail and a large estimated at 50 meters (164 feet) wide, with boulders and debris observed drifting away via telescopes like Hubble. Spectroscopic observations indicate that Dimorphos shares a similar composition with Didymos, classified as an rich in silicates and metals, akin to meteorites, though pre-impact data suggested a boulder-strewn surface lacking fine . The DART impact changed the moonlet's shape from an oblate to a triaxial , with approximate dimensions of 177 m × 174 m × 116 m, and ongoing analyses from ground-based telescopes and the ESA's mission, launched in October 2024 and set to arrive in 2026, continue to refine understanding of its internal structure and the ejecta dynamics.

Discovery and nomenclature

Discovery

The binary nature of the near-Earth asteroid (65803) Didymos, serving as the primary body in the system, was first indicated through photometric observations conducted in November 2003 at the Ondřejov Observatory in the by Petr Pravec and Petr Kušnirák. Lightcurve analysis revealed periodic brightness variations consistent with mutual eclipses and occultations caused by a small companion orbiting the primary. These initial photometric findings were confirmed later that same month through radar observations using the by Jean-Luc Margot and collaborators, which directly imaged the binary configuration and resolved the . The provided the first direct of the companion's existence and enabled the estimation of its orbital parameters, including a semi-major axis of approximately 1.18 km. Detection proved difficult owing to the satellite's small size and low , which rendered it faint compared to Didymos and resulted in subtle lightcurve perturbations that demanded precise, extended observation campaigns to distinguish from rotational effects alone.

Naming

Upon its discovery in 2003, the satellite of the asteroid was provisionally designated S/2003 (65803) 1, following the standard nomenclature for natural satellites of minor planets established by the (IAU). This designation reflected the year of observation and the parent body's number in the catalog. Informally, it was also referred to as Didymos I or Didymoon during early studies. The official name "Dimorphos" was approved by the IAU's for Small Bodies (WGSBN) on 23 June 2020, in recognition of its role as the target of NASA's (DART) and ESA's missions. The name was proposed by Kleomenis Tsiganis, a planetary scientist at and a member of the DART science team. Derived from the Greek word dimorphos, meaning "having two forms," it alludes to the satellite's anticipated change in orbital characteristics before and after the planned kinetic impact by DART in 2022. This naming complements the parent asteroid's designation as Didymos, from for "twin," which was chosen to highlight the binary nature of the system discovered in 1996. Together, the names emphasize the duality of the pair, both in their physical companionship and the transformative objectives of the planetary defense missions targeting them.

Physical characteristics

Size and shape

Prior to the DART impact, Dimorphos was modeled as an oblate with principal axes measuring approximately 177 m × 174 m × 133 m, corresponding to a volume-equivalent of about 160 m. These dimensions were derived from ground-based observations and lightcurve analysis conducted in the years leading up to the mission, revealing a compact, slightly flattened morphology wider than it was tall. The irregular, non-spherical form suggested a rubble-pile structure, consistent with its low overall density and the gravitational aggregation of debris, though the surface appeared relatively smooth compared to more rugged small asteroids. Following the DART impact in September 2022, subsequent observations refined the shape model, indicating a subtle reshaping of Dimorphos into a more triaxial ellipsoid, often described as oblong or watermelon-like, with elongation along one axis. Updated photometric and radar data from 2023 and 2024, including analysis from the Hera mission planning, showed minimal change in overall volume, with a post-impact volume-equivalent diameter of approximately 160 m and negligible reduction (<0.01%) due to ejecta loss (~0.02% of mass); the shape change results from momentum transfer and internal reconfiguration rather than significant erosion. This slight alteration preserved the asteroid's compact scale while enhancing its elongated profile, highlighting the impact's role in redistributing surface material without substantially altering bulk dimensions. Dimorphos's pre- and post-impact morphology aligns with that of other small near-Earth asteroids, such as those in the 100–200 m range, which often exhibit elongated, rubble-pile forms resulting from rotational disruption and reaccumulation around a larger parent body like Didymos. Unlike more monolithic small bodies, its structure underscores the prevalence of loosely bound aggregates in this size class, where tidal and rotational forces maintain irregular yet stable shapes.

Surface features

The surface of Dimorphos is characterized by a predominantly smooth, boulder-strewn terrain with a scarcity of large craters, as documented through high-resolution imagery captured by the DART spacecraft's DRACO camera and the LICIACube CubeSat during the mission's approach and impact in September 2022. This topography suggests a relatively young surface, possibly resurfaced by ongoing processes related to its binary system dynamics, with boulders ranging from a few meters to over 6 meters in diameter dominating the landscape and indicating a rubble-pile composition. Crater size-frequency analysis indicates Dimorphos's surface age is approximately 300,000 years, consistent with its formation from Didymos debris (as of 2024). Notable among these features is an equatorial ridge, which may have formed through rotational instabilities or material accretion during Dimorphos's origin from the parent body Didymos approximately 300,000 years ago. Small impact craters, typically 10-20 meters in diameter, are sparsely distributed, with at least 12 identified in pre-impact images, reflecting limited exposure to major collisional events. The overlying regolith layer is estimated to be 1-10 meters thick, based on analyses of boulder tracks and surface mobility, supporting the interpretation of Dimorphos as a loosely aggregated body with minimal internal cohesion. Following the DART kinetic impact, significant alterations to the surface were observed, including the generation of a massive plume that extended tens of thousands of kilometers and redistributed material across the . At the impact site, located between two prominent boulders (Atabaque and Bodhran), a depression approximately 40-60 meters wide formed, accompanied by localized resurfacing due to the mobilization and redeposition of and boulders. These changes were inferred from variations, analyses, and imaging, with ground-based and observations from 2022 to 2024—such as those from Hubble and ESO facilities—revealing ongoing dispersal and subtle shape modifications consistent with global deformation. Approximately 37 large boulders (up to ~7 meters across) were observed ejected at speeds up to 52 meters per second, with additional smaller , further altering the surface texture without evidence of deep excavation. Dimorphos exhibits no signs of prominent geological activity, such as cryovolcanism or tectonic resurfacing, aligning with its structure as a low-strength, loose aggregate held together primarily by and weak inter-particle forces. This passive surface evolution underscores the asteroid's recent formation and the dominant role of external impacts and tidal interactions in shaping its .

Composition and density

Spectroscopic observations of Dimorphos in the visible to near-infrared range (0.55–2.5 μm) indicate a surface composition consistent with L/LL ordinary chondrites, dominated by such as and . These spectra reveal absorption features around 1 μm and 2 μm attributable to the iron-bearing silicates in these analogs, with no strong evidence for significant hydrated minerals or organics on the surface. The reflectance properties align closely with those of the primary asteroid Didymos, suggesting a shared compositional heritage. Post-impact analyses constrain the of Dimorphos to lower than ~2.4 g/cm³ (range 1.5–2.4 g/cm³), derived from orbital dynamics, volume measurements, and impact modeling. This low implies a highly porous internal structure, characteristic of a rubble-pile body with macroporosity around 34–38%, indicating substantial void (20–40%) within loosely aggregated rocky material. The boulder population on the surface, modeled as having grain densities of 3.2–3.6 g/cm³, further supports this porous framework, with limited cohesive strength (less than a few pascals). The estimated mass of Dimorphos is about 5 × 10⁹ kg, calculated from its and dimensions inferred via and lightcurve analysis prior to the DART impact. This value aligns with perturbations in the binary system's orbital parameters and post-impact ejecta modeling. Formation models propose that Dimorphos originated as a fragment of Didymos through rotational mass shedding, where rapid spin-up ejected that reaccumulated into the moonlet, consistent with its low metal content and primitive silicate-rich composition indicative of an early solar system . This process involved the loss of a small (~1%) of Didymos's , forming Dimorphos in its current orbit without significant capture from external . The rubble-pile structure and spectral similarity reinforce this binary evolution scenario over alternative captured-body hypotheses.

Orbital and rotational properties

Orbit around Didymos

Dimorphos orbits its , Didymos, in a synchronous, near-circular path characterized by a semi-major axis of 1.19 ± 0.03 km and an eccentricity of approximately 0.04. This configuration results in an of 11.9217 ± 0.0002 hours, during which Dimorphos completes one full revolution around Didymos. The lies nearly in the equatorial plane of Didymos, promoting long-term stability through tidal interactions that maintain the secondary's position relative to the primary. The synchronous nature of the means Dimorphos maintains the same face toward Didymos throughout its cycle, a state achieved via in the . This locking contributes to the overall dynamics of the Didymos-Dimorphos pair, where the mutual of 11.92 hours contrasts with Didymos's faster period of about 2.26 hours, influencing the system's photometric variability and gravitational interactions. The slight eccentricity introduces minor variations in distance, but the remains stable without significant perturbations prior to external intervention. Following the DART mission's kinetic impact on September 26, 2022, the of Dimorphos was shortened by 33.25 ± 0.025 minutes as of 2024 measurements from ground-based lightcurve and observations, reducing the period to approximately 11 hours 22 minutes. Recent 2025 studies indicate an additional ~30-second shortening due to ongoing system evolution toward equilibrium, for a total change of approximately 33 minutes 45 seconds as of October 2025. This alteration also reduced the semi-major axis to 1.144 ± 0.070 km and initially induced a post-impact eccentricity of about 0.028 ± 0.016, which decayed to near zero within ~70 days, though the retained its overall synchronous and stable characteristics.

Rotation

Dimorphos rotates with a period synchronous to its orbital period around Didymos (11.92 hours pre-impact), as expected from in the , confirmed by pre-impact lightcurve analysis of mutual events observed between 2003 and 2021. This synchronization indicates , where the same hemisphere of Dimorphos consistently faces Didymos throughout its orbit. The spin axis of Dimorphos is closely aligned with the vector of the , exhibiting a small obliquity that supports stable . The position is at 320.6° and −78.6°, corresponding to a retrograde inclination of 168.6° relative to the plane, or an effective tilt of approximately 11° from alignment with the pole. Pre-impact observations and dynamical models reveal no significant tumbling or non-principal , consistent with the equilibrium expected from tidal interactions in a mature system. The stability arises from the close proximity and mutual gravitational influence between Didymos and Dimorphos, which dampen any deviations from synchronous over time. Following the DART impact in September 2022, numerical simulations predict a minor excitation of Dimorphos's spin state due to transfer from the collision and , potentially introducing non-principal axis or tumbling (with roll up to 45°, pitch up to 20°, yaw up to 25°) while maintaining an average synchronous period; however, such depends on post-impact ratios and may not occur at estimated values. Ground-based lightcurve observations post-impact, including 2024-2025 data, confirm no drastic period change beyond the orbital alteration, with overall rotational stability observed despite potential minor perturbations from reshaping.

Exploration and scientific study

Ground-based and telescopic observations

Ground-based and telescopic observations of Dimorphos before the DART mission focused on techniques to infer its physical and orbital properties within the Didymos . These efforts, spanning imaging, photometry, and , provided essential constraints on Dimorphos's , , , and composition, though limited by the moonlet's faintness and small relative to Didymos. Radar observations played a key role in resolving the binary structure and estimating shapes. The captured the first images of the system on November 23–24, 2003, using delay-Doppler imaging at 12.6 cm wavelength, which confirmed Dimorphos's presence by detecting separate echoes from the primary and secondary components during close approach. These data yielded an initial estimate of Dimorphos's at approximately 160 m and revealed its orbital motion around Didymos. Photometric lightcurve campaigns over multiple apparitions further characterized Dimorphos's and , including refinements from observations in and later. Observations from 2003 to , coordinated by Petr Pravec at Ondřejov Observatory and involving facilities like , analyzed composite lightcurves of the system to detect mutual events such as eclipses and occultations. These events, occurring when Dimorphos passed in front of or behind Didymos, allowed determination of the moonlet's sidereal of 11.92 hours and low eccentricity of 0.03 ± 0.01, with Dimorphos in synchronous . Extensive coverage during the 2019–2020 apparitions, using telescopes from 0.6 m to 10 m apertures, refined these parameters to uncertainties below 1 second for the period, essential for DART targeting, and enabled a 3D shape model of Dimorphos as an oblate spheroid approximately 170 m in . The mutual parameters included Dimorphos's semi-major axis of about 1.2 km. Spectroscopic observations classified the Didymos system, with spectra dominated by the primary, as S-type based on visible and near-infrared features indicative of ordinary chondrite-like silicates. Pre-DART near-IR spectra from NASA's Infrared Telescope Facility (IRTF) showed a moderately red slope consistent with S-complex asteroids, while (VLT) observations with X-Shooter in 2021 confirmed the through absorption bands near 1 and 2 μm linked to and . No distinct pre-impact spectrum of Dimorphos alone was obtained due to its flux contribution being less than 5% of the system total. Despite these advances, ground-based methods faced inherent limitations, including radar resolutions of approximately 30 m per pixel that precluded detailed surface mapping or boulder-scale features on Dimorphos. Photometric and spectroscopic data also suffered from blending with Didymos, restricting insights into the moonlet's individual composition and geology until spacecraft flybys.

DART mission

The Double Asteroid Redirection Test (DART) was a NASA planetary defense demonstration mission designed to test kinetic impact as a method for altering the trajectory of a potentially hazardous asteroid. Launched on November 24, 2021, aboard a SpaceX Falcon 9 rocket from Vandenberg Space Force Base in California, the mission targeted the binary asteroid system (65803) Didymos and its moonlet Dimorphos. The spacecraft traveled for nearly 10 months before arriving at the target system on September 26, 2022, when it intentionally collided with Dimorphos at a relative speed of approximately 6.6 km/s, about 11 million kilometers from Earth. This impact marked the first deliberate alteration of an asteroid's orbit by human means, validating the kinetic impactor technique for future deflection efforts against near-Earth objects. The DART spacecraft, developed and operated by the (APL) under NASA's , was a box-shaped impactor with a mass of approximately 610 kg at launch, including about 110 kg of propellant for trajectory corrections. Key to its success was the Didymos Reconnaissance and Asteroid Camera for Optical navigation (), a high-resolution imager based on the Long Range Reconnaissance Imager from the mission, which enabled autonomous navigation via the SMART Nav system during the final approach. captured images down to seconds before impact, providing real-time data on Dimorphos's size, shape, and surface features to refine targeting. Accompanying the main spacecraft was LICIACube, a 6U developed by the (ASI) in collaboration with Argotec, deployed from DART on September 11, 2022—about 15 days prior to impact—to observe the collision from a safe distance of roughly 50-55 km. Equipped with two optical cameras (LEIA and LUKE) for visible and near-infrared imaging, LICIACube documented the ejecta plume, crater formation, and early dynamical effects on Dimorphos, complementing DRACO's onboard observations. The impact occurred in Dimorphos's southern hemisphere, as confirmed by the sequence of images transmitted in the final minutes, which revealed a rugged surface dominated by a dense field with over 950 identifiable boulders ranging from decimeters to several meters across. These images, streamed live to , showed the spacecraft's approach toward a of irregular rocks and possible ridges, highlighting the moonlet's loosely consolidated rubble-pile suitable for transfer during deflection. The mission's execution achieved its core objective, demonstrating that a like DART could successfully navigate to and strike a small, fast-moving celestial target with precision.

Impact effects and orbital changes

The DART impact on September 26, 2022, generated a prominent plume from Dimorphos, consisting of dust and boulders expelled at high velocities. Observations from the SOAR Telescope in captured the plume forming a comet-like tail exceeding 10,000 kilometers in length shortly after impact, with the tail extending up to 70,000 kilometers over subsequent weeks as material continued to disperse. The ejected mass was estimated at 1.3–2.2 × 10^7 kilograms, equivalent to 0.3–0.5% of Dimorphos's total mass assuming a of 2,400 kg/m³, significantly enhancing the overall transfer beyond the spacecraft's direct . This contributed to a momentum amplification factor of 2–4, as the plume's pushed Dimorphos more effectively than the impactor alone. The impact did not produce a traditional deep but instead caused widespread surface disruption due to Dimorphos's highly porous, rubble-pile . Hydrocode simulations indicate the excavation site spanned approximately 40–60 meters in , with excavated to depths of about 15 meters, though the low cohesion (a few Pascals to tens of kilopascals) led to shallow features and extensive redistribution rather than a confined bowl-shaped . Global reshaping occurred as recoiling boulders and debris, totaling up to 8% of the asteroid's mass moving below , altered Dimorphos's overall form from a near-spheroidal shape to one with increased elongation, potentially raising its from 1.02 to 1.2. Ground-based observations from 2022 to 2025, including lightcurve photometry from telescopes like SOAR, confirmed the impact's orbital perturbations around Didymos. The mutual shortened by 33.0 ± 1.0 minutes, from 11 hours 55 minutes to about 11 hours 22 minutes, with the post-impact orbit becoming slightly eccentric at e ≈ 0.028 ± 0.016. This change, validated through mutual event timing and , implies a velocity reduction of 2.70 ± 0.10 mm/s for Dimorphos. The momentum enhancement factor β, defined as the ratio of total momentum transferred to the impactor's momentum, was determined to be 3.6 ± 0.6 based on plume modeling and orbital , assuming Dimorphos's of 2,400 kg/m³; broader ranges yield β between 2.2 and 4.9. This value demonstrates the efficacy of kinetic impactors for deflection, as the ejecta plume amplified the effect by a factor exceeding 3, providing critical for future planetary defense strategies.

Future missions

The European Space Agency's (ESA) Hera mission represents the primary planned follow-up to NASA's DART impact on Dimorphos, aimed at conducting a detailed in-situ investigation of the system. Launched on , 2024, aboard a rocket from , is scheduled to arrive at the Didymos-Dimorphos system in November 2026, an earlier timeline than initially planned due to refined trajectories informed by post-DART data analysis. The mission, part of ESA's Space Safety Programme, will orbit Didymos and perform close-proximity observations of Dimorphos over approximately six months to characterize the kinetic impact site's effects and validate planetary defense techniques. Hera carries two CubeSats for extended monitoring: Milani, which will conduct and to analyze surface composition, and , equipped with the low-frequency instrument JuRA for subsurface sounding to probe Dimorphos's internal structure, gravity field, and potential site properties. is designed to detach and attempt a controlled on Dimorphos, providing the first measurements inside an . These CubeSats will enable detailed studies beyond Hera's main orbiter capabilities, including radio experiments to augment and determinations. Key objectives include high-resolution mapping of the DART impact crater on Dimorphos to assess its size, shape, and distribution; precise measurement of Dimorphos's and through orbital tracking and radio to evaluate momentum transfer efficiency from the impact; and analysis of the binary system's dynamics, including any lingering plume remnants and changes in orbital parameters. These investigations build on DART's precursor demonstration of deflection by kinetic impactor. As of November 2025, remains on track following a successful Mars flyby in March 2025, which provided gravitational assist and imaging opportunities of the planet and its moon Deimos. has contributed through the selection of participating scientists in 2024 to support and instrument calibration, enhancing international collaboration on the mission. No additional standalone missions to Dimorphos are currently approved beyond and its CubeSats, though proposals for future extended monitoring via smallsats have been discussed in planetary defense planning.

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