25143 Itokawa

25143 Itokawa /iːtoʊˈkɑːwə/ (provisional designation 1998 SF36) is a sub-kilometer near-Earth object of the Apollo group and a potentially hazardous asteroid. It was discovered by the LINEAR program in 1998 and later named after Japanese rocket engineer Hideo Itokawa.[1] The peanut-shaped S-type asteroid has a rotation period of 12.1 hours and measures approximately 330 meters (1,100 feet) in diameter. Due to its low density and high porosity, Itokawa is considered to be a rubble pile, consisting of numerous boulders of different sizes rather than of a single solid body.

25143 Itokawa
Asteroid Itokawa imaged from 8 kilometers by JAXA's Hayabusa spacecraft in 2005
Discovery [1]
Discovered byLINEAR
Discovery siteLincoln Lab's ETS
Discovery date26 September 1998
Designations
(25143) Itokawa
Pronunciation/iːtoʊˈkɑːwə/
Japanese: [itokaɰa]
Named after
Hideo Itokawa (糸川)
(Japanese rocket scientist)[2]
1998 SF36
Apollo · NEO · PHA [1][3]
Orbital characteristics [3]
Epoch 27 April 2019 (JD 2458600.5)
Uncertainty parameter 0
Observation arc20.38 yr (7,443 d)
Aphelion1.6951 AU
Perihelion0.9532 AU
1.3241 AU
Eccentricity0.2801
1.52 yr (557 d)
288.88°
0° 38m 48.48s / day
Inclination1.6214°
69.081°
162.82°
Earth MOID0.0131 AU (5.10 LD)
Physical characteristics
Dimensions535 m × 294 m × 209 m[4]
Mean diameter
313 m[5]
330 m[3]
350 m[6][7]
Mass(3.51±0.105)×1010 kg[4]
(3.58±0.18)×1010 kg[8]
Mean density
1.9±0.13 g/cm3[4]
1.95±0.14 g/cm3[8]
12.132 h[6][9]
0.23[7]
0.283±0.116[5]
0.36±0.22[10]
0.53[11]
SMASS = S (IV)[3]
Sqw [12] · S [13] · Q [6]
18.61[14] · 18.95 (R)[15]
19.00[13] · 19.2[1][3]
19.48[6][7] · 19.51±0.09[5]

    It was the first asteroid to be the target of a sample return mission, the Japanese Hayabusa space probe, which collected more than 1500 regolith dust particles from the asteroid's surface in 2005. After its return to Earth in 2010, the mineralogy, petrography, chemistry, and isotope ratios of these particles have been studied in detail, providing insights into the evolution of the Solar System. Itokawa is the smallest asteroid ever photographed and visited by a spacecraft.

    Discovery and naming

    Itokawa was discovered on 26 September 1998 by astronomers with the Lincoln Near-Earth Asteroid Research (LINEAR) program at Lincoln Laboratory's Experimental Test Site near Socorro, New Mexico, in the United States. It was given the provisional designation 1998 SF36. The body's observation arc begins with its first observation by the Sloan Digital Sky Survey just one week prior to its official discovery observation.[1] The minor planet was named in memory of Japanese rocket scientist Hideo Itokawa (1912–1999), who is regarded as the father of Japanese rocketry.[1][16] The official naming citation was published by the Minor Planet Center on 6 August 2003 (M.P.C. 49281).[17]

    Orbit and classification

    Itokawa belongs to the Apollo asteroids. They are Earth-crossing asteroids and the largest dynamical group of near-Earth objects with nearly 10,000 known members. Itokawa orbits the Sun at a distance of 0.95–1.70 AU once every 18 months (557 days; semi-major axis of 1.32 AU). Its orbit has an eccentricity of 0.28 and an inclination of 2° with respect to the ecliptic.[3] It has a low Earth minimum orbital intersection distance of 0.0131 AU (1,960,000 km), which corresponds to 5.1 lunar distances.[3]

    Left: orbital diagram of Itokawa on December 2006. Right: animated orbits of Itokawa (green) and Earth (blue) around the Sun.

    Hayabusa exploration

    In 2000, it was selected as the target of Japan's Hayabusa mission. The probe arrived in the vicinity of Itokawa on 12 September 2005 and initially "parked" in an asteroid–Sun line at 20 km (12 mi), and later 7 km (4.3 mi), from the asteroid (Itokawa's gravity was too weak to provide an orbit, so the spacecraft adjusted its orbit around the Sun until it matched the asteroid's). Hayabusa landed on 20 November for thirty minutes, but it failed to operate a device designed to collect soil samples. On 25 November, a second landing and sampling sequence was attempted. The sample capsule was returned to Earth and landed at Woomera, South Australia on 13 June 2010, around 13:51 UTC (23:21 local). On 16 November 2010, the Japan Aerospace Exploration Agency reported that dust collected during Hayabusa's voyage was indeed from the asteroid.[18]

    Surface features

    Names of major surface features were proposed by Hayabusa scientists and accepted by the Working Group for Planetary System Nomenclature of the International Astronomical Union.[16] Also, the Hayabusa science team is using working names for smaller surface features.[19][20] The following tables list the names of geological features on the asteroid.[16] No naming conventions have been disclosed for surface features on Itokawa.

    Craters

    Ten impact craters on the surface of Itokawa were named on 18 February 2009.[21]

    Crater Coordinates Diameter
    (km)    		 Approval
    Year    		 Eponym Ref
    Catalina17°S 14°E0.022009Catalina Station (astronomical observatory) in Arizona, United StatesWGPSN
    Fuchinobe34°N 91°W0.042009Fuchinobe in Sagamihara, JapanWGPSN
    Gando76°S 155°Wn.a.2009Gando, Canary Islands; Spanish launch facilityWGPSN
    Hammaguira18°S 155°W0.032009Hammaguir, Algeria; abandoned French launch site and missile testing range in the Sahara desertWGPSN
    Kamisunagawa28°S 45°E0.012009Kamisunagawa, town in Hokkaido Japan, where a microgravity test facility is locatedWGPSN
    Kamoi6°N 116°W0.012009Japanese town of Kamoi in Yokohama, location of the NEC TOSHIBA Space Systems Ltd. factoryWGPSN
    Komaba10°S 102°E0.032009Komaba in Meguro, Japan, where the Institute of Space and Astronautical Science is locatedWGPSN
    Laurel1°N 162°E0.022009U.S. city of Laurel in Maryland, where APL/JHU is locatedWGPSN
    Miyabaru40°S 116°W0.092009Radar site of the Uchinoura Space Center in JapanWGPSN
    San Marco28°S 41°Wn.a.2009San Marco platform, an old oil platform near Kenya that served as a launch pad for Italian spacecraftWGPSN

    Regiones

    Regio or regiones are large area marked by reflectivity or color distinctions from adjacent areas in planetary geology. The following regiones have been named on Itokawa.[16][21]

    RegioNamed after
    Arcoona RegioArcoona, Australia
    LINEAR RegioLincoln Near-Earth Asteroid Research
    MUSES-C RegioMUSES-C, name of the Hayabusa probe prior to launch
    Ohsumi RegioŌsumi Peninsula
    Sagamihara RegioSagamihara, a town in Japan where Institute of Space and Astronautical Science is located
    Uchinoura RegioUchinoura, a town in Japan (now part of Kimotsuki), the location of Uchinoura Space Center, Hayabusa launch site
    Yoshinobu RegioLaunch site in the Tanegashima Space Center, Japan

    Physical characteristics
    Schematic of Itokawa's two lobes separated from each other. Their divergent densities suggest that these were stand-alone bodies that came into contact later on, making the rubble pile also a likely contact binary.[22]
    Preliminary shape model of Itokawa based on radar observations by Goldstone and Arecibo[23]

    Itokawa is a stony S-type asteroid. Radar imaging by Goldstone in 2001 observed an ellipsoid 630±60 meters long and 250±30 meters wide.[23]

    The Hayabusa mission confirmed these findings and also suggested that Itokawa may be a contact binary formed by two or more smaller asteroids that have gravitated toward each other and stuck together. The Hayabusa images show a surprising lack of impact craters and a very rough surface studded with boulders, described by the mission team as a rubble pile.[4][24] Furthermore, the density of the asteroid is too low for it to be made from solid rock. This would mean that Itokawa is not a monolith but rather a rubble pile formed from fragments that have cohered over time. Based on Yarkovsky–O'Keefe–Radzievskii–Paddack effect measurements, a small section of Itokawa is estimated to have a density of 2.9 g/cm3, whereas a larger section is estimated to have a density of 1.8 g/cm³.[4][25]

    Rotation period and poles

    Since 2001, a large number of rotational lightcurves of Itokawa have been obtained from photometric observations. Analysis of the best-rated lightcurve by Mikko Kaasalainen gave a sidereal rotation period of 12.132 hours with a high brightness variation of 0.8 magnitude, indicative of the asteroid's non-spherical shape (U=3). In addition, Kaasalainen also determined two spin axes of (355.0°, −84.0°) and (39°, −87.0°) in ecliptic coordinates (λ, β).[6][9] Alternative lightcurve measurements were made by Lambert (12 h),[26] Lowry (12.1 and 12.12 h),[27][28] Ohba (12.15 h),[29] Warner (12.09 h),[30][lower-alpha 1] Ďurech (12.1323 h),[31] and Nishihara (12.1324 h).[15]

    Composition

    The 26 August 2011 issue of Science devoted six articles to findings based on dust that Hayabusa had collected from Itokawa.[32] Scientists' analysis suggested that Itokawa was probably made up from interior fragments of a larger asteroid that broke apart.[33] Dust collected from the asteroid surface is thought to have been exposed there for about eight million years.[32]

    Scientists used varied techniques of chemistry and mineralogy to analyze the dust from Itokawa.[33] Itokawa's composition was found to match the common type of meteorites known as "low-total-iron, low metal ordinary chondrites".[34] Another team of scientists determined that the dark iron color on the surface of Itokawa was the result of abrasion by micrometeoroids and high-speed particles from the Sun which had converted the normally whitish iron oxide coloring.[34]

    2018 Hayabusa results

    Two separate groups report water in different Itokawa particles. Jin et al. report water in low-calcium pyroxene grains. The water's isotope level corresponds with inner Solar System and carbonaceous chondrite water isotope levels.[35] Daly et al. report "OH and H
    2    O    " apparently formed by implantation of solar wind hydrogen. The rims of an olivine particle "show an enrichment of up to ~1.2 at % in OH and H20".[36] The water concentrations of the Itokawa grains would indicate an estimated BSI (Bulk Silicate Itokawa) water content in line with Earth's bulk water, and that Itokawa had been a "water-rich asteroid".[37]

    2020 Hayabusa results

    At the 2020 Lunar and Planetary Science Conference, a third group reported water and organics, via a third Hayabusa particle- RA-QD02-0612, or "Amazon." Olivine, pyroxene, and albite contain water. Isotopic compositions indicate a clear extraterrestrial origin.[38]

    See also

    Notes
    1. Lightcurve plot of (25143) Itokawa, Palmer Divide Observatory (716) by B. D. Warner (2004). Summary figures at the LCDB.

    References
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    2. Schmadel, Lutz D. (2006). "(25143) Itokawa [1.32, 0.28, 1.6]". Dictionary of Minor Planet Names – (25143) Itokawa, Addendum to Fifth Edition: 2003–2005. Springer Berlin Heidelberg. p. 188. doi:10.1007/978-3-540-34361-5_2203. ISBN 978-3-540-34361-5.
    3. "JPL Small-Body Database Browser: 25143 Itokawa (1998 SF36)" (2019-02-04 last obs.). Jet Propulsion Laboratory. Retrieved 25 February 2019.
    4. Fujiwara, A.; Kawaguchi, J.; Yeomans, D. K.; Abe, M.; Mukai, T.; Okada, T.; et al. (June 2006). "The Rubble-Pile Asteroid Itokawa as Observed by Hayabusa". Science. 312 (5778): 1330–1334. Bibcode:2006Sci...312.1330F. doi:10.1126/science.1125841. PMID 16741107. S2CID 206508294. Retrieved 25 February 2019.
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    6. "LCDB Data for (25143) Itokawa". Asteroid Lightcurve Database (LCDB). Retrieved 15 August 2017.
    7. Sekiguchi, T.; Abe, M.; Boehnhardt, H.; Dermawan, B.; Hainaut, O. R.; Hasegawa, S. (January 2003). "Thermal observations of MUSES-C mission target (25143) 1998 SF36". Astronomy and Astrophysics. 397: 325–328. Bibcode:2003A&A...397..325S. doi:10.1051/0004-6361:20021437.
    8. Abe, Shinsuke; Mukai, Tadashi; Hirata, Naru; Barnouin-Jha, Olivier S.; Cheng, Andrew F.; Demura, Hirohide; et al. (June 2006). "Mass and Local Topography Measurements of Itokawa by Hayabusa". Science. 312 (5778): 1344–1349. Bibcode:2006Sci...312.1344A. CiteSeerX 10.1.1.885.4729. doi:10.1126/science.1126272. PMID 16741111. S2CID 37768313.
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    12. Thomas, Cristina A.; Emery, Joshua P.; Trilling, David E.; Delbó, Marco; Hora, Joseph L.; Mueller, Michael (January 2014). "Physical characterization of Warm Spitzer-observed near-Earth objects". Icarus. 228: 217–246. arXiv:1310.2000. Bibcode:2014Icar..228..217T. doi:10.1016/j.icarus.2013.10.004. S2CID 119278697.
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