25143 Itokawa

25143 Itokawa

Image of Itokawa from the Hayabusa spacecraft
Discovery[1]
Discovered by	LINEAR
Discovery site	Lincoln Lab's ETS
Discovery date	26 September 1998
Designations
MPC designation	(25143) Itokawa
Pronunciation	/ˌiːtoʊˈkɑːwə/ Japanese: [itoꜜkawa]
Named after	Hideo Itokawa[2]
Alternative designations	1998 SF36
Minor planet category	Apollo NEO PHA[1][3]
Orbital characteristics[3]
Epoch 27 April 2019 (JD 2458600.5)
Uncertainty parameter 0
Observation arc	20.38 yr (7,443 d)
Aphelion	1.6951 AU
Perihelion	0.9532 AU
Semi-major axis	1.3241 AU
Eccentricity	0.2801
Orbital period (sidereal)	1.52 yr (557 d)
Mean anomaly	288.88°
Mean motion	0° 38m 48.48s / day
Inclination	1.6214°
Longitude of ascending node	69.081°
Argument of perihelion	162.82°
Earth MOID	0.0131 AU (5.10 LD)
Physical characteristics
Dimensions	535 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]
Synodic rotation period	12.132 h[6][9]
Geometric albedo	0.23[7] 0.283±0.116[5] 0.36±0.22[10] 0.53[11]
Spectral type	SMASS = S (IV)[3] Sqw[12] S[13] Q[6]
Absolute magnitude (H)	18.61[14] · 18.95 (R)[15] 19.00[13] · 19.2[1][3] 19.48[6][7] · 19.51±0.09[5]

25143 Itokawa (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.

It was the first asteroid to be the target of a sample-return mission, of the Japanese space probe Hayabusa, which collected more than 1500 regolith dust particles from the asteroid's surface in 2005. Since 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 was the smallest asteroid to be photographed and visited by a spacecraft prior to the DART mission to Dimorphos in 2022.

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]

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 in December 2006. Right: animated orbits of Itokawa (green) and Earth (blue) around the Sun.

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]

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.

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

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]

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]

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][a] Ďurech (12.1323 h),^[31] and Nishihara (12.1324 h).^[15]

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]

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 H₂O" 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 H₂O".^[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]

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]

A further report by Daly's group was published which supported the theory that a large source of Earth's water has come from hydrogen atoms carried on particles in the solar wind which combine with oxygen on asteroids and then arrive on Earth in space dust. Using atom probe tomography the study found hydroxide and water molecules on the surface of a single grain from particles retrieved from the asteroid Itokawa by the Japanese space probe Hayabusa.^[39][40]

Dust ponds are identified in the asteroid. They are a phenomenon where pockets of dust are seen in Celestial bodies without a significant atmosphere. Smooth deposits of dust accumulate in depressions on the surface of the body (like craters), contrasting from the Rocky terrain around them. ^[41] In the Sagamihara and Muses-Sea regions of the asteroid dust ponds were identified. Dust particles had a size varying from millimeters to less than a centimeter.

• 162173 Ryugu, target of sample-return mission Hayabusa2, the successor to Hayabusa
• 101955 Bennu, target of NASA sample-return mission OSIRIS-REx
• (341843) 2008 EV5, target of cancelled NASA sample-return mission Asteroid Redirect Mission

• Normile, D (30 April 2010). "Spunky Hayabusa Heads Home With Possible Payload". Science. 328 (5978): 565. Bibcode:2010Sci...328..565N. doi:10.1126/science.328.5978.565. PMID 20430991.

Wikimedia Commons has media related to 25143 Itokawa.

• Hayabusa's Scientific and Engineering Achievements during Proximity Operations around Itokawa (JAXA press release)
• Gazetteer of Planetary Nomenclature
• MIT's LINEAR asteroid named for Japan's 'Dr. Rocket' (MIT press release)
• JAXA Hayabusa official page
• Earth impact probability of the Asteroid (25143) Itokawa to be sampled by the spacecraft Hayabusa (paper abstract)
• Astronomy Picture of the Day: Approaching Asteroid Itokawa, A Robot's Shadow on Asteroid Itokawa, The Missing Craters of Asteroid Itokawa, Smooth Sections on Asteroid Itokawa
• Special issue: Hayabusa at Itokawa, Science, Vol. 312, no. 5778, 2 June 2006
• Initial Scientific Results of Hayabusa’s Investigation on Itokawa ~Summary of the Special Issue of "Science"Magazine~ (ISAS/JAXA press release)
• Hot Topic: Hayabusa – Dust from Itokawa, Science, Vol. 333, no. 6046, 26 August 2011
• Animated model of Itokawa rotating (in anaglyph form for use with red-blue glasses)
• Itokawa in enhanced color (From this presentation)
• List of the Potentially Hazardous Asteroids (PHAs), Minor Planet Center
• PHA Close Approaches To The Earth, Minor Planet Center
• List Of Apollo Minor Planets (by designation), Minor Planet Center
• 25143 Itokawa at NeoDyS-2, Near Earth Objects—Dynamic Site
  • Ephemerides · Observation prediction · Orbital info · MOID · Proper elements · Observational info · Close approaches · Physical info · Orbit animation
• 25143 Itokawa at the JPL Small-Body Database
  • Close approach · Discovery · Ephemeris · Orbit viewer · Orbit parameters · Physical parameters