|Discovered by||Annibale de Gasparis|
|Discovery date||April 5, 1853|
|Alternative names||1947 BA; 1955 OH|
|Minor planet category||Main belt (Themis)|
|Epoch July 23, 2010 (JD 2455400.5)|
|Semi-major axis||468.1 Gm
|Orbital period||2021 d (5.54 yr)|
|Average orbital speed||16.76 km/s|
|Longitude of ascending node||35.99°|
|Argument of perihelion||107.7°|
|Mass||1.13±0.43×1019 kg 
|Mean density||2.78±1.35 g/cm³ |
|Equatorial surface gravity||0.15+0.08
−0.07 m/s² 
−20 m/s 
|Rotation period||0.34892 d (8 h 23 min)|
|Spectral type||C  (B-V=0.68)|
|Absolute magnitude (H)||7.08|
24 Themis is one of the largest main-belt asteroids. It is also the largest member of the Themistian asteroid family. It was discovered by Annibale de Gasparis on April 5, 1853. It is named after Themis, the personification of natural law and divine order in Greek mythology.
Discovery and observations
24 Themis was discovered on 5 April 1853 by Annibale de Gasparis of Naples, though it was given its name by fellow Italian astronomer Angelo Secchi. The asteroid was named after Themis, the Greek goddess of law. Gravitational perturbations in the orbit of Themis were used to calculate the mass of Jupiter as early as 1875.
On 24 December 1975, 24 Themis had a close encounter with 2296 Kugultinov with a minimum distance of 0.016 AU (2,400,000 km). By analyzing the perturbation of Kugultinov's orbit due to the gravitational pull of Themis, the mass of Themis was determined to be approximately 2.89×10−11 solar masses.
Orbit and rotation
Themis is in an elliptical orbit around the Sun with an eccentricity of 0.1306 and an inclination of 0.76°. It has an orbital period of 5.54 years. The distance between Themis and the Sun ranges from 2.71 AU at perihelion and 3.55 AU at aphelion, with a mean distance of 3.1302 AU. Themis is part of the Themis family of asteroids, which is located in the outer part of the main belt. The family consists of a core of large objects surrounded by a cloud of smaller objects; 24 Themis is a member of the core.
On October 7, 2009, the presence of water ice was confirmed on the surface of this asteroid using NASA’s Infrared Telescope Facility. The surface of the asteroid appears completely covered in ice. As this ice layer is sublimated, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface.
Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the Moon. The presence of ice on 24 Themis supports this theory. Because of its proximity to the sun (~3.2 AU), the widespread ice on the surface of 24 Themis is somewhat unexpected. The surface ice may be replenished by a sub-surface reservoir of water or impact gardening—a lunar phenomenon in which the moon overturns surface material at a rate of 1 m/ Gyr.
An alternative mechanism to explain the presence of water ice on 24 Themis is similar to the hypothesized formation of water on the surface of the Moon by solar wind. Trace amounts of water would be continuously produced by high-energy solar protons impinging oxide minerals present at the surface of the asteroid. The hydroxyl surface groups (S–OH) formed by the collision of protons (H+) with oxygen atoms present at oxide surface (S=O) can further be converted in water molecules (H2O) adsorbed onto the oxide minerals surface. The chemical rearrangement supposed at the oxide surface could be schematically written as follows:
- 2 S-OH → S=O + S + H2O
- 2 S-OH → S–O–S + H2O
where S represents the oxide surface.
Water presence on other celestial bodies:
- "JPL Small-Body Database Browser: 24 Themis". 2010-03-17 last obs. Retrieved 2010-05-27.
- Baer, James; Steven R. Chesley (25 June 1999). "Astrometric masses of 21 asteroids, and an integrated asteroid ephemeris" (PDF). Celestial Mech Dyn Astr (Springer Science+Business Media B.V. 2007) 100 (2008): 27–42. Bibcode:2008CeMDA.100...27B. doi:10.1007/s10569-007-9103-8. Retrieved 24 October 2008.
- Michalak, G. (2001). "Determination of asteroid masses". Astronomy & Astrophysics 374 (2): 703–711. Bibcode:2001A&A...374..703M. doi:10.1051/0004-6361:20010731. Retrieved 7 November 2008.
- (Mass of Themis 0.12 / Mass of Ceres 4.75) * Mass of Ceres 9.43E+20 = 2.38E+19
- Schmadel, Lutz D. (2003). Dictionary of minor planet names (5th ed.). Springer. p. 17. ISBN 978-3-540-00238-3.
- "Our Astronomical Column". Nature 13 (316): 48. 18 November 1875. Bibcode:1875Natur..13...47.. doi:10.1038/013047d0.[dead link]
- García, A. López; Medvedev, Yu. D.; Fernández, J. A. Moraño (1997). "Using Close Encounters of Minor Planets for the Improvement of their Masses". Dynamics and Astrometry of Natural and Artificial Celestial Bodies. Poznań, Poland: Kluwer Academic Publishers. pp. 199–204. ISBN 978-0-7923-4574-9.
- The Astronomical Almanac. United States Naval Observatory and United Kingdom Hydrographic Office. 2011. p. G2. ISBN 978-0-7077-4103-1.
- "Dictionary of Astronomy". Oxford Dictionary of Astronomy. Oxford University Press. 2010-05-27. p. 528.
- Cowen, Ron (8 October 2009). "Ice confirmed on an asteroid". Science News. Archived from the original on 12 October 2009. Retrieved 9 October 2009.
- Atkinson, Nancy (8 October 2009). "More water out there, ice found on an asteroid". International Space Fellowship. Archived from the original on 11 October 2009. Retrieved 11 October 2009.
- Campins, Humberto; Hargrove, K; Pinilla-Alonso, N; Howell, ES; Kelley, MS; Licandro, J; Mothé-Diniz, T; Fernández, Y; Ziffer, J (2010). "Water ice and organics on the surface of the asteroid 24 Themis". Nature 464 (7293): Pages:1320–1321. doi:10.1038/nature09029. PMID 20428164.
- Rivkin, Andrew S.; Emery, Joshua P. (2010). "Detection of ice and organics on an asteroidal surface". Nature 464 (7293): 1322–1323. Bibcode:2010Natur.464.1322R. doi:10.1038/nature09028. PMID 20428165.
- More Water Out There, Ice Found on an Asteroid | International Space Fellowship
- Asteroid Themis has 'frosted surface'
- NASA detection of Water Ice and organics on the surface of 24 Themis