243 Ida

**243 Ida** is a Main belt asteroid with pictures taken by the _Galileo_ spacecraft on August 28, 1993. It was the first binary asteroid to be found and is a member of the Koronis family of asteroids.

## Discovery and name

Ida was found by Johann Palisa on September 29, 1884 in Vienna. It is named after Ida, a Cretan nymph in Greek mythology who lived on a mountain that has her name (see Mount Ida, Crete).

## Moon

Ida has a small moon, Dactyl, which was found on February 17, 1994 by _Galileo_ mission member Ann Harch, while examining the delayed image downloads. The moon was given the designation **S/1993 (243) 1** and later named **(243) Ida I Dactyl**. The pictures where it was first seen were taken on 28 August 1993, hence the designation. The moon was reported on 12 March 1994.

Dactyl is only 1.4 km in diameter. Some researchers believe that Dactyl formed from debris blown off from Ida because of cratering, while others suggest that Ida and Dactyl formed as a pair a billion or more years ago when Ida's parent body was disrupted. Both of these theories present difficulties that are unresolved at this time. Since Dactyl was found, other sightings have suggested that asteroid moons are common.

Although Ida's dimensions, measured along the principal axes (based on its rotation) are 53.6×24.0×15.2 km, the best-fit ellipsoid measures 60.0×25.2×18.6 km. Since its mass is well known thanks to its little moon, one can calculate that its surface gravity can be between 0.0031 and 0.0324 m/s². The rotation axis is within one degree of the shorter dimension axis, which means the centrifugal effect can reach a value as high as 0.0042 m/s² — at the tips of its longest axes, Ida is actually under tension.

## Related pages

- Comet
- Asteroid

## References

- P. C. Thomas, M. J. S. Belton, B. Carcich, C. R. Chapman, M. E. Davies, R. Sullivan, and J. Veverka; _The shape of Ida_, Icarus, Issue 1, No. 120, pp. 20–32, 1996.
- M. E. Davies, T. R. Colvin, M. J. S. Belton, P. C. Thomas, J. Veverka, The Galileo Imaging Science Team, _The North Pole Direction and the Control Network of the Asteroid 243 Ida_, Bulletin of the American Astronomical Society, Vol. 26, p. 1154, June 1994
- Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites, 2000
- [ftp://ftp.lowell.edu/pub/elgb/astorb.html The Asteroid Orbital Elements Database]
- Minor Planet Discovery Circumstances
- IAUC 6082 announcing Dactyl's naming

## Infobox (planet)

- **bgcolour**: #FFFFC0
- **minorplanet**: yes
- **name**: 243 Ida
- **image**: 243 ida.jpg
- **caption**: Galileo spacecraft, Galileo image of 243 Ida. The tiny dot to the right is its moon, Dactyl.
- **discoverer**: Johann Palisa
- **discovered**: September 29, 1884
- **alt names**: A910 CD; 1988 DB<sub>1</sub>
- **mp category**: Main belt (Koronis family, Koronis)
- **epoch**: October 22, 2004 (Julian day, JD 2453300.5)
- **semimajor**: 428.025 Giga, Gmetre, m (2.861 Astronomical unit, AU)
- **perihelion**: 408.207 Gm (2.729 AU)
- **aphelion**: 447.843 Gm (2.994 AU)
- **eccentricity**: 0.046
- **period**: 1767.724 day, d (4.84 Julian year (astronomy), a)
- **inclination**: 1.138degree (angle), °
- **asc node**: 324.218°
- **arg peri**: 108.754°
- **mean anomaly**: 245.469°
- **avg speed**: 17.60 km/second, s
- **satellites**: Dactyl (moon), Dactyl
- **dimensions**: 53.6×24.0×15.2 kilometre, km
- **mean radius**: 15.7 km
- **mass**: 4.2×10<sup>16</sup> kilogram, kg [http://www.psi.edu/pds/archive/astdata/DENSITY/densbest.tab <sup>1</sup>]
- **density**: 2.6 g/cubic centimetre, cm³ [http://www.psi.edu/pds/archive/astdata/DENSITY/densbest.tab <sup>1</sup>]
- **surface grav**: 0.0109 m/s²
- **escape velocity**: 0.0185 km/s
- **rotation**: 0.1931 d (4 h 37 min) [http://www.astrosurf.com/aude-old/map_files/AstVarMAP01-2003.htm <sup>2</sup>]
- **spectral type**: S-type asteroid, S
- **abs magnitude**: 9.94
- **albedo**: 0.2382 [http://www.psi.edu/pds/archive/astdata/ALBEDOS/albedos.tab <sup>3</sup>]
- **temp name1**: Kelvin
- **mean temp 1**: ~158
- **max temp 1**: 229
- **temp name2**: Celsius
- **max temp 2**: -45°