(243) Ida and Dactyl

compiled by Wm. Robert Johnston
last updated 25 November 2008

Orbital elements and other data (Assumed or derived values in parenthesis, my estimates in italics and parenthesis. Source identifications in brackets, see this link for sources):

orbital data, primary (osculating elements) [JPL]:
semimajor axis a: 2.86162755840448 AU
orbital period P: 4.8409 y (=1768.2 d)
eccentricity e: 0.0461649374183922
perihelion distance q: 2.72952070125599 AU
aphelion distance Q: 2.99373442 AU
inclination i: 1.13746499957824°
argument of perihelion omega: 108.550476474476°
ascending node OMEGA: 324.209454531122°
mean anomaly M: 306.752252986874°
perihelion passage TP: 2006-05-06.5271778
Epoch: 18 AUG 2005
data arc: 1885-2005 (1163 obs.)

orbital data, secondary:
semimajor axis a: 108 km [A3a]
semimajor axis/primary radius a/Rp: (6.9)
orbital period P: 1.54 d [A3a]
eccentricity e: (>=0.2?)

other data, primary:
diameter: 31.3 ± 1.2 km, mean [B96a]
59.8 x 25.4 x 18.6 km [S03a]
max./min. dimensions: 55.8 km, 14.8 km [S03a]
absolute magnitude H: 9.94 [JPL]
rotation period: 0.1930680 ± 0.0000003 d [B93a]
(=4.633632 ± 0.000007 h)
amplitude delta M: 0.45-0.86 [MPb]
pole direction : 348.76° ± 7.5°, 87.10° ± 0.4° [D96a]
color index U-B: 0.416 ± 0.037 [SBa]
color index B-V: 0.802 ± 0.009 [SBa]
color index V-R: ?
color index R-I: ?
slope parameter G: (0.15) assumed
geometric albedo: 0.20 ± 0.04 [V96a]
mass: (4.2 ± 0.6)x1016 kg [B96a]
GM: 0.0030 km3/s2 [JPL]
density: 2.7 ± 0.4 g/cm3 [P97b]
type: S [SBd]

other data, secondary:
diameter: 1.4 km, mean [V96a]
1.6 x 1.4 x 1.2 km [V96a]
diameter ratio Ds/Dp: (0.045)
component magnitude difference: (13.5)
rotation period: ?
geometric albedo: 0.21 ± 0.02 [V96a]

--(243) Ida--discovery and notes:

Primary discovered 29 September 1884 from Vienna, Austria, by J. Palisa. Alternate designations A910 CD, 1988 DB1.

Companion discovered in images from 28 August 1993 taken by the Galileo probe during its flyby. The images were transmitted back to Earth somewhat later, and the satellite was recognized 17 February 1994 and reported 12 March 1994 by M. Belton and R. Carlson on behalf of the Galileo SSI and NIMS teams.

This image of Dactyl was taken by Galileo on 28 August 1993 from a range of 3900 km. Resolution is about 39 meters. The south pole is near the center of the unlit portion on the left. The largest crater, on the terminator (at 138°E, 39°S), is 280 meters across and has been named Acmon. A second 160-meter diameter crater at 220°E, 46°S (in shadow at lower center) is named Celmis. (Image credit: NASA/JPL)

Dactyl's orbit is poorly constrained by the available Galileo imagery. Rather, by assuming Dactyl to be in a bound orbit, constraints on Ida's mass have been found.

A shape model for Dactyl was obtained from Galileo imagery by Veverka et ali [V96a]; the maximum topographic variation from the best fit ellipsoid is 0.13 km. No evidence of rotation of Dactyl was obtained. Together with the fact that Dactyl's long axis is oriented towards Ida, this is consistent with Dactyl being in synchronous rotation with respect to Ida. Dactyl appears to be, in general, similar in composition to Ida; some slight differences, including a higher pyroxene/olivine ratio, are suggested [V96a].

See more information and links at Asteroid/Comet Connection Catchall Catalog page

--Links, more technical:

--Links, less technical:

--Links to ADS abstracts:

© 2001-2005, 2008 by Wm. Robert Johnston.
Last modified 25 November 2008.
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