Asteroids with Satellites Database--Johnston's Archive

(243) Ida and Dactyl

compiled by Wm. Robert Johnston
last updated 28 May 2014
under construction--testing upgrades!

--Orbital and physical data--

(See this page for definitions of quantities; source codes in brackets, see this link for full references; *A, *D, and *E indicate assumed, derived, and estimated values, respectively.)

dynamical type, primary: main belt asteroid


orbital data, primary (osculating elements) [JPL] :
semimajor axis a: 2.862471602 AU
orbital period P: 4.843061483 yr (= 1768.9282070 d)
eccentricity e: 0.041485093
perihelion distance q: 2.74372170 AU
aphelion distance Q: 2.981221504 AU
inclination i: 1.1322337°
argument of perihelion ω: 110.31757°
ascending node Ω: 324.02846°
mean anomaly M: 236.460083°
perihelion passage TP: 2016 Jan 20.0367
Epoch: 2014 May 23
data arc: 1885-2013 (2209 obs.)
Earth MOID: 1.75787 AU
Jupiter Tisserand invariant TJ: 3.3


orbital data, secondary:
binary dynamical type:L [*VO]
semimajor axis as: 108 km [M03b]
separation/primary radius as/rp:6.9 [*D]
separation/Hill radius as/rH:0.014 [*D]
orbital period Ps: 1.54 d [M03b]
eccentricity es: 0.2 [M03b]
normalized ang. mom. αL: 1.11 [*VO]

other data, system (combined):
absolute mag. H: 9.94 [MPC]
slope parameter G: 0.15 [*A]
effective diameter dE: 31.4 ± 1.2 km [T96a]
geometric albedo: 0.20 ± 0.04 [V96a]
color index U-B: 0.416 [T95a]
color index B-V: 0.802 [T95a]
taxonomic type: S (SMASSII) [JPL]
S (Tholen) [JPL]
mass m: 4.2x1016 ± 6x1015 kg [B96a]
density ρ: 2.6 ± 0.5 g/cm3 [B96a]
Hill radius rH: 7900 km [*D]

other data, primary:
diameter dp: 31.4 ± 1.2 km [T96a]
dimensions :59.8x25.4x18.6 km [*S03a]
geometric albedo :0.20± 0.04 [V96a]
rotation period RPp: 4.633632 ± 0.000007 h [B93a]
amplitude in mag., rotational ΔM: 0.655 ± 0.205 [*MPb]
pole direction : 384.76±7.50° ; 87.10±0.40° ; [*D96a]


other data, secondary:
diameter ds: 1.4 km [V96a]
dimensions :1.6x1.4x1.2km [V96a]
diameter ratio ds/dp: 0.045 [*D]
component mag. difference ΔM: 6.7 [*D]
geometric albedo :0.21± 0.02 [V96a]
rotation period RPs: ?

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

Primary discovered 1884 Sep 29 from Vienna, Austria by J. Palisa. Alternate designation(s): 1988 DB1, A910 CD. Permanent number assigned 1884.

Named for the Greek mythological nymph.

Shape model for Ida published [*].

Companion discovered 1993 Aug 28 by M. Belton, R. Carlson, Galileo SSI Science Team using observations from Galileo spacecraft. Announced 1994 Mar 12 [I5948]. Provisional designation S/1993 (243) 1. Permanent designation I Dactyl, assigned 1994 Sep 26; named for the Greek mythological creature [I6082].

Specifically, Dactyl was 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.

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].

--Links, more technical:

--Links, less technical:

--Links to ADS abstracts:


© 2001-2014, 2014 by Wm. Robert Johnston.
Last modified 28 May 2014.
Return to Home. Return to Astronomy and Space. Return to Asteroids with Satellites.