Galileo's flyby of 243 Ida in August 1993 led to the discovery of a sm
all satellite, Dactyl, some 85 km from the asteroid's center. From Ear
th at mean opposition, the satellite is a V = +20.3 mag object (some 6
.7 magnitudes fainter than Ida). Forty-seven images of the satellite a
t 18 different observing times were played back, including one multico
lor sequence in which the satellite is resolved adequately to distingu
ish surface markings (similar to 105 m/pxl) and three higher resolutio
n single-color views (89, 39, and 24 m/pxl). The satellite, mean radiu
s = 0.7 km, is an elongated, but not angular body with principal diame
ters of 1.6 x 1.4 x 1.2 km. In the highest resolution view, the longes
t axis points approximately in the direction of Ida, and its shortest
axis is perpendicular to the orbital plane. The spin period is slow (>
8 hr?) and may be synchronous. The satellite shows no conspicuous sha
rp edges and is much less irregular in shape than Ida. Limb profiles a
re remarkably smooth over distances of 200-300 m. The geometric albedo
s of the two objects are similar (0.20 vs 0.21), as are the 0.4-1.0-mu
m colors. Like Ida, Dactyl is an S-asteroid, but has a slightly deepe
r 1-mu m band than Ida (by 5-8%). While no identical regions (in color
) are seen on Ida, the color difference is consistent with color varia
tions reported within the Koronis family and may be due to a slightly
higher pyroxene/olivine ratio on the satellite. More than a dozen crat
ers ranging from less than or similar to 90 to 280 m diameter are visi
ble in the best image (39 m/pxl at 470 phase). The largest contains an
off-centered, positive relief feature some 75 m across. The image inc
ludes an intriguing crater chain, but no grooves, ridges, or sharp edg
es are evident. In terms of limb roughness, Dactyl is much smoother th
an Ida, but comparable to the two satellites of Mars, Phobos and Deimo
s. While the satellite's origin is uncertain, a likely scenario would
have the satellite date from the breakup of the Koronis family. It is
interesting that crater densities on the satellite are similar to thos
e on Ida itself. (C) 1996 Academic Press, Inc.