18. Ida and Dactyl
Dynamics of an Asteroid

Note: Click on Solar System Exploration: Galileo Legacy Site (NASA-JPL) to get this image at its highest resolution.

This color-enhanced picture shows both asteroid 243 Ida and Dactyl (Ida's moon).
Ida (left) is about 56 kilometers (35 miles) long and Dactyl (right) is about 1.5 kilometers (1 mile) across in this view.
Although appearing to be "next" to Ida, Dactyl is actually in the foreground, about 80 kilometers (48 miles) closer to the spacecraft than Ida is.
The brighter areas, appearing bluish in the picture, suggest a difference in the abundance or composition of iron-bearing minerals in those areas.
Dactyl is not identical in spectral properties to any area of Ida in view here, though its overall similarity in reflectance and general spectral type suggests that it is made of basically the same rock types.
About 14 minutes before closest approach on August 28, 1993.
Range, 10,500 kilometers (6,500 miles).

This color picture is made from a sequence of images in which Dactyl, Ida's moon, was originally discovered.

This picture is made from images through the 4100-angstrom (violet; 16 millionths of an inch), 7560 ─ (infrared; 30 millionths of an inch) and 9680 ─ (infrared; 38 millionths of an inch) filters.
The color is enhanced in the sense that the camera is sensitive to near-infrared wavelengths of light beyond human vision; a natural color picture of this asteroid would appear mostly gray.
Shadings in the image indicate changes in illumination angle on the many steep slopes of this irregular body as well as subtle color variations due to differences in the physical state and composition of the soil.

Combining this data with further imaging data and more detailed spectra from Galileo's near-infrared mapping spectrometer, may allow scientists to determine whether the larger parent body -of which Ida, its moon and some other asteroids are assumed to be fragments- was a heated, differentiated object, or was made of relatively unaltered primitive chondritic -i.e. having small, rounded grains about 1 to 5 mm (0.04 to 0.2 inches) in their interiors- material.

Shortly after the discovery, Galileo scientists found it unlikely that the moon is a passing body caught in Ida's gravity.
They also doubt that the moon is a piece of Ida knocked loose by a smaller projectile, especially since Ida and Dactyl's bulk compositions differ slightly.
Instead, scientists are theorizing that the two are siblings of a "family" of asteroids formed hundreds of millions of years ago when a larger, 100-kilometer-wide asteroid was shattered in a great collision.
Instead of fragments shooting straight out from the impact, the exploding asteroid may have produced jets of material carrying two or more objects out together. Those objects could then be captured, gravitationally, around each other.

Thus, a family of asteroids could have been created as a result of such an impact.
Ida belongs to the Koronis family that travels in the main Asteroid Belt between Mars and Jupiter.
Gaspra, the asteroid visited by Galileo in October 1991, is a member of the Flora family.

The Dynamics of an Asteroid

(excerpted from "Solving for Dactyl's Orbit and Ida's Density" in The Galileo Messenger #35; NASA-JPL, December, 1994)

[ Although Ida and Dactyl were photographed near each other, scientists needed to use detective work and some circumstantial evidence to prove that Dactyl was a satellite.
This article shows that finding Dactyl's orbit and period was far from straightforward, and that the result also tells us something about Ida's mass and composition!

Galileo's discovery that Ida has a satellite (now known as Dactyl) suggests that satellites orbiting asteroids may be a commonplace occurrence.
Dactyl and Ida appear in 47 Galileo images.
Their locations in these images were used to estimate Dactyl's orbit and Ida's bulk density, which is of great interest because it may indicate whether Ida is composed of rocks that have been thermally processed deep within a collisionally destroyed planetesimal.
Density calculations were based on an Ida volume of 16,100 cubic kilometers, which was determined from an accurate model of the shape of Ida based on Galileo's images.
While Dactyl's orbit is of interest, its greatest significance is in providing the first accurate estimate of an S (stony)-type asteroid's density -another achievement by Galileo!

It became clear almost immediately that the mass (or, for a given volume, density) of Ida could not be solved at the same time as Dactyl's orbit.
Instead, a series of Dactyl orbits were generated for a range of Ida mass/density values -from 1.5 to 4.0 grams per cubic centimeter.
For each density value, there is a unique orbit; over this range of densities, these orbits differ greatly.
For Ida densities less than about 2.1 grams per cubic centimeter, the orbits are just barely hyperbolic.
For higher Ida densities, the orbits are elliptical with a large apoapsis (farthest point from Ida), a periapsis (nearest point to Ida) of around 80-85 km, and periods that range from just over a day to many tens of days.
At a density of about 2.8 grams per cubic centimeter, the orbit is nearly circular (about 82 by 98 km) with a period of about 27 hours.
For even higher densities, the elliptical orbits have apoapses of about 95-100 km, with periapses that decrease with increasing density.
For an Ida density greater than about 2.9 grams per cubic centimeter, the periapsis is less than about 75 km and the period is less than 24 hours.

Dynamical studies show that orbits with periapses less than about 75 km from Ida are unstable and either collide with, or escape from, Ida; thus, orbit solutions that correspond to an Ida density of about 2.9 grams per cubic centimeter or greater, are not physically possible.

At the other extreme, hyperbolic and even highly elliptical orbits around Ida are very unlikely.
The observed speed of Dactyl around Ida for any of the orbit solutions is no more than about 10 m/s, about the speed of a fast run or a slowly thrown baseball.
Calculations indicate that the chance of a random piece of asteroidal material the size of Dactyl passing by Ida at that speed, just when Galileo was observing it, is about 2 x 10^-17.
In addition, if Dactyl were in a hyperbolic or highly elliptical orbit, it should have been seen by the Hubble Space Telescope (HST) when it observed the region around Ida over an 8-hour period on April 26, 1994.
HST would have easily seen Dactyl had it been more than about 700 km from Ida.
Combining these two restrictions gives a preliminary estimate for Ida's density of 2.2 to 2.9 grams per cubic centimeter, and shows that Dactyl is, in fact, a satellite of Ida's.
Allowing for a ▒12% uncertainty in the modeled volume of Ida, increases the range to 2.0 to 3.2 grams per cubic centimeter.

This density range is surprisingly limited and suggests that Ida is fairly porous and/or made of fairly light rocks.
This result excludes several classes of dense igneous rocks that had previously been suggested as the primary components of Ida.
Clearly, Ida is neither all ice, nor all iron.

Further work on the long-term stability of orbits that fit these observations, as well as a more precise analysis of the images themselves, may lead to a better determination of both the density of Ida and the orbit of Dactyl.
These, combined with other ongoing work involving the color, spectral properties, and geology of Ida's surface are expected to lead to major advances in our knowledge of the nature of asteroids and what they can tell us about the birth of the planets.

Next slide: Highest-Resolution Image of Dactyl

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