19. Highest-Resolution Image of Dactyl

How Do You Name an Asteroid?
Cratering and the Earth
What Have We Learned About Asteriods From Galileo's Flybys?

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

This image is the most detailed picture of Dactyl, the newly discovered moon of asteroid Ida.
Dactyl is surprisingly round, measuring about 1.2 x 1.4 x 1.6 kilometers (0.75 x 0.87 x 1 miles).
More than a dozen craters larger than 80 meters (250 feet) in diameter are clearly evident.
The larger crater on the terminator is about 300 meters (1,000 feet) across.
August 28, 1993. Range, 3,900 kilometers (2,400 miles).

Shuttered through the camera's broadband clear filter as part of a 30-frame mosaic designed to image the asteroid Ida itself, this frame fortuitously captured the previously unknown moon just over 4 minutes before the spacecraft's closest approach to Ida.
Features as small as about 78 meters (250 feet) are visible on the surface of the moon.
At the time this image was shuttered, Ida was about 90 kilometers (56 miles) away from the moon, outside this frame to the left and slightly below center.

The large number of craters indicates that the moon has suffered numerous collisions from smaller solar system debris during its history.
Craters are seen on all solid bodies in the solar system in which weathering doesn't occur, and even a few there as well.
Micrometeoroid bombardment is a form of weathering - that's why the features are softened and a regolith (an outer layer of fine soil and loose rocks) exists.

How do you name an asteroid?

Just like a newborn baby, Ida's tiny moon needed a name. But, unlike a human infant, the moon's name would have to be approved by the International Astronomical Union (IAU).
As discoverers of the moon, the Galileo Project had the honor of recommending a name to the IAU, which approved the name Dactyl in the fall of 1994.

Members of the Galileo flight team came up with some suggestions of their own, including the lighthearted "Ho" (thus favoring one state over 49 others) and "Lupino" (the late Ida Lupino being a well-known actress in the 1930's and 40's).
The Galileo Project invited the general public to suggest names, and it was one of those suggestions that gave Dactyl its name.

The name is derived from the Dactyli, a group of Greek mythological beings who lived on Mount Ida, where the infant Zeus was hidden from his father Cronus, who knew that Zeus could one day depose him.
In some accounts, Zeus - whose Roman name is Jupiter - was raised by the nymph Ida and protected by the Dactyli, who danced around the cave in which Zeus was hidden to make noise which would drown out the infant Zeus's crying.
Modern parents, of course, prefer to use the stereo.

Cratering and the Earth

Cratering affects the Earth's environment, too.
The Earth's atmosphere protects us from the multitude of small debris, the size of grains of sand or pebbles, thousands of which pelt our planet every day, and from most stony meteoroids up to about 10 meters in diameter.
But larger falls can have a serious impact: for example, the 1908 Tunguska event (which leveled 2,200 square kilometers of Siberian forest) was a stony meteorite in the 100-meter class.
The famous meteor crater in northern Arizona, some 1,219 meters (4,000 feet) in diameter and 183 meters (600 feet) deep, was created 50,000 years ago by a nickel-iron meteorite perhaps 60 meters in diameter.
Falls of this class occur once or twice every 1,000 years.

There are now over 100 ring-like structures on Earth recognized as definite impact craters.
Most of them are not obviously craters, their identity masked by heavy erosion over the centuries, but the minerals and shocked rocks present make it clear that impact was their cause.

What have we learned about asteroids from Galileo's flybys?

Although scientists are still working with data from Galileo's asteroid flyby data, we have discovered a great deal so far about asteroids.

The main thing noticable about Gaspra's surface are major depressions, indentations, ridges, and craters.
Some of these craters are as wide as 1.5 kilometers - indicating, for example, an impact with a body 100 meters (330 feet) in size, traveling 5 kilometers per second (3.1 miles per second).
Gaspra has probably existed in its present form for the last 200 to 500 million years.
Another initial observation is that the albedo (or reflectivity) is relatively homogeneous, although there are noticeable variations in albedo and color at about the 10 percent level.
Some data suggest compositional differences between Gaspra's northern and southern hemispheres.

Since the gravitational force is so low, when something impacts with Gaspra, most of the dust and material from the resulting collision fly off the asteroid.
With these conditions, scientists expected to see sharply defined craters and ridges, since there should have been little, if any, surface rubble (called regolith).
What is surprising about Gaspra, then, is its subdued appearance.
There is a small layer of regolith on the surface.
As yet, scientists cannot tell how deep this layer may be, but initial estimates are from a few centimeters to a few meters.
Scientists did note, with relief, that the Dust Detector registered no increase in dust impacts during the encounter, indicating a relatively "clean" environment around the asteroid.

Other data from the Gaspra flyby suggested that the asteroid may have a magnetic field.
If Gaspra has a permanent magnetic moment, the finding would have bearing on its thermal history, have implications for the history of the magnetic field of the early solar system, and give us more reason to believe some asteroids are very rich in iron or iron-nickel alloys of considerable economic value.

Analysis of Dactyl's possible orbit provided the first accurate estimate of an S-type (stony iron type) asteroid's density - another achievement by Galileo!
Preliminary estimates of Ida's density are in the range of 1.9 to 3.2 grams per cubic centimeter.
This density range is surprisingly well constrained and suggests that Ida is fairly porous and/or made of fairly light rocks.
This result already excludes several classes of dense igneous rocks that had previously been suggested as the primary components of Ida's composition.

Further work on the stability of Dactyl's possible orbits, as well as a more precise analysis of Dactyl, 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.

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Updated: November 24 '96

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