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Last Month's Talk: A Review

by Pamela Mydock
Asteroids, impact craters and the first asteroid discovered with its own natural satellite featured at August meeting.

Dr. Daniel D. Durda, Research Associate of the Lunar and Planetary Laboratory of the University of Arizona, was the guest speaker at the meeting of the Alachua Astronomy Club Tuesday evening. His topic was "Ida and Dactyl: Big Craters and Little Moons," the discovery story of the first asteroid in our solar system with its own natural satellite. Durda presented images taken from the Galileo spacecraft as well as artist renderings of asteroids.

Durda was in Gainesville for the International Astrophysical Union Colloquium, sponsored by the University of Florida Department of Astronomy. Durda is just one of the many astronomers from around the globe who came to Gainesville for the three day event, to study interplanetary particles and asteroids. "Gainesville feels like home to me," said Durda, who spent many years as a student at the University of Florida. He spends his time now writing code to make computer models that fit the data received from spacecraft such as Galileo. "This is what astronomers do now, analyze data. I haven't looked through a telescope for two years."

The asteroid 243 Ida is named after a cave on Mount Ida on the Mediterranean island of Crete. The small satellite has been named Dactyl, after the Dactyli, mythological lizard-like creatures who inhabited the mountain. Legend has it that the ancient Greek god Zeus (known as Jupiter to the Romans) was raised as a child in this cave and was protected by the creatures. Space, however, may not be the best place to raise your kids. The asteroid's rocky surface has hills and craters, but no rivers or trees, not to mention much gravity. Ida orbits the Sun between the planets Mars and Jupiter, where it is very cold. Ida is estimated to be about 56 kilometers long and 24 kilometers wide, roughly shaped like a 35 mile long baking potato. Dactyl is almost spherical, about like a one mile wide garbanzo bean.

Asteroid 243 Ida was discovered over 100 years ago but her satellite was seen for the first time in 1993, after the space probe Galileo did a fly-by on the way to Jupiter. It has been speculated that the tiny moon unit may be a chunk of the asteroid that broke free after a collision with another asteroid. Or it may be a collection of very loose particles and dust that came together gradually. Ida and Dactyl offer physical evidence of asteroid doublets, a configuration which has in the past resulted in double impact craters on both the Earth and the Moon.

Durda plans to publish the results of his research soon. He explained that it was very difficult to keep quiet about the discovery of Dactyl when it was first detected in the returning images from Galileo. "We had to be sure what it was before the news was released to the press." The odds that Dactyl was not a satellite of Ida, and was just passing by when Galileo snapped the images, are about "ten million trillion to one," according to Durda.

The process of naming the craters and peaks on both Ida and Dactyl is not yet complete. Names will primarily include those of caves and grottoes on Earth. Durda plans to include names from around North Central Florida as well.

In the years since he graduated and left Gainesville, Durda has learned to paint. He now can express his excitement over various physical concepts that can not be adequately photographed by using airbrush and more traditional painting techniques.

"It started as a way to relax at the end of a long day, using the left side of my brain. I needed to exercise the right side of my brain. It turned out to be very enjoyable and now I am an official card-carrying member of the International Association of Astronomical Artists." Durda plans to use his own paintings to illustrate the results of his astronomical research.

[Note: See Dan Durda's Space Art Gallery. Also see the AAC's space art web page. Ed.]

At the end of his presentation, amateur astronomers asked about the possibility of asteroids hitting the Earth. Durda answered by saying, "So far we have only actually detected a few hundred objects..., but it is projected that there are possibly a few thousand that are at least one kilometer across." He suggested that our knowledge of nearby space is incomplete and stressed the importance of continuing funding for projects such as NEAR, Near Earth Asteroid Rendezvous, which is scheduled for a fly-by of asteroid Eros in 1997. In order to be able to divert an asteroid from a collision path with the Earth, first you have to understand what they are made of and what holds them together. Durda offered small consolation to the audience when he said that catastrophic collisions only happen "about once in every two million years."

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