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First Evidence of Asteroid Moon Shown

March 24, 1994|ROBERT LEE HOTZ | TIMES SCIENCE WRITER

NASA scientists at the Jet Propulsion Laboratory on Wednesday released the first evidence of a moon orbiting an asteroid, a discovery that could yield clues to the creation of the solar system and how the traffic jam of orbiting rocks between Mars and Jupiter came into being.

The moon, a mountain of stone about a mile in diameter, appears to circle an asteroid the size of Los Angeles, called Ida. The National Aeronautics and Space Administration released a photograph of the asteroid and its newly discovered moon.

"This is the first clear detection of a satellite of an asteroid," said Clark R. Chapman at the Planetary Science Institute at Caltech in Pasadena. "There have been hints before.

"One reason this is important is that asteroids have played a role in shaping our own planet. We need to understand these objects that rain down on us," he said.

Such small pairs of asteroids and their moons may be responsible for unusual double craters found in Canada, Siberia and on the surface of the moon and Mars, astronomers and scientists said Wednesday.

"At first we worried it might be scrambled data, but most of us were believers," said imaging team member Ann Harch, who first spotted the object near Ida in a photograph transmitted to Earth by the agency's Galileo probe. Aesthetically, it is just so beautiful to think of all these objects dancing out there together.

"Discovery. . . . It's great!" she said.

Science Coordinator Marcia Segura, who helped confirm the moon's existence with measurements from an infrared sensor also on board Galileo, said, "At first we were kind of blase about it. Then we were really jazzed. Really excited. The adrenalin is still pumping. It comes in waves."

So far the only thing NASA scientists can say for certain about Ida's moon is that it exists--hanging in space, about 62 miles from the larger asteroid, well within its sphere of gravity. Both lie in the belt of more than 50,000 cold, dusty, meteor-pocked rocks between the orbits of Mars and Jupiter.

But it may be months before scientists can determine its composition, its orbit around Ida or how fast it is moving. They do know that for an object in the deep freeze of space, it is relatively warm--about 99 degrees Fahrenheit below zero.

"I am sorry I can't be definite (now) but the indications are that the satellite is the same type of material as Ida," said Robert W. Carlson, the JPL scientist in charge of the infrared mapping sensor on Galileo. "It is so similar that it may even be part of the same rock Ida is--a true daughter of Ida."

NASA scientists said Wednesday they suspect the moon and Ida were formed at the same time, but they don't know when.

Ida, a battered rock shaped like a sweet potato, belongs to the Koranis family of asteroids. Scientists think the group was formed when a very large asteroid perhaps 180 miles in diameter broke up several hundred million years ago, creating a constellation of smaller orbiting fragments of rock.

Galileo project scientists said they think that the moon may be a smaller fragment from the break-up of that parent body, or that it may be a chip off the asteroid Ida itself caused by a more recent collision. They said it was virtually impossibly that the moon just happened to wander by the larger asteroid and be captured by its gravity field.

One thing they hope to learn from Ida and its moon is how such small orbiting bodies interact within the asteroid belt, under conditions that resemble those during the formation of the solar system.

Now that they have found one moon around an asteroid, NASA scientists said, they will step up the search for others. "If you find one, then it is probably not the only one. We will be taking a look for others," said Michael J.S. Belton, a space imaging expert from the National Optical Astronomy Observatories in Tucson.

The new moon has been provisionally designated "1993 (243) 1," meaning that it is the first natural satellite discovered in 1993 near Ida, which in turn was the 243rd asteroid discovered over the past two centuries. Eventually, the International Astronomical Union will give the moon its formal name.

All About Ida

* Background: The moon was photographed by Jupiter-bound space probe Galileo last Aug. 28, about 14 minutes before its closest approach to the asteroid. The photograph was transmitted to Earth earlier this year.

* Size: Ida is about 35 miles long, and its moonlet, which some astronomers are calling "Baby Ida," is about one mile long.

* Location: There are more than 50,000 asteroids orbiting in a belt between Mars and Jupiter. Sun-scorched, meteor-pocked, and devoid of life, they range in size from a few feet in diameter to more than 600 miles across. Often they collide, producing debris that falls to Earth as meteorites.

* Significance: NASA scientists hope that their observations of Ida and its moon will help them better understand the formation of the solar system.

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