The gathering could be the largest in the history of the society, which was founded in August 1933 at the Field Museum in Chicago. Organizers expect as many as 350 scientists from around the world to attend the meeting, which will convene at the Hotel Inter-Continental, 505 N. Michigan Ave.

"Because the society actually was founded in Chicago, we thought it would be appropriate to bring it back to Chicago during the millennial year," said meeting co-chair Meenakshi Wadhwa, curator of meteorites at the Field Museum and a research scientist/lecturer at the University of Chicago.

The society's founding president, the late Frederick Leonard, was a 1918 University of Chicago graduate who grew up in a row house near 51st Street in Chicago's Kenwood neighborhood.

"He used to set up his telescope in the backyard and look at the heavens," said meeting co-chair Andrew Davis, a research scientist at the University of Chicago's Geophysical Sciences Department. Roy Clarke Jr., research chemist and curator emeritus of meteorites at the Smithsonian Institution will present a talk on Leonard titled, "Before He Knew Meteorites."

Highlighting this year's meeting are special sessions on current planetary missions and future sample-returns, and on interstellar dust grains that were formed before the birth of the solar system 4.5 billion years ago.

The special session on current missions and future sample returns will include talks regarding two missions that have recently made significant discoveries: the Near Earth Asteroid Rendezvous Shoemaker mission and Mars Global Surveyor.

"In the past, the Meteoritical Society has focused mainly on laboratory studies of meteorites," Wadhwa said. "However, in recent years, society members have become more interested and involved in spacecraft missions, particularly those that are sending back images and data from places in our solar system where meteorites may have originated."

NEAR Shoemaker began orbiting asteroid 433 Eros on Feb. 14-at times as close as 12 miles- and is providing scientists with their closest look ever of an asteroid. In May, using its X-ray/gamma-ray spectrometer, NEAR Shoemaker tentatively matched the chemical composition of Eros to chondritic meteorites, the most primitive rocks in the solar system.

The finding begins to resolve a long-standing mystery in meteoritics. The characteristics of chondrites, the most common type of meteorite found on Earth, previously did not seem to match the most common type of asteroid.

Speaking first about the best images and infrared data obtained by NEAR Shoemaker will be Cornell University astronomer Joe Veverka. Then astrophysicist Jacob Trombka of NASA's Goddard Space Flight Center will present additional mission data regarding the asteroid's chemical composition.

Following Trombka will be Michael Carr of the U.S. Geological Survey in Menlo Park, Calif., who will talk about "Mars at a Scale of One Meter: Water, Wind and Ice." Some scientists have interpreted recent images from Mars Global Surveyor as evidence for the relatively recent existence of water on Mars. Carr and others have proposed alternative explanations for the intriguing images.

Presentations also will cover five future missions: The Stardust comet sample-return mission; the Genesis solar wind sample-return mission; the CONTOUR mission that will visit three comets; the MESSENGER mission to Mercury; and the MUSES-C asteroid sample-return mission. Stardust, launched in 1999, is scheduled to return comet samples in January 2006. The other missions have yet to be launched.

The special session on interstellar dust grains will focus on grains that were thrown out of stars billions of years ago, before the sun's birth, then got mixed into clouds and interstellar dust that collapsed to form the solar system.

"We know from studying these grains that they come from different kinds of stars," Davis said. The grains come from red giants, stars once like the sun that at the end of their lifetimes grow huge, drop in temperature and shed much of their mass; from supernovae, stars that end their lives in a huge explosion; and from novae, stars that undergo a series of less violent explosions.

Research on such grains, which have been transported to Earth via meteorites, reveals new details about how stars evolved billions of years ago to produce the elements that formed Earth and all other objects in the solar system.

Michael Pellin of Argonne National Laboratory in Illinois will be among this session's speakers. Pellin and his collaborators at the University of Chicago's Enrico Fermi Institute have been precisely measuring the isotopic compositions of heavy elements found in meteoritic dust grains. Isotopes are different varieties of an element, such as silicon, that differ only in the number of neutrons in their nuclei. Argonne is the only laboratory in the world that can perform these measurements using a technique called resonant ionization mass spectrometry.

"It took a long time to get to this stage. Now we're in a position to really probe the insides of stars," said Davis, a member of the collaboration.

Martian meteorites will figure prominently at the Chicago meeting. A new Martian meteorite found near Los Angeles late last year will be the topic of six papers. "It's clearly different from the others," Davis said. "It's most likely the sort of thing to come out of a volcano on Mars."

Three additional papers will continue the scientific debate over the Antarctic meteorite that allegedly contains evidence for microscopic life on Mars.

A report on a new lunar meteorite will be among the papers presented during a session devoted to asteroids and the moon. Tim Fagan of the University of Hawaii at Manoa and his co-authors, including the University of Chicago's Robert Clayton, will describe a meteorite found in northwest Africa in October 1999. The meteorite resembles rocks of volcanic origin collected during the Apollo 12 and 15 missions.

Another session will include talks about a newly confirmed impact crater in Botswana, Africa. The crater measures more than two miles in diameter and was created approximately 180 million years ago. Other presentations will cover new findings from Arizona's Meteor Crater and from the meteorite impact on Mexico's Atlantic coast that scientists have linked to the dinosaur extinction.

One of the final sessions will include two presentations regarding rare findings of tiny water samples in meteorites. "This is four-and-a half-billion-year-old water," Davis said.