Planetary scientists at The University of Arizona in Tucson will begin field surveys tomorrow morning, June 15th, for possible fragments of a small asteroid that exploded in the southern Arizona sky a week ago.
Hundreds of eyewitness reports from across the state and neighboring states have enabled UA planetary scientist David A. Kring to identify an area in the vicinity of Gila Bend and Casa Grande where any possible fireball fragments might be found. Kring and his colleagues estimate that the object was between the size of a football and a desk before it exploded.
The team will survey an area west of Stanfield Road first. This area, although unpaved, is easily accessible to all types of vehicles.
If nothing is found in that area, the team will go to another area farther south and west. This second location requires 4-wheel drive vehicles.
University of Arizona
June 12, 1998
Planetary scientist David Kring has received phone calls from hundreds of eyewitnesses, each of which has provided a unique perspective on the fireball event. These reports have been made by people from across the state — from Prescott, Payson, Crown King, Cave Creek, Wickenburg, several cities within the Phoenix metropolitan area, Yuma, Ajo, Gila Bend, Casa Grande, Catalina, Tucson, Bisbee, Douglas, Willcox, Safford, Clifton, Globe. Reports have also come from Gallup and Silver City, N.M., as well as several localities in southern California.
These reports have been tremendously helpful, Kring says. "The outpouring of public support for this scientific endeavor has been outstanding."
Several eyewitnesses were able to provide Kring with good compass bearings to the location of the object's terminal burst, including an observation made by UA astronomer Joe Montani. Based on these observations, the location of the explosion has been narrowed to the vicinity of Gila Bend and Casa Grande. In a map (available from Kring or from UA News Services, 520-621-1877), the uncertainty in the observations is reflected in a series of expanding error ellipses.
Today, scientists were also able to exclude the possibility the burst was produced by spacecraft debris that re-entered the Earth's atmosphere. A European scientist who tracks this type of debris determined that the spacecraft debris on decaying orbits closest to the observed fireball involve an Iridium rocket and debris from the Mir space station.
However, if these objects had re-entered the Earth's atmosphere, they would have crossed the southern Arizona skies approximately 14 hours earlier or 14 hours later, respectively, than when the fireball was observed. Consequently, the fireball probably was produced by a small asteroid colliding with Earth.
Asteroids typically hit the top of the atmosphere with speeds of 25,000 to 55,000 miles per hour. They are decelerated in the atmosphere as they continue to slam into the gas in the atmosphere. At those speeds, the asteroid quickly heats up and ionizes nearby gas, causing it to glow. A thin layer of material around the asteroid is also melted and begins to slough off. Eventually, the object flew so low in the atmosphere that the impacting pressure with the gas overwhelmed the strength of the asteroid which caused it to catastrophically explode.
It still is not clear whether any portion of the asteroid survived the explosion, Kring says. However, he and his colleagues plan to begin field surveys beneath the point of the terminal explosion next week in hopes of finding meteorite fragments on the ground. Their hope is to collect new samples of oneof our planetary neighbors as valuable evidence to further understand the geologic evolution of our solar system.
Kring says that any samples found should be thought of as Arizona's meteorite, "because it was observed falling by so many people across the state, and because the point of the explosion could not have been determined without the aid of so many people."
If a meteorite is found, it would be only the second meteorite observed to fall in Arizona history, he added. The last witnessed fall occurred in 1912, when a small, one-meter diameter asteroid exploded near Holbrook, showering the ground with over 14,000 stones. A similar shower of meteoritic stones also occurred 20,000 years ago near Kingman. (Contact UA News Services for a previous news release about this event.)
Although two-to-three fist-size meteorites fall in Arizona each year, they are rarely seen falling and are even more rarely found.
Plans are being made to begin a field survey for this asteroid on Monday.