Disintegrating asteroid’s dust monitored for first time; Sandia optical sensors contributed

The object — a little less than 10 meters across — entered Earth’s atmosphere on Sept. 3, 2004, traveling at 13 kilometers per second.

The space-based infrared sensors of the US Department of Defense detected it at an altitude of 75 kilometers, descending off the coast of Antarctica.
DOE visible-light sensors built by Sandia noticed the intruder when it became a fireball — thus identifying itself as an asteroid — at approximately 56 kilometers above Earth,

Five infrasound stations, built to detect nuclear explosions anywhere in the world, registered its acoustic waves; these were analyzed by researchers at Los Alamos National Laboratory.
NASA’s multispectral polar orbiting sensor imaged the debris cloud formed by the disintegrating space rock.
It was one of the largest meteoroids to have entered the Earth’s atmosphere in the past decade. (Later analysis showed that its original solar orbit is similar to that of near-Earth asteroids of a particular family, the Aten group.)

Some 7.5 hours after the initial observation, a cloud of anomalous material was detected in the upper stratosphere over Davis Station in Antarctica by ground-based lidar.

These were the first direct measurements ever made of such meteoritic “smoke.”

Something unusual about the cloud

“We noticed something unusual in the data,” says Andrew Klekociuk, a research scientist at the Australian Antarctic Division. “We’d never seen anything like this before, [a cloud that] sits vertically and things blow through it. It had a wispy nature, with thin layers separated by a few kilometers. Clouds are more consistent and last longer. This one blew through in about an hour.”

There was certainly something unusual about the cloud. It was too high for ordinary water-bearing clouds (32 kilometers instead of 20 km) and too warm to consist of known manmade pollutants (55 degrees warmer than the highest expected frost point of human-released solid cloud constituents). The cloud could, of course, have been made of dust from a solid rocket launch, but the asteroid’s descent and the progress of its resultant cloud had been too well observed and charted; the pedigree, so to speak, of the cloud was clear.

What was really unusual about the cloud was the size of its particles. Computer simulations agreed with sensor data that the particles’ mass, shape, and behavior identified them as asteroid constituents roughly 10 to 20 microns in size.

Micron-sized particles are big enough to reflect sunlight, cause local cooling, and play a major role in cloud formation.
Scientists formerly had paid little attention to dust from meteoroids, assuming that the burnt matter disintegrated into nanometer-sized particles that did not affect Earth’s environment. Some researchers (and science fiction writers) were more interested in the damage that could be caused by the intact portion of a large asteroid striking Earth.

“Our observations suggest that [meteoroids exploding] in Earth’s atmosphere could play a more important role in climate than previously recognized,” write Klekociuk and other researchers, including Sandia’s Dick Spalding (5740), in a paper published last week in the journal Nature (Aug. 25 issue).

Klekociuk, along with researchers from the University of Western Ontario, the Aerospace Corp., LANL, and Sandia had found evidence that dust from the asteroid burning up as it descended through Earth’s atmosphere formed a cloud of micron-sized particles significant enough to influence local weather in Antarctica.

Volcanic eruptions from the sky

Says Dee Pack of Aerospace, “This asteroid deposited 1,000 metric tons in the stratosphere in a few seconds, a sizable perturbation.” Every year, he says, 50 to 60 meter-sized asteroids hit Earth.

Micron-sized meteoroid dust could be a factor in climate simulations because meteroids entering Earth’s atmosphere are extremely reduced by the fireball caused by the friction of their passage. The solid mass reduced to dust may be as much as 90 to 99 percent of the original asteroid.

Peter Brown at the University of Western Ontario, initially contacted by Klekociuk, helped analyze data and did theoretical modeling. He points out that climate modelers might have to extrapolate from this one event to its larger implications.
“[Meteoroid dust could be modeled as] the equivalent of volcanic eruptions of dust, with atmospheric deposition from above rather than below,” he says. The new data on micron-sized particles “has much greater implications for [extraterrestrial visitors] like Tunguska.” He was referring to an asteroid or comet that exploded 8 kilometers above the Stony Tunguska River in Siberia in 1908. About 2,150 square kilometers were devastated, but little formal analysis was done on the atmospheric effect of the dust that must have been deposited in the atmosphere.

Preventing nuclear war

The capabilities of defense-related sensors to distinguish between the explosion of a nuclear bomb and an asteroid fireball that releases similar amounts of energy — in this case, about 13 kilotons — could provide an additional margin of world safety. Without that information, a country that experienced a high-energy asteroid burst that penetrated the atmosphere more deeply might lead a hair-trigger military response unit to believe either that its country has been attacked or that a nearby country is testing a nuclear weapon.

The Sandia sensors’ primary function is to observe nuclear explosions anywhere on Earth. Their evolution to include meteor fireball observations came when Dick Spalding recognized that ground-based processing of data might be modified to record the relatively slower flashes due to asteroids and meteoroids. Sandia computer programmer Joe Chavez (5724) wrote the program that filtered out signal noise caused by variations in sunlight, satellite rotation, and changes in cloud cover to realize the additional capability. The Sandia data constituted a basis for the energy and mass estimate of the asteroid, says Dick.
Longer research papers being prepared from the same data for other journals are expected to discuss possible negative effects on the planet’s ozone layer, says Pack.