* Astronomy

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Antarctic Fireball (2004)

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Antarctic Meteor

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The Davis LIDAR (Light Detection and Ranging) is a remote sensing instrument which profiles atmospheric density, temperature and wind velocity as a function of altitude. It operates in a manner akin to radar; pulses of laser light are transmitted into the sky, and the weak 'light echo' scattered back to the instrument from atmospheric gases and aerosols is collected and analysed.
The LIDAR, developed by the Australian Antarctic Division in collaboration with the Adelaide University, was installed during the 2000/01 summer.

Laser scatter caused by the dust cloud, 28-31 km up, over about one hour.

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Lead author, Dr Andrew Klekociuk, from the Australian Antarctic Division said that early last September, a physicist at Australia's Davis station in Antarctica had prepared his monitoring instrument, known as LIDAR, for keeping watch on atmospheric activity during the long night ahead.

"Just as observation of the stratosphere began a strange signal was recorded from 30 kms overhead. Our physicist thought his preparation of the optics may have been amiss so fitted a filter but the signal persisted for another 30 minutes. What he didn't know at the time was that seven hours earlier an asteroid had crashed to Earth in another part of Antarctica, about 1500 kms west of Davis. The closest it got to human habitation was around 900 kms west of Japan's Syowa station" - Dr Andrew Klekociuk.

Shortly after the LIDAR observations it was revealed that the event had also been picked up by the global network of satellites and a range of other instruments.

But the most detailed evidence of the trail of dust, carried by strong winds around Antarctica, has been captured by the LIDAR at Davis station.

Dr Klekociuk said that it was thought that the asteroid had come from what is known as the Aten group somewhere between Venus and Earth, ranging anywhere up to 46 million kms from the sun. Measuring roughly 10 metres it is the biggest body to enter Earth's atmosphere in the past decade.

Its travel time from entering Earth's atmosphere 75 kms up until it landed? Just five seconds.

Scientists believe that the asteroid's original size was close to that of a small house weighing a thousand tonnes and that if it had not broken up on entry into the atmosphere its effect on impact would have been that of the bomb dropped on Hiroshima.

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Image from the Aqua satellite of the meteors dust trail one hour after the meteoroid exploded. The image has been specially processed to reveal the dust trail, as well as the surface shadows and scattered sunlight from the trail.
The image is centred at latitude 69 south, longitude 27 East

"The size of the dust cloud in the stratosphere was 200 kms by 75 kms. Had a cloud that size passed over the sun the light would have dimmed by around 20 per cent. Inevitably particles contained in the dust cloud have fallen to Earth and samples from all three Australian Antarctic continental stations -- Davis, Casey and Mawson -- have been retrieved for analysis at the Australian Antarctic Division"

Dr Klekociuk said that these analyses will enable scientists to validate models of atmospheric circulation. The timing and location of the event will also allow for testing theories relating to the impact of large meteorites on ozone and climate.

"While there were no obvious short-term associated changes in regional climate or ozone levels, the longer term implications are still being evaluated" - Dr Andrew Klekociuk.

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A meteor hit the Earth's atmosphere a year ago with a force at least as powerful as the Hiroshima bomb, according to a scientific study published today.
Infrared sensors in space first picked up the massive rock - about the size of a house and weighing one million kilogrammes or roughly 1,000 tonnes - as it hurtled towards Earth at an altitude of 75km.

The meteor was ripped apart by the huge pressure and heat produced as it flew through the atmosphere, but the resulting fireball produced a debris trail extending from 56km to 18km above the ocean near Antarctica.

Scientists in Australia, Canada and the United States were able to use data from the US defence and energy departments infra-red and visible light sensors to carry out one of the most detailed analyses of the destruction of a meteor, which has been published in the journal Nature.
They warned that such meteor strikes could play a much greater part in climate change than previously thought.
The resulting dust cloud, they said, contained larger than expected particles which would damage the ozone layer, reflect sunlight and cause raindrops to form in clouds.

The report said: "Sensors detected a large meteor in flight at an altitude of approximately 75km on 3 September.
"Subsequently, the emissive debris trail from the fireball was measured, extending from 56 to 18km altitude.
"Five infrasound stations detected acoustic gravity waves from the fireball, with the furthest detection being 13,000km from ground zero."

The researchers calculated that the meteor released energy equivalent to between 13 and 28 kilotons of TNT high explosive - at least as much as the nuclear bomb that destroyed Hiroshima in 1945.
The dust clouds produced by this kind of meteor strike had been thought to contain incredibly small particles just nanometres across, but the scientists said this strike resulted in particles much larger, at up to 20 micrometres.

And they said: "This dust is likely to have atmospheric residence times of weeks to months. Micrometre-sized aerosols play a crucial role in climate-forcing effects, as well as in ozone depletion.

"The conventional view is that (meteoric dust) is of low significance in climate forcing, primarily because nanometre-sized particles dominate the size spectrum. In light of our findings, this view requires further investigation."

Dr Matthew Genge, an expert on meteors at Imperial College London, said there were about five similar meteor strikes on Earth's atmosphere every year.

"It's certainly true no-one has got a really good look at one of these events. However, they are not really something we should worry about. These things tend to destroy themselves in the atmosphere and don't reach the ground " - Dr Matthew Genge.

The Antarctic meteor would have been about six or seven metres across. Meteors usually impact on the ground if they are more than ten metres across.

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A thousand-ton meteor exploded in the atmosphere a year ago, releasing the power of a nuclear bomb and leaving behind an unusual dusty trail that has raised questions about the ways extraterrestrial material finds its way to Earth's surface and the geological record.
The meteor exploded over Antarctica on Sept. 3, 2004, with the energy of 13,000 to 28,000 tons of TNT, roughly equivalent to the atomic bomb that destroyed Hiroshima in 1945.
Yet it wasn't eye-witnessed by anyone and never reached the Earth's surface. Instead, its smoky aftermath was picked up in routine measurements by atmospheric researchers in Antarctica who use lasers to probe the polar atmosphere for dust particles, called aerosols.

"We were probably the only ones to know of (the meteoroid)," said Andrew Klekociuk of the Australian Antarctic Division in Kingston, Tasmania, Australia.

What clued them in was the discovery of a cloud of dust higher than any particles should be.

"This was unusual," said Klekociuk of the initial dust observations.

He immediately suspected it was the remains of a meteoroid. And after checking their equipment to make sure it wasn't an instrument error, he sent word to other scientists in other countries who had access to data from satellites and infra-sound stations to see if any other instruments had picked up the meteoroid.
As it turned out, the house-sized chunk of falling rock had been detected elsewhere by other instruments, but hadn't been identified as such until Klekociuk started asking around. A report on the meteoroid by Klekociuk and his colleagues appears in the Aug. 25 issue of Nature.

Satellites picked up the meteoroid blazing down into the atmosphere from 56 to 18 kilometres high.
Infrasound stations picked up a very low-frequency boom from the explosion as near as McMurdo Station in Antarctica, and as far away as Germany, Klekociuk said.

"It's pretty remarkable that it reached all the way to 13,000 kilometres," he said of German infrasound detection.

All the data was used in calculations that helped the researchers discover the impressive size of the meteoroid.
But more important than the event itself is the dust it left behind, said Klekociuk.
The meteoroid's dust adds a new wrinkle to an ongoing debate about which kind of meteoroids leave the most extraterrestrial material in the geological record and ice core records seen on the ground: Is it the occasional big fiery bodies or an ongoing rain of zillions of tiny interplanetary dust particles (IDPs) falling to Earth?

Many researchers had thought that big meteoroids like last year's would leave telltale small particles of dust, just a few nanometres across and far smaller than IDPs.
But the meteoroid dust measured by the Australian light detection and ranging instruments (LIDAR) instruments at Davis in Antarctica was 400 to 1,000 nanometres across: bigger than thought and comparable to IDPs, explains Klekocuik.

So are IDPs seen in ice cores and sediments really IDPs, or the remnants of larger bodies that burned up in Earth's atmosphere? The answer is not yet clear, said NASA researcher Kevin Zahnle.

"The people who study the asteroids and meteors have long thought that the big bodies were the most important," said Zahnle. "The people who study IDPs seem to think that the IDPs are more important"

"It's actually pretty hard to tell on ordinary time scales, although if you think of hundred-million-year intervals and the big bodies include monsters like the K-T event," said Zahnle, referring to the meteorite impact that coincided with the extinction of the dinosaurs, "it is inevitable that the big bodies win."

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