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Desert Fireballs

An Operational Desert Fireball Network camera station

An Operational Desert Fireball Network camera station on the Nullarbor, with satellite link and solar panel power source. Photo courtesy Geoff Deacon

By Alex Bevan, Philip Bland & Pavel Spurný

An intelligent camera system has been set up to track and recover meteorites in the Nullarbor.

The stuff from which many meteorites are made is 4560 million years old, and has remained virtually unaltered since its formation. Mainly representing debris left over after the planets were constructed, meteorites carry a unique record of the earliest events during the birth of the Solar System.

Meteorites can be fragments of rock, metal and mixtures of rock and metal mostly broken from asteroids in solar orbits between Mars and Jupiter. These broad groupings, however, belie the diversity of rocks that fall to Earth and are recovered.

Our best estimate is that the world’s meteorite collections contain samples from at least 135 different asteroids, each having had an independent history since the formation of the Sun and its system of orbiting planets. A small army of planetary scientists in universities and museums around the world are currently involved in deciphering the cryptic clues presented by meteorites to understand the chemical and physical processes that led to the construction of the Solar System.

Of the more than 50,000 meteorites in collections around the world, the vast majority are chance finds. Over the past 300 years or so, only about 1100 worldwide have actually been seen falling before being quickly recovered. Of these, the phenomena associated with the fall of only 12 have been photographed, enabling the orbits of the objects that gave rise to the meteorites to be determined.

Networks of All-Sky Cameras, designed to observe fireballs, calculate orbits, triangulate fall positions and recover meteorites, have been established in the USA and Canada, while the European Network has logged over 40 years of operation. Although hundreds of fireballs associated with large (>100 gram) meteorites have been observed, remarkably only four meteorites were recovered. The orbits of others were determined from chance video recordings.

This poor success rate is explained simply by the location of the networks. Lush vegetation in central Europe, for example, makes locating small meteorites extremely difficult. Until now, no camera network has been established in an area, such as a desert, where meteorites can be recovered easily.

The newly established Desert Fireball Network in the Western Australian Nullarbor s now providing fundamentally important information about the nature and origin of meteorites. Through an international collaboration between Imperial College London, the Ondrejov Observatory in the Czech Republic and the Western Australian Museum in Perth, construction of a trial network comprising four fireball observatories was completed in 2007 and enjoyed almost immediate success. A meteorite fall photographed in July 2007 was later recovered within 100 metres of the landing site predicted by the network.

Why is the Nullarbor such a special place? Certainly no more meteorites fall there than anywhere else on the Earth, but a lack of vegetation and the way that the Nullarbor’s pale limestone sharply contrasts with generally dark meteorites create an ideal combination for meteorite spotting. The climate of south-western Australia is another advantage, allowing around 200 clear nights per year for observations.

Today, four satellite-monitored cameras specifically designed to operate in extreme desert conditions have been deployed in the Nullarbor. Orbits are calculated from fireballs, and meteorite fall positions over an area of approximately 200,000 km2 are determined for later recovery.

Data from the current network indicate that three or four meteorite falls are detected per year. In the sparsely vegetated Nullarbor we expect to recover a significant proportion of those photographically recorded meteorite falls, which will dramatically increase the record of meteorites with known orbits.

Although we can analyse meteorites in collections to gain clues to the origins of our Solar System, we only have an approximate knowledge of where most meteorites come from. Surveys of light reflected from asteroids reveal a diversity of bodies, each with a distinct inferred surface mineral make-up. An expanded collection of meteorites with known orbits would allow us to relate some samples to specific regions or bodies, providing us with a spatial context for interpreting meteorite composition.

Even if an object has no motion relative to the Earth, gravitational attraction will cause it to enter the atmosphere at a minimum velocity of 11.2 km/s (the Earth’s “escape” velocity). Friction in the atmosphere causes the surface of the body to melt, and the air around it to become electrically charged. The resulting fireballs can give rise to spectacular visual displays.

In order to calculate the orbit of an observed meteorite fall accurately, the event has to be photographed by a number of specialist cameras. From this record we can determine the time of the event, the atmospheric trajectory and the point in the atmosphere where the fireball extinguished (usually 10–30 km above the Earth’s surface). By triangulation, and consideration of the atmospheric conditions at the time of the fall, an area on the ground can be identified to search for the surviving material.

Most importantly, the speed of the object in the atmosphere can be measured. This vital piece of information can be used to determine the orbit of the object in space that gave rise to the meteorite fall.

The speed of the object in the atmosphere is relative to the Earth, but in order to calculate the orbit of the object in space we need to know the speed of the object relative to the Sun at the Earth’s distance from the Sun. Taking into account the Earth’s own orbital speed (29 km/s) and the rate of rotation of the Earth (around 0.5 km/s at the Equator) it is possible to work back to obtain the dimensions and shape of the orbit of the infalling body.

In all cases measured previously, the orbits of bodies that have given rise to recovered meteorite falls are highly elliptical, with their furthest distance from the Sun in the asteroid belt between Mars and Jupiter. This is strong evidence, albeit circumstantial, that the majority of meteorites we see falling to Earth are bits that have broken from asteroids.

On 20 July 2007, two cameras from the Nullarbor Desert Fireball Network detected the fall of a meteorite. At 19 hours 13 minutes 53.2 seconds (±0.1 s) Universal Time, a fireball was recorded low on the horizon east of the network area. This was a less than ideal observation, but thanks to the high resolution imaging system of the cameras the atmospheric trajectory, luminosity of the fireball, orbit, and impact position were determined precisely. The record also indicated that the body broke in the atmosphere to give multiple surviving fragments — a common feature of meteorite falls.

The successful recovery of three fragments of the Bunburra Rockhole meteorite fall represents a number of scientific firsts. While it is only the fifth predicted meteorite fall in history, it is the first known meteorite from an Aten-type asteroid orbit, the first basaltic achondrite (a kind of igneous rock) with a known orbit, and the first instrumentally observed meteorite fall in the Southern Hemisphere. Moreover, it is the first documented meteorite fall from a relatively small object that produced a short-lived fireball with a terminal height of 30 km.

The Bunburra Rockhole meteorite has been full of surprises, not least of which is its orbit. The Aten asteroids are a group of near-Earth asteroids named after the first of the group to be discovered (2062 Aten) on 7 January 1976 by the famous astronomer Eleanor Helin. Half of their largest orbital dimension is less than the distance from the Earth to the Sun. However, because the orbits of asteroids can be highly elliptical, the orbit of an Aten asteroid need not be entirely contained within Earth’s orbit. In fact, nearly all known Aten asteroids have orbits with their greatest distance from the Sun beyond the Earth’s orbit, as was the case with the Bunburra Rockhole meteorite.

Even more exciting is that the meteorite itself is extremely unusual. Although many other similar basaltic meteorites are known and have been linked tentatively to asteroid 4 Vesta and related bodies called the V-type asteroids as their parents, Bunburra Rockhole seems to have come from a different region of space. Although its chemical composition is not too different from other meteorites of its kind, the isotopic make-up of its oxygen sets it aside.

There are three stable isotopes of oxygen, with atomic weights of 16, 17 and 18 that occur in the approximate proportions 99.8%, 0.04% and 0.2%, respectively. Because of the abundance of oxygen on Earth and its importance in rock formation, analysis of these stable isotopes provides important information that helps scientists to determine relationships between many natural materials.

When the ratios of the heavier isotopes of oxygen to light oxygen-16 in different samples from Earth are plotted against each other they lie on a line with a slope of exactly one half. All Earth samples lie on this line, as do samples from the Moon. This is strong evidence that the Earth and the Moon are not chance associates, but formed from the same oxygen source in the same region of the Solar System. Any samples formed from another source of oxygen would lie on different lines.

This is the case for Bunburra Rockhole. Not only did it form in a different region of the Solar System to the Earth and the Moon, it (or at least parts of it) also formed in a different parent body from other similar basaltic igneous meteorites. So much excitement has the meteorite generated in the planetary science community that, currently, researchers in 21 institutions around the world are studying the meteorite.

To date, the Desert Fireball Network has recorded more than 550 fireballs. Of these, multi-station observations have enabled the precise calculation of 150 atmospheric trajectories and orbits. This is the first set of data for Southern Hemisphere fireballs, and it is likely that a new, active meteor shower has also been discovered.

Of the events recorded on multiple stations, around 11 may have resulted in meteorites, not all of which are recoverable. Four of these are probable falls with masses of 10–100 grams, five are certain falls with terminal masses greater than 100 grams, and one had an initial mass of

20 tonnes. Unfortunately this latter fall, which may have had a cometary origin, fell into the watery grave of the Great Australian Bight. One certain fall, and two probable falls are, as yet, unrecovered and lie in easily searchable areas of the Nullarbor. Three additional recent events are almost certainly recoverable falls, and calculations to allow us to locate them are in progress.

With the continued successful operation of the network and the recovery of the Bunburra Rockhole meteorite, we have achieved a major milestone for this initial phase of the project. Having demonstrated the undoubted viability of the project, the Desert Fireball Network in the remote and sometimes climatically hostile Nullarbor is set to be a major contributor to Solar System research.

Alex Bevan is Head of Earth & Planetary Sciences at the Western Australian Museum. Phil Bland is a Principal Research Fellow in the Department of Earth Science and Engineering at Imperial College London. Pavel Spurný is Head of the Department of Interplanetary Matter at the Astronomical Institute of the Academy of Sciences in the Czech Republic.