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Planetary Science

Catch a falling star

The European Fireball Network has recovered a meteorite after photographing its violent entry into the Earth's atmosphere. This is only the fourth such recovery, and analysis points to a surprising past for this primitive object.

Meteoroids, or rocks from space, are seen travelling through the Earth's atmosphere as fireballs — extremely bright shooting stars or meteors — and any surviving debris is scattered as meteorites. For more than 40 years, networks of cameras have been primed to capture images of the incoming fireballs, and pinpoint the landing sites of their meteorites, with the aim of tracing the precise orbit and likely origin of the meteoroid. But in all that time, meteorites have been recovered from only three detected fireballs. On page 151 of this issue1, Spurný, Oberst and Heinlein report the fourth instance of meteorite recovery, from the European Fireball Network — the project that also made the first recovery. All four meteorites, with known paths to their final resting place, are unique tracers in time and space of the history of our Solar System — although the latest has also thrown up a conundrum.

In the early years of the twentieth century, Ernst Opik led expeditions to the southwest deserts of North America to make systematic visual observations of meteors. He initially concluded that most meteors were of interstellar origin. In New Mexico, in the late 1940s and early 1950s, Fred Whipple and colleagues made photographic studies using a rotating shutter in front of fast cameras to measure the meteors' velocities. In what is some of the most precise work even by today's standards, these experiments showed that most meteors in fact originated from comets, but that a substantial fraction were in asteroid-like orbits — that is, the furthest points of their orbit from the Sun (their 'aphelia') fell in the asteroid belt, between Mars and Jupiter.

Shortly thereafter, astronomers began to classify asteroids on the basis of the spectrum of light that they reflected2. Although some order could be divined from the colour groupings, chemical analysis of meteorites was revealing more about the history of the Solar System than the asteroids could. For example, from the classification of meteorites it became apparent that there are only tens to hundreds of parent bodies for the thousands of meteorite specimens in collections around the world3. Historically, attempts to match the spectra of meteorites to specific parent asteroids have not been easy. But it was realized that if one could pick up a fresh meteorite, look skywards, and know precisely where it came from, a big piece of the puzzle might be uncovered. As a result, several networks of cameras, spanning areas of a million square kilometres, were set up in Europe, Canada and the United States. The intention was to photograph fireballs to calculate their orbits and find the meteorites that dropped from them. But the booty from these networks, despite years of sky-watching, has been disappointingly small — just four meteorites in 43 years.

On the evening of 7 April 1959, Zdenek Ceplecha, founder of the European Fireball Network, was watching television when a tremendous fireball lit up the walls of his living-room. He immediately ran to the television set and adjusted its brightness level to produce the same level of illumination of the walls. Calibrating this later, he was able to estimate the fireball's brightness as being of magnitude −19, much brighter than the full Moon.

That historic night, his fireball network had photographed and tracked the meteor's path through the atmosphere, and its orbit in the Solar System was calculated. Because the fireball's atmospheric trajectory was known precisely, an estimate could be made of where meteorites might have fallen. A search was mounted and two days later a farmer found a 4.5-kg piece in his field near Příbram, Czechoslovakia. The total mass of meteorite recovered at Příbram amounted to 5.8 kg, from an initial mass of perhaps a tonne (estimated from the fireball's brightness). Příbram is classified as an H5 chondrite — a primitive, very old, undifferentiated stone containing small spheres of silicates (chondrules) reflecting the make-up of the presolar nebula that became the Solar System. Its orbit had an aphelion in the main asteroid belt and its perihelion was inside the orbit of the Earth, one-fifth of the way to the Sun.

On the night of 3 January 1970, another brilliant fireball lit up the sky over the central United States. As bright as the full Moon (magnitude −12), the meteor was photographed by the Prairie Fireball Network, and its orbit determined. On 10 January, the network team found a 10-kg meteorite lying on top of the snow in the middle of a road near Lost City, Oklahoma. Guided by a local resident who reported seeing glowing red lights and hearing a thud nearby, they found three other pieces, raising the total recovered mass to 17 kg, from an estimated pre-atmospheric mass of 75–200 kg. The Lost City meteorite is also classified as an ordinary H5 chondrite, and its orbit took it from the main belt to just inside the orbit of the Earth.

On 5 February 1977, another fireball of magnitude −12 was reported by an airline pilot flying over Saskatchewan, Canada. Members of the Meteorite Observation and Recovery Project recruited local youths to search for meteorites in the snow, and they found the first specimen near Innisfree, Alberta, on 17 February. Five more pieces were found in April after the snow had melted, and a farmer turned up another three. The total weight of all nine specimens came to 4.6 kg, from an estimated pre-atmospheric mass of 20–40 kg.

And that was it. No other meteorites had been recovered by fireball networks, until now. As Spurný et al.1 report, the fourth meteorite was found on 14 July 2002, 6 km from the famous castle of Neuschwanstein (Fig. 1), after being photographed on 6 April 2002 by the successor to the original European Fireball Network. The fireball was of magnitude −17 and carved a luminous path 91 km long across the sky. Amazingly, it has been found to have an orbit identical to the Příbram meteorite, the first ever recovered (Fig. 3 on page 152). And yet, unlike Příbram (a fairly ordinary chondrite), Neuschwanstein is classified as an 'EL6 enstatite chondrite': it differs significantly from the other three in that it is more reduced (with less FeO in its silicates), it has lower ratios of Mg/Si and (Ca, Al, Ti)/Si, and it has a coarser granularity with little or no evidence of chondrules. Furthermore, Neuschwanstein has a cosmic-ray exposure age — a measure of the length of time that the body has been travelling through space — of 48 million years, compared with 12 million years for Příbram. So it seems that the meteor stream in which these objects were flowing is more heterogeneous than is usually assumed.

Figure 1: Shooting star.
figure1

JOSEF BECK/GETTY IMAGES

In April 2002, the Streitheim camera of the European Fireball Network in southern Germany took this time-lapse photograph (top right), capturing the 91-km trail of the meteor as it arrived in the Earth's atmosphere. Three months later a 1.75-kg meteorite (below), part of the fireball's debris, was recovered near the fairytale castle of Neuschwanstein, Bavaria. Spurný et al.1 have analysed the trajectory of the meteor and the chemical make-up of the meteorite. Despite the similarity of its orbit to an earlier recovered meteorite, the Neuschwanstein specimen is chemically very different, suggesting that there is a greater variety of objects in the meteor stream than had been thought.

Looking skyward, knowing where these rocks came from, we may begin to understand something of the early history of our Solar System. Zdenek Ceplecha, now retired, must be delighted to see this new success of his fireball network.

References

  1. 1

    Spurný, P., Oberst, J. & Heinlein, D. Nature 423, 151–153 (2003).

  2. 2

    Chapman, C. R., Morrison, D. & Zellner, B. Icarus 25, 104–130 (1975).

  3. 3

    Burbine, T. H., McCoy, T. J., Meibom, A., Gladman, B. & Keil, K. in Asteroids III (eds Bottke, W. F., Cellino, A., Paolicchi, P. & Binzel, R. P.) 653–667 (Univ. Arizona Press, Tucson, 2003).

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Correspondence to Jack Drummond.

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