Catching a Falling Star

Meteor Caught in the Act. Click image for larger view. Credit: ESO

On most nights, the European Southern Observatory’s Very Large Telescope looks at objects light years away. Supernovas, galaxies, and other vast distant objects are photographed and analyzed by various instruments on the telescope. But earlier this year, a meteor only 100 kilometers away passed between the telescope and its usual field of study.

At the time of this event, the VLT was taking spectroscopic exposures of a supernova in a distant galaxy. So when the meteor passed within the narrow field of view of the telescope’s spectrograph, the VLT recorded the spectrum of the meteor trail.

Meteor spectra have been obtained before during photographic star spectra surveys. But this is the only meteor spectrum recorded with a large telescope and a modern spectrograph.

"We really hit the jackpot", says ESO astronomer Emmanuel Jehin. "Chances of capturing a meteor in the narrow slit of the FORS1 spectrograph are about as big as for me winning the national lottery."

Because the VLT is tuned to observe objects far out in space, it focuses at infinity. The meteor, being only 100 kilometers above the telescope, therefore appears out of focus in the field of view.

The meteor emission spectrum results from collisions between air molecules, which are knocked into high speeds after first colliding with the meteor. The spectrum indicates the temperature of the meteor trail was about 4,600 degrees Celsius (8,312 Fahrenheit).

A composite of Leonid meteor images recorded by a CCD camera onboard the MSX satellite

The meteor’s spectrum includes oxygen and nitrogen atoms and nitrogen molecules. The spectrum didn’t have carbon emission lines, however, even though carbon would have been expected. The scientists say that absence of carbon in the spectrum puts constraints on the role of meteor-induced atmospheric chemistry when life started on Earth.

"We calculated that these lines should have been visible if all atmospheric carbon dioxide in the meteor path was dissociated into carbon and oxygen atoms, but they were conspicuously absent," says Peter Jenniskens of the SETI Institute, lead author of the paper just published in the journal Meteoritics and Planetary Science.

The VLT is located at the Paranal Observatory in Chile. The meteor passed overhead on the night of May 12, 2002, and was captured in a brief photographic exposure of 1/50 of a millisecond. The meteor was estimated at magnitude -8, or nearly as bright as the first-quarter Moon.

The VLT’s camera – called the Mini All-Sky Cloud Observation Tool (MASCOT) – typically takes 90-second exposures every 3 minutes. The main purpose of MASCOT is to monitor the clouds over Paranal, but it also observes serendipitous events like meteor showers, atmospheric phenomena, and man-made satellites.

The meteor may have been a part of the Southern May Ophiuchid meteor shower, which appears just east of the bright star Antares. This shower contributes only one or two meteors per hour, but it was one of the stronger showers that particular night.

Spectra of a meteor. Click image for larger view.

"At first, the bright trace across the supernova spectrum was a puzzle, but then I realized that the spectroscopic signature was that of our atmosphere being bombarded," says astronomer Remi Cabanac of the Catholic University of Santiago de Chile. "We asked around to see if others in our country had witnessed the meteor, but it seems we at the VLT were the only ones, perhaps not too surprising as Paranal is located in the middle of the empty desert."

When meteors orbit the sun, they are called meteoroids (larger rocks orbiting the sun are called asteroids). It is only when these small rocks enter the Earth’s atmosphere that they are called meteors. They make a dramatic and beautiful entrance, burning up and producing a short glowing trail in the night sky that rarely lasts more than a second or two. Most meteors are completely destroyed at altitudes between 80 and 110 kilometers, but those that make it to the ground are given yet another name: meteorites.

Many meteoroids originate as fragments of asteroids and appear to be unaltered since the formation of the solar system. Based on the peculiar composition of some meteorites, we know that a small fraction of meteoroids originate from the Moon, Mars, or the large asteroid Vesta. They result from major impacts on these bodies that blasted rock fragments into space. These fragments then orbit the sun and may eventually collide with the Earth.

Comets are another important source of meteoroids. After many visits near the sun, a comet’s "dirty-snowball" nucleus of ice and dust decays and fragments, leaving a trail of meteoroids along its orbit. Some "meteoroid streams" cross the Earth’s orbital path, and when our planet passes through them some of these particles enter the atmosphere. The outcome is a meteor shower – the most famous being the "Perseids" in the month of August and the "Leonids" in November.

More information: Meteoritics and Planetary Science, Vol. 39, Nr. 4, p. 1, 2004 ("Spectroscopic anatomy of a meteor trail cross section with the ESO Very Large Telescope", by Peter Jenniskens (SETI Institute, USA), Emmanuël Jehin (ESO), Remi Cabanac (Pontificia Universidad Catolica de Chile), Christophe Laux (Ecole Centrale de Paris, France), and Iain Boyd (University of Michigan, USA).

Related Web Pages

2001 Astrobiology Results from Leonids Missions
Asteroids, Comets, and Meteors Site
Meteor Organizations
Great Impact: Part I
Great Impact: Part II
Great Impact: Part III
Great Impact: Part IV
Impact Hazards Website
NASA/JPL Near Earth Object Program