Catching a Falling Star: ESO's Very Large Telescope Obtains UniqueSpectrum of a Meteor (Forwarded)

ESO Education and Public Relations Dept.

----------------------------------------------------------------------------
Text with all links and the photos are available on the ESO Website at URL:
http://www.eso.org/outreach/press-re.../pr-19-04.html
----------------------------------------------------------------------------

Contacts

Peter Jenniskens
SETI Institute
Mountain View, California
Phone: +1 650 604 30 86
Email:

Emmanuël Jehin
ESO, Chile
Phone: +56 2 463 30 54
Email:

For immediate release: 30 July 2004

ESO Press Release 19/04

Catching a Falling Star

ESO's Very Large Telescope Obtains Unique Spectrum of a Meteor

Summary

While observing a supernova in a distant galaxy with the FORS
instrument on ESO's Very Large Telescope at the Paranal
Observatory (Chile), astronomers were incredibly lucky to obtain
serendipitously a high quality spectrum of a very large meteor in
the terrestrial atmosphere.

The VLT spectrograph provided a well calibrated spectrum,
making it a reference in this field of research. From this spectrum,
the temperature of the meteor trail was estimated to be about
4600 degrees centigrade.

The serendipitous spectrum reveals the telltale meteor emissions
of oxygen and nitrogen atoms and nitrogen molecules. The VLT
spectrum was the first to reveal the far red range where carbon
emission lines are predicted; the absence of the lines puts
constraints on the role of atmospheric chemistry when life started
on earth.

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

PR Photo 22a/04: Meteor Caught in the Act (MASCOT)
PR Photo 22b/04: Spectrum of a Meteor (FORS1/VLT)
PR Photo 22c/04: Details of the Meteor Spectrum (FORS1/VLT)

Astronomers' luck

ESO PR Photo 22a/04

Meteor Caught in the Act (MASCOT)

Captions: ESO PR Photo 22a/04 shows the trail of a bright
meteor, photographed by the Mini All-Sky Cloud Observation Tool
(MASCOT) at the ESO Paranal Observatory. MASCOT consists of
a small CCD camera behind a fish-eye objective. It typically takes
90s exposures every 3 minutes and helps astronomers inside the
VLT Control Room to keep an eye on the sky. The main purpose
of MASCOT is to monitor the clouds over Paranal but it also
observes from time to time serendipitous events like meteor
showers, atmospheric phenomena, artificial satellites, etc. This
image was obtained by MASCOT on August 25, 2002 and shows
a meteor caught in the act. (Note that this is not the meteor whose
spectrum was recorded). The Milky Way is also clearly visible in
the centre.

A popular saying states that when you see a meteor, you may
make a wish. While astronomers cannot promise that it will be
realised, a team of astronomers [1] have indeed seen a dream
come true! On May 12, 2002, they were lucky to record the
spectrum of a bright meteor when it happened -- by sheer chance
and against all reasonable odds - to cross the narrow slit of the
FORS1 instrument on the ESO Very Large Telescope.

At the time of this unlikely event, the telescope was performing a
series of 20-minute spectroscopic exposures of a supernova in a
distant galaxy in order to establish constraints on the dark energy
content of the Universe (see e.g. ESO PR 21/98). Thanks to its
enormous light-collecting and magnifying power, the VLT recorded
the spectrum of the meteor trail perpendicular to its path on one of
these exposures.

"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."

Meteor spectra have on occasion been obtained serendipitously
during photographic star spectra surveys. But this is now maybe
the only meteor spectrum recorded with a large telescope and a
modern spectrograph. The spectrum covers the wavelength range
from 637 to 1050 nm, which is dominated by emissions from air
atoms and molecules in the meteor path and teach us about the
collision processes in the wake of a meteoroid.

The rapid motion of the meteor across the sky resulted in a very
brief exposure while crossing the narrow spectrograph slit -- only
1/50 of a millisecond! -- and despite the relative brightness of the
meteor it was only thanks to the VLT's great light-gathering power
that any record was procured. The meteor was estimated at
magnitude -8, or nearly as bright as the first-quarter Moon.

Although it is not possible to be sure from which shower this meteor
belongs, a possible candidate is the Southern May Ophiuchid
shower which appears from a direction just east of the bright star
Antares. The shower contributes only one or two meteors per hour
but was one of the stronger showers of that night.

Telltale emissions

ESO PR Photo 22b/04 ESO PR Photo 22c/04

Spectrum of a Meteor Details of the Meteor Spectrum
(FORS1/VLT) (FORS1/VLT)

Captions: ESO PR Photo 22b/04 shows the spectrum of a bright
meteor, as observed serendipitously by the multi-mode FORS 1
instrument on the ESO Very Large Telescope during the night of
May 12-13, 2002, in front of a photo of the VLT enclosures and
with a meteor trail inserted in the sky (montage). The position of
the meteor trail on the narrow slit of FORS (not to scale) is also
indicated. The lower panel shows the spectrum of the meteor,
following removal of the supernova spectrum and before (up) and
after (down) removal of the spectrum of the night sky by image
processing. Several emission lines from colliding Oxygen and
Nitrogen atoms (sharp emissions) and molecules (broad
emissions) are visible. ESO PR Photo 22c/04 illustrates details of
the extracted VLT meteor spectrum (solid line): the intensity (in
arbitrary units) is shown as a function of the wavelength. The
dashed line is a theoretical model of the spectrum of air heated to
a temperature of 4600 degrees at an altitude of 95 km.

"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."
And unfortunately for the astronomers, the MASCOT all-sky
camera (e.g. PR Photo 22a/04) was not yet in operation at that
time.

The VLT spectrograph provided a well calibrated spectrum of the
meteor emission, making it a reference in this field of research. The
meteor emission results from collisions between air molecules,
knocked to high speeds after initial collision with the meteoroid.

Closer inspection of the spectrum revealed about 20 telltale meteor
emissions of oxygen and nitrogen atoms and nitrogen molecules
(see PR Photo 22b/04 and 22c/04). The ratio of atomic and
molecular emissions could be used as a "thermometer" to measure
the conditions in the meteor-induced hot gas in the wake of the
meteoroid, by means of laboratory measurements and meteor
models that calibrate the VLT data.

From here to infinity

"To our surprise, we found the meteor trail to be wider than
expected and also that the meteor's heat appeared evenly
distributed in the trail, with the temperature varying only from about
4,570 to 4,650 degrees across the trail," says meteor specialist,
astronomer Peter Jenniskens of the SETI Intitute, who analysed the
data together with Christophe Laux of the Ecole Centrale Paris
(France) and Iain Boyd of the University of Michigan at Ann Arbor
(USA). "We later realised that this was due to the fact that, as seen
by the VLT, the meteor trail was out of focus, even though it was
100 kilometres away!"

The VLT is indeed focussed at infinity, which is perfect for most
astronomical objects that it routinely observes. But not for
meteoroids entering the atmosphere above Paranal. A point at 100
kilometres distance will appear as a small circle of diameter 15
arcsec at the VLT focal plane. This corresponds to roughly half of
the maximum apparent diameter of Mars in the evening sky! It is
the same effect as when you try to photograph your children with a
forest in the background. If you focus your camera on the distant
forest, then (in most cases) your children will be out of focus. Or to
put this in another way, the VLT is clearly not very suited to observe
ships passing by on the Pacific Ocean, just 12 km from Paranal!

No Trace of Carbon

The meteor spectrum also provided a first view of such an object in
the near-infrared window between wavelengths 900 and 1050 nm.
This spectral region contains relatively strong lines of atomic
carbon, but no such emissions were detected.

"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," says Jenniskens, "but they were
conspicuously absent". This observation is important because it
sets new constraints on the efficiency of meteor-induced
atmospheric chemistry at the time when life began on our planet.

Appendix: Cosmic showers

Meteoroids are small grains of rocks orbiting the Sun. Far smaller
than asteroids, they make their presence known to us in a dramatic
and beautiful way when they enter earth's atmosphere and burn up,
producing a short glowing trail in the night sky, rarely lasting more
than a second or two -- a meteor. Most meteoroids are completely
destroyed at altitudes between 80 and 110 km, but some of the
bigger ones make it to the ground. Here they may be collected as
meteorites.

Many meteoroids originate as fragments of asteroids and appear to
be unaltered since the formation of the Solar System, some 4500
million years ago. 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 obviously
result from major impacts on these bodies which 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 and perhaps
the most spectacular. After many visits near the Sun, a comet
"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 orbit and when our planet passes
through them, some of these particles will enter the atmosphere.
The outcome is a meteor shower -- the most famous being the
"Perseids" in the month of August [2] and the "Leonids" in
November.

Thus, although meteors are referred to as "shooting" or "falling
stars" in many languages, they are of a very different nature.

More information

The research presented in this paper is published in the journal
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 P. Jenniskens et al.).

Notes

[1] The team is composed of 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).

[2] The maximum of the Perseids is expected on August 12 after
sunset and should be easily seen.

----------------------------------------------------------------------------
ESO Press Information is available on Receive email notification
the WWW at about important news from ESO -
http://www.eso.org/outreach/press-rel/. subscribe to the
----------------------------------------------------------------------------
(c) ESO Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
----------------------------------------------------------------------------