Scientists from the Naval Research
Laboratory (NRL) report that the first ever far-ultraviolet (UV)
image of a meteor has been obtained by the Global Imaging Monitor
of the Ionosphere (GIMI) instrument on board the DoD Space Test
Program's Advanced Research and Global Observation Satellite
(ARGOS). The image was taken on November 18, 1999, during the
annual Leonid maximum, which in 1999, lasted from November 16
- 18.
At the time of the exposure,
the ARGOS spacecraft was about 20° south of the equator,
over the south Pacific Ocean; however, the viewing direction
(and the ARGOS altitude of 833 km) was such that the meteor itself
was much closer to the equator.
Dr. George Carruthers, NRL's
GIMI principal investigator, reports the scientific significance
of the observation, saying, "To our knowledge, this is the
first observation of a meteor entry to the atmosphere in the
far-UV spectral range. Such an entry cannot be observed from
Earth's surface or from aircraft because of its absorption by
the lower atmosphere. Ground-based observations of meteors cannot
detect many of the important elements and compounds expected
to be present in meteoroids."
"The first observation from
space of a meteor, by GIMI in UV light, adds another dimension
to the handful of previous space observations of meteors,"
notes Dr. Noah Brosch of the Wise Observatory and the Department
of Astronomy and Astrophysics at Tel Aviv University in Israel.
Dr. Brosch explains, "Satellites view the Earth continuously
and rarely detect extremely bright fireballs. These detections
are mostly in the visible or near-infrared spectral domains.
The GIMI observation is the first such space experiment to be
done in the UV."
Commenting on the practical application
of this discovery, Brigadier General (sel) S. Pete Worden, USAF
Deputy Director for Command and Control says, "As our civil,
commercial and national security use of space continues to increase,
natural phenomena that can disrupt satellite operations cause
growing concern. The Leonid meteor storm is such a phenomenon.
NRL's impressive data promises to provide a unique new tool to
understand the true composition and structure of these meteors.
This is vital information if we are to predict and mitigate future
meteor-induced problems to our space operations."
Abundances of various elements
and compounds found in meteors are known to be highly variable
among the types of meteorites which have been recovered on the
ground, but the latter are not representative of the range of
meteoroids as expected to be present in the solar system, since
only the most refractory and/or massive meteoroids survive entry
to the atmosphere. Once we have an opportunity to analyze our
data and examine other images for events of this type, says Carruthers,
we may establish the feasibility of using far-UV spectroscopic
instruments to more accurately measure the compositions of incoming
meteoroids.
Because Earth's lower atmosphere
strongly absorbs far-UV radiation in the wavelength range observed
by GIMI (131-200 nanometers, or 1310-2000 Angstroms), the scientific
team estimates that in order to be observable, the meteor had
to have been at an altitude well above 100 kilometers. Since
its entry velocity was probably in excess of 60 km/sec (i.e.
much higher than Earth escape velocity of 11.2 km/sec), the energy
was probably available for producing emission in the far-UV at
relatively high altitudes, due to excitation of the atmosphere
and/or meteoric constituents.
The most likely emission source,
Dr. Carruthers says, is nitric oxide (NO) which is produced and
excited by dissociation of molecular nitrogen and its subsequent
reaction with atomic oxygen, producing emission in the 190-200
nm wavelength range. However, if the meteor is of carbonaceous
composition, far-UV emissions of carbon monoxide (CO) and atomic
carbon may be produced as well.
GIMI is one of nine primary experiments
on the ARGOS mission, which launched into a polar orbit on February
23, 1999 to study space weather. GIMI's principal objective is
to obtain simultaneous wide-field FUV/EUV images of ionospheric
and upper atmospheric emissions, covering large areas of the
earth from a low-earth orbit. The GIMI images will be used to
determine chemical densities [O+, nighttime O2, NO and N2] on
a global basis and to detect disturbances in the ionosphere that
are caused by auroral activity, gravity waves and foreign materials
from meteors, suspected "ice comets," rocket exhausts
and chemical releases. In between the atmospheric observations,
GIMI is gathering data for an all-sky survey of stars and data
on celestial diffuse sources at far-ultraviolet wavelengths.
The GIMI instrument has two cameras
for simultaneous observations of selected targets. Camera 1,
which is sensitive in the 75-110 nm ranges is primarily being
used for observations of the dayside ionosphere, auroras, and
stellar occultations, and for star field surveys. Camera 2 is
sensitive in the 131-160 and 131-200 nm far-UV wavelength ranges
and is used for observations of the nightside ionosphere, airglow,
stellar occultations, star field surveys, and also gas releases
and rocket plumes at night.
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