NRL Instrument Makes First UV Observation of Meteor in Space

2/9/2000 - 9-00r
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Meteor image and caption

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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