Results of ESA's 1999 Leonids Campaign presented at International Conference
26 April 2000Why is it that meteors - popularly known as shooting stars - are seen ingreater numbers from an aircraft than from the ground? It's an absorbingtale, according to ESA's Detlef Koschny, one of the speakers at theInternational Leonid Multi-Instrument Aircraft Campaign (Leonid-MAC)Workshop, which is currently being held in Tel Aviv, Israel.
For the past two years, the Leonid meteor shower has attracted considerable attention from both the scientific community and the general public. Last November's apparition was blessed with favourable viewing conditions over much of Europe, and scientists from the European Space Technology and Research Centre (ESTEC) in The Netherlands were on hand to witness the event.
Simultaneous ground-based observations were made from two observing sites located on mountain tops in southern Spain. Meanwhile, ESA scientist Michael Schmidhuber, armed with a meteor camera, was cruising above the Mediterranean on board a specially modified U.S. Air Force Boeing 707.
After months of analysing the data from these experiments, preliminary results from the ESTEC team have established that the meteor count was 5 - 10 times higher from the aircraft than from the ground locations.
227 meteors were seen over a period of 10 hours 6 minutes from Calar Alto Observatory, while 138 were recorded over 9 hours 13 minutes from the Sierra Nevada site. In contrast, 136 meteors were observed from the aircraft over a mere five minutes near the peak of the meteor storm (1.47 - 1.52 GMT on 18 November 1999). Why was this?
ESTEC scientist Detlef Koschny today told the Leonid-MAC meeting that he believes it is the result of improved viewing conditions at high altitude.
According to Dr. Koschny, it doesn't really matter whether you look towards the horizon or towards the zenith from ground-based observatories. The number of meteors you see will be more or less the same.
At first glance, this seems rather surprising. Since meteors burn up at 80-120 km above the Earth, more meteors should, in theory, be visible over the same area of sky if the observer looks towards the horizon. This is because the observer is looking through a larger volume of atmosphere than if he or she looks well above the horizon.
Unfortunately, the greater thickness of atmosphere also results in increased scattering and absorption of light. This is the reason why we see red sunrises and sunsets. So the two effects more or less cancel each other out, and the number of observed meteors varies very little with viewing angle.
Suppose, however, that the observer is lucky enough to be inside an aeroplane which is cruising at an altitude of 10 km, where the air is thin. This time, there is considerably less air along the line of sight when he or she looks towards the horizon, and so the atmosphere 'swallows' less light. There is also the added bonus that the atmosphere is slightly darker from high altitude. As a result, more meteors are recorded.
Dr. Koschny concluded, "It was worth placing a camera on an aircraft because the transparency of the atmosphere is so much better at high altitude. This meant we were able to see much fainter meteors in large numbers."
Although predictions are not promising for the November 2000 shower, Detlef Koschny is hoping to repeat his experiment in the Far East in 2001.
"It will not be dark in Europe at the predicted time of the storm," he explained.
The Leonid-MAC Workshop is sponsored by NASA, the U.S. Air Force, and the Israel Space Agency. Scientists from all over the world will be discussing the recent Leonid observing campaigns and other Leonid storm studies. Apart from reports of observations from the November 1998 and 1999 campaigns, astronomers are putting forward their plans for the future years.
The 1999 Leonid Multi-Instrument Aircraft Campaign (Leonid-MAC) was the first of its kind, and involved two research aircraft provided by the U.S. Air Force. Eight observing windows, four on either side, were available in the EC-18 (a modified Boeing 707) aircraft. This was the home of ESTEC scientist Michael Schmidhuber for several days during November 1999. On the NKC-135 aircraft, 20 upward-looking ports oriented at different angles for maximum coverage of the sky were available for other observers.