Wednesday, November 17, 1999 Published at 15:48 GMT
Voyage through a comet's trail
The 1998 Leonids sparkled over Canada
By BBC Science's Dr Chris Riley on board Nasa's Leonid mission
Early on the morning of Thursday 18 November, in its annual encounter with the dusty trail of comet Tempel Tuttle, the Earth will collide with a trillion fragments of ice - sending shooting stars hurtling through the night sky.
If forecasts are right, it should be an exceptional show.
The comet responsible for spreading this dust across the Earth's orbit came by in February last year and by November satellite operators and interested sky watchers were bracing themselves for a meteor onslaught. The show arrived some 14 hours earlier than predicted, and with only a couple of hundred shooting stars visible each hour it barely disrupted the 700 plus satellites in Earth orbit.
With Dr Rob McNaught of the Australian National University and Professor Vacheslav Emel'yanenko of South Ural University, Chelyabinsk, Russia, Asher has scrutinised historical accounts of past showers and storms that followed previous visits from the comet.
Filaments of dust
They have discovered patterns to the meteor displays which suggest that Tempel Tuttle's debris cloud is in fact a tangled mass of ancient filaments of dust - each one thrown off the comet during a different passage past the Sun.
"A narrow trail can last a few centuries before being scattered into the Leonid stream as a whole and so there could be a dozen of these dust trails in existence at one time," explains Asher.
Last year, the Earth crashed through a filament thrown off Tempel Tuttle in 1333. Since its creation six centuries ago, all the fine debris has been blown away by the solar wind - leaving only the big bits which caused the spectacular show of fireballs in our skies. This year Asher reckons we will be passing through a filament of dust thrown off in 1899.
This one should have more fine material as it is a lot younger and fresher than the filament we passed through last year. More fine material means a potentially higher number of meteors. According to Asher we went through this same filament of debris in 1966 when 40 shooting stars a second lit up the skies over Arizona.
Because of the poor track record of predicting the Leonids in the past, satellite operators are not taking advance evasive actions this year. Instead, they will be waiting on the night to see if a potentially dangerous storm breaks, and space agencies around the globe will be keeping watch for them.
Nasa and the US Air Force have once again mounted a mission to fly under the breaking storm this week, guaranteeing a ringside seat for the spectacle. Fifty scientists are taking to the skies on board two aircraft normally employed for military research.
For mission scientist Dr Peter Jenniskens, his dream of flying through comet Tempel Tuttle's dusty trail has come true again.
"This year is our best chance at observing a storm," he says. "I'm expecting anything from 50 to 10,000 meteors an hour."
During the night of the 17 November the mission will fly west from Tel Aviv to the Azores, prolonging the night.
By dawn on the morning of the 18th, scientists on board the Nasa flights should know whether Asher's models are right and if we are any closer to accurate forecasting of the multi-billion dollar menace of space weather.
Chris Riley is flying with Nasa and you can read his account of what happened by coming back to the Sci Tech section of BBC News Online.