LEONID 1999 OBSERVATIONS

NASA Space Science News for November 16, 1999

Leonids Environment Operations Center monitors meteor activity -- A global team of observers coordinated by the US Air Force and the University of Western Ontario will provide meteor counts for satellite operators. Data summaries will be posted at http://www.LeonidsLive.com for members of the public.

NASA Ames Research Center, Moffett Field, CA

Nov. 15, 1999

ASTROBIOLOGISTS FLYING HIGH TO STUDY LEONID METEORS

Current information about the Leonid MAC Astrobiology mission.

European Space Agency

12 November 1999

Campaign to catch thousands of falling stars

Michael has been enlisted by the ESA Space Science Department in ESTEC (Netherlands) to participate in an international project to study the famous Leonid meteor shower. If all goes well, he should be able to witness one of the most majestic sights in nature -- a meteor storm to rival the most glorious of man-made firework displays.

Over a period of four nights around the predicted peak of meteor activity, he will climb aboard a specially equipped NASA Boeing 707 with scientists from the United States and Europe for the photo opportunity of a lifetime.

Unfortunately, Michael has been given a full research programme, so he won't have much time to admire the view.

"There are windows in the ceiling of the aeroplane through which we can point our cameras," said Michael. "The cameras will be fixed to a kind of rail so that they can stand alone to take video images of the shower."

In order to avoid getting a stiff neck, Michael will be given a useful little gadget to wear on his head.

"The view of the night sky seen in my camera will be fed to a Video Head Display so that I can see everything in the camera's field of view," he explained. "We say what we see, then someone notes it down, and every 15 minutes or so the meteor count is forwarded to a NASA ground station."

Counting meteors is an important job. Although most meteors are no larger than a grain of sand, the Leonids travel so fast (40 times faster than a rifle bullet) that if they plough into a satellite they can cause serious damage. The results from Michael's vigil will be fed to satellite operators around the world so that they can take measures to protect their satellites if they see a storm building.

After a rehearsal on 15-16 November, the work will start in earnest the following night. However, the most exciting time of all will come on 17-18 November, when the number of meteors is expected to reach its peak.

"Our observations will last much longer on this night," said Michael. "The plane will fly against the Earth's rotation so that we stay in darkness for the longest time possible."

So how does he feel about his mission to catch a shower of falling stars?

"Although I'm a keen amateur astronomer, I've never done anything like this before," he admitted. "However, I'm used to counting meteors and know what to look for."

"Curiously, I've never actually seen the Leonids," he added. "I looked last year, but the cloud cover was too bad."

If you want to read about Michael's adventures, extracts from his diary will be published on this Web site during each day of the Leonid meteor campaign.

Diary of the ESA Space Science Department Leonid99 team.

NASA Space Science News for November 11, 1999

NASA wants you! (to help observe the Leonids) - You don't have to be a rocket scientist to observe the 1999 Leonid meteor shower, but you can help out scientists by sending your observations to Science@NASA. This article includes observing tips for sky watchers in the northern and southern hemispheres, instructions for scientific meteor counting, and a place to submit your pictures and meteor counts after the shower.

ROYAL ASTRONOMICAL SOCIETY

10 November 1999

LOOK OUT FOR THE LEONID METEOR SHOWER

Professor Bailey is putting his confidence in the work of his colleague at Armagh, Dr David Asher, who collaborated with Rob McNaught of the Australian National University. They believe they have discovered enough about the location in space of the dust streams responsible for the meteors to give 2.08 a.m. on 18th November, give or take 5 minutes, as the time for the peak of the display.

The number of meteors is more difficult to assess. Dr Asher says, "It's marginal as to whether the meteor activity will reach storm level in 1999, but however strong it turns out to be, European longitudes are ideally placed for observing the outburst". His best estimate is a maximum of 20 meteors a minute visible to a single observer in ideal conditions under a clear, dark sky (conditions rarely experienced by casual observers). Professor Iwan Williams of Queen Mary and Westfield College, London, who has also done research on the Leonids, is more cautious, but said "Most models lead us to expect a better display than last year". Neither Asher nor Williams expects anything like the spectacular storm of 1966, when the rate reached 40 a second for a brief period.

Professor Bailey comments, "It is sometimes said that comets are like cats: they have tails and are unpredictable. If that's the case, predicting a meteor storm has to be about as easy as herding cats! But Asher and McNaught believe they have discovered how to do it. The 1999 Leonids will be a serious test of their method."

Minimising the Danger to Satellites

Apart from knocking a spacecraft off alignment or causing physical damage, such collisions can also generate a cloud of plasma which may cause electrical shorts or damage a spacecraft's sensitive electronics.

This threat is not simply theoretical. In 1993, a European Space Agency satellite called Olympus spun out of control, possibly as the result of an electrical disturbance caused by the impact of a particle from the Perseid meteor shower. There are currently more satellites in orbit around the Earth than ever before, all of which pose a tempting target for one of nature's miniature missiles.

Fortunately, impacts with spacecraft are quite rare, but satellite operators around the world will be monitoring the situation very closely and taking a variety of precautions.

"There could be a lot of activity, but we just don't know for sure," commented Dr Walter Flury of the European Space Operations Centre (ESOC) at Darmstadt in Germany. "It's better to take precautions now than be sorry later."

The European Space Agency's Space Science Department will provide information on meteor numbers to ESOC every 15 minutes. Using this data and radar counts from other sources, ESOC will be able to issue a security alert, warning spacecraft operators to power down their spacecraft or turn them away from the storm.

One of the largest targets, the NASA-ESA Hubble Space Telescope will be manoeuvred so that its mirrors face away from the incoming meteors and its solar arrays are aligned edge on to them during the Leonids' predicted peak.

Apart from reducing the exposed area of giant solar arrays, operators may shut off power to vulnerable electrical components of satellites. In the case of ESA's two European Remote Sensing (ERS) satellites, all of the science instruments will be switched off during the peak of the Leonid activity.

Even spacecraft located some distance from the Earth may be at risk. ESA's Solar and Heliospheric Observatory (SOHO) studies the Sun from a vantage point 1.5 million kilometres away, but it, too, will be turned so that its main navigational aid, the star tracker, is pointing out of harm's way.

Expecting the unexpected

Meteor watchers awaiting the Leonid shower last year (1998) were taken by surprise when a spectacular display of bright meteors occurred 16 hours before the predicted time for the maximum of the shower. However, the explanation for this phenomenon was discovered afterwards by David Asher, Mark Bailey, and Professor Vacheslav Emel'yanenko of South Ural University, Chelyabinsk, Russia, and was published in April (see RAS Press Notice 99/09). They showed that the bright meteors were seen when Earth passed through a dense arc-shaped trail of particles shed from Comet Tempel-Tuttle in the year 1333.

NOTES

Leonid meteors are dust particles that have come off Comet Tempel-Tuttle. Most of this dust is still following the comet fairly closely in space. The comet takes 33 years to complete an orbit around the Sun, and planet Earth ploughs through its main dust trail when the comet returns to our vicinity every 33 years. In the years when this happens, a strong shower or storm takes place. Particularly intense storms were recorded in 1833, 1866 and 1966. In the years between returns of the comet, a very small number of Leonid meteors are seen in mid-November.

Some meteor showers produce about the same rate of meteors around the same date every year. Regular annual showers happen when the dust from a comet has spread around the whole of the comet's orbit, something that takes place gradually over a long period of time. An example is the Orionids, a shower in late October each year caused by dust from Halley's Comet.

The Leonids are so-called because the trails of the meteors belonging to the shower appear to radiate out from a point in the constellation Leo. But this is an effect of perspective. In reality, the meteor particles enter the atmosphere along parallel tracks from the same direction in space.

People who wish to observe the Leonids are recommended to wrap up in warm clothes and find a cloud-free, dark site away from city lights, preferably with a good view towards the north-eastern horizon. Between about 11 p.m. and dawn, they can expect to see rapidly moving shooting stars anywhere in the north-eastern sky, emanating from the 'sickle' (a backwards question mark) made up by the stars in the head of the constellation Leo.

Air Force News Service

9 Nov 1999

Edwards to ferry NASA scientists around world in 8 days to study Leonid

EDWARDS AIR FORCE BASE, Calif. (AFPN) -- After a weak celestial show in 1998, NASA's Peter Jenniskens dreamed of chasing Leonid meteor storms once again in 1999.

"This will be our last shot at it for a century," he said after last year's effort. "The mission we have in mind would circle the world, and do that in just a few days."

He's getting his wish with two Edwards airplanes: a modified KC-135 tanker called the Flying Infrared Signature Technology Aircraft, or FISTA, and an EC-18 that normally serves as an Advanced Range Instrumentation Aircraft, or ARIA.

Both aircraft, which belong to the 452nd Flight Test Squadron, and 25 airmen will ferry 50 scientists on an eight-day, 18,000-mile journey that will take them from the Mojave Desert to Europe to the Middle East and back. The researchers' intent: to gather data during a natural fireworks show called Leonid.

A Leonid meteor shower occurs every November when Earth passes close to the orbit of comet Tempel-Tuttle. Usually not much happens, according to NASA officials. Earth plows through a diffuse cloud of old comet dust that shares Tempel-Tuttle's orbit, and debris burns up harmlessly in the atmosphere.

Typical Leonid meteor events consist of only 10 to 20 shooting stars per hour. But every 33 years, that meek shower surges into a full-fledged storm, when thousands of shooting stars rain down from the sky hourly.

That's what Jenniskens and his crew hope to witness on this trip.

The two-ship formation leaves here Nov. 13 with the first stop being a "gas and go" at McGuire Air Force Base, N.J., said Capt. Jeff Lampe, aircraft commander for FISTA. From there it's on to Royal Air Force Mildenhall, England, where they'll launch late Nov. 16 for a seven-hour mission to Tel Aviv, Israel, hoping to capture a streaking light display in clear, dark skies.

The next night they will leave on their main flight, an eight-hour mission to Lajes Field, the Azores, a small island several hundred miles off the coast of Portugal. It's there scientists believe they will follow a trail of thousands to tens of thousands of meteors per hour.

On this route, the two Edwards planes will fly 100 miles parallel to each other, giving researchers "an almost stereoscopic (three-dimensional) viewing," said Maj. Tracy Phelps, commander of the EC-18.

Finally, the team will fly another seven-hour mission from Lajes to Patrick AFB, Fla., Nov 18-19, and then return home Nov. 20.

With powerful telescopes scattered throughout the world, some people might wonder why take such a time-consuming trip. The answer: Only an airborne mission can bring scientists to the right place at the right time to view Leonid, and guarantee clear weather. Moreover, using both the FISTA and C-18 allows scientists to measure meteor trajectories and orbits in space along with triangulating data.

Indeed, this mission centers on two Edwards aircraft serving as observation platforms for cameras and investigative instruments. Therefore, both planes have undergone modifications for the journey, including installation of optical windows, special camera gear and antenna mounts.

And besides helping collect data for NASA, the C-18 also will downlink real-time video for Air Force Space Command.

Capt. Jon Haser participated in last year's Leonid event and will be going again this year. He said the crews didn't get the expected meteor storm.

"It was sporadic, but they were some persistent trails that lasted five seconds or so. Hopefully, the sky's alive this time."

Maybe he will get to witness what James Young of the Jet Propulsion Laboratory's California Table Mountain Observatory did in 1966, when the last great Leonid storm occurred. He remembers a heaven "absolutely full" of meteors. Young called it a "sight never imagined ... and never seen since."

IMAGE CAPTIONS:

[Image 1]
An EC-18 aircraft from the 452nd Flight Test Squadron, Edwards Air Force Base, Calif., will transport NASA scientists overseas to study the Leonid meteor storm. The EC-18, which normally serves as an Advanced Range Instrumentation Aircraft, or ARIA, also will downlink real-time information to Air Force Space Command during the storm.

[Image 2]
Capt. Jamie McKeon, left, Capt. Jon Hasser, Capt. Jeff Lampe and Maj. Tracy Phelps plan the 452nd Flight Test Squadron's eight-day mission for the Leonid meteor shower. The Edwards Air Force Base, Calif., unit is flying NASA scientists overseas to study the Leonid meteor storm in modified EC-18 and KC-135 aircraft. (Photo by Dennis Taylor)

Marshall Space Flight Center
Huntsville, AL

Nov. 9, 1999

NASA Engineers to Operate Center for Monitoring Leonids Shower; Scientists to Launch Balloon for Clear View of Leonids Meteors

In a separate effort to learn more about these dazzling fireballs, NASA scientists will launch a balloon to record meteor images and sounds -- and maybe even catch a piece of a "shooting star."

A Leonids shower happens every year when Earth passes close to the orbit of the comet Tempel-Tuttle and the debris left in the comet's path. As Earth travels through the comet dust, the debris burns up in Earth's atmosphere, and observers typically see about 10 to 20 shooting stars an hour. But some experts predict this year's annual shower may turn into a "storm" -- a spectacular display of 1,000 meteors per hour or more.

To monitor any increases in meteor activity, the Leonid Environment Operations Center at NASA's Marshall Space Flight Center in Huntsville, Ala., will be staffed 24-hours a day from the afternoon of Nov. 15 until the shower has passed on Nov. 19.

Marshall Center engineers will coordinate and distribute pre-Leonid storm information and real-time observations about Leonid activity, intensity and potential threat to NASA and U.S. Air Force spacecraft. A worldwide network of radar and optical observation sites sponsored by the Air Force and operated by the University of Western Ontario will send information to the Leonid Environment Operations Center at Marshall, where scientists and engineers will analyze the information and distribute it to satellite operators.

"NASA, the Air Force, the University of Western Ontario and several other organizations have teamed together to provide these space weather updates to keep spacecraft operators well informed so they can best protect our satellites," said Dr. Jeff Anderson of the Marshall Center's Engineering Directorate. "Monitoring the Leonid meteor stream also provides a rare look at a natural phenomenon that will continue to grow in importance as more and more satellites orbit our planet, and we venture deeper into space."

Although a typical meteor is smaller than a grain of sand, it travels more than 40 times the speed of a bullet. The Leonids are the fastest of all the meteor streams, fast enough to circle the globe in less than 10 minutes. Meteor impacts can impair satellites and their sensitive sensors.

Space weather forecasts are not something one hears on the nightly news because they are tricky at best. Will this year's annual Leonids display be just a shower -- a few to a few hundred shooting stars per hour? Or will it be a storm -- a few thousand to a few hundred thousand meteors an hour?

"In 1998, the world watched and nothing happened," said Anderson. "But in 1966, most folks in the western U.S. were sleeping while one of the most spectacular displays in history was going on over their heads."

It is because many experts are predicting a storm this year that the Marshall Leonid Operations Center is being staffed around the clock during the shower. Just last year, the comet Tempel-Tuttle visited our inner solar system, depositing a dense cloud of debris. But because Earth crossed the comet's orbit too soon after the comet's passage, there was no storm -- just a strong shower.

In 1999, the Earth will pass only 68,200 miles (110,000 kilometers) from the comet debris cloud, making a storm more likely.

In another activity, Marshall scientists will work to give the public a clearer view of the streaking fireballs. Weather permitting, they will launch a 10-foot (3-meter) diameter weather balloon from Marshall's Atmospheric Research Facility at approximately 12:30 a.m. CST, early Thursday morning Nov. 18. The balloon will ascend approximately 20 miles (32 kilometers), carrying a sensitive camera for capturing pictures of the meteors. During the flight from around 12:30 a.m. to 3:30 a.m. CST, both still and low-resolution television from the onboard camera can be viewed online at the Science Directorate's Web site at http://www.leonidslive.com.

Last year, more than one million people tuned into the live Web cast or saw the replay the next day on the Web site. This year a new feature will be a recording device that sends back sounds of meteors from space. Visitors to the Web site will be able to hear the "whistlers" and other bizarre noises that meteors make as they interact with ionized gas or plasma in the Earth's atmosphere. Scientists hope to use this radio receiver to record very low frequency electromagnetic emissions below 10 kHz emitted by the meteors.

A capture device on the balloon may even bring back a meteor particle. Scientists are still analyzing data from an aerogel-collecting device that was flown last year to capture bits of comet Tempel-Tuttle. The meteoroid capture device on the upcoming flight uses xerogel, a close relative of aerogel, and a variety of low-density acrylic materials.

"It works like flypaper," said Dr. John Horack, an astronomer at the Marshall Center. "We expose these materials to the air up in the stratosphere while the meteor shower is under way. When tiny particles strike the exposed xerogel, they stick."

IMAGE:
NASA Photo #9906509 : To keep their satellites safe, this month NASA, the U.S. Air Force and the University of Western Ontario will operate the first center for around-the-clock monitoring of the annual Leonids meteor shower. From left, NASA engineers Dr. Bill Cooke, Dr. Jeff Anderson and Dr. Rob Suggs discuss the meteoroid approach angles at the Leonid Environment Operations Center at NASA's Marshall Space Flight Center in Huntsville, Ala. Engineers will coordinate and distribute pre-Leonid storm information and provide updates to satellite operators about Leonid activity, intensity and potential threat to spacecraft. (NASA Marshall Space Flight Center Photo by Emmett Given)

NASA Space Science News for November 3, 1999

Leonids on the Moon: When the Leonid meteor shower strikes on November 18, Earth won't be the only place in the cross hairs. The Moon will also pass very close to the debris stream of comet Tempel-Tuttle. Leonid meteorite impacts on the Moon might be visible from Earth and provide a means for long-distance lunar prospecting.

Near-Live Leonid Watching System

A Brief Background on the Leonid Meteors:

As it approaches the inner solar system and is heated by the sun, the comet replenishes its path with tiny bits of material eroded away by the solar wind and radiation. As Earth travels in its path around the sun and encounters this debris stream, the small grains of material in this stream slam into Earth's upper atmosphere at a very high rate of speed becoming incandescent and leaving an ionized and luminous trail that we see as meteor or "falling star".

On an average year, 15 to 20 Leonid meteors per hour can be seen, depending on your local viewing conditions. During a storm year, anything can happen as history has shown. The last major Leonid Meteor Storm occurred on November 16, 1966, peaking for observers in the mid-western United States. Hourly meteor rates were estimated to be as high as 144,000! Historical accounts dating back to the 1833 and 1866 Leonids are fascinating to read! The 1899 and 1932 Leonids were largely missed and it is suspected that Jupiter may have altered the meteor stream's orbit for those years. Studies also suggest 2000, 2001 or even 2002 could be much better than normal years, with significantly higher meteor counts than normal years! The 1999 Leonid Meteor Shower is an event that has been long anticipated by the astronomy community.

Predictions:

If the peak occurs during the earlier predicted time, Asia and Europe will be positioned favorably with the radiant high overhead. If it occurs a few hours past the later predicted time, the western portions of Europe and Africa, along with the East Coast of the United States, will be positioned favorably. This is certainly an event not to be missed and it would be well advised to look for the Leonids during the early morning hours of November 17th and November 19th, due to the uncertainty.

A Scenario For US East Coast Observers:

George Varros

NASA Space Science News for November 1, 1999

NASA Meteor Balloon Rises Again: NASA scientists and ham radio amateurs are teaming up for a weather balloon flight to the stratosphere during the Leonid meteor shower on November 18, 1999. The balloon will transmit a live webcast of the meteor shower from an altitude of 100,000 ft or more, far above any bad weather or obscuring clouds. The transmission will include audio from a VLF radio receiver designed to capture natural radio signals from the meteors.

Marshall Space Flight Center

Leonids in The Crystal Ball

Most experts agree that 1999 is a likely year for a Leonids meteor storm.

"Good evening space weather lovers! Last night Earth was hit by a high-pressure solar wind stream. It's expected to persist for 3 or 4 more days producing a 50% chance of mid-latitude aurora. But the big news today is the 1999 Leonid meteor shower. Experts are predicting a big storm on November 18th with up to 100,000 shooting stars per hour. Of course, we could be off by a couple of years. The storm might hit in 2001 instead. Or maybe not at all! Hey, if predicting these things were easy we wouldn't need experts!"

One day, space weather forecasts like this could be commonplace. As our society comes to rely on satellites, cell phones, and other space-age gadgets, forecasting solar storms and meteor showers can be just as important as knowing the chances of rain tomorrow. Three weeks from now we may be treated to a very visible reminder of space weather when the Leonid meteor shower strikes on November 18, 1999.

What's the probability of significant meteoroid precipitation? That's what stargazers and satellite operators everywhere would love to know.

Most experts would agree that predicting the Leonids can be tricky. To understand why it's helpful to know the difference between a "meteor shower" and a "meteor storm." Simply put, meteor showers are small and meteor storms are big. Meteor showers produce a few to a few hundred shooting stars per hour. Meteor storms produce a few thousand to a few hundred thousand meteors per hour. A meteor storm, like a total solar eclipse, ranks as one of Nature's rarest and most beautiful wonders.

A Leonid meteor shower happens every year around November 17 when Earth passes close to the orbit of comet Tempel-Tuttle. Usually not much happens. The Earth plows through a diffuse cloud of old comet dust that shares Tempel-Tuttle's orbit, and the debris burns up harmlessly in Earth's atmosphere. A typical Leonid meteor shower consists of a meager 10 to 20 shooting stars per hour.

Every 33 years something special happens. Comet Tempel-Tuttle swings through the inner solar system and brings a dense cloud of debris with it. For 3 or 4 years after its passage the Leonids can be very active. In 1966 for instance, over 100,000 meteors per hour were seen from parts of North America. Curiously, there isn't a full-fledged storm every time Tempel-Tuttle passes by. Sometimes there's simply a stronger-than-average shower. Sometimes nothing happens at all!

Will there be a storm in 1999? (Probably, yes.)

The period of maximum activity during the 1998 Leonid shower took place about 12 hours before the earth crossed Tempel-Tuttle's orbital plane. The early activity caught many observers by surprise, but it was business as usual for the unpredictable Leonids. Rainer Arlt of the International Meteor Organization noted that while the maximum activity came early, there was a secondary maximum when the Earth passed the comet's orbit. This pattern is similar to that observed in 1965, the year that preceded the great Leonids storm of 1966. In his report, Bulletin 13 of the International Leonid Watch: The 1998 Leonid Meteor Shower, Arlt wrote:

[T]he radar, visual, and photographic records of the 1965 Leonids indicate an activity profile which resembles that of the 1998 Leonids. Even the low population index seems comparable. Judging from these phenomenological facts, we may expect 1999 to show a similar shape of activity as in 1966. The actual maximum meteor numbers are hardly predictable.

Instead, there was an stunning display of shooting stars over western North America. This recollection by James Young at JPL's Table Mountain Observatory in California gives a sense of what the storm was like:

"This very noteworthy [1966] meteor shower was nearly missed altogether... There were 2-5 meteors seen every second as we scrambled to set up the only two cameras we had, as no real preparations had been made for any observations or photography. The shower was expected to occur over the European continent.

The shower peaked around 4 a.m., with some 50 meteors falling per second. We all felt like we needed to put on 'hard hats'! The sky was absolutely full of meteors...a sight never imagined ... and never seen since! To further understand the sheer intensity of this event, we blinked our eyes open for the same time we normally blink them closed, and saw the entire sky full of streaks ... everywhere!"

Joe Rao, a Leonids expert who lectures at New York's Hayden Planetarium, also advocates 1999 as possibly the best year for a storm during this 33 year cycle. Writing for Sky &Telescope he says:

Based on what happened last November, I will venture a prediction. If a meteor storm is to take place at all, 1999 would appear to be the most likely year for it to happen. But even if this year's Leonids are richer in number, observers should not expect the same high proportion of fireballs that were seen in 1998. Instead, a more even mix of bright and faint meteors is likely. [ref]

Rao is also a meteorologist for News 12 Westchester, which seems a suitable occupation for predicting meteor showers.

In 1999, the Earth will pass nearly three times as far from the comet's orbital path as it did in 1966 and more than six times farther than it did during the great storm of 1833. If the peak of the Leonids arrives exactly when the Earth passes through the comet's orbital plane, Donald Yeomans of JPL gives 01:48 UT on November 18, 1999 as the most likely time for the 1999 maximum [ref]. That would make Europe and West Africa the best places to watch the show. However, Leonid meteor showers frequently arrive much earlier or later than predicted, so any place on the globe could be favored.

If the peak of the Leonids occurs over Europe or the Atlantic Ocean, then observers in the USA could be in for an unusual treat. The Leonid radiant would just be rising over North America at the time. In the eastern US sky watchers would see a large number of earth-grazing meteors skimming horizontally through the upper atmosphere. "Earth grazers" are typically long and dramatic, streaking far across the sky.

To look or not to look, that is the question

All sorts of conjectures were made by all sorts of people ... We may learn of this that, when men are in a high state of excitement, their testimony must be taken with many grains of allowance.

From a first-hand account of the 1833 Leonid Meteor Shower. by Elder Samuel Rogers

If 1999 is the year, when should you look? Most experts predict that the Leonids peak will occur between 0100 and 0400 Universal Time on November 18th. However, it is important to remember that such predictions are always uncertain. The 1998 Leonid fireball display occurred nearly 16 hours before the predicted maximum! No matter where on Earth you live, the morning of November 18 will probably be the best time to look for Leonids in 1999. This is true even if morning where you live occurs much earlier or later than 0100-0400 UT.

Conventional wisdom says that meteor observing is always best between midnight and dawn local time on the date of the shower (November 18 in this case). For a shower or storm like the Leonids that might be relatively brief, it is best to start watching no later than midnight. In fact, when the author of this story went outside last year at midnight to view the 1998 Leonids, the shower was already well underway! With this in mind you may decide it's a good idea to begin observing even earlier, say, 10 p.m. on November 17.

In the coming weeks Science@NASA will post more stories about the Leonids with observing tips for meteor watching with the naked eye, video cameras and other types of recording devices. One thing seems sure, no matter where you live: The Leonids are coming and, on Nov 18, 1999 the place to be is outside, looking up!

Air Force News Service

26 Oct 1999

'Glowworms in sky' studied at Starfire Optical Range

Between Nov. 16 and 19, scientists will gather at Starfire to train a variety of instruments on the Leonid meteor shower. Grains of sand and dust from the comet Temple Tuttle produce these meteors. Named for the constellation Leo, from which they appear to emanate, these meteors vaporize in the Earth's atmosphere.

According to Dr. Jack Drummond, the laboratory's Directed Energy Directorate astronomer, the Leonid meteors leave behind trails which, unlike ordinary meteors that fade in a matter of seconds, can last up to an hour and are still unexplained.

"I call these lingering meteor trails 'glowworms in the sky' since they are not only visible for minutes by chemical reactions, but are twisted by the winds into serpentine shapes, appearing like snakes or worms," Drummond explained.

Scientists hope to gather enough information to explain one of the more curious atmospheric phenomena -- What makes the meteor trails glow?

The glow, called chemiluminescence, is the production of light from chemical reactions similar to bioluminsescence, the same kind of glowing reaction found in biological entities such as fireflies and their larvae, glowworms.

Drummond, who last year guided lasers onto the trails when they appeared last November, said the lingering meteor trails are self-luminescent.

"They do not shine by reflected moonlight or sunlight," he said, adding, "but the exact chemical reactions involved are unknown."

The scientists will direct a lidar, a laser that operates at visible wavelengths to gather data, onto the lingering trails. This instrument, on loan from the University of Illinois, was steered onto meteor trails last year to probe the meteors' compositions.

The data gathered by the lidar, which is attached to the 3.5-meter telescope at Starfire, indicates that although sodium may be involved in a catalytic reaction with ozone to produce sodium airglow, it is not the principle emission from the trails.

This year, additional instruments will be used to try and identify the emission lines. Three spectrographs will be aimed at the trails to try and determine the chemical reactions that are responsible for the glow.

A spectrograph divides light into a color spectrum, revealing bright (emission) and dark (absorption) lines produced by elements and molecules. The operation of two spectrographs, one from the University of Arizona and one from the University of Illinois, will study light visible to the naked eye. The third spectrograph, from Aerospace Corporation, will study light in the infrared at wavelengths that are not visible to the naked eye.

An electronic charge-coupled device camera and video camera will be used to record the changing appearance of the trails. Shown at several conferences, last year's 13-minute video, "blew the audiences away," according to Drummond. He said most people have never seen or heard of these 'glowworms in the sky' which are characteristic of the Leonid Meteor showers.

The scientists hope to answer why the 'glowworms' are peculiar to the Leonid storm periods, which occur every 33 years, and why they are rarely seen at other times. They wonder if it may imply something about the composition of the parent comet.

Although this year's shower is expected to peak over Europe on the morning of Nov. 18, it is hoped that some of the fireball components of the storm will be visible locally one or two days before or after the main peak. Here, on a dark quiet mountaintop on the southern end of Kirtland Air Force Base, N.M., scientists will be watching, waiting and hoping to unravel a mystery occurring 60 miles overhead.

Images supporting this release

ROYAL ASTRONOMICAL SOCIETY

30 August 1999

RAS Web

FINE LEONID METEOR DISPLAYS PREDICTED THROUGH TO 2002

In the early hours of 17th November last year (1998), meteor watchers awaiting the Leonid shower were taken by surprise when a spectacular display of bright meteors occurred 16 hours before the predicted time for the maximum of the shower. The explanation for this phenomenon was discovered by Dr Asher and his colleagues Professor Mark Bailey of Armagh Observatory, and Professor Vacheslav Emel'yanenko of South Ural University, Chelyabinsk, Russia, and was published in April (see RAS Press Notice 99/09). They showed that the bright meteors were seen when Earth passed through a dense arc-shaped cloud of particles shed from Comet Tempel-Tuttle in the year 1333 and they proved for the first time that meteoroid streams can have complex braid-like structures within them. This work pointed the way to more precise predictions of the timing and intensity of meteor showers, such as those Asher and McNaught are now making for the Leonids.

The latest analysis, covering Leonid meteor storms over the past two hundred years, shows that the peak times of the strongest storms and sharpest outbursts are predictable to within about five minutes. The technique involves mapping the fine `braided' structure of the dense dust trails within the Leonid meteoroid stream. Although comet Tempel-Tuttle, the 'parent' of the Leonid stream, passed close to the Earth in 1998, Asher and McNaught predict strong meteor storms in both 2001 and 2002. 1999 and 2000 will be less spectacular, but good. In 1999, observers at European longitudes are favoured, and may see up to 20 meteors a minute (in ideal conditions under a clear, dark sky) at around 2 a.m. on the morning of November 18th.

Meteors, popularly known as 'shooting stars', can be seen on any night, given a sufficiently clear, dark sky. They are produced by the impact on the Earth's atmosphere of small dust grains released from comets. Most meteors arrive in 'showers' at fixed times of the year, when the Earth passes close to the orbit of the parent comet. But occasionally - just a few times a century - a phenomenon known as a meteor storm occurs. During a storm, meteors appear at astonishing rates, sometimes several per second. The most famous example, the incredible Leonid display of 1833, is credited with starting the serious scientific study of meteors.

Good news for meteor observers can be a concern for satellite operators. A satellite can be disabled by the impact of even a small dust grain. While the hazard from man-made space debris is well known, the danger from meteoroids has been more difficult to assess. Prior knowledge of the detailed structure of the Leonid stream is potentially of immense value. Satellite operators could use this information to take appropriate avoiding action and minimise the risk. With this new work, McNaught and Asher have defined the structure of the Leonid dust trails more accurately than ever before.

NOTES

What are the Leonid meteoroid stream and the Leonid meteor shower?

When do the most intense outbursts occur?

Why are some longitudes favoured?

Can damage to satellites occur?

History of this work

A few months after developing the technique, McNaught and Asher extended their work to permit estimates of meteor rates (in addition to predicting storm timings), and applied it to forthcoming encounters of the Earth with Leonid dust trails. There is no doubt that 2001 and 2002 will provide opportunities to witness exceptional Leonid meteor storms.

The fact that something out of the ordinary is expected in both 2001 and 2002 had in fact been published more than a decade ago, by two researchers, Kondrat'eva and Reznikov, in Kazan, Russia. The English translation of their paper did not come to the notice of many western researchers.

ROYAL ASTRONOMICAL SOCIETY

15 April 1999

SURPRISE 1998 LEONID DISPLAY WAS A LARGE BLAST FROM THE PAST

The Leonid meteor shower occurs between 15 and 21 November each year, with peak activity on the night of the 17/18 November. These meteors are produced when small dust particles ejected from Comet Tempel-Tuttle enter the Earth's atmosphere at high speed and burn up. Comet Tempel-Tuttle moves around the Sun in an elliptical orbit taking approximately 33 years for a complete revolution. Its orbit is similar to that of Halley's Comet, and so Comet Tempel-Tuttle is classified as a Halley-type short-period comet. Owing to the large angle between the Earth's orbit and the comet's (162 degrees), the dust grains collide almost head-on with the Earth, and hit the atmosphere at about 71 kilometres per second. At this speed, a one-centimetre particle carries the same amount of energy as a speeding truck on a motorway.

Every 33 years or so, when Comet Tempel-Tuttle passes near to the Earth, the intensity of the Leonid display is greatly enhanced because the stream of dust grains is more densely packed close to the comet. Meteor 'storms' have been seen many times during the past thousand years, notable events being those of 1799, 1833, 1866 and 1966. The earliest record of Leonid meteors dates back to the year 899.

November 1998 saw astronomers preparing for a possible meteor storm during the night of 17/18 November. Although a moderately strong peak was observed as predicted, the meteor shower as a whole was dominated by the appearance of hundreds of exceptionally bright meteors, known as fireballs, more than 16 hours ahead of the predicted peak.

The intensity and duration of this exceptional event indicated that the Earth must have passed through an extremely dense, narrow stream of large dust grains and particles, having sizes ranging up to several centimetres. The timing suggested that these particles occupied an orbit somewhat different from the main stream of small grains, and that they left the comet's nucleus many hundreds of years ago. But in that case, it is necessary to explain why the stream has held together so tightly for so long.

To solve the problem, Dr David Asher and his co-workers calculated the motion of large dust grains ejected from the comet at each of the last 42 occasions when it made its closest approach to the Sun. (Comets release very little dust, if any, when they are far from the Sun's heat.) They checked each case to see whether any of the particles could explain the fireballs seen in 1998, and identified September 1333 as the time when most of the observed particles were released. These particles did not spread out in space because of a dynamical process known as a resonance. (A similar process gives rise to the fine structure seen in Saturn's rings.)

Many comets and asteroids swing around the Sun in orbits that are simple multiples of the orbital period of Jupiter, the most massive planet in the solar system and the biggest disturbing influence on cometary orbits. Comet Tempel-Tuttle is no exception to this rule, having entered one of these 'resonant' orbits as long ago as the seventh century AD. For every fourteen revolutions of Jupiter, Comet Tempel-Tuttle makes five, and the same relation holds true for the largest dust particles gently released by the comet.

The large grains therefore have average orbital periods very close to that of the comet, and are kept in step by the influence of Jupiter. Instead of spreading around the whole orbit, they occupy a rather short arc, leading to the formation of a dense strand of large particles, distinct from the 'normal' storm strands of small particles, ahead of and behind the comet. The structure of the meteoroid stream close to the comet can be visualized as rather like a telephone wire, made up of many separate, narrow strands. These form a complex, braided structure of material within the broader, ribbon-like meteoroid stream.

The calculations by David Asher and co-workers showed that in November 1998 most of the resonant arcs missed the Earth by a wide margin, but the arc of particles released in 1333 cut right through the Earth's orbit, and the calculated time for when this happened matched the observed fireball maximum to the hour.

This remarkable result is the first observational demonstration of one of the most important dynamical features of meteoroid streams associated with Halley-type short-period comets. The work highlights the presence of fine structure *within* meteoroid streams, and suggests important new avenues for research. For example, by observing the variations in meteor rates close to the peak of a shower it may be possible to infer the precise distribution in space of the meteor-producing strands. Variations in meteor rates may be correlated with changes in the meteor brightness distribution to infer the history of mass loss by a comet over many revolutions around the Sun.

The researchers are not expecting a repeat performance of bright fireballs in November this year (1999). All the resonant strands in the meteoroid stream will be well past Earth in space. However, a strong 'normal' display is likely, peaking at about 2 a.m. on November 18th, due to meteoroids ejected from Comet Tempel-Tuttle in the years 1866, 1899 and 1932, which have not yet had time to disperse around the comet's orbit.

Leonid 1998 observations

All-sky photograph of 156 Leonids taken by the Astronomical and Geophysical Observatory (AGO) at Modra, Slovakia

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