(Een Nederlandstalig artikel over het mogelijk zandstralen van satellieten door de Leoniden verscheen in het novembernummer 1998 van maandblad KIJK.)
14th Air Force
Vandenberg AFB, California
November 18, 1998
Uncertain about the severity of the storm, 14th Air Force space operations crews spent much of the past several months preparing for possible storm damage to Air Force satellites. Crews practiced comprehensive techniques to limit storm damage and honed their responses to satellite hazards.
"We prepared for the worst and were pleased the storm did not directly threaten our space assets," said Major General Gerald Perryman, commander of 14th Air Force and component commander, U.S. Air Force Space Operations, U.S. Space Command. "America absolutely depends on its commercial and military space assets. We erred on the side of caution to protect those assets and are pleased to report that our space forces are on-station and healthy."
Each November the Earth passes through the debris trail of comet Tempel- Tuttle; but every 32-33 years it passes through the densest portion of the comet's debris, which significantly increases the number of meteoroids near the Earth.
The last Leonids storm occurred in 1966 when there weren't as many satellites on orbit or the technology to gather detailed meteoroid data. Predicting the number and location of Leonids meteoroids is not easy, according to Air Force officials. This year, impact expectations ranged from negligible to serious. As a result, satellite operation crews prepared for the worst. Even so, Air Force officials did not expect any DoD satellites to lose complete capabilities during the storm, but were ready to carry on vital space missions before, during and after the storm.
Fourteenth Air Force encompasses all U.S. Air Force space forces. Its operations include global ballistic missile warning, space surveillance, satellite control, space-based navigation and communications, and spacelift generation and range operations.
November 12, 1998
The Russian space station Mir will be turned so that the smallest surface possible is exposed to the threat of the Leonid meteor storm. To be safe, however, the two cosmonauts will board the Soyuz escape capsule when the shower reaches its peak. On Wednesday, November 11, cosmonauts Gennady Padalka and Sergei Avdeyev performed a 6-hours spacewalk, installing French equipment to catch the flow of Leonid meteorites. The "meteorite trap", that is attached to the outside of Mir, will be collected by French astronaut Jean-Pierre Haignere in February 1999 when he makes a trip to Mir.
Space Telescope Science Institute, Baltimore, MD
November 9, 1998
Using the brilliant glow of a distant quasar located near the southern boundary of the constellation Aquarius, Hubble will probe galaxy formation and the distribution of matter in space. The Hubble data will become immediately available to the astronomical community.
The meteor storm is an expected downpour of thousands of meteors zooming by Earth. They pose a small but potential threat to Hubble and other satellites, say experts. The meteors appear to come from the direction of the zodiacal constellation Leo the Lion, and hence the storm is called the Leonids.
For a 10-hour period at the peak of the storm, estimated to be at approximately 2:43 p.m. Eastern Standard Time on the 17th, the telescope will be oriented with its aft bulkhead facing into the direction of the meteoroid stream. Hubble's solar panels will lay flat, or parallel to the meteoroid flow.
Though most Leonid meteoroids are smaller than a grain of sand, they zoom across space at a menacing 155,000 miles per hours. A speck-sized meteoroid can pack the wallop of a .22 caliber bullet as it pierces the spacecraft hull.
Still, even at the peak of meteor activity the density of particles in any given region of space is extremely low. So, project scientists predict that Hubble has less than a 1-in-10,000 chance of being hit by a particle large enough to pierce it's aluminum skin.
Smaller meteoroids vaporizing on impact create a plume of plasma that can short-circuit spacecraft electronics. However, a short circuit on Hubble is unlikely because its electronics are housed inside aluminum boxes that also serve as a meteoroid shield.
The Space Telescope won't be idle during the shower. STScI director Steven Beckwith is making his discretionary observing time available so astronomers can still observe the heavens while the orbiting observatory is aimed away from the meteoroid barrage.
Hubble will be aimed at a quasar, the bright core of an active galaxy, approximately 10 billion light-years away. Hubble won't be studying the quasar itself but the surrounding galaxies, protogalaxies and primordial hydrogen clouds between us and the quasar. The quasar is so brilliant, it is like a searchlight shining through fog.
Strung along billions of light-years, like beads on a string, the gas clouds will be detectable in the way they subtract certain colors or frequencies of the quasar's light. The observation will help determine whether the clouds are cold primeval hydrogen or are sites of ongoing star formation which have been enriched with heavier elements.
Hubble's Space Telescope Imaging Spectrograph will take a long-exposure picture to identify galaxies along the sight, and divide the light into a rainbow of colors (a spectral image) to determine galaxy distances. This is accomplished by measuring how the light has been stretched or redshifted by the universe's expansion.
Follow-up spectroscopic observations with large ground-based telescopes and high-resolution spectrographs will measure the quasar light directly and identify the distance of the intervening gas clouds.
The redshifts of the gas clouds from the ground-based data will then be matched with the redshifts of the galaxies along the line of sight seen in the HST data. These combined observations will allow astronomers to see if galaxies are associated with these invisible clouds.
The comet made its last closest passage to the sun in late February of this year. Warmed by the sun, the icy comet nucleus spewed a great deal of dust into space as its ices melted. These dust particles appear as meteors when they enter Earth's atmosphere and burn up from friction.
The stunning estimates of as many as 10,000 meteors during the 1-hour storm are based on prior meteor storms that have occurred when the comet has returned to Earth during the past 2 centuries.
Because the comet has a 33-year period, the last shower was on November 17, 1966. A brief, 20 minute burst in meteor activity -- as seen from the central and western United States lit up the skies with 40 meteors per second!
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).
This release and an illustration is available on the Internet
Goddard Space Flight Center
Greenbelt, MD 20771
Nov. 9, 1998
The annual Leonid shower -- this year a storm -- is expected to be unusually intense because the Earth is crossing Comet Tempel-Tuttle's orbital path at a time when the comet has recently passed by. This happens once every 33 years when Tempel-Tuttle makes its closest approach to the Sun. The Sun's radiation boils bits of dust and sand off the comet, littering its path with debris.
Where possible, controllers will change the orientation of satellites to reduce the possibility that one of these tiny particles (1 to 100 microns in size, or about the size of a small sand grain) will strike and disable a spacecraft. However, Leonid storms pose a greater than usual threat to spacecraft not only because of the many tiny meteors (thousands per hour) hitting our atmosphere, but also the tremendous velocities of the particles.
As the Earth moves across the comet's trail, Leonid particles will enter the planet's atmosphere. Like two freight trains hurtling at one another on the same track, the distance between the massive debris cloud and the Earth closes at a mind-boggling 45 miles per second, or over 200 times the speed of sound. In contrast, Perseid meteors reach speeds of about 37 miles per second, and typical daily meteors achieve velocities of about 12 miles per second.
On spacecraft where it is practicable, high voltage systems that supply instruments will be turned off, or ramped down, to safeguard against the potential for electrical damage as a result of the satellite's plunge into the debris cloud. The tiny meteors can hit the spacecraft like a sandblaster and disintegrate, creating a cloud of electrically charged plasma. Under the right conditions, this plasma cloud can set off a chain reaction causing a massive short circuit. The loss of the European Space Agency's Olympus communications satellite in 1993 was attributed to a strike from the Perseid shower, and the resulting plasma discharge that zapped the spacecraft's delicate electronics.
The 22 NASA spacecraft under Goddard's control -- from the 24,500 pound Hubble Space Telescope to the 25-year old, 800 pound IMP-8 satellite -- will be continuously monitored during the peak of the storm, and some maneuvered to provide the greatest protection possible from debris.
"Each individual mission and project team reviewed its procedure for dealing with this annual phenomena, and has a specific implementation plan for the Leonid meteor storm," said Philip E. Liebrecht, Associate Director for Networks and Mission Services. "Each spacecraft has an operating plan that balances the risk of taking specific defensive measures against the risk of taking no action. We've had independent review teams assess our plans, and I think we are doing everything prudent and practicable to ensure the safety of our spacecraft."
The Leonid meteor shower arrives every November. It takes its name from the constellation Leo, the area of the sky where the meteors appear to originate. The shower's small particles are completely vaporized high in the Earth's atmosphere, and present no danger to the Earth's surface or to aircraft.
Historically, the most active Leonid showers occur during the first two years following the comet's closest approach to the Sun. This last occurred on Feb. 28, 1998. This year's outburst is projected to be less severe than that observed in the last 33-year cycle, which occurred in 1966. The peak time for the Leonid meteor storm will be Nov. 17, sometime between 11:43 a.m and 5:43 p.m. Eastern Standard Time.
For the past several weeks, engineers at Goddard have been reviewing the status of all the spacecraft under their control and developing ways to reduce exposure to the meteor storm. In general, the health of these spacecraft will be monitored before, during and after the storm, and commands to a number of the spacecraft will be stopped or curtailed during this period.
The Hubble Space Telescope will be maneuvered so that its mirrors face away from the storm. Its solar arrays will be rotated so only the edges are exposed to oncoming particles. Controllers won't turn Hubble off during the storm, but rather use the 10-hour period that Hubble is maintained in this attitude to take a long-exposure picture (for more on this, check out http://www.stsci.edu/ftp/proposer/leonid.html).
Some spacecraft, like the Tropical Rainfall Measuring Mission, are already in the ideal orientation for the storm, and only an adjustment to position the solar arrays "edge on" to the storm will be needed. The Rossi X-ray Timing Explorer's instruments will be turned off to protect the spacecraft's high voltage devices from a potential massive short circuit similar to what happened to Olympus.
For the Advanced Composition Explorer, the solar arrays will be rotated, and high voltage supplies for instruments will be ramped down. Since the center of the Leonid stream is closer to the L-1 orbit (1 million miles from the Earth toward the Sun) than to Earth, ACE will see an even more intense storm than Earth-orbiting satellites.
Risk reduction procedures will be followed for other spacecraft including the Extreme Ultraviolet Explorer, Compton Gamma Ray Observatory, Upper Atmosphere Research Satellite, Total Ozone Mapping Spectrometer, Fast Auroral Snapshot, Solar Anomalous Magnetospheric Particle Explorer, Transition Region and Coronal Explorer, WIND, POLAR, Solar and Heliospheric Observatory, Interplanetary Monitoring Platform and Earth Radiation Budget Satellite.
The Tracking and Data Relay Satellites will be maintained in their full operational mode, as these spacecraft are vital to provide the communications link to and from other spacecraft during the peak storm period.
Flight control teams for all of Goddard's operational spacecraft have been briefed on the meteor storm and have developed contingency plans to react to any damage sustained during the storm. In addition, all available command and control capabilities will be on alert for possible use in an emergency, and subsystem engineers will be on standby for consultation if there are any problems resulting from the storm.
Applied Physics Laboratory
Johns Hopkins University
Each November 17 the Earth and its swarm of orbiting satellites passes through the Leonid meteor stream, which originates from the wake of Comet 55P/Tempel-Tuttle. In most years the Leonid shower (so named because the meteors appear to come from the direction of the constellation Leo) is unimpressive. But once every 33-1/4 years -- this year - the Earth passes through Tempel-Tuttle's path just after the comet has made its closest approach to the sun, when dust, sand, and other materials have been freshly boiled up from the comet's surface by the sun's radiation and left in its trail.
NASA technicians are changing the orientation of spacecraft and ramping down their high voltage supplies to reduce the risk of damage as they plunge through the massive cloud of Leonid particles at a speed of 45 miles per second for approximately six hours. They are maneuvering the Hubble Space Telescope so its mirrors face away from the storm and adjusting other spacecraft so their solar arrays will meet the storm "edge-on."
At most risk, scientists say, will be the Advanced Composition Explorer (ACE) spacecraft designed and built by The Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Md. Launched in August 1997, ACE is in orbit at a point 1 million miles from Earth toward the sun, a position that will take it closest to the center of the comet's wake as we pass through.
ACE carries nine instruments designed to study energetic particles from the sun, interplanetary space, and regions beyond. But none of these instruments is designed to look at the tiny dirt and dust particles the spacecraft will soon encounter. In fact, they'll be ducking their heads.
Mary Chiu, ACE Program Manager at APL, says, "We hope for the best, but we really can't predict what will happen. The probability of getting hit by a particle that could cause damage is still fairly small, but, depending on the size of any given particle and where it might hit, the possibility for problems exists."
To the extent possible, NASA will maneuver ACE so that the backs of solar arrays will face the meteor storm and three of the most sensitive instruments -- the Solar Energetic Particle Ionic Charge Analyzer, the Solar Isotope Spectrometer, and the Cosmic Ray Isotope Spectrometer -- will be pointed away from it.
"We're standing by to help the NASA team analyze and assess any problems that might result from the storm," Chiu says. "But at the same time, we're crossing our fingers in hopes for an uneventful day on November 17."
This dramatic encounter with the Leonid meteor storm will not be visible to amateur stargazers in America, who, like sensitive instruments on NASA spacecraft, will be on the back side of the Earth as it roars through the cloud of particles.
The Applied Physics Laboratory is a not-for-profit laboratory and independent division of The Johns Hopkins University. APL conducts research and development primarily for national security and for nondefense projects of national and global significance. APL is located midway between Baltimore and Washington, D.C., in Laurel, Md.
ESA Science News
6 Nov 1998
Minute grains of dust create the glowing heads and tails that make comets famous. A trail of dust traces the orbit of each comet, and when the Earth encounters a comet trail the result is a meteor shower. Comet Tempel-Tuttle has just refreshed its dust trail on a visit to the Sun's vicinity, which it makes every 33 years.
The Leonids approach the Earth from the direction of the constellation Leo. As a precaution, the Hubble Space Telescope will turn its back on Leo for ten hours around the predicted peak of the Leonid event, which is at about 20:30 CET on 17 November. Astronomers will take the opportunity to look for undiscovered galaxies in the opposite direction in the sky. Any disturbances caused to the 11.6-tonne Hubble spacecraft by the Leonid dust impacts will be recorded for analysis by dust specialists. One of the teams chosen for this study includes ESA and UK scientists and is headed by John Zarnecki of the University of Kent.
Zarnecki comments: "It seems like doing an experiment with the crown jewels. But Hubble is a fantastically accurate star pointer, so we should detect wobbles due to quite small impacts. We hope to check our theories about the numbers of grains of different masses. But I'd hate to see any harm come to Hubble," Zarnecki adds. "Or any other spacecraft for that matter."
If the rate is again 15,000 per hour, a spacecraft presenting a target of 10 square metres to the Leonid storm is likely to receive one hit penetrating aluminium to a depth of 0.4 millimetre. A larger spacecraft has a greater chance of being hit by a more penetrating dust grain. Operators are therefore advised to turn their spacecraft to present as small a target as possible, and to try to ensure that sensitive parts do not face the meteor stream.
"Bullet-like damage caused by large particles is only part of the story," says Walter Flury of ESOC's mission analysis section. "Fine grains are far more numerous and can sand-blast optical systems, thermal blankets and solar cells. And in a cloud of charged particles created by the impacts, lightning- like discharges can cause faults in the electronic systems of the spacecraft. The very high speed of the Leonids aggravates that risk, so it may be advisable to switch off sensitive equipment. Damage due to electrical discharges may be the most serious hazard from the Leonids."
Predictions are very uncertain and effects are very chancy, so one recommendation is simply to reinforce the spacecraft operation teams on 17 November, to cope with any emergency that arises. The direction of arrival of the Leonids is favourable for satellites in one respect. The dust grains will come from a direction almost at right angles to the direction of the Sun. Flat solar panels in their normal orientation, facing the Sun, present only a narrow edge as a target for the Leonids.
Controllers of ESA's Earth observation satellites ERS-1 and ERS-2 will switch off the instruments during the hazardous period to reduce the risk of electrically-induced damage. ESA's solar spacecraft SOHO, stationed 1.5 million kilometres out in space, is likely to experience an even stronger storm of Leonids than satellites in the Earth's vicinity. Measures to reduce the hazard may include rotating the spacecraft to screen vital equipment, and switching off scientific instruments.
Next year's appearance of the Leonids, in November 1999, will be best seen from Europe, and it could be bigger than this year's event. For the same reason, the risk posed by the Leonids to spacecraft will recur at that time. ESA scientists will be rehearsing this year for ground-based observations of the Leonids next year, from southern Spain.
Controllers were less lucky in August 1993 when a dust grain from Comet Swift-Tuttle, in the Perseid meteor stream, was probably to blame for knocking out ESA's Olympus telecommunications satellite after four years of operation. Although it remained intact, Olympus lost so much thruster fuel in trying to correct its attitude that it became unmanageable. More direct knowledge of dust impacts on spacecraft came from examining part of the original solar array of the Hubble Space Telecope, provided by ESA, which was returned to Earth in the first refurbishment mission in December 1993. The solar cells were pitted by many small dust impacts.
ESOC report on Meteor showers
Meteor trail animation
General information: Leiden Leonid site (mirroring a NASA site)
International Meteor Organization
ESA science site: updates on the Leonid event as it affects ESA science activities
14th Air Force experts explain upcoming meteor storm
Centre for Research in Earth and Space Technology
Though the risk of damage from the Leonids meteor storm is considered slight, a number of international satellite operators, including those of the United States Department of Defence, the European Space Agency and the Canadian Space Agency, are working with CRESTech to reduce the odds of impact from the natural space debris forming the comet trail into which Earth will soon pass.
The Leonids are a swarm of metoroids that intercept Earth's orbit to some degree every year, usually with little more fanfare than a spectacular night of watching shooting stars. This year, however, the storm will reach levels not seen in over thirty years as Earth travels directly into a path of interplanetary flotsam flung by the four-kilometer-wide Comet Tempel-Tuttle.
That last peak occurred in 1966, at a time when only a handful of small scientific satellites were in operation. Currently, there are estimated to be well over 600 operational satellites in Earth's orbit, transmitting signals 24 hours a day for services such as search-and-rescue, entertainment broadcasts, telecommunications, as well as the global-positioning system (GPS) used by commerce, transportation industries and military forces, among others.
"In the last 30 years, people in technologically advanced countries have developed a strong dependence on a wide variety of satellite services," explains Peter Brown, the University of Western Ontario astronomer leading the CRESTech science team. "Unfortunately, if even one satellite was disabled during the storm, it may not only be a multi-million dollar disaster for its owners but may disrupt services for up to millions of clients."
Peter Brown is organizing the Canadian science team providing the hurricane- style forecast of the shower's activities to satellite operators around the world during the peak night. Clients include the United States Space Command, the European Space Agency, the Canadian Space Agency, Canada's Department of National Defence and several other government and commercial organizations.
There is a general consensus within the scientific community that some enhanced activity will be realized due to the next shower, however useful predictions are not possible due to limited historic data. During the last major meteor storm, in 1966, satellites were much smaller and less numerous than today. Today there are estimated to be between 600 and 750 operational satellites in Earth's orbit. While many space-faring nations, predominantly the United States, have devoted considerable resources to measuring and understanding "space weather" phenomena caused by solar activity and other aspects of the space environment, meteoroid activity has not figured prominently in this effort.
The hazards to spacecraft result not only from the large number of meteoroids, particularly those of smaller masses, but from the high speed of the Leonid meteoroids (72 kilometres per second). The effects of such impact may range from infrastructure damage to the more serious threat of electro-static discharge generated from such an impact. Such charges could interact directly with a spacecraft's electrical systems and essentially "short-circuit" the satellite. The latter effect has been proposed as the likely explanation for the loss of ESA's Olympus telecommunications platform in 1993. All satellites, both military and civilian, at all orbital altitudes are at risk from this natural threat.
Extensive work has been performed by the University of Western Ontario in London, Ontario, under the funding auspices of the Toronto-based Centre for Research in Earth and Space Technology (CRESTech), to model the physical processes resulting in the appearance of this storm. This unique model allows for predictive capabilities so long as appropriate observational data are available to constrain the initial conditions. However, observational data is not yet sufficient to provide accurate forecasts of the shower over the next five years. Observations made in 1997 indicate that, and agree with, the recent discussions at an American Institute of Aeronautics and Astronautics/Aerospace Corporation Conference proposing that activity level may reach as high as 10,000 naked-eye observable meteors per hour.
Organization's participating in the operational portion of CRESTech's program will receive real-time data on the storm's build-up rate, peak and general activity. It is expected that these data will be used by some satellite operators to take one of several courses of action in the event that peak activity reach dangerously high levels. Among those actions are: changing a spacecraft's attitude (i.e. pointing direction) to avoid being hit in crucial or delicate areas; powering down an endangered spacecraft so as to avoid a massive electrical discharge; or riding out the storm with enhanced ground- station teams in order to react quickly to storm-generated anomalies.
Instruments: The 1998 campaign will employ two methods of data collection: radar and Low-Light-Level Television (LLTV) observations. Both are capable of measuring the smallest meteoroids (which are the population relevant to the space hazard) and each can be used to establish their physical parameters. The Mongolia site will host two LLTV sites (each with approximately five camera), and the Australian site will host one radar site as well as one LLTV site (with two cameras).
Science Team: The CRESTech science team includes Dr. Jim Jones, Dr. Alan Webster, Dr. Kerry Ellis, Dr. Wayne Hocking, Dr. Martin Beech, Dr. Robert Hawkes, Ms. Margaret Campbell and Mr. Peter Brown, whom acts as the Project Manager. This represents the largest group of meteor specialists in the world with a most varied list of strengths. Dr. Hawkes (Mount Allison University, Sackville, NB) is the world expert in application of video technology to meteor observations and heads the LLTV component of the program working along with Ms. Campbell (University of Western Ontario). Drs. Jones, Webster, Hocking (UWO) and Ellis (CRC Ottawa) are world leaders in meteor radar systems, observations and analysis. Dr. Beech (University of Regina) and Mr. Brown (UWO) have studied and adapted models of the stream and liaise with the satellite community in an effort to understand satellite effects of the storm. No other single group has the equivalent breadth of scientific expertise in this field.
HEADQUARTERS, U.S. SPACE COMMAND
PETERSON AFB, CO 80914-3190
October 6, 1998
The annual Leonid shower -- this year a storm -- is expected to have an intensity not seen in more than three decades. Even so, the event could provide a dramatic "light show" for some parts of the world, particularly East Asia and the western Pacific region.
The Leonid meteors originate from the debris released from the Comet Tempel-Tuttle which completes an orbit around the Sun every 33 years, leaving a trail of debris such as dust and other tiny particles. The Comet passed perihelion, its closest approach to the Sun, early in 1998, setting the stage for probable meteor storms in 1998 and 1999.
Conditions exist for encountering larger than normal numbers of meteors -- "shooting stars" -- streaking through Earth's upper atmosphere at rates of thousands per hour. Leonid meteors will disintegrate upon entering Earth's atmosphere and pose no threat to aircraft or the Earth's surface.
Leonid meteors travel at about 45 miles per second compared to about 12 miles per second for typical meteors. This risk of physical or electrical damage to near-Earth spacecraft will be greater than normal.
Space operations crews have developed comprehensive strategies to limit the potential effects of the storm. Crews have anomaly resolution procedures in place that are based on years of experience and numerous recovery actions. Several contingency plans exist that deal with specific anomalies for each constellation of spacecraft.
NASA and the U.S. Air Force Space Command will conduct studies of the 1998 Leonid storm and will use these data in forecasting the potential 1999 storm.
Additional information on the expected Leonid meteor storm can be found on the worldwide web at:
Simple software to calculate the probability of impacts by Leonid meteors on spacecraft in Earth orbit can be found on the worldwide web at:
NASA Headquarters, Washington, DC
Johnson Space Center, Houston, TX
U.S. Space Command Public Affairs
On November 17, 1998, the annual Leonid meteor shower is expected to arrive with an intensity not seen for more than three decades. U.S. Government agencies, led by NASA and the Department of Defense, have been studying the potential risk of the Leonids to spacecraft in near Earth orbit out to the L1 location, 1 million miles from the Earth toward the Sun. Projections are the 1998 Leonids will not reach the levels of the 1966 meteor storm but will present an elevated, though not serious, threat to spacecraft in the vicinity of the Earth during a period of about half a day. However, the 'light show' visible in some parts of the world, particularly East Asia and the western Pacific region, may be dramatic.
Due to the changes in the relative positions of the Earth and Comet Tempel-Tuttle, Leonid meteor storms do not occur every 32-33 years. The Leonids did not reach storm levels in 1900 or 1933, but did reach major storm proportions in 1966. The 1998 outburst is likely to be much lower than that observed in 1966. In fact, the stream of Leonid meteors for all sizes, ranging from dust to sand-like particles, will probably not exceed the typical daily meteor exposure. However, the higher velocity of the Leonid meteors (~45 miles per second) as compared to the typical daily meteors (~12 miles per second) means the risk of physical or electrical damage to near Earth spacecraft will be greater than normal.
The kinetic energy of a Leonid meteor will be 13-18 times that of a typical meteor of the same mass. However, since the mass of Leonid meteors are expected to be much less than that of typical meteors, the Leonids energy level should be equivalent to normal exposure of a few hours to days. The electrical discharge potential of the Leonids during the 12-hour period centered around the peak activity may be equivalent to as much as months to years of normal daily exposure.
30 SEPTEMBER 1998
Every year, around mid-November, the Earth crosses the orbit of a comet called Tempel-Tuttle and passes through debris the comet has shed. This burns up in the upper atmosphere as a meteor shower. Every 32 to 33 years, the Earth runs into an especially dense cloud of debris, turning the shower into a storm. At the peak of the last storm, in 1966, the skies above North America were lit up by 5000 meteors in just 20 minutes.
Astronomers are now bracing themselves for the next Leonid storm, predicted to reach a peak around 17 November. Communications and other satellites could be threatened by the bombardment -- and both NASA and the Russian Space Agency have postponed launches until the danger has passed.
No one knows just how bad the damage will be. For example, astronomers can't predict with certainty exactly where the densest part of the debris cloud is. Now Duncan Steel, an astronomer with Spaceguard Australia in Adelaide, has thrown another variable into the equation. If his model of the chemical composition of the Leonid meteors is correct, attempts to observe the approaching meteors may detect only a few per cent of them.
Steel says that data gathered during the recent visits by comets Hale-Bopp and Hyakutake reveal that the dust these comets gave off was rich in volatile organic compounds. If the same is true of the cometary debris that forms the Leonids, most of the meteors may be invisible. This is because if they are made of highly volatile material, many will burn up at relatively low temperatures -- too low to leave behind glowing trails detectable from the ground. Cool-burning meteors will also emit relatively few electrons, and that will make them invisible to ground-based radars, which can only spot electron-dense trails.
"If small meteoroids in storms are largely composed of organics, then none of the data collected to date gives a realistic assessment of the hazard level," says Steel, whose conclusions are published this week in the journal Astronomy and Geophysics (vol 39, p 24).
Current estimates put the risk of a serious impact between a meteor and a large satellite at about one in a thousand. Steel says his study suggests that this "seriously underestimates" the hazard. "If I am right, the economic loss caused by the Leonids may be immense," he says.
Other astronomers agree that the reliability of the storm predictions depends crucially on the composition of the meteors. "Steel's paper is very interesting -- though whether it is actually correct is another matter," says Iwan Williams of Queen Mary and Westfield College, London. "We may know after the Leonids next month."
Steel's advice is not to rely too heavily on satellite communication and navigation systems in the coming month. "I would not depend for my life on the Global Positioning System being fully functional on 18 November," he says.
Author: Robert Matthews
New Scientist issue 3rd October 1998
The Aerospace Corporation
El Segundo, California
June 8, 1998
But he said the effects on spacecraft are expected to be minimal, despite the fact the storm "will be the largest such threat ever experienced by our critical orbiting satellite constellations."
Ailor, director of the Center for Orbital and Reentry Debris Studies established last year at The Aerospace Corporation, presented his testimony during a hearing titled "Asteroids: Perils and Opportunities." He was invited to appear before the Subcommittee on Space and Aeronautics, a panel of the House Committee on Science, by U.S. Rep. Dana Rohrabacher (R-Calif.), subcommittee chair.
"It is possible," Ailor told the subcommittee, "that some satellites will be damaged, but the most likely source of damage will not be from a rock blasting a hole in a satellite, but rather, from the creation of a plasma, or free electric charge on the spacecraft. The charge could cause damage to computers and other sensitive electronic circuits on board the spacecraft, and ultimately cause the spacecraft to fail. For example," Ailor said, "during the 1993 Perseid meteor shower, it was determined that the Olympus communications satellite was damaged by a meteor strike and went off the air shortly thereafter as a result of an electrical failure."
Ailor pointed out that, "The latest information on the coming Leonid meteoroid storm was presented at the Leonid Meteoroid Storm and Satellite Threat Conference sponsored by Aerospace and the American Institute of Aeronautics and Astronautics in Manhattan Beach, California, on April 27 and 28.
"The primary recommendations from the conference," Ailor reported, "were that, while it is very unlikely that the storm will have any major effect on satellites, the 'A-team' of controllers should be on duty during the ... storm, and operators should check the state of health of their satellites frequently, looking primarily for electrical anomalies and glitches. It was also recommended that, if possible, satellites be oriented so that sensitive components are shielded from the oncoming stream of particles, and that recovery plans be in place should there be a spacecraft system failure during the storm."
Ailor said Aerospace collected information on spacecraft anomalies experienced during the 1997 Leonid shower and will be collecting similar information for the 1998 and 1999 events. "This information will help us plan for the 1999 Leonid and future meteoroid storms. It may also help us to understand whether additional safeguards against the meteoroid impact threat should be included in future spacecraft designs," Ailor said.
Ailor's full testimony is on the Internet at http://www.aero.org/leonid.
NOTE: Dr. David Lynch of The Aerospace Corporation presented information on this subject at a press briefing at the AAS meeting today. An image from the AAS press briefing is available at http://www.aas.org/meetings/press/meteors.html.
Dr. David K. Lynch of The Aerospace Corporation told reporters of the potential threats to orbiting spacecraft by the Leonid meteoroids, expected around November 17, 1998, and the mitigation strategies adopted by spacecraft operators (June 8, 1998). AAS photo by Larry Marschall.
PG News: Astronomers debate effects of meteor event on satellites
BBC News Sci/Tech: Scientists warn of meteor storm