Air Force Space Command News Service
50th Space Wing Public Affairs
November 19, 1999
As a matter of policy AFSPC does not comment on the status of individual satellites. Command officials said it may be several weeks before an anomaly caused by a Leonids impact may emerge.
Just as the Global Positioning System satellites still provide accurate, instantaneous and reliable information to civilian users throughout the world, AFSPC military satellites continue to provide unequalled support to the men and women in America's uniform.
From Cooperative Research Centre for Satellite Systems (CRCSS)
Space Industry News, Issue 84, September 1999
Research in meteor science was well established before the space age. A big worry was the potential threat meteoroids could pose to satellites or humans in space. But experience has shown the threat was mostly slight.
However, several times each century, meteor activity has increased to intense levels, called storms. Although storms are related to the dust released from specific comets, their occurrence has been largely unpredictable. With their potential catastrophic consequences for satellites, much attention is now being focused on meteor storms.
Meteor storms result from Earth passing through dense narrow streams of debris from comets. Comet Temple-Tuttle produces the Leonid meteor storms. Storms have been largely unpredictable, but closely related to the orbital period of the parent comet; they occur within a few years of the comet passing Earth. The most intense Leonids occurred 1833 and 1966. The 1966 storm was the only one within the space age. No satellite damage occurred, but few satellites were in orbit then.
As comets approach the Sun, the increasing solar radiation sublimates ices which are swept away by the solar wind. Fine dust grains are released with the gasses.
The size of particles in a comet's visible dust tail are too small to produce naked-eye meteors. Particles causing visible meteors are only slightly affected by solar radiation wind and begin in orbits close to the comet's. The particles eventually scatter around the orbit.
The traditional approach to predicting meteor storms involved examining the correlation between comets' orbital geometry, and the historical dates and times of storms. The best this approach could do was suggest likely storm-years, and estimate peak intensity to within a day, or maybe a few hours. This is inadequate to assess the potential risk or organise evasive action.
We know the solar wind has some influence on the particles producing visual meteors. The radiation counteracts the Sun's gravitational attraction, so particles orbit the Sun more slowly. This is why storms occur in years shortly after passage of the parent comet. The exact time lag for the main bulk of the particles depends on the range of masses and ejection velocities, and the number of orbital revolutions before encounter.
David Asher and I have checked Earth's predicted times of approach to these dust trails. We could back-predict the peak times to within 5 minutes of the observed times, for all years when the maxima were timed (1866, 1867, 1869, 1966 and 1969). We also developed a model of dust trail density; our model closely fits the observed rates for all storm years (1833, 1866, 1867, 1869 and 1966).
We believe we can predict storms to within 5 minutes, and perhaps 50% in visual rates. Our model predicts the main activity will occur between 1999 and 2002, with the highest rates in 2001 and 2002. We predicted no activity from dust trails in 1998.
The highest activity over the next few years will be due to encounters with young dust trails:
Additional dust trail encounters in these years will be at different times with lower rates.
The geometry of encounter with the dust trails results in the more southerly parts of Earth experiencing the peak first. It then moves roughly northwards at around 600 km/min., leaving the northern extreme about 22 minutes later.
Given this model, and the small uncertainty in the predicted maximum time, there are specific consequences in mitigating the threat to satellites.
For a satellite that can be maneuvered to any point in its orbit at the maximum time, and having the appropriate orbital geometry, the two obvious strategies are to place the satellite in the "Leonid shadow" produced by the Earth, or at the point in the orbit furthest from the center of the Leonid dust trail. With the maximum of a Leonid storm being around 30 minutes, the period in shadow can be very significant. If the satellite orbit is oriented perpendicular to the dust trail, the rates furthest from the dust trail can be a small fraction of those closest to the dust trail. Combinations of these, and additional strategies, can result in a substantial reduction of the overall risk.
[Robert McNaught works in the Research School of Astronomy and Astrophysics at the Australian National University, studying the discovery and tracking of Near-Earth objects. He previously worked in optical satellite tracking.]
European Space Operations Centre
European Space Agency
November 15, 1999
The objective of this note is to provide background information about the expected Leonid meteor shower around 18 November 1999 and to give advice for spacecraft operators what kind of precautions can be taken to minimize the risk of damage to operational spacecraft. In this note updated predictions for the expected time and intensity of the Leonid 1999 meteor shower are given.
European Space Agency
15 November 1999
One of the most famous meteor showers is known as the Leonids, so-called because their light trails all seem to originate from the constellation of Leo. The Leonid meteors are associated with dust particles ejected from Comet P/55 Tempel-Tuttle, which pays periodic visits to the inner Solar System once every 33.25 years.
` Actually, the Leonids appear every year between November 15-20, when the Earth passes very close to the comet's orbit. However, the numbers on view vary tremendously. In most years, observers may see a peak of perhaps 5-10 meteors per hour around 17 November.
But, roughly every 33 years, the Leonids generate a magnificent storm, when thousands of them illuminate the night sky: they are renowned for producing bright fireballs which outshine every star and planet. Their long trails are often tinged with blue and green, while their vapour trains may linger in the sky like enormous smoke rings for 5 minutes or more.
Although the incoming particles are small, ranging from specks of dust to the size of small pebbles, the Leonids glow brightly because they are the fastest of all the meteors. A typical Leonid meteor, arriving at a speed of 71 km/s (more than 200 times faster than a rifle bullet), will start to glow at an altitude of about 155 km and leave a long trail before it is extinguished.
"In hindsight people found out that actually in 1965 we saw a similar display ... these fireballs the night before and then not quite the activity that was expected at the real maximum and you might know that in 1966, that was when the real storm occurred" said Detlef Koschny, scientific coordinator of the ESA's Leonids observation campaign. "So that was one year after. So some scientists say, 'Ok, for this year we really expect a big storm or we hope; you can never be sure."
So what about this year? In 1999, although the Earth will reach Tempel- Tuttle's orbit 622 days after the comet passed by, the distribution of its dust ribbons means that a notable display is still possible. One encouraging sign is that the 1998 shower was similar to that of 1965, the year before the storm of 1966. Most astronomers are not expecting a comparable display in 1999, but a spectacular show cannot be ruled out.
Activity will probably reach a peak on the night of 17-18 November, though earlier fireballs are always a possibility. Nothing will be visible until the 'sickle' of Leo rises above the eastern horizon around 22.30 GMT. At first, the fainter meteors will be swamped by light from the first quarter Moon, but once this sets soon after midnight, conditions should be ideal as long as the sky is cloud free.
The maximum activity should occur around 02.00 GMT, at the time when the Earth passes closest to the comet's orbit. At this time, Leo will be well above the horizon over Western Europe.
Some scientists predict that 2000 or 2001 may provide even better viewing opportunities for the Leonids, but no one can be sure if these unpredictable cosmic travellers will live up to expectations.
"We just know from past history that, in the two years after the perihelion of Comet Tempel-Tuttle, there is enhanced activity," said Dr. Walter Flury of the European Space Operations Centre (ESOC).
"A storm is possible, but these things are very uncertain," he added. "Predictions are based on models of the way material is distributed along the comet's orbit. But the models are quite inaccurate. We just don't have enough information."
There are two main reasons why scientists study meteors: the clues they hold about the formation of the planets and the potential threat they pose to Earth-orbiting satellites:
If the storm does materialise, scientists from ESA's Space Science Department intend to be ready. Armed with a variety of equipment, including image-intensifier video cameras, CCD cameras with wide-angle lenses and a spectrograph, they are planning an observational campaign at two observatories in southern Spain (Calar Alto and Sierra Nevada) from 11 to 19 November.
The main science goals are:
There will also be an ESA scientist with a meteor camera on board an aircraft operated by the American SETI (Search for Extraterrestrial Intelligence) Institute. Results from the meteor count experiments will be sent to ESOC in Germany so that spacecraft operators can determine the level of threat posed by the space dust.
This threat is not simply theoretical. In 1993, an ESA's satellite called Olympus spun out of control due to an electrical disturbance caused by the impact of a particle from the Perseid meteor shower.
The situation is further complicated by the fact that 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. Despite this spacecraft population explosion, few, if any, satellites are likely suffer significant problems from meteors, even during a storm. Researchers estimate that the chance of one getting hit by a Leonid meteor is only about 0.1 percent.
This low hit rate was born out by an absence of damage during the 1998 Leonids event. Nevertheless, driven by uncertainty over the future of their high-tech hardware, satellite operators will once again be taking precautions to protect their multi-million Euros charges this November.
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. These precautions will continue for several Earth orbits, a duration of seven hours, 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, or switch them off entirely 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 roll so that its main navigational aid, the star tracker, is pointing out of harm's way.
"There could be a lot of activity, but we just don't know for sure," commented Walter Flury. "It's better to take precautions now than be sorry later."
ESOC issued a warning to all satellite operators in August, explaining key protective measures to be taken such as switching off the payload and modifying the angle of the solar panels in relation to the Leonids storm.
By November 17, 1999 ESOC will have secured both earth observation satellites ERS-1 and ERS-2 by switching off the payload. The risk does not only lie in the damage caused by direct impact but rather in plasma discharge that could disrupt the functioning of the on-board electronics. Teams of operations experts will be on shift throughout the night to assess any potential damage and take necessary action.
Information on the Leonids flux and events at ESOC.
ESA Science News
11 Nov 1999
But the storm so eagerly awaited by astronomers is also making spacecraft controllers take precautions. Like a ship caught in a tempest, ESA's Solar and Heliospheric Observatory (SOHO) will try to stay as safe as possible during the meteor storm.
The scintillating effect produced by the friction between the comet's dust particles and the upper layers of our atmosphere should be visible in North America, Europe, Africa, and part of Asia.
With the Earth remaining at a distance of about 1 million kilometres from the comet's path, the danger for SOHO and other satellites may be considered minimal, say scientists. But given that the flow of dust particles is hundreds of thousands of kilometres wide, space agencies and satellite operators have decided the risk should not be disregarded.
The loss of ESA's Olympus communications satellite in the early 1990s was thought to have been caused by the Perseid meteor shower.
The relative velocity between our planet and the particles left behind by comet Tempel-Tuttle will be about 70 km/s, that is more than 250 000 km/h. "At that speed, if you had a hit by a discrete particle, it would pierce right through the spacecraft like a bullet," says Bernhard Fleck, SOHO Project Scientist.
During the coming peak of activity of the Leonids, as many as 1000 meteors per hour are expected to strike the Earth's atmosphere.
To avoid the danger of a sandblasting effect on the optical systems of its instruments, the protective doors on SOHO will be commanded to close.
In 1986, the camera on ESA's Giotto spacecraft suffered -- as expected -- severe damage as it passed in front of Halley's comet nucleus. Since cometary dust could cause similar problems to SOHO's vital star trackers, ground controllers will command the spacecraft to rotate 120 degrees around its axis. The star trackers are used by the spacecraft to determine its position in space as it observes the Sun from its vantage point 1.5 million kilometres from Earth.
Ten of SOHO's 12 scientific instruments will also be turned off, to minimise the risk that any instrument hit by a particle would be severely damaged by the combination of incandescent debris from the impact and the high-voltage power supply.
During the peak of the storm, SOHO will be placed in a safety configuration, known as the Coarse Roll Pointing mode. The 10 instruments will remain switched off for a period of almost 70 hours starting 16 November.
An image supporting this release is available.
NASA Science News for November 10, 1999:
Heads Up! - The upcoming Leonids meteor shower (Nov. 17-18) is predicted to be the biggest in decades and perhaps for the next century. While we are safe on the ground, satellite operators are concerned that even small impacts could short-circuit satellites. NASA will coordinate a team that helps track changes in the shower that could be a storm.
ROYAL ASTRONOMICAL SOCIETY
10 November 1999
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."
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.
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.
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 Space Command News Service
10 Nov 1999
PETERSON AIR FORCE BASE, Colo. (AFPN) -- Air Force satellites are expected to sail through the potentially worst meteor shower in 33 years; however, Air Force Space Command people are not leaving anything to chance.
AFSPC and other national agencies have been diligently working on a host of plans and operations that will ensure critical communication, navigation and surveillance systems stay operational.
This year marks what is likely to be the last in the 33-year cycle of the comet Tempel-Tuttle that produces what is commonly known as the Leonid meteor shower. The height of the Leonid meteor shower will be the evening of Nov. 17 in most of North America.
Tempel-Tuttle has been orbiting the sun opposite the Earth for nearly 2,500 years, but only poses a potential threat to the Earth three years out of each 33-year cycle said Lt. Col. Don Jewell, AFSPC's deputy chief scientist.
The comet travels 43 to 45 miles per second, relative to Earth's orbit, leaving a huge trail of dust through which the earth travels when their paths cross. The Temple-Tuttle is one of the fastest comets known to man.
The increased speed makes the comet's particles more dangerous to space satellites. The debris trail of the comet contains particles from 0.04 to 0.40 of an inch in size.
Radiation hardening gives military satellites greater protection than civilian satellites from the flying debris. Although the comet does contain particles up to 0.40 of an inch in size, the chances of one of those hitting a satellite are very small, said Jewell.
"If one of those hit a satellite, it would be like a bullet hitting a satellite and certainly it would damage it," said Jewell. "We don't anticipate that happening, but we have to plan for it."
And planning is exactly what's been going on since last year's Leonid storm.
"This year we are focusing on refining the Leonid plan that was developed last year," said Lt. Col. Doug Hine, 14th Air Force's chief, space operations branch. "We took lessons learned and ensured satellite and ground systems are prepared to weather the storm."
The Leonid storm of 1966 was the last time the meteor shower impacted the nations' space assets. The potential harm was minimal though because America only had 50 or 60 satellites in orbit then, said Jewell.
The real concern came 32 years later, in 1998, when the United States had several hundred military satellites in orbit, operating everything from early missile warning to the Global Positioning System.
"Essentially, it was a non-event," said Jewell. Three civilian satellites were damaged, but all military space assets were left unharmed.
There is not expected to be any damage to military satellites this year either, said Jewell, but the Air Force is prepared for the worst.
The Air Force began planning for the 1998 Leonid storm a full year in advance. Hundreds of thousands of dollars were allocated for Leonid preparation, but the money ran out and the analysis of the data could not be completed, said Hine.
This year, the Air Force and other sister agencies are poised to spend $2 million to proactively protect U.S. space assets, said Hine.
Special electro-optical video equipment will be set up at sites in Hawaii, Florida, the Canary Islands, Kwajalein Atoll in the Marshall Islands and at two sites in the Negev Desert, Israel, to record the storm as it develops. The video signals go to the Marshall Space Flight Center in Huntsville, Ala. The center will give real-time storm warnings when necessary.
The Air Force will also monitor the storm 24-hours a day Nov. 16 through 18 through mobile multi-frequency high-frequency radar deployed in Canada.
Additionally, aircraft are being deployed Nov. 16 to perform high-altitude observation and collect data on the Leonid storm.
"We don't want to downplay this. We plan for the worst and hope for the best," said Hine. "Air Force Space Command has experienced and well-trained crews who are prepared and ready to respond to any problems that may surface during the storm."
The good news is, when Nov. 17th is over, so are the short-term Leonid worries. "We won't have any problems with it again for about another 30 years," said Jewell.
So what's the advice for people Nov. 17? Sit back and enjoy because "it ought to be a beautiful show," said Jewell.
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.
The Leonid meteor display is associated with the Earth's passage through the Leonid stream. This stream consists of the debris of Tempel-Tuttle, a comet that orbits the Sun about every 33 years. Tempel-Tuttle is the source of debris that gives rise to the Leonid meteor shower that peaks around Nov. 17th each year.
To prepare for the event, NOAA's engineers are modifying configurations for all three types of environmental satellites. NOAA operates the Geostationary Operational Environmental Satellites (GOES), the Polar-orbiting Operational Environmental Satellites (POES), and satellites in the Defense Meteorological Satellite Program (DMSP). The satellites are operated by controllers in NOAA's Suitland facility. The satellites provide weather and environmental data vital to forecasting the weather and monitoring the environment.
Spacecraft configuration changes will include the alignment of on-board solar arrays to a position parallel to the storm radiant (or direction) for specific POES and DMSP satellites that, due to their orbital configuration, present a large solar array cross section to the storm radiant. In addition, specific sensors on-board the GOES and DMSP satellites will be reconfigured to better sense attitude or local space environmental disturbances that could be the result of meteoroid impacts. Although some command uploads may be moved to reduce commanding during the predicted peak, satellite data and products will not be impacted.
In addition to the configuration changes, engineering staff will augment the normal operational support during the 24-hour period centered on the predicted storm peak.
For more information on NOAA's satellite operations, visit: http://www.oso.noaa.gov/operations/
The Aerospace Corporation
The company plans to provide updated information every 15 minutes from observers around the world. This information can be accessed at http://www.leonidstorm.com. Included at this site is information on how observers who wish to participate can register.
William Ailor, Ph.D., principal director of the company's Center for Orbital and Reentry Debris Studies, said it is hoped that the information will help satellite owners and operators determine when it is prudent to switch from defensive to normal operations.
During peak activity some operators plan to take measures to safeguard their satellites. Options include turning off sensitive components, turning solar panels away from the stream of meteoroids, orienting satellites to minimize exposure and other measures.
Scientists predict the Leonids event will reach storm level this year with peak activity of 1,000 meteors or more per hour predicted. Meteors are flashes of light created when meteoroids burn up as they slice through the Earth's atmosphere.
Last year some 300 meteors per hour were observed at peak levels. A number of satellite owners and operators who attended the Leonids Storm and Satellite Threat Conference sponsored by The Aerospace Corporation and the American Institute of Aeronautics and Astronautics in May 1999 reported that sensors aboard their satellites detected impact with the tiny meteoroids, but no satellite was reported seriously damaged.
The Leonids occur with intensity every 33 years or so when the Earth passes through the most dense part of the debris stream created by the comet Tempel-Tuttle.
For further information on the Leonid meteoroid storm, please visit: "Understanding the Leonid Meteor Storms"
Nov 01 1999
The storm, consisting of tiny sand-like particles traveling faster than 140,000 mph, is essentially a cloudy tail of space debris from the comet Temple-Tuttle.
Although there was some concern when the earth first passed through the comet's tail in 1998, the Air Force said scientists expect this year to be the largest display from Temple-Tuttle since 1966.
Full story here.