What are the Leonids?
Each year, the night sky is illuminated by dozens of meteor showers. During these showers, fragments of cosmic debris leave glowing trails as they are incinerated during entry to the Earth's upper atmosphere. We see them as short-lived trails of light streaking across the sky.
Most meteors are caused by cosmic dust burning up as it enters the Earth's upper atmosphere. The dust comes from giant dirty snowballs called comets. For most of their elliptical (egg-shaped) orbits, comets remain in deep freeze, far from the Sun. When they approach the Sun, their icy surfaces are warmed and start to vapourise, generating powerful jets of gas and dust which spurt into space. The dust which is ejected lingers around the comet for a while, but eventually spreads out around its orbit.
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.
The meteors 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. In recent times, the most memorable of these storms occurred in 1833, when tens of thousands lit up the heavens over North America.
Unfortunately, not all of the comet's appearances are marked by such wondrous sights. While meteors appeared in large numbers during the last perihelion passage of Tempel-Tuttle in 1966, the previous viewing opportunity of 1933 proved to be a damp squib.
The reason for this spasmodic and unpredictable behaviour is that, although the main stream of debris trails for millions of kilometres behind the comet, it is not very wide, perhaps 35,000 km across. Within this narrow stream, the dust ejected during each of the comet's close approaches to the Sun forms a series of separate ribbons. Their characteristics vary considerably. Generally, the most recent dust streamers are thin and dense, while the older material, which has had time to spread out, forms wider, less densely populated bands.
The location of the stream also changes with time as the gravity of the planets, especially Jupiter, exerts an influence. Sometimes the Earth ploughs right into a dense stream of debris, causing a storm of bright meteors. Sometimes it misses almost all of the tightly confined dust trail, so very few meteors are seen.
The Leonids 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.
The reason for this high speed encounter is that, like their parent comet, the particles travel around the Sun in a direction which is almost directly opposite to the orbital motion of the Earth. The result is a head on collision between the planet and the dust.
The unpredictable Leonids
Based on past behaviour, a meteor storm was predicted for 1998 or 1999. Last year, some very bright fireballs appeared unexpectedly 18 hours before the predicted maximum. They were associated with a previously unknown dust band which had been shepherded into a narrow stream by Jupiter's gravity. Unfortunately, although there was also a peak in meteor activity at the predicted time, their trails were not very bright and hard to see with the naked eye.
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 on 18 November, 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.
Light trails left by fast-moving meteors may be seen in any part of the sky, but, if traced backwards, they will all seem to originate in the same place - the constellation of Leo. However, appearances are deceptive. Although they appear to spread out like spokes of a wheel, the trails are actually parallel to each other. They just seem to splay out because of our viewing perspective, just as railway lines appear to diverge as they come closer to us.
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 visitors 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.
"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."
ESA scientists seek to study the Leonids.
If the storm does materialise, ESA scientists 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:
· determine the varying rates in the number of meteors and their magnitudes (visual brightness).
· study the physical properties of individual meteors by measuring their light output and changing velocity, then compare these to other meteor streams.
· use the 1.5 m telescope at the Sierra Nevada Observatory to perform spectroscopy of persistent trains and so determine their composition.
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 the European Space Operations Centre (ESOC) in Germany so that spacecraft operators can determine the level of threat posed by the space dust.
Meteors, comets and space missions
There are two main reasons why scientists study meteors: the potential threat they pose to Earth-orbiting satellites, and the clues they hold about the formation of the planets.
Although they are very small, the tremendous speed of the Leonids means they pack a mighty punch. Apart from knocking a spacecraft off alignment or causing physical damage in the form of an impact crater, such collisions can also generate a cloud of plasma (gas composed of neutral and electrically charged particles) which may cause electrostatic discharges 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.
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 dollar charges this November.
"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."
The ESA Space Science Department will provide information on meteor numbers every 15 minutes for the European Space Operations Centre (ESOC) at Darmstadt in Germany. 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. 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. 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. At the same time, their power levels will be monitored and measures will be taken to reduce the possibility of electrical discharges and unexpected changes in attitude.
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.
Meteors, comets and the Rosetta Mission
Meteors are also objects of fascination for purely scientific reasons. Most of the particles which produce meteors have been ejected by comets passing through the inner Solar System. Since comets are thought to be left-overs from the formation of the planets, studies of meteors allow scientists to learn more about the physical and chemical characteristics of their 4.5 billion year-old parents.
However, there are limits to such ground-based observations. The only way to study comets at first hand is to send a spacecraft to study them at close quarters. ESA's Giotto spacecraft paved the way with the first close flyby of a comet (Halley) in 1986. An even more ambitious and exciting project is now being planned by ESA - the Rosetta mission to Comet Wirtanen. Scheduled for launch in 2003, Rosetta will spend eight years circling the Sun before it closes in on Wirtanen's icy nucleus. After entering orbit around the tiny comet, it will drop a small lander onto its black nucleus to tell us about such things as its composition, temperature and density.
Over the next two years, the mother spacecraft will orbit just 1 km above the nucleus, monitoring the changes which take place as it heads towards the Sun and starts to vapourise. If the spacecraft survives its long trek through space and its buffeting from gas and dust jets spurting from the nucleus, it will make the first detailed record of the transformation that takes place when a comet switches from frozen inactivity to boiling effervescence.