Leonid Meteor Shower Prospects for UK and Europe
Professional and amateur skywatchers worldwide are awaiting the night of 17th/18th November with considerable anticipation because of the possibility that a spectacular meteor shower will take place. But what can we really expect to see in the UK and rest of Europe?
According to the best data available, Europe is likely to experience a good meteor shower, but not a truly exceptional one - perhaps up to 100 meteors per hour if we are lucky. The best time to look will be between 1 a.m. and dawn in the early hours of 18th November. A storm of many thousands of meteors per hour could occur, but it is much more likely to be seen in the Far East - China, Thailand, Japan - than in Europe.
Forecasting Metoer Showers
Predictions of a meteor storm in 1998 are based on the fact that exceptional displays of the Leonid meteors - so-called because they appear to radiate from a point in the sky within the constellation Leo - tend to recur every 33 years or so. There is not always a great storm, however, such as the one in 1966 when observers in parts of the USA for a short time saw meteors at a rate of 40 per second.
But forecasting meteor showers is not a precise business, unlike predicting eclipses, for example, for which the exact times and circumstances can be calculated in advance. The time when a meteor shower will peak, and the maximum rate at which meteors will appear to rain down, can never be anticipated with great certainty. They are something of a celestial lottery.
For that reason, it is well worth looking out for meteors in the early hours of the 18th, if skies are clear, even from the UK. There is a slim chance of something exceptional, but a modest display at least is on the cards, and meteors are easy to observe. They are best seen with the naked eye and, during a shower, they can streak across almost any part of the sky, as long as the radiant point is above the horizon.
If a Leonid storm takes place, it is unlikely to last more than an hour or so, but the gentler background shower carries on for a day or two. According to the experts the expected peak time of any storm is most likely to be about 7.45 p.m. (GMT). If this is correct, the storm would be finished several hours before the constellation Leo rises above the horizon in the UK.
What Are Meteors?
Meteors are caused by small fragments of material, mostly no larger than a grain of sand, which burn up as they enter Earth's atmosphere at high speed - around 71 kilometres (45 miles) per second in the case of the Leonids.
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 cloud when the comet returns to our vicinity every 33 years. In the years when this happens, a strong shower or storm takes place. In the years in between, 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.
Looking ahead to 1999, Comet Tempel-Tuttle will still be relatively nearby and some astronomers are predicting that the Leonid meteor display could be better next year than this. If that were to happen, then Europe is expected to be the ideal location.
Do The Leonids Present Any Hazards?
Most of the Leonids weigh about 1 millionth of a gram - not much more than a particle of cigarette smoke. Normally, objects this size would pose no threat to spacecraft. However, when they are travelling many times faster than a bullet from a high velocity rifle, the threat increases significantly.
Since the velocity of the meteor impacts is affected by a spacecraft's motion as it orbits the Earth, hits could occur at any speed between 65 and 80 km (40 and 50 miles) per second. These could result in some physical damage in sensitive areas as well as electrical short circuits, plasma discharges, and computer malfunctions, which may be sufficiently serious to disable a satellite. A form of sand-blasting can erode outer surfaces such as thermal blankets, mirrors and solar cells. Large impacting particles may even knock a satellite out of its normal position, as happened to the European Space Agency's Giotto spacecraft during its 1986 flyby of Haley's Comet.
"These microparticles could penetrate a fairly weak spacecraft skin," said Professor Tony McDonnell of the Unit for Space Sciences and Astrophysics at the University of Kent in Canterbury. However, the most likely form of damage is to vulnerable power systems. "Perhaps a handful of satellites could have unusual electrical anomalies," said McDonnell.
Past evidence suggests that the risks are fairly low. During the past four decades, only one spacecraft, the European Space Agency's Olympus satellite, is known to have been disabled by a (Perseid) meteor. Furthermore, no spacecraft were damaged by the 1966 Leonid storm. On the other hand, there are now more than 500 spacecraft orbiting the Earth, over 10 times as many as in the mid-1960s.
"The biggest uncertainty is the hourly rate (of arrival)," said Professor McDonnell. "If this reaches 150,000 per hour, there will be all sorts of damage, but there may only be 1,800 per hour."
While the probability of any satellite being hit is thought to be less than 0.1%, many spacecraft operators are taking no chances. The Space Shuttle mission that carried John Glenn was deliberately timed to avoid the Leonid shower. Cosmonauts on the Mir space station do not have the luxury of choosing their flight window. While the Mir station presents a large target for the Leonids, no serious damage is expected. However, the two crewmen may play safe by moving into the Soyuz lifeboat at the peak of the shower.
Fortunately, the direction from which the particles approach the Earth is almost perpendicular to the direction of the Sun. This means that the chance of a direct hit will be substantially reduced since most satellites will already have their solar panels aligned edge-on to the shower.
Further damage-limitation measures have been recommended by the European Space Operations Centre operated by the European Space Agency. These include turning spacecraft so that their most vulnerable systems are not in the direct line of fire; switching off high voltage systems; and putting a team of ground controllers on alert in case of emergencies.
In the case of the Hubble Space Telescope, its all-important mirror will be turned away from the shower during observations of distant galaxies. Most of the scientific instruments on the European ERS-1 and ERS-2 Earth observation satellites and the Solar and Heliospheric Observatory (SOHO) will be powered down and placed in 'sleep' mode during the shower. SOHO and the American Advanced Composition Explorer (ACE), which are located 1.5 million km sunward of the Earth, will be particularly at risk since the main stream of meteors is expected to pass much closer to them than any of their Earth-orbiting brethren. Indeed, the trail of Leonids will actually travel between the Earth and these two solar observatories.
More Information about the Leonid Meteors may be found at the following Web Sites:Leonid '98 Meteor Outburst Mission Homepage (P. Jenniskens - NASA) http://www-space.arc.nasa.gov/~leonid/1998.html
Leonid Information for the Beginning and Advanced Meteor Enthusiast (G. Kronk) http://medicine.wustl.edu/~kronkg/leonids.html
SKY Online's Meteor Page (Sky & Telescope magazine) http://www.skypub.com/sights/meteors/meteors.html
Sky & Telescope magazine (based in Boston, USA) has available for distribution:
, phone 00 1 617-864-7360 x148). Fax for both is 00 1 617-576-0336.