LOOK OUT FOR THE LEONID METEOR SHOWER
The 1999 Leonid meteor shower should produce a fine display in the early hours of the 18th of November, Professor Mark Bailey of Armagh Observatory will tell the Royal Astronomical Society's meeting on Friday 12th November. Research done at Armagh, and by other experts around the world, suggests that an unusually strong shower is on the cards this year when Earth passes through streams of dust that have been shed by Comet Tempel-Tuttle.
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
A reliable method of predicting strong meteor showers such as the Leonids has real practical value. The Leonids are the fastest of all meteors, plunging into the Earth's upper atmosphere at 71 km/s - 40 times the speed of a rifle bullet. Despite their small size, the tremendous speed of the Leonids means they pack a mighty punch.
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
Since meteor showers - the Leonids especially - are given to springing surprises, it will definitely be worth watching out on the nights of both 16/17 and 17/18 November, any time between 11 p.m. and dawn, though after midnight is best.
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.
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 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.
CONTACTS FOR THIS RELEASE
MORE INFORMATIONon the Armagh Observatory Leonid web site: