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(meteorobs) Leonid Meteor Shower Prospects for UK and Europe
Royal Astronomical Society
For immediate release: 10 November 1998
Ref. PN 98/23
Issued by:
Dr Jacqueline Mitton
RAS Public Relations Officer
Office & home phone: Cambridge ((0)1223) 564914
Mobile phone: 0370 386133
FAX: Cambridge ((0)1223) 572892
E-mail: jmitton@dial.pipex.com
and
Peter Bond
Space Science Advisor
Phone: (0)1483-268672
Fax: (0)1483-274047
E-mail: 100604.1111@compuserve.com
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 Halley'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.nasadot gov/~leonid/1998.html
Leonid Information for the Beginning and Advanced Meteor Enthusiast
(G. Kronk)
http://medicine.wustldot edu/~kronkg/leonids.html
SKY Online's Meteor Page (Sky & Telescope magazine)
http://www.skypub.com/sights/meteors/meteors.html
Visual Material
Sky & Telescope magazine (based in Boston, USA) has available for
distribution:
* still images from the 1966 Leonid storm,
* artistic renderings
* video of the 1997 Perseid and Geminid meteor showers
* a broadcast-quality, 1-minute animation of why the Leonids occur by
artist Don Davis.
Anyone wanting these for reproduction or broadcast can contact Irene
Szewczuk (irenes@skypub.com, phone 00 1 617-864-7360 x127) or Kelly Beatty
(kbeatty@skypub.com, phone 00 1 617-864-7360 x148). Fax for both is
00 1 617-576-0336.
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