[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

The Leonids Are Back. Is A Storm Brewing?

European Space Agency
Press Information Note Nr. 17-99
Paris, France				15 November 1999

THE LEONIDS ARE BACK. IS A STORM BREWING?

Will the night sky be illuminated with thousands of 'shooting stars' --
known to scientists as the Leonid meteors -- on the night of 17-18
November? No one knows for sure, but some experts are predicting a
dramatic display over Western Europe which will rival any millennium
celebrations. 

What are the Leonids? 

Each year, the night sky is illuminated by dozens of meteor showers. During
these showers, pieces of cosmic debris -- mostly coming from giant dirty
snowballs called comets -- 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.

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. 

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. 

"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:

1. ESA Scientists Seek to Study the Leonids

Since the Leonids ejected from a comet and since comets are thought to be
left-overs from the formation of the planets, studying the Leonids helps
scientists to learn more about the physical and chemical characteristics
of their 4.5 billion year-old parents. 

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: 

* To determine the varying rates in the number of meteors and their
  magnitudes (visual brightness). 

* To study the physical properties of individual meteors by measuring their
  light output and changing velocity, then compare these to other meteor
  streams. 

* To 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 ESOC in Germany so that spacecraft operators can determine the
level of threat posed by the space dust.

2. Hazards to spacecraft: how ESA is preparing 

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, 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. 

The press is welcome to attend the press briefing that will take place on
Thursday 18th November at 8:00 am at ESOC after a night's watch. Please
contact Jocelyne Landeau-Constantin, Public Relations Office,
Tel +496151902696, Fax +496151902961 for accreditation. Possibility
for interviews is available as from 17th November 1999.

More information, including live images and reports, and an 'Ask the
experts' forum, can be found on the ESA Science Leonids 99 website at:
     http://sci.esa.int/leonids 99

Information on the Leonids flux and events at ESOC is available at:
     http://www.esoc.esa.de/pr/leonids.php3

For further information please contact:

ESA Public Relations Division
Tel: +33 (0)1.53.69.71.55
Fax: +33 (0)1.53.69.76.90

Further information on the ESA science programme and a subscriber service
for news can be found on the Worldwide Web at: http://sci.esa.int

More information about ESA at http://www.esa.int

----------
Archives located at:
http://www.meteoritecentral.com/list_best.html

For help, FAQ's and sub. info. visit:
http://www.meteoritecentral.com/mailing_list.html
----------