AS YOU read this, astronomers will be making final preparations for what they
hope will be the most spectacular celestial display for decades. If they’re
right— and it is a big “if”—on the night of 17 November, the sky
will come alive with thousands of shooting stars. The Leonid meteors are making
their comeback, and they look set to take us by storm.
If people associate shooting stars with a particular time of year, then it is
early August. Under clear, balmy skies you can’t fail to spot a meteor roughly
once a minute. These are members of the Perseid shower: tiny dust grains from a
comet called Swift-Tuttle. This giant dirty snowball tramps around the Solar
System like a leaky wheelbarrow, depositing debris as its ice sublimes away.
When the Earth encounters this debris every August, it streams down into the
upper atmosphere and burns up, producing the streaks of light that you see.
Although the meteors come in on parallel paths, they seem to originate from a
point—the radiant—just as the sides of a straight road seem to
diverge from a point on the horizon. The radiant is in the constellation of
Perseus, so the meteors are called Perseids.
The Perseids are the most active of about a dozen meteor showers visible
throughout the year, all caused when the Earth intersects the orbit of a
litter-dropping comet. By contrast the Leonids—whose radiant is in the
constellation of Leo, the lion—are generally among the least impressive
showers, yielding just 8 to 10 meteors an hour. But every so often, the lion can
roar.
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In the early hours of 13 November 1833, Americans were treated an amazing
display. Astronomy writer Agnes Clarke described how “a tempest of falling stars
broke over the Earth. At Boston, the frequency of meteors was . . . about half
that of flakes of snow in an average snowstorm”. At the height of the storm,
about 10 000 meteors per hour were falling, causing commotion in the streets and
waking the nation.
In 1863, Yale professor Hubert Newton tracked the history of similar showers
back nearly a thousand years to AD 902, when Chinese astronomers reported that
“stars fell like rain”. The historical record was patchy, but Newton discovered
that there had been significant storms in 934, 967, 1037, 1202, 1366, 1533 and
1799, and deduced that the meteor storms had a periodicity of around 33 years.
And in 1866, when Ernst Tempel in France and Horace Tuttle in the US
independently discovered a comet with an orbital period of about 33 years,
Italian astronomer Giovanni Schiaparelli soon established that the orbit of the
debris in the Leonid stream exactly matched that of the newly discovered comet.
As if to set the seal on this new science of meteor astronomy, the 1866 Leonids
produced a spectacular storm over Europe, with 5000 meteors an hour.
No show
So in 1899, all eyes were on the sky for the widely publicised storm of
shooting stars expected to coincide with the return of Comet Tempel-Tuttle. But
almost nothing happened. “This is the worst blow ever suffered by astronomy in
the eyes of the public,” wrote American meteor expert Charles Olivier. Worse was
to come. In 1932, not only was the shower weak, but the parent comet also failed
to materialise. Astronomers concluded that it had suffered the fate of Biela’s
Comet, which broke in two in 1846 and later vanished.
However, rates of meteors began to climb slowly in the early 1960s, and in
1965 astronomers rediscovered Comet Tempel-Tuttle as it streaked past the Earth,
coming almost as near as the Moon. It was the closest approach since 1833, and
the following year history repeated itself. The 1966 Leonid meteor storm
erupted, once again, over America and took even the most cautious observers by
surprise.
“I’m conservative, but I’d estimate 50 000 an hour,” said an astronomer who
saw the shower from Flagstaff, Arizona. Others estimated there were as many as
200 meteors per second—nearly a million meteors an hour. For two hours,
the sky rained shooting stars. Some witnesses said that looking towards the
meteors’ radiant gave them an amazing feeling of travelling through space and
hitting the meteors head-on—which indeed they were.
But while American astronomers were exultant about the 1966 Leonids, it was a
different story elsewhere. Astronomers in Europe waited up all night, to no
avail. They were too early to see the peak flux of the meteor swarm, confirming
what astronomers already suspected: the densest part of the Leonids stream is
incredibly narrow, just 35 000 kilometres thick. The Earth swept through it in
an hour, and if you were in the wrong place—or in daylight—you
missed out.
So the Leonids’ unpredictability and their showiness when they do oblige are
linked. The stream shed by some comets is very broad, ensuring that the Earth
will travel through part of it every year, producing a predictable shower such
as the Perseids in August. But Comet Tempel-Tuttle’s particles are packed into a
narrow stream: either you miss and see nothing, or you hit and get it all. The
spectacle is enhanced since the comet orbits the Sun in the opposite direction
from Earth, so its meteors hit us head-on. Travelling at 71 kilometres per
second, the Leonids are the fastest and brightest meteors of all.
Why is the debris stream so narrow? One reason could be that Tempel-Tuttle is
a relative newcomer to the inner Solar System, and there has not been time for
its debris to spread around its orbit. But comet expert Iwan Williams of Queen
Mary and Westfield College in London has a more exotic explanation.
Tempel-Tuttle has a long, thin orbit that takes it from near Earth right out
to Uranus. The comet’s orbit and that of Uranus are locked in a relationship
called a resonance: for every five orbits of the comet, Uranus goes around the
Sun twice. This results in a gravitational tug-of-war between planet and comet
that, Williams calculates, periodically removes a third of the meteor stream.
But the comet hasn’t been close to Uranus lately, so the stream may be not be
too depleted or so sharply confined. Williams thinks this means the forthcoming
Leonid shower will be worth watching, but not in the class of `66. “It’ll be
impressive, rather than sensational,” he predicts. “About 10 000 meteors an
hour. “That’s still nearly 200 per minute.
The narrowness of the debris stream is just one reason why the Leonids are so
unpredictable. There also appear to be separate “ribbons” within the stream,
corresponding to different parts of the comet that have crumbled away in the
past. These may be more sharply confined, and might even miss the Earth
altogether.
Off course
Then there is the distribution of the dust coming off the comet. Don Yeomans,
a leading comet expert at NASA’s Jet Propulsion Laboratory in Pasadena, has
mapped its location. He found that most of the dust is behind the comet and
outside its orbit, implying that the dust is strongly affected by the pressure
of sunlight and also perturbed by the gravity of the planets, both notoriously
changeable influences. The no-show of the Leonids in 1899 was because the meteor
swarm passed close enough to Saturn in 1870 and Jupiter in 1898 to be slightly
deflected.
Comets, too, are notoriously fickle. The jets of gas they spurt out when
close to the Sun act like rocket thrusters that push the comet in the opposite
direction—but because the areas of weakness where the gas breaks through
are randomly distributed, you can’t predict exactly how a comet will stray. And
because comets are so insubstantial—the nucleus of Tempel-Tuttle is an icy
ball just 1.8 kilometres across—they are at the gravitational mercy of the
planets, particularly giant Jupiter and Saturn.
At least we know that the comet has returned to the inner Solar System. It
was spotted on 10 March 1997, passed Earth on 17 January 1998, and came closest
to the Sun on 28 February. We will cross the debris stream on 17 November. NASA
scientists talk about this as “the mission to Comet Tempel-Tuttle”. But as Peter
Jenniskens of NASA’s Ames Research Center in California points out: “During this
mission, we do not go to the comet, but the comet comes to us.”
NASA is planning an ambitious airborne assault on the Leonids, along with
participants from the Britain, Japan, the Netherlands and the Czech Republic.
Two planes bristling with detectors will take advantage of the shower to learn
more about the meteors, the comet, Earth’s upper atmosphere, and even the theory
that the building blocks of life arrived from space. The planes can go to the
area of the world where this year’s Leonids are expected to peak, over Japan and
China, and will be able to climb above any clouds that obscure the sky.
One of the planes, the Electra, will carry a laser radar system (lidar),
while the other has 20 upward-looking ports for imaging the meteors and
recording their spectra. The Electra also has a microwave link, allowing images
from the HDTV cameras on board to go out live on NASA Television.
The lidar will monitor the mesosphere, a mysterious region of the atmosphere
extending from 90 to 120 kilometres above the Earth’s surface where auroras and
rare noctilucent clouds appear. As the meteors burn up in this layer, the paths
taken by the debris will reveal much about mesospheric winds.
The meteors themselves will be analysed spectroscopically by the light they
emit, allowing astronomers to sample the composition of dust ejected from its
parent comet. Triangulation between the two planes will reveal the precise
trajectories of individual meteors, revealing just how dust boils away from the
comet’s nucleus. Measurements of the brightness of the most luminous
meteors—produced by the most massive dust particles that escape from the
comet—will allow a more accurate estimate of the comet’s mass. And the
HDTV images will allow much more reliable counts of the meteor rates than is
possible from the ground.
There are even schemes afoot to collect meteoric dust directly, although it
is often hard to distinguish between meteoric debris and volcanic ash.
Researchers from the University of New Mexico plan to fly a balloon with a
modified vacuum cleaner to gather the tiny dust particles. Higher still, a
French device on the Mir space station will attempt to trap Leonid dust in
aerogel—but the Leonids move so fast that they may simply explode on
impact.
Indeed, the speed of the incoming Leonids may prove to be a serious hazard to
spacecraft in orbit. Although each meteoroid is smaller than a grain of sand and
sometimes thinner than a human hair, its momentum delivers as much energy as a
.22 bullet. In 1993, a Perseid meteor struck the European Space Agency’s Olympus
satellite and destroyed its directional control. Now 500
spacecraft—including communications satellites in geostationary
orbit—could be at risk.
Cosmic sandblast
Duncan Steel, an astronomer with Spaceguard Australia, thinks the hazard may
be even greater than expected
(This Week, 3 October, p 21).
Studies of the comets Hale-Bopp and Hyakutake reveal that a great deal of their dust is in the
form of volatile organic compounds which burn up at low temperatures—and
are therefore invisible from the ground. If Tempel-Tuttle’s debris stream is
also laden with invisible dust, then scores of satellites could be sandblasted
by the storm.
Surprisingly, it’s not the sandblast effect that causes the worst damage,
although meteors have pitted space shuttle windscreens and holed solar panels. A
colliding meteor evaporates some of the matter in the surface of the target,
creating a plume of plasma that can short-circuit the sensitive electronics in
the satellite. The Leonids’ high speeds make them particularly dangerous: the
Hubble telescope, which suffered minor damage during the 1993 Perseid shower,
will protect itself by turning away from the stream of particles, an option
being considered by satellite owners.
But here on Earth we will be hoping for a spectacular storm. So far, the
signs are good. The rates have been increasing over the past few years and
peaked at 150 per hour in 1997, despite observations being hampered by strong
moonlight. And the geometry of the comet’s approach to us is very similar to
that in 1799, when thousands of meteors were seen over South America and
Florida.
Despite the unpredictability of comets, there is surprising unanimity among
the different groups of experts. The consensus is that 2000 to 10 000 meteors
per hour will be seen over China, Japan, the Philippines, Vietnam and Thailand
on 17 November at about 1943 hours universal time (or 2.43 am on 18 November in
Bangkok). This is when we are predicted to cross the stream, and Southeast Asia
will be ideally placed, with the radiant in Leo well above the horizon. The peak
of the shower is expected to last an hour and a half, and many of the meteors
should be very fast and extremely bright—a few as brilliant as the planet
Venus.
All is not lost, however, for those in the West. Because of the uncertainty
about structure of the meteor stream, astronomers are warning that a storm could
strike almost anywhere. What’s more, because of the distribution of dust that is
trailing Comet Tempel-Tuttle, next year and even perhaps 2000 also look set to
yield Leonid storms. In 1999, the sharply peaked maximum favours Europe and
North Africa, at 0140 hours Universal Time on 18 November. And although some
experts reckon that the 1999 Leonids may be down on the 1998 rates, at 5000 per
hour, one estimate puts the rates as high as 100 000 per hour.
In all of this, one thing is certain. If we miss these Leonid storms—or
if they miss us—a repeat is very unlikely. As Yeomans revealed at the
Kyoto Congress of the International Astronomical Union last year, the stream is
slipping away from us. The precession of its orbit means that it will no longer
intersect Earth’s. Before about AD 900, there was no opportunity to see the
Leonids, as the stream was just too far away. “And after next century,” says
Yeomans, “there’ll be little chance again. It’s a matter of urgency to observe
˛Ô´Ç·É.”
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Further reading:
The Heavens on Fire: The Great Leonid Meteor Storms by Mark Littmann (Cambridge, 1998); -
Mission to Comet Tempel-Tuttle website:
http://www-space.arc.nasa.gov/~leonid