810-Lancaster Fanning, Saturday, January 11,1986
Halley's:
WASHINGTON - A
“magnificient old conflagration,”
Mark Twain thought. When last
seen, it was a fiery ball streaking
across the night sky like some
shooting star. Or so it was
popularly believed in 1910.
Comet Halley has come back as
a dirty snowball that barely moves
among the stars from night to
night. It is no less a marvel.
“The snowball theory cut down
on the hysteria,” says Fred L.
Whipple, the renowned 79-year-old
Harvard astronomer who in the
late 1940 s frist described comets as
great masses of icy substances,
embedded with cosmic dust or
meteoric particles. They are
probably the stuff of the origin of
the 4.6 billion-year-end solar
system.
Malevolent ‘Broom Stars’
More has been learned about the
nature of comets since Halley’s
spectacular 1910 appearance than
in all the centuries since its first
recorded sighting in 240 B.C. by the
Chinese, who thought of comets as
“broom stars” sweeping the
heavens of evil and sending
disasters down to Earth.
As much or more is expected to
be discovered before Halley’s next
visit in 2061. Its 1986 passing,
although among its dimmest, will
be the most-watched astronomical
event in history.
As part of the International
Halley Watch, 950 scientists in 50
countries, 400 to 500 serious
amateur astronomers, and 100 of
the world’s largest telescopes will
be focused on the comet.
“We’re even trying to cover the
oceans as much as possible by
setting up an island network, aA we
can get a record of observations
every 15 degrees around the
globe,’’ explains Murray Geller,
U.S. deputy director of/the Halley
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Watch,
For the first time, a flotilla of
spacecraft-two Russian, two
Japanese, and one European—is
speeding toward Halley’s. The
European Space Agency’s Giotto
mission is the most daring,
targeted to come within 300 miles
of the comet’s nucleus on March 13
and take the first pictures of it U.S.
spacecraft will be distant ob
servers.
For most earthlings, the
celestial show will be less than
dazzling, because of the comet and
planet’s relative positions. In 1910,
Earth nearly had a close encounter
with Halley’s tail. This time, when
the comet is brightest in March
and April, it will pass beneath the
plane of Earth’s orbit. On April 11,
when it will be closest to Earth, it
will be 39 million miles away. Tlie
best views will be south of the
equator.
In the Northern Hemisphere,
Halley’s will be most visible in
January after sunset and in mid-
March before sunrise. (In
December it looked like a fuzzy
teardrop.) For best viewing, it is
necessary to go to a hilltop beyond
the range of the millions of city
lights that astronomers complain
have “polluted” the skies since
1910.
First Orbit Calculated
Of all the 700 to 800 known
comets, Halley’s was the first
whose elliptical orbit around the
sun was calculated and whose
faithful return, every 75 to 76
years, was noticed and predicted.
English astronomer Edmond
Halley (rhymes with valley) first
recognized the phenomenon in
1705. Proof didn’t come until after
his death, when a German farmer,
an amateur astronomer, spotted
the comet on Christmas night 1758,
reappearing as Halley had
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foretold. Thereafter it has carried
Halley’s name.
Its fame, brightness, predic
tability, and long life (it shows no
signs of “burning out”) have made
Halley’s a star among comets in
attracting scientific attention.
Today the primary focus is on the
head of the heavenly body and its
icy nucleus, an irregularly shaped
sphere estimated to be about four
miles in diameter.
According to Whipple’s theory,
now generally accepted, a comet is
a conglomerate of frozen sub
stances; water, ammonia,
methane, carbon dioxide, and
hydrogen cyanide.
When a comet nears the sun,
some ice evaporates, and the dust
within it is blown off. Halley’s
already has lost some of its 1910
crust. A haze of dust, called a
coma, forms around the nucleus.
As a comet gets closer to the sun,
its coma grows bigger, sweeping
into a tail that may stretch millions
of miles, always away from the
sun.
A comet actually has two tails,
which sometimes appear as one.
Halley’s tail may extend 50 million
miles, but all its matter could fit
into a suitcase.
A snowball in sunlight - some of
its ice turning directly to gases -
is really like a small jet engine,
Whipple realized. When it leaves
the sun, its tail shrinks, and it
becomes a frozen ball once again.
Where do comets come from?
They are thought to originate in the
“Oort Cloud,” a huge spherical
shell that surrounds the sun at the
outer reaches of the solar system.
Named for the Dutch astronomer
who first envisioned this comet
nursery in 1950, the vast cloud is
estimated to contain at least 100
billion orbiting comets.
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Six Discoveries Annually
Comets remain there un
disturbed unless the gravity of a
passing star pulls them out toward
the sun, making them visible from
Earth. About a half dozen new
comets are discovered each year,
says astronomer Brian G. Mar
sden, director of the Central
Bureau of Astronomical
Telegrams of the International
Astronomical Union.
“There may be a thousand times
as many comets in an intermediate
part of the solar system as there
are in the Oort Cloud - perhaps a
quadrillion,” Marsden says. “It’s
impossible to know how many are
out there. We have no way to detect
them.”
Comets are believed to be lef
tovers of the materials from which
the sun and planets were formed.
“Comets have spent most of that
tune in the deep freeze of outer
space. They are the best way to
find out about the origin of the
solar system. We are placing our
hopes on comets,” says
astronomer Jay M. Pasachoff of
Williams College.
One of the most intriguing
unanswered questions, Whipple
says, is “the possibility that
comets brought life-giving
elements to the surface of the
Earth...that some of the atoms in
our bodies may have come from
comets...that we ourselves are
made of comets.”
The Space Age has carried
scientists closer than ever to a true
understanding of comets.
Ultraviolet-light detectors have
shown that most of the icy nucleus
is frozen water and that an in
visible hydrogen cloud surrounds
it. Radar has proved that the core
is a solid snowball. Radio-tracking
has revealed new molecules.
A spacecraft caught up with a
comet for the first time last Sep
tember, when the U.S. In
ternational Cometary Explorer
flew through the tail of Comet
Giacobini-Zinner.
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The five spacecraft en route to
March flybys of Halley’s should
send back clues about the size and
appearance of its nucleus, the
atomic composition of the
chemical factory of gases close to
the nucleus, and the amount of
cosmic dust.
Man may never again lose
contact with Halley’s. Cameras on
the space telescope, to be launched
in early 1987, possess the
technology to “follow Halley’s all
the way out to its farthest point
beyond Neptune,” says John C.
Brandt of the National Aeronautics
and Space Administration. But, he
cautions, “the cameras may not
live up to expectations and may not
be used for Halley’s.” *
After Halley’s-what? The next
major breakthrough, Whipple
says, would be snatching a piece of
a comet and bringing it back to
Earth. “If we could analyze the
isotopes, we could answer a lot of
questions about time, place of
origin, and life-giving elements,”
he says. A second Giotto satellite
may undertake such a mission.
NASA had plans, dropped from
the 1986 budget, to rendezvous with
a comet in the 1990 s and fly along
with it. There’s even some talk of
ultimately putting a man on a
comet, possibly before Halley’s
return in 2061.
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