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NASA Science News for June 27, 2003

Uncharted Meteors

The solar system is littered with clouds of dust--some of them
uncharted. Earth might encounter one such cloud this Friday, June
27th. 

June 27, 2003: In 1967, NASA's Mariner 4 spacecraft was cruising
through the solar system, not far from Earth, when something
unexpected happened. 

"Mariner 4 ran into a cloud of space dust," says Bill Cooke of the
Marshall Space Flight Center Space Environments Team. "For about 45
minutes the spacecraft experienced a shower of meteoroids more
intense than any Leonid meteor storm we've ever seen on Earth." The
impacts ripped away bits of insulation and temporarily changed the
craft's orientation in space. 

Fortunately, the damage was slight and the mission's main objective--
a flyby of Mars--had been completed two years earlier. But it could
have been worse. 

"There are many uncharted dust clouds in interplanetary space. Some
are probably quite dense," says Cooke. Most of these clouds are left
behind by comets, others are formed when asteroids run into one
another. "We only know about the ones that happen to intersect
Earth's orbit and cause meteor showers such as the Perseids or
Leonids." The Mariner 4 cloud was a big surprise.

"Of all NASA's Mars spacecraft, Mariner 4 was the only one we've sent
with a micrometeoroid detector," he continued. During its journey to
Mars and back, the detector registered occasional impacts from
interplanetary dust grains--as expected. The space between the
planets is sprinkled with dust particles. They're harmless in small
numbers. But when Mariner 4 encountered the cloud "the impact rate
soared 10,000 fold," says Cooke. 

Mapping these clouds and determining their orbits is important to
NASA for obvious reasons: the more probes we send to Mars and
elsewhere, the more likely they are to encounter uncharted clouds. No
one wants their spaceship to be surprised by a meteor shower hundreds
of millions of miles from Earth. 

Much of Cooke's work at NASA involves computer-modeling of cometary
debris streams--long rivers of dust shed by comets as they orbit the
sun. He studies how clumps form within the streams and how they are
deflected by the gravity of planets (especially giant Jupiter). He
and his colleagues also watch the sky for meteor outbursts here on
Earth. "It's a good way to test our models and discover new streams,"
he says. 

One such outburst happened on June 27, 1998. Sky watchers were
surprised when hundreds of meteors streamed out of the constellation
Bootes over a few-hour period. Earth had encountered a dust cloud
much as Mariner 4 had done years earlier. 

The meteors of 1998 were associated with a well-known meteor shower
called the June Bootids. Normally the shower is weak, displaying only
a few meteors per hour at maximum. But in 1998 it was intense.
Similar outbursts had occurred, with no regular pattern, in 1916,
1921, and perhaps 1927. 

The source of the June Bootids is comet 7P/Pons-Winnecke, which
orbits the Sun once every 6.37 years. The comet follows an elliptical
path that carries it from a point near the orbit of Earth to just
beyond the orbit of Jupiter. Pons-Winnecke last visited the inner
solar system in 2002. The comet's dusty trail is evidently clumpy.
When our planet passes through a dense spot in the debris stream, a
meteor shower erupts. 

Meteor forecasters D.J. Asher and V.V. Emel'yanenko (MNRAS 331, 1998,
126) have calculated that the meteors seen in 1998 might return in
2003, although 2004 is more likely. "That's why watching for June
Bootids this year is important: any activity now may herald another
outburst in 2004," notes Robert Lunsford, Secretary-General of the
International Meteor Organization, who is encouraging people to
monitor the sky this week. 

"The shower's peak is expected to occur on Friday, June 27th, near
1930 universal time (3:30 p.m. EDT)," says Lunsford. Although the
timing favors sky watchers in the Middle East and southern Asia,
North Americans and Europeans might see some meteors, too. 

Bill Cooke offers this advice: "Step outside after sunset on Friday
and look straight up. The constellation Bootes will be almost
directly overhead." If the shower is active, sky watchers will see
one or two meteors flying out of Bootes every minute. 

"You probably won't see a thing other than Bootes itself," Cooke
cautions. On the other hand, you might spot a genuine meteor
cloud ... uncharted no longer. 

Visit Spaceweather.com for updates about this year's June Bootid
meteor shower. 

Credits & Contacts
Author: Dr. Tony Phillips 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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