The sky literally will be falling Nov. 17, but, with any
luck, satellites owned by cable companies won't be part of the celestial shrapnel
raining down.
Scientists predicted that one of the most intense meteor
events of a lifetime will happen either this fall or next, when the P/Temple-Tuttle comet
swings past the earth on its annual visit. Each year, debris from this space traveler
enters the earth's atmosphere, creating an inordinate number of shooting stars known
as the Leonid meteor showers.
Every 33 years or so, the Leonid peaks in intensity as it
passes extra close to the sun, resulting in a 100-fold or greater increase in space
nuggets falling from the comet.
This phenomenon represents quite a spectacular sight for a
stargazer, but lately, the Leonids have been a bit of a concern for cable companies with
multimillion-dollar communications gear floating in space. This surge of meteoric activity
means greater odds that one of these satellites could be hit by heavenly hunks of rock.
But scientists said that even with such increased activity,
it is highly unlikely that any satellite will take a debilitating hit and, with a few
precautions, most spacecraft should escape unscathed.
Scientists recorded the most intense meteor
"storm" in history during the last peak of the Leonids, in 1966, when 150,000
meteors per hour were spotted streaking from the sky. During most annual Leonid events,
between 15 and 20 meteors fall per hour. During the other notable annual meteor event, the
Perseids, between 40 and 50 fall per hour.
Donald Yeomans, senior research scientist at the National
Aeronautics and Space Administration's Jet-Propulsion Laboratory in Pasadena, Calif.,
estimated that this year's Leonid event will not reach the intensity of the one in
1966. He said perhaps a few-hundred to 5,000 meteors will fall per hour, qualifying the
event as a meteor "shower." The Leonids usually only last between a few hours
and one day.
"The chances of a satellite sustaining a hit in the
few hours during the peak of the Leonid showers is about the same as the chance that a
satellite would be hit from a random meteor during the course of a year," Yeomans
said. "It's an interesting chance to see celestial fireworks, but I don't
see it as a threat in any way."
Other scientists agreed that the next Leonid peak likely
won't reach epic proportions. Peter Jenniskens, a research scientist at NASA's
Ames Research Center in Mountain View, Calif., predicted around 1,000 to 10,000 meteors
per hour for the Leonids this year or next, with a slim possibility of as many as 50,000.
Scientists with The Center for Research in Earth and Space
Technology in London, Ontario, however, expected five to 10 functional spacecraft to be
hit by the Leonid meteoroids in 1999 or 2000. These scientist placed a 0.01 percent to 1
percent probability of impact during a storm. There are more than 500 satellites in orbit.
The risk of a meteor hitting a satellite could increase
10-fold during the Leonid meteor shower, Jenniskens said.
He added, however, that satellite operators can greatly
reduce the odds of their craft being hit -- or being hit in a vulnerable area, such as
optics -- by turning the thin edge, or backside of the satellite, toward the origin of the
debris. Unlike random meteors, the trajectory of the Leonid shower is predictable and
consistent. All of the meteors travel along the same parallel paths.
"It would be prudent for operators to present the
safest side of the satellite to the meteor shower," Yeomans said.
Yeomans admitted that no one can know for certain what
P/Temple-Tuttle will bring.
"Nothing's for sure when talking about
comets," he said.
In particular, predicting how much the sun will deteriorate
the comet is extremely difficult. Every 33 years, when the comet passes unusually close to
the sun, heat and other forces from the sun break up the comet.
Yeomans said predicting just how much the comet will break
up is like trying to estimate the damage from a car accident before it happens.
For sure, satellite operators do need to worry about such
things. Most particles from the P/Temple-Tuttle comet will be the size of a grain of sand.
In fact, they are made up of silicate, a sandlike substance. But although they are small,
these particles pack a wallop. They travel at 70 kilometers per second, or around 158,000
miles per hour. In comparison, a bullet travels at only 1 kilometer per second.
"That's a whole lot faster than anything
we're used to on Earth," Yeomans said.
Such blazingly fast sand grains can take out a satellite by
creating a plume of plasma on impact with the craft. The plasma cloud can then short out
the electronic circuits in a craft if it gets inside the craft's protective skin.
NASA will not conduct any space-shuttle missions during the
Leonids.
Although the Leonids pose a greater hazard to satellites
for a few hours, satellites have proven durable. Scientists believe that only one
satellite has been taken out by a meteor in all of the years since Sputnik.
During a Perseid shower in 1993, a satellite was reportedly
hit by a meteoroid and knocked out of action by a plasma plume.
Another satellite was reportedly hit by a meteor grain,
which temporarily caused the craft to lose equilibrium.
However, Robert Zitter, senior vice president of technology
operations for Home Box Office, said meteors are one of the least of his worries about
satellite failure.
"I don't think that the Leonids are any greater
risk than what we are already prepared for," Zitter said. "Quite frankly, the
risk of being hit by anything is minimal."
The biggest risk to satellites is during start-up, when
certain components can become damaged. Then, near the end of a satellite's life,
equipment failure is always a concern. Also, solar flares, such as the major ones that
occurred earlier this decade, can cause signal interference and electronic difficulties.
And in general, satellites can suffer electronics failures at any time from faulty
manufacturing.
Zitter said his company only came close to losing a
satellite once, but that was due to a technician's error while piloting the satellite
from the ground.
Still, entertainment companies such as cable programmers
have more revenue than ever dependent on satellites.
"We have a lot of money riding on satellites,"
Zitter said.
And when a satellite breaks, it is very difficult to fix,
and it usually takes two years to send up a replacement.
HBO builds in redundancies to its satellite-communications
network to ensure continued communications. It spreads its signals over three satellites
so that if one fails, the network can keep most of its programming going. HBO also has
access rights to other birds, which it can use in an emergency.
"We have enough backups to make sure that whatever
happens, it won't affect our customers," Zitter said.
Regardless of whether the P/Temple-Tuttle's visit this
year or next creates a storm or only a light shower of meteors, cable operators with
satellites can rest easy that they won't have to worry about such a surge of
celestial debris for the rest of their lives.
Jenniskens said that during the next two peaks of the
Leonids -- 33 and 66 years from now -- the comet will be passing much further away from
the earth, greatly lessening the Leonid meteor activity. The comet will not move closer to
the earth until 99 years from now.
"If something is to happen, it has to happen this year
or next," Jenniskens said.
