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 November 5, 1999: The Earth is
headed for a close encounter with the orbit of a newly discovered
comet. Found by the Lincoln Near-Earth Asteroid Research (LINEAR)
team on May 12th, Comet LINEAR (C/1999 J3) brightened to 9th
magnitude as it made its closest approach to the Sun in September.
On November 11, 1999 at 1941 UT, Earth will arrive at a point
in space just 0.0115 AU from the orbit of the comet, 40 days
after the comet itself was in the area. If there is a residual
swarm of dusty debris left over from the comet's passage, we
could be in for an unexpected meteor outburst -- the first known
apparition of the 'Linearids.'
The International Meteor Organization (IMO)
has issued a call
for meteor observations during a 48 hour period centered
on November 11. Linearid meteors -- if they materialize -- will
appear to emanate from the bowl of the Big Dipper. The shower's
radiant will be near right ascension 11h40m
and declination +53 degrees.
LINEAR
C/1999J3 is a long period comet. The most recent
orbital elements indicate that it returns to the inner solar
system only once every 63,000 years. All of the well-known annual
meteor showers such as the Perseids and Leonids are produced
by comets with much shorter periods. Short period comets pass
by the sun over and over, building up a substantial cloud of
dusty debris that causes meteor activity when the Earth passes
close to the comet's orbit. For example, Comet Tempel-Tuttle
-- the parent of the Leonid meteor shower -- has a period of
only 33 years. Comet LINEAR has such a long period that dust
ejected during close passes by the Sun probably dissipates before
the comet returns to the vicinity of Earth. If this is the case,
November 11 may not herald a particularly impressive shower.
Nevertheless, astronomers say it's still worth watching.
Click for visual observing tips or radio
observing tips!
"As to what type of activity might be expected," says
meteor expert Joe Rao, "it should be noted that the 1985
Giacobinids briefly produced an outburst of 600 to 800 [meteors
per hour] from Japan, with the Earth following comet Giacobini-Zinner
to its node by just 26.5 days. The separation between the orbit
of the comet and Earth was 0.033 -- or three times the separation
between the upcoming case of Earth and C/1999J3."
"In 1933, when a major Giacobinid storm (3000 - 29000 meteor
per hour) occurred, these values were 80 days and 0.005 a.u.
Earth is following comet LINEAR to its descending node by ~39.9
days. Hence, the situation regarding the prospective Linearids
is roughly midway between the two above cases."
These similarities are encouraging, says Rao, but they may not
be significant. "Giacobini-Zinner is a well-known short-period
comet of 6.5 years and has been observed to circle the Sun on
many other occasions, whereas LINEAR is a newly discovered long-period
object of ~63,000 years. In addition, the dust-distribution surrounding
LINEAR is completely unknown."
The phase of the Moon will be just past new on November 11, so
observers outside of urban areas will enjoy very dark skies.
Unfortunately for North Americans, Earth makes its closest approach
to comet LINEAR's orbit at 2:41 p.m. EST (1941 UT) when it is
broad daylight across the United States. If the meteor shower
occurs at this time, then observers in Asia will be favored.
However, meteor showers are notoriously difficult to predict
-- much like weather on Earth -- and interested observers everywhere
should be prepared to watch for activity between midnight and
dawn local time on November 10, 11, and 12. For more information
about how to observe the Linearids visually, click
here.
Radio Linearids
In the United States, where the peak of the Linearid shower
might take place during the day, radio observations may provide
the only way to "see" these meteors.
 Linearid meteoroids
will strike Earth's atmosphere at a velocity of about 52 km/s.
When fast-moving meteoroids like these plummet through our atmosphere
they heat and ionize the air in their path. The luminous ionized
trails are not only visually striking (if seen at night) but
they also reflect radio waves. During a major meteor shower,
radio signals from TV stations, RADAR facilities, and AM/FM transmitters
are constantly bouncing off short lived meteor trails. For those
who know how to listen, it's easy to hear the echoes.
Above: Artist's concept of radio
meteor observing. Meteor trains reflect transmissions from stations
that are over the horizon, and normally impossible to detect.
Whenever a meteor passes by with the correct geometry, listeners
hear a brief "ping" on the receiver's loudspeaker.
more
information from the North American Meteor Network.
One of the advantages of radio observing is that meteors can
be detected when skies are cloudy or during daylight. Radio observing
has some advantages at night, too. The human eye can only see
shooting stars brighter than 6th magnitude, but radio methods
can detect meteors that are at least 5 times dimmer.
 If you're interested in detecting
radio Linearids, the procedures are relatively simple. You'll
need a good commercial radio receiver and an aerial. Although
meteor trains can reflect radio waves at almost any frequency,
the best frequencies to try are usually between 50 and 120 MHz.
Many observers use a common FM radio tunable between 88 and 108
MHz and a Yagi FM/TV antenna. During a meteor shower tune your
receiver to a distant transmitter between 200 and 1000 miles
away. Commercial radio stations, TV stations, and radar transmitters
are all suitable if located at the correct distance. Under normal
circumstances the transmitter should be difficult or impossible
to detect, but when a meteor intervenes the signal hops over
the horizon and a brief fragment of the transmission can be heard.
Depending on the type of the transmitter it might sound like
a tone, a bit of music or voice, or simply noise. Contact lasts
for as long as the meteor train persists, usually from 100 milliseconds
to a few seconds.
Stan Nelson of Roswell, NM, captured this
radar echo from a meteor on April 11, 1999. He used an ICOM
R8500 communications receiver to monitor 217 MHz transmissions
from the Navy Space Surveillance Radar located in Lake Kickapoo,
TX. 217 MHz would normally be considered a poor frequency for
meteor observations, but the tremendous power of the Naval Space
Surveillance radar (NAVSPASUR) more than compensates for its
less-than-optimum transmission frequency. NAVSPASUR is an excellent
transmitter for meteor observers across the southern United States.
For more information about meteor observing with NAVSPASUR, please
see the Dec. 1998 Science@NASA article The
Ghosts of Fireballs Past. To learn more about radio meteor
observing in general, see the North American Meteor Network radio
meteor tutorial.
How to View the Linearids
Experts aren't sure what time is best to try observing the
Linearids because so little is known about the distribution of
dust around the parent comet. Earth will pass closest to the
comet's orbit on November 11 at 1941 UT, or 2:41 p.m. Eastern
Standard Time. If the shower peaks at that time, observers in
Asia will be favored. In North America it will be broad daylight.
Since so little is known about this new comet, it is probably
unwise to place too much confidence in the orbit crossing time
1941 UT. The author recommends monitoring the sky between local
midnight and dawn on November 10, 11, and 12. ("Local midnight"
means midnight wherever you live.) The the best time to look
for meteors is almost always just before dawn. That's when the
Earth's orbital motion is headed most directly into the cometary
debris stream.
Above:The rate of meteor activity
is usually greatest near dawn because the earth's orbital motion
is in the direction of the dawn terminator. Earth scoops up meteoroids
on the dawn side of the planet and outruns them on the dusk side.
Linearids
at a Glance
- The meteor shower
could be active from November 4 through November 18.
- Maximum activity is
expected at 1941 UT on November 11, 1999.
- The radiant is at
RA=11h40m, DEC=+53o
- Atmospheric velocity=52
km/s
- The phase of the moon
will be just past new during the expected activity maximum.
Current Moon Phase
Updated every 4 hours. |
The Linearids -- if they materialize -- will appear to
radiate from one of the most familiar asterisms in the Northern
Hemisphere, the Big Dipper. Even novice stargazers can usually
find it by first locating the north star and then looking nearby
for the ladle-shaped arrangement of bright stars. The shower's
radiant is near Phecda, a 2nd magnitude star in the lower left
corner of the Dipper's bowl. At local midnight, the radiant point
will be just 15 degrees above the horizon as seen from mid-latitude
sites in the northern hemisphere. The low altitude of the radiant
will make some meteors difficult to see. However, the situation
will improve. By 3 am the radiant will rise to an altitude of
35 degrees, and then to 60 degrees by dawn. The phase of the
Moon will be just past new on November 11, so observers outside
of urban areas will enjoy very dark skies.
You won't need binoculars or a telescope to observe Linearid
meteors, the naked eye is usually best for seeing "shooting
stars," which often streak more than 45 degrees across the
sky. The field of view of most binoculars and telescopes is simply
too narrow for good meteor observations.
 Experienced observers suggest
the following viewing strategy: Dress warmly. Bring a
reclining chair, or spread a thick blanket over a flat spot of
ground. Lie down and look up somewhat toward the north star.
Meteors can appear in any part of the sky, although their trails
will tend to point back toward the radiant, pictured in the sky
map below.
The red dot shows the
region of the sky from which the Linearid meteors might emanate.
This point, called the radiant, is really an optical illusion
- the meteors are moving along parallel paths, but appear to
come from a single point, just as a stretch of parallel railroad
tracks will appear to meet at a point on the horizon. |
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