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A Surprise November Meteor Shower?


A surprise November meteor shower?
Marshall Space Flight Center
http://science.nasa.gov/newhome/headlines/ast05nov99_1.htm

On November 11, 1999 Earth will pass close to the orbit of newly-discovered
Comet LINEAR C/1999J3. The result could be a new meteor shower -- the
Linearids.

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.

"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.

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 a 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.

                          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.

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.

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