A Surprise November Meteor Shower?

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A surprise November meteor shower?

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 11^h40^m 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=11^h40^m, DEC=+53^o
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. Web Links

Comet LINEAR C/1999J3 - From Gary Kronk's Comets & Meteors Web Site

Leonids Live! -site of the live webcast of the 1999 Leonids

North American Meteor Network - home page

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April's Lyrid meteor shower -- Apr. 21, 1999. The oldest known meteor shower peaks this year on April 22.

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Leonid Sample Return Update -- Apr. 1, 1999. Scientists will describe initial results from a program to catch meteoroids in flight at the NASA/Ames Leonids Workshop April 12-15, 1999.

The Ghost of Fireballs Past -- Dec. 22, 1998. RADAR echoes from Leonid and Geminid meteors.

Bunches & Bunches of Geminids -- Dec. 15, 1998. The Geminids continued to intensify in 1998

The 1998 Leonids: A bust or a blast? -- Nov. 27, 1998. New images of Leonid fireballs and their smokey remnants.

Leonids Sample Return payload recovered! -- Nov. 23, 1998. Scientists are scanning the "comet catcher" for signs of Leonid meteoroids.

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The Leonid Sample Return Mission -- Nov. 16, 1998. NASA scientists hope to capture a Leonid meteoroid and return it to Earth.

Great Expectations: the 1998 Leonid meteor shower -- Nov. 10, 1998. The basics of what the Leonids are and what might happen on November 17.

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