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Meteors Down Under

Debris from Halley's comet will put on a sky show for southern observers in early May

astroKoala BearMay 3, 1999: It's a fact of Nature that Koala bears, kangaroos and other denizens of the southern hemisphere rarely have a favorable view of major meteor showers. Most cometary debris streams in the inner solar system are arranged in such a way that that they tend to produce shooting stars mainly over the northern hemisphere. The well-known Leonids display is relatively easy to see from southern latitudes, but that shower is only intense at 33 year intervals when the parent comet Tempel-Tuttle passes close to Earth.

Throughout the year meteor enthusiasts "down under" are able to view some of the more intense northern displays over the horizon, and there are a number of minor showers averaging 5 -15 shooting stars per hour. Without a doubt, however, the highlight of the meteor observing season is the eta Aquarids. Each year around May 5 the eta Aquarids reach their peak with 30 to 50 meteors per hour visible from below the equator. It's the best annual shower in that part of the world.

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If you don't live in the southern hemisphere you might be inclined to disregard the eta Aquarids. After all there are many annual showers that are more intense and easier to view. Sky watchers at northern mid-latitudes will typically see only about 5 to 10 per hour, and above latitude 45 degrees they are practically impossible to see at all. Nevertheless, there is something special about these meteors that makes them worth watching no matter where you live. Every eta Aquarid meteoroid is a tiny piece of Halley's comet!

How to view the eta Aquarids

Halley's comet is among the most famous of all heavenly bodies. It orbits the sun once every 76 years and has been seen on each visit to the inner solar system since 240 BC. Many of the apparitions have been spectacular and some even play a role in human history. In 1066 the comet was so bright that it terrified millions of Europeans and was widely credited with the Norman victory at the Battle of Hastings. Halley's Comet of 1456 followed the 1453 invasion of Constantinople by the Turks, and was thought by some to be a gesture of heavenly support for their victory. There are accounts that Pope Calixtus III excommunicated the comet and ordered prayers to prevent the Turks from taking over all of Europe.

In 1910 Earth passed through Halley's tail, prompting widespread fears that cyanogen gas from the comet would poison the atmosphere. Fortunately comet tails are very tenuous and Earth suffered no ill effects from the encounter. Although the wispy nature of comet tails was already well known to astronomers in 1910, sales of gas masks that year were brisk.

Left: This image of Comet Halley's nucleus was taken by the Giotto spacecraft during a flyby on March 13, 1986. Scientists estimate that about 10% of the surface was boiling off into space. The stuff that boiled off Halley in 1986 may one day be seen again during an eta Aquarid meteor shower.

Mark Twain was born in 1835 when Halley's comet was in the sky. Later in life he began to say that "I came in with Halley's Comet in 1835. It is coming again next year (1910), and I expect to go out with it. It will be the greatest disappointment of my life if I don't...." During the spectacular return of 1910 he died [refs. 1, 2].

Each time Halley's comet swings by the sun, solar heating evaporates about 6 meters of ice and rock from the nucleus. The debris particles, usually no bigger than grains of sand, gradually spread along the comet's orbit until it is almost uniformly filled with tiny meteoroids. Twice a year when Earth passes by the debris cloud there is a meteor shower. The first, on May 5th, is called the eta Aquarids and the second, on October 21st is called the Orionids. Sometimes both showers are referred to as "Halleyids" after the parent comet.

So, although Halley's comet won't return to Earth until 2061, skywatchers can glimpse bits of the comet in just a few days when the eta Aquarid meteor shower peaks.

the orbit of Halley's cometLeft: Comet Halley moves around the sun in an elliptical retrograde orbit, opposite to the direction of Earth's motion. As a result Halleyid meteoroids and the earth approach one another at high speed, much like two cars in a head-on collision. Although the meteoroids are very small and light they give rise to bright shooting stars because they speed through the atmosphere at nearly 66 km/s (148,000 mph). The size of Earth's orbit is exaggerated by a factor of two in this diagram.

In 1985 scientists enjoyed the closest-ever view of comet Halley and its meteoroids when five spacecraft from Russia, Japan, and the European Space Agency were sent to rendezvous with the comet. The ESA's Giotto probe captured close-up color pictures of Halley's nucleus showing jets of solar-heated debris spewing into space. In fact, just 14 seconds prior to its closest approach, Giotto was hit by a small piece of the comet which altered the spacecraft's spin and permanently damaged the camera. Most of the instruments were unharmed, however, and Giotto was able to make many scientific measurements as it passed within 600 km of the nucleus.

Right: An artist's rendering of the European Space Agency's Giotto probe.

Some of the most important measurements came from Giotto's 'mass spectrometers', which allowed scientists to analyze the composition of the ejected gas and dust. It's widely believed that comets were formed in the primordial Solar Nebula at about the same time as the sun. If that's true, then comets and the Sun would be made of essentially the same thing -- namely light elements such as hydrogen, carbon and oxygen. Objects like Earth and the asteroids tend to be rich in heavier elements like silicon, magnesium, and iron. True to expectations, Giotto found that light elements on comet Halley had the same relative abundances as the Sun. That's one reason why the tiny meteoroids from Halley are so light. A typical debris particle is about the same size as a grain of sand, but it is much less dense, weighing only 0.01 gram.

How to View the eta Aquarids

The best times to view the eta Aquarids in 1999 are during the early morning hours of May 5 and May 6. The constellation Aquarius rises above the horizon at about 2:30 am. The the best time to look for meteors will be between about 4 a.m. and dawn. That's when the local sky is pointing directly into the meteoroid debris stream (see the diagram below), and also when the elevation of the radiant exceeds 15 degrees as seen from mid-latitudes. The nominal maximum of the eta Aquarids occurs at 11 UT on May 6, but the shower's peak is broad and there should be enhanced meteor activity between 10 UT on May 5 and 11 UT on May 6. The bright gibbous moon will make all but the brightest meteors difficult to see on both days.

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.

eta Aquarids
at a Glance
  • The meteor shower is active from April 21 until May 12.
  • Maximum activity is expected at 1100 UT on May 6, 1999.
  • The radiant is at RA=22h28m, DEC=-1o
  • Atmospheric velocity=66 km/s
  • Average magnitude 2.4 - 3.1

Current Moon Phase

 

Updated every 4 hours. 
The constellation Aquarius, pictured in the sky map below, peeks above the southeastern horizon at approximately 2:30 a.m. local time at mid latitudes in both hemispheres. The radiant of the eta Aquarids is located near Fomalhaut, a 1.3 magnitude star in the constellation Piscis Austrini. Fomalhaut will be below the horizon for most northern hemisphere observers, but it is a good finder star for skywatchers in the south. In Sydney, Australia, for example, Fomalhaut will be visible at 4 a.m. at an elevation of +25 degrees, just above the radiant of the meteor shower. All of the stars in Aquarius are fairly dim -- the brightest is Sadalmelik at magnitude 3.2. The bright light of the waning gibbous moon on May 6 will cause many stars in Aquarius to be difficult to see.

You won't need binoculars or a telescope to observe eta Aquarid meteors, the naked eye is usually best for seeing meteors 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 south. 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 eta Aquarid meteors 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. The constellations are shown as they would be seen by an observer in the southern hemisphere. Northern skywatchers should rotate the image 180 degrees.
Web Links

The eta Aquarids - From Gary Kronk's Comets & Meteors Web Site

The Orionids - From Gary Kronk's Comets & Meteors Web Site

North American Meteor Network - home page

Leonids Live! -site of the live webcast of the 1998 Leonids

Related Stories:

Tuning in to April meteor showers -- Apr. 27, 1999. Amateur astronomers capture radio echoes from fiery meteors in April 99.

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

A Wild Ride to the Stratosphere in Search of Meteors -- Apr. 14, 1999. The payload from the NASA Meteor Balloon has been recovered.

Meteor Balloon set for Launch -- Apr. 9, 1999. NASA scientists prepare to launch a weather balloon designed to capture micrometeoroids in the stratosphere.

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.

Early birds catch the Leonids -- Nov. 19, 1998. The peak of the Leonid meteor shower happened more than 14 hours earlier than experts had predicted.

A high-altitude look at the Leonids -- Nov. 18, 1998. NASA science balloon catches video of 8 fireballs.

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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Dr. John M. Horack , Director of Science Communications
Author: Dr. Tony Phillips
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NASA Oficiaicial: Ron Koczor