May 2, 2000 -- May 5, 2000 is a
red-letter day for many astronomy enthusiasts thanks to the upcoming alignment of five
planets with the Sun and the Moon. Unfortunately, the alignment
won't produce much of a sky show. The Sun will be right in the
middle of the cluster -- blinding sunlight will make it nearly
impossible to see the other members of the planetary get-together.
If that sounds like discouraging news and you're thinking it
might not be worth star gazing this Friday evening, wait! There
could be a sky show on May 5 after all -- the annual eta Aquarid
meteor shower.
Right: This fanciful picture by Duane
Hilton shows an eta Aquarid meteor streaking across an aurora-filled
sky. No auroral storms are predicted for May 5, but such
displays are becoming more common with the approach of solar
maximum.
"This week will provide one of the few good views of a meteor
shower this year," says Robert Lunsford, the North American
Coordinator for the International Meteor Organization. "Moonlight
will spoil most of the major meteor showers in 2000, but the
eta Aquarids will occur with the moon near new and out of the
way."
The nominal peak of the eta Aquarids occurs near 1700
UT on May 5, but "the display will not have a sharp peak
of activity .... instead good rates will occur for a week centered
on May 5."
Experts expect the eta Aquarids to produce 15 to 20 shooting
stars per hour for lower-latitude observers in the northern hemisphere
and up to 60 per hour in the southern hemisphere. The best times
to look will be in the hours before dawn on Thursday, Friday
and Saturday, May 4-6.
The eta Aquarid shower isn't the most important annual meteor
display for northern skywatchers -- the Leonids, Perseids, Geminids
and Quadrantids are all more intense -- but it is interesting
for another reason. Every eta Aquarid meteoroid that streaks
across the sky on May 5 is a tiny piece of history's best known
cosmic snowball -- Halley's Comet.
Our
planet passes close to the orbit of Halley's comet twice a year,
once in May and again in October. Although the comet itself is
very far away -- presently beyond the orbit of Jupiter -- tiny
pieces of Halley are still hanging around the inner solar system.
These particles are leftovers from Halley's close encounters
with the Sun every 76 years; each time the comet returns 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. When these meteoroids strike Earth's
atmosphere they produce the eta Aquarid meteors in May and the
Orionid meteors in October.
Above: Comet Halley moves around the Sun in an elliptical
retrograde orbit, opposite to the direction of Earth's motion.
As a result, eta Aquarid 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 comet itself won't return to the inner solar system until
2061. The size of Earth's orbit is exaggerated by a factor of
two in this diagram.
Like most meteor showers, the eta Aquarids are named after
the constellation containing the shower's radiant, a point in
the sky from which the shooting stars appear to stream. Because
there are several annual showers that come from the direction
of Aquarius, the May 5th shower is called the eta Aquarids
(the radiant is very close to the star eta Aquarius) to
distinguish it from the others. The eta Aquarid's sister shower
in October is called the Orionids, from the constellation Orion.
meteoroid -- a tiny space rock (usually a speck
of debris from a comet or asteroid) while it is in space or flying
through Earth's atmosphere
meteor -- a streak of light in the sky caused by a disintegrating
meteoroid
meteorite -- a rock from space that hit the ground because
it did not burn up entirely in the atmosphere. |
The radiant of the eta Aquarid display never climbs very
far above the horizon in the northern hemisphere. That makes
it a better shower for stargazers south of the equator. Although
the rate of shooting stars is not very high in the north, skywatchers
can hope to see some spectacular "earth grazers" while
the radiant is low on the horizon. These are meteors that skim
horizontally through the upper atmosphere. "Earth grazers"
are typically slow and dramatic, streaking far across the sky.
Meteorites hitting the Moon?
When the Earth passes through Comet Halley's meteoroid stream,
so will the Moon. There's no air on the Moon so meteoroids don't
burn up in the atmosphere as they do here on Earth. Instead,
they simply plummet to the ground, disintegrating with a brief
flash of light on the Moon's surface. Scientists calculate that
a rocky meteoroid the size of a grapefruit or larger could produce
a 3rd magnitude flash, easily seen by the naked eye or through
binoculars.
During
the 1999 Leonid meteor storm, several amateur and professional
astronomers were on the lookout for signs of meteoroids hitting
the moon. Few really expected to see anything, but surprised
observers captured half a dozen flashes on videotape. It turns
out that the Moon passed through a dense region of the Leonid
meteoroid stream, resulting in numerous visible impacts.
Above: This video of a lunar meteorite impact was captured
by David Dunham on the night of the 1999 Leonid meteor shower.
At peak brightness, the flash was about 3rd magnitude. [more
information from LunarImpact.com]
The eta Aquarid meteor shower may offer another opportunity to
spot lunar meteoroid strikes. The average space density of particles
in the eta Aquarid debris stream is probably 100 to 1000 times
less than that of the Leonid stream. Nevertheless, there are
indications of dense filaments within the eta Aquarid stream
[ref]
that might carry large numbers of meteoroids. If the Moon passes
directly through one of these, flashes could be visible.
"The eta Aquarid debris stream will
hit the Moon near its western (sunlit) limb this year,"
says Prof. George Lebo at the University of Florida Department
of Astronomy. "The best place to watch for meteor strikes
will probably be about midway between the Moon's sub-Earth point
(the middle of the moon's disk) and the slender sunlit crescent.
Because the Moon is nearly New there will be lots of dark terrain
where flashes might be visible."
Watching for meteorite strikes on May 5 will be difficult, continues
Lebo, because the Moon will be so close to the Sun. On the days
that follow, however, the Moon will rapidly move away -- by May
6, it will set two hours after the Sun, and by May 10 the Moon
will still be above the horizon 6 hours after sunset.
Although the eta Aquarids peak around May 5, activity is high
for about a week centered on the maximum. Brian Cudnick, coordinator
of the Lunar
Meteoritic Impact Search Program sponsored by the American
Association of Lunar and Planetary Observers (ALPO), is organizing
a watch for eta Aquarid impacts from May 6 through 10, 2000.
The ALPO website includes a lunar blank and report form that
observers can use to plot and report impact sightings.
For more information about lunar meteorite impacts and how to
make scientifically useful recordings of meteorite flashes, visit
LunarImpact.com.
Naked-eye
eta Aquarid Observing Tips
The
nominal peak of the eta Aquarids shower is 1700 UT on May 5.
The shower is expected to produce up to 20 meteors per hour for
northern hemisphere observers and 40 to 60 meteors per hour in
the southern hemisphere. The difference results from the fact
that the radiant is higher above the horizon in the south than
it is in the north. The eta Aquarids are usually the best annual
meteor shower for southern sky watchers.
The eta Aquarids are noted for unpredictable behavior, with
secondary peaks that occur before or after the nominal maximum
[ref].
Scientists attribute this to filamentary structures within the
cometary debris stream. Brief outbursts of 40 - 60 (or more)
meteors per hour could occur anytime during the week centered
on May 5. No matter where you live, the best times to watch will
be during the hours before dawn. That's when your sky will be
headed directly into the densest part of the meteoroid stream
(see diagram below).
The
eta Aquarid radiant peeks above the eastern horizon at approximately
2:30 a.m. local time at mid-latitudes in both hemispheres. The
radiant 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 and westward of the shower's radiant.
All of the stars in Aquarius are fairly dim -- the brightest
is Sadalmelik at magnitude 3.2.
Left: This image shows the area of sky around the eta
Aquarid radiant (indicated by a red dot) as seen from Sydney,
Australia at 4 a.m. on May 5, 2000. At 4 a.m. local time -- that
is to say, when it is 4 in the morning where you live -- the
following statements will be true: The radiant will be about
30 degrees above the eastern horizon if you live at a mid-latitude
site south of the equator. The radiant will be about 15
degrees above the eastern horizon if you live at a mid-latitude
site north of the equator. Northern observers should rotate
this sky map by 180 degrees to see the arrangement of stars in
their hemisphere.
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 20 degrees across the sky. The field of
view of most binoculars and telescopes is simply too narrow for
casual meteor observations.
- The meteor shower
is active from April 21 until May 12.
- Maximum activity is
expected around 1700 UT
on May 5, 2000.
- The radiant is at
RA=22h28m, DEC=-1o
- Atmospheric velocity=66
km/s
- Typical magnitude
= 3
|
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 east. Meteors can appear in any part of the sky, although
their trails will tend to point back toward the radiant.
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. |