How a comet can cause a meteor shower
The Earth often passes through the trail crumbs left behind by comets.
Meteors — popularly referred to as "shooting stars" — are generated when debris enters and burns up in Earth's atmosphere. But astronomers have found that some meteor swarms correspond closely to the orbits of known comets.
In the wake of the recent close pass of Comet Hartley 2, it is interesting to note that our Earth is currently interacting with the cosmic leftovers from a far more famous icy wanderer: Halley's Comet.
And more meteors spawned by comets are expected to give skywatchers a cosmic show in upcoming weeks.
Right now, in fact, we are sweeping through some of the dust that was shaken loose from Halley's Comet as it runs along its gigantic orbital loop from the vicinity of the sun all the way out to Neptune.
These tiny comet crumbs ram into our atmosphere at high speeds to create shooting stars, which we call the Orionid meteor shower.
There are actually two points along Halley's path, where it comes relatively near to our orbit. One of these points corresponds to early May and causes a meteor display that emanates from the constellation Aquarius, the Water Carrier. The other point lies near the late October part of our orbit to produce the Orionids.
In May we meet the "river of rubble" shed by the comet on its way outward from its nearest approach to the sun, while in October we encounter the part of the meteor stream moving inward toward the sun.
The meteors are moving through space opposite or contrary to our orbital direction of motion. That explains why both the Aquarids and the Orionids hit our atmosphere very swiftly at 41 miles (66 km) per second – only the November Leonids move faster.
Another distinguishing characteristic that October's Orionids share with the May Aquarids is that they start burning up very high in our atmosphere — possibly because they are composed of lightweight material.
This means they likely come from Halley's diffuse surface and not its core.
Moon muscles in on meteor show
Unfortunately, 2010 is a poor year for the Orionids thanks to the presence of the moon,which turns full today (Oct. 22).
It will flood the sky all night long with its brilliant light right on through the weekend. So although the Orionids are currently near their peak, producing hourly rates of around 20 per hour, most of these streaks of light will likely be obliterated by the bright moonlight.
Still, an exceptionally bright Orionid, darting from out of the region of the constellation Orion (from where we get the name "Orionid"), might still be glimpsed.
The best time to look would be in the early hours of the morning; the stars of Orion dominate the southern sky around 4 a.m. local time. After this weekend, Orionid activity will begin to slowly descend. The last stragglers usually appear sometime in early to mid-November.
But by then, the moon will no longer be a factor and another meteor shower will take center stage.
The Taurids arrive
The Taurid meteors, sometimes called the "Halloween fireballs," show up between mid-October and mid-November. But between Nov. 5-12 will likely be the best time to look for them this year, taking into account both their peak of activity and the fact that the moon will be new on Nov. 5, offering no interference whatsoever.
Even when the moon widens to its first quarter phase on Nov. 13, it will still set before midnight, leaving the rest of the night dark for meteor watchers.
After the moon sets, some 10 to 15 meteors may appear per hour. They are often yellowish-orange and, as meteors go, appear to move rather slowly.
The Taurid meteor shower's name comes from the way these meteors seem to radiate from the constellation Taurus, the Bull, which sits low in the east a couple of hours after sundown and is almost directly overhead by around 1:30 a.m.
In the case of the Taurids, they are attributed to debris left by Comet Encke, or perhaps by a much larger comet that upon disintegrating, left Encke and a lot of other rubble in its wake.
The Taurids are actually divided into the Northern Taurids and the Southern Taurids. This is an example of what happens to a meteor stream when it grows old.
So what was originally one stream now diffuses into a cloud of minor streams and isolated particles in individual orbits. These cross Earth's orbit at yet more widely scattered times of the year and coming from more scattered directions until they are entirely stirred into the general haze of dust in the solar system.
The two radiants lie just south of the Pleiades. So during the first couple of weeks of November, if you see a bright, slightly tinted orange meteor sliding rather lazily away from that famous little smudge of stars, you can feel sure it is a Taurid.
Mark your calendars
Because its orbit passes rather close to Earth's you might be wondering if Comet Hartley 2 can produce any kind of meteor display. The answer is yes, although at the present time, we could hardly call it a meteor shower.
We currently pass closest to the orbit of Hartley 2 around Nov. 2, but at the moment our respective orbits are too widely separated to produce much more than a few meteors during the course of an entire night's watch.
However, if the calculations of Russian astronomer Mikhail Maslov are correct, Hartley 2 is destined to provide a spectacular meteor display for us in the distant future.
Using a method which he calls "vertical trails," Maslov forecasts that in the year 2055, we will encounter considerable amounts of comet debris shed by Hartley 2 from 1985 right on through the first half of the 21st century.
On Nov. 2, 2055, between 9 and 10 a.m. GMT (Universal Time), Maslov predicts a display of meteors with "high brightness."
They will appear to emanate from the constellation of Cygnus, the Swan, not too far away from the beautiful double-star Albireo. This could turn out to be a true "storm" of meteors, producing anywhere from 1,000 to 1,300 per hour.
Especially to all of our younger readers, I can only say, mark your calendars!
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.