Comet Hartley 2 Could Deliver 'Bonus' Meteor Shower
These two fireballs with orbits similar to the comet Hartley 2 were observed on Oct. 16. 2010 by cameras in western Ontario (left) and the southeastern USA (right). The fireballs may have been caused by meteors from the comet.
Credit: UWO/NASA/Bill Cooke [Full Story]

The icy Comet Hartley 2 put on an interesting show during an unusually close pass by Earth last month and is poised to be visited by a NASA space probe on Thursday (Nov. 4). Now, there may be an unexpected bonus: a new meteor shower.

The new meteor shower, which astronomers have billed the ?Hartley-ids,? could brighten the night skies in early November, with the peak times occurring between Nov. 1 and Nov. 3. [Photos of Comet Hartley 2]

In fact, some skywatching cameras have revealed two fireballs that may be the result of Hartley 2 meteors ? or maybe not. Here is the story:

Recipe for a meteor shower

As a comet nears the sun, particles are shed from its nucleus. Thus its orbit is not an imaginary path through space like Earth's, but a continuous trail of dust moving in the same direction as the comet. 

Every time Earth crosses one of these dust trails, it stops millions of orbiting particles, and alert observers might catch a glimpse of some of these plunging through our atmosphere from their location; air friction vaporizes the particle with a flash of light.

This may start at about 80 miles (nearly 129 km) above the Earth and end a second or two later at about 40 miles (64 km).

Strictly speaking, the word "meteor" is used only for this brief luminous lifetime. When still out in space, the particles are called meteoroids.

A meteor that rivals Jupiter or Venus in brightness is a fireball. One that breaks up along its luminous flight across the sky ? often with a strobe-like flash ? is called a bolide. Some are so large that parts of them reach the ground and become geologic specimens called meteorites.

In the case of Comet Hartley 2, the possibility of it spawning a meteor shower are practically nil: the orbits of the comet and Earth never intersect or approach each other close enough for that dusty comet material to interact with our atmosphere. 

But recently, something took place which made one meteor expert stop to reconsider the possibility.

Fiery coincidence?

On Oct. 16, a pair of NASA all-sky cameras caught an unusual fireball streaking across the night sky over Alabama and Georgia. It was bright, slow, and strangely similar to another fireball that passed over eastern Canada less than five hours earlier.

The Canadian fireball was recorded by another set of all-sky cameras operated by the University of Western Ontario. Because the fireballs were recorded by multiple cameras, it was possible to triangulate their positions and determine their orbits.

This led to Bill Cooke of NASA's Meteoroid Environment Office to make a remarkable conclusion:  "The orbits of the two fireballs were very similar. It's as if they came from a common parent."

As it turned out, the orbits of the two fireballs were not only similar to one another, but also roughly similar to the orbit of Comet Hartley 2. 

"It makes me wonder," Cooke said, adding that "thousands of meteoroids hit Earth's atmosphere every night. Some of them are bound to look like ?Hartley-ids? just by pure chance."

Comet crumbs from space

So while the odds are still long for any kind of significant meteor activity from Comet Hartley 2, perhaps it is not entirely out of the question. 

In fact, 13 years ago I had discussed the possibility of a meteor shower from Hartley 2 on MeteorObs,  an Internet forum for meteor observers.

On Oct. 27, 1997, I posted a message which indicated that the orbit of the comet was probably too far removed from the Earth?s to create any meteors. The two orbits are currently separated by 6.2 million miles (10 million km).  

Like other comets, Hartley 2 leaves a dusty trail of material in its wake. This trail might widen to perhaps a million miles or more in width after it has circled the sun a few times, but in the case of Hartley 2, this still amounts to only a fraction of the present distance between the orbits of the earth and the comet. 

So at first glance it doesn?t appear that any prospective meteor stream from comet Hartley 2 would be wide enough to even reach the vicinity of Earth.

What if there IS a meteor shower?

Let?s play devil?s advocate for a moment and assume an observable meteor shower from Comet Hartley 2 is a lock.

Because Hartley 2 is a member of Jupiter's comet family, it periodically passes close enough to that giant planet to have its orbit perturbed by Jupiter's gravitation. In fact, my own calculations show that over the past 40 years, Jupiter has notably altered the orbit of Hartley 2 at least three times: in Dec. 1993, Dec. 1982 and May 1971. 

And during the years 1985 and 1991, the distance separating the Earth?s orbit from the orbit of the comet was less than half what it is now. The dust that was shed by Hartley 2 during those years likely has orbited the sun three or four times and now just might be passing close enough to our orbit to perhapsencounter the Earth and produce meteors. 

As it turns out, Earth is rapidly approaching that part of its orbit where we will be passing closest to the orbit of Comet Hartley 2. 

We will, in fact, be nearest on the nights between Nov. 1 and Nov. 3. More interestingly, the source of any prospective meteors ? Comet Hartley 2 itself ? will have passed through this region of space just a week before Earth's own arrival. 

Since the greatest concentration of dusty material usually is in the general vicinity of a comet, the Nov. 1 to Nov. 3 timeframe offers skywatchers the best chance of catching a view of a Hartley-id meteor.

Some observing tips

If you want to see if there might be any meteor activity that can be traced back to Comet Hartley 2, here are a few tips: 

First, any meteors belonging to the comet will appear to emanate from out of the constellation of Cygnus, the Swan.

At this time of year, Cygnus is almost directly overhead as darkness falls and does not set below the northwest horizon until around 3 a.m. local time. So, it will be in view for much of the night.  

In addition, during the key Nov. 1 to Nov. 3 interval, the moon will be a waning crescent and won?t rise until after 3 a.m. local time. So it will offer no interference.

Also, because Comet Hartley 2 and the Earth are traveling around the sun in the same direction, any Hartley-id meteor would appear to move very slowly across the sky. 

Their entry speed into our atmosphere would be equal to only 7 miles per second (11.3 kps); about as slow any meteor can be. Contrast this to the annual November Leonid meteor shower, whose particles are moving in a direction opposite to the Earth and hit our atmosphere head-on at 45 miles per second (72.4 kps).

Leonid meteors usually flash across our line of sight in less than a second, but a Hartley-id meteor may take many seconds to pass across the sky. Such slow meteors often display hues of red or yellow orange in contrast to the white or blue-greens that the Leonids are known for. 

And lastly, keep your expectations low. 

Don't expect to step outside and immediately see meteors. Since the Earth may literally be right on the edge of the comet's dust trail, consider yourself fortunate if you see any Hartley-ids at all.

In fact, Russian astronomer, Mikhail Maslov has made his own calculations for possible Hartley-id meteors in 2010. He concluded that "activity is not expected." 

NASA?s Cooke is also skeptical.

"It's probably going to be a non-event," Cooke said.

But then again, as the old saying goes, "nothing ventured, nothing gained." 

"This sets up nicely," Cooke added.  "Cool weather, waning moon, and the possibility of seeing a new shower.  I will definitely be out observing if the weather cooperates."

How about you?

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.