10 August 2013
The Hairless Star
Imagine if you will a mountain far larger than Everest or K2. Now imagine that this mountain is made of ice;
ice with a leavening of stony debris and dust—gravel and particles of
fine grit. Color it the darkest shade of charcoal, with perhaps a slight
reddening, and then lift it into the sky. If your imagination can reach,
place it far from earth—out in the vast spaces beyond the planets, on a
curving path that takes it even farther from the faded Sun, a path that will
not return it to the warmth of the inner Solar System until the year 2125.
This is Comet 109P/Swift-Tuttle*. Too far from the
heat and light-pressure of the Sun to display tail or even the dusty crown of coma,
109P tumbles through space on the outbound leg of its 133 year-long orbit,
naked, dark, invisible against the background stars. Darkened by
uncounted centuries of exposure to Sol’s ultraviolet radiation, it has
long-since been lost to sight by Earth-bound observers with even the greatest
of telescopes. Farther and farther from the warm center of our solar
neighborhood, Comet 109P journeys alone. But not forgotten.
Yesterday’s Comet
Comet Swift-Tuttle was probably observed by Chinese
astronomers in the first century BC, and certainly it has appeared many times
in our skies over the centuries, but it acquired its hyphenated name in July 1862,
when independently discovered by Lewis Swift and Horace Parnell Tuttle. It’s
orbit was calculated and soon researchers realized that this “hairy star”
had appeared in the heavens before.
Comet 109P is also part of my own history; in the early
1990s I was able to observe its passage through our skies with my portable telescope
and binoculars, and so I have a small but definite proprietary feeling
regarding this frozen visitor from afar. It is one of the few dozen
comets I have been fortunate enough to view over the forty+ years I’ve been
stargazing, beginning with Comet Kohoutek in the early 1970s.
I’ll never see the bloom of coma, the fan-shaped tail
of Swift-Tuttle again; my own journey through life is quite short when compared
to that of this lonely voyager. But I will see the orphaned children of
109P, and soon.
The Dusty Trail
Like all active comets, Swift-Tuttle leaves behind pieces of
itself as it travels its long orbital track around the Sun. In the heat
and pressure from our star the crust of the comet loses infinitesimal portions
of its mass; sublimation of its shell of ice from a solid to gaseous state
results in a slow dispersal of its solid mass along the portion of its orbit
that crosses the inner solar system, dust and ice particles slowly spreading
out along its track and following in the wake of the parent body with only the
tiniest of differences in velocity. Comet 109P has been pursuing nearly
the same orbit—with only occasional perturbations caused by the gravities
of planets passed along the way—that this slow process has spread the lost
mass along the entirety of Swift-Tuttle’s orbit.
Imagine this—not only is the 26-kilometer-across mass
of 109P continuing faithfully to follow this well-defined pathway around the
Sun, but that same pathway is also inhabited by the debris that the comet has
generated over the countless millennia! The individual orbits of the
billions upon billions of particles of dust and icy residue vary slightly, so
there is some spreading-out; some are lost forever through perturbation by the
giant planets, others simply wander away due to miniscule differences in momentum,
and many, many are channeled by gravitational variations into twisted ribbons
of matter, invisible streams of dust and ice. Unseen they rush through
space, in general following the orbit of their parent body. These
particles are known to science as Meteoroids.
Tonight’s Meteors
Comet Swift-Tuttle’s orbit is an ellipse that drives
deep into the solar neighborhood, and compared to the open vastness of the
outer system our warm central region, close to the Sun, is a crowded
superhighway of planets and asteroids; it is remotely possible that 109Ps orbit
will intersect that of one or more of these bodies at some point. And
when it does…
Now we come to the most wonderful co-incidence of our story;
the intersection of one of those ribbons of dust and ice—a meteoroid “stream”—with
a planet near and dear to us--Earth. For the past few weeks our world has
been driving deeper into the spread-out debris of Comet 109P, being struck by
more and more particles over recent nights, and as I write this we near the
central, most dense portion of Swift-Tuttles’ stream. Because of
the same effect of relative motion that makes snowflakes seem to streak toward
your windshield from a certain point in the distance, this stream of debris
appears to drive toward us from the direction of the constellation Perseus; for
this reason we call this collision of planet and meteoroid stream the “Perseid”
meteor shower.
When the particles of space debris dive into our planet’s
atmosphere they are known as “meteors”; striking the thin upper air
at incredibly high speeds they begin to slow, transferring their kinetic energy
they heat the surrounding molecules of gas to incandescence. This is the
bright streak you see—a “shooting star”. The Meteoroid
is not “burning up” as much as breaking-up; the increasing air
pressure overcomes the objects tensile strength as it shatters into smaller and
smaller pieces until—in most cases—only the finest dust remains to
sift through the lower atmosphere to the ground. The rare fragment that
makes it intact to the ground is known as a Meteorite.
The Sons of Perseus
The Perseids are one of the years’ most impressive and
reliable meteor showers—the “old faithful” of annual
celestial fireworks shows. Each year, beginning in the last week of July
and strengthening into the second week of August, the bombardment begins;
millions of tiny bits of matter striking our planet’s upper
atmosphere. Most are too faint for us to see; the vast majorities are
detectable only by the “hiss” of radio static to be heard on certain
frequencies. Many are seen as quick “shooting stars”, and a
tiny fraction—a few thousands—will light up the night sky and draw
appreciative gasps from all observers. To see a bright “fireball”,
drawing a green or yellow trail behind it, is one of the most sought-after
experiences of the meteor watcher.
The next two nights; 11-12 and 12-13 August, are forecast to
be the critical times to observe the Perseids; from a dark location with a wide
view of the sky it may be possible to observe 50 to 80 meteors an hour after
midnight, and even before that time there should still be a pretty impressive display.
I’d like to invite you all to go out under the August skies and meet the
wayward children of Comet 109P Swift-Tuttle; they’ve come a very, very
long way. Weather permitting, I’ll be out on deck to greet them.
Tom Epps
Operations Chief
USNS Alan Shepard
Persian Gulf
