Where the Heavenliest of Showers Come From
By Dr.
Ethan Siegel
You
might think that, so long as Earth can successfully dodge the paths
of rogue asteroids and comets that hurtle our way, it's going to be
smooth, unimpeded sailing in our annual orbit around the sun. But
the meteor showers that illuminate the night sky periodically
throughout the year not only put on spectacular shows for us,
they're direct evidence that interplanetary space isn't so empty
after all!
When
comets (or even asteroids) enter the inner solar system, they heat
up, develop tails, and experience much larger tidal forces than they
usually experience. Small pieces of the original object—often
multiple kilometers in diameter—break off with each pass near the
sun, continuing in an almost identical orbit, either slightly
ahead-or-behind the object's main nucleus. While both the dust and
ion tails are blown well off of the main orbit, the small pieces
that break off are stretched, over time, into a diffuse ellipse
following the same orbit as the comet or asteroid it arose from. And
each time the Earth crosses the path of that orbit, the potential
for a meteor shower is there, even after the parent comet or
asteroid is completely gone!
This
relationship was first uncovered by the British astronomer John
Couch Adams, who found that the Leonid dust trail must have an
orbital period of 33.25 years, and that the contemporaneously
discovered comet Tempel-Tuttle shared its orbit. The most famous
meteor showers in the night sky all have parent bodies identified
with them, including the Lyrids (comet Thatcher), the Perseids
(comet Swift-Tuttle), and what promises to be the best meteor shower
of 2014: the Geminids (asteroid 3200 Phaethon). With an orbit of
only 1.4 years, the Geminids have increased in strength since
they first appeared in the mid-1800s, from only 10-to-20 meteors per
hour up to more than 100 per hour at their peak today! Your
best bet to catch the most is the night of December 13th, when they
ought to be at maximum, before the Moon rises at about midnight.
The
cometary (or asteroidal) dust density is always greatest around the
parent body itself, so whenever it enters the inner solar system and
the Earth passes near to it, there's a chance for a meteor storm,
where observers at dark sky sites might see thousands of
meteors an hour! The Leonids are well known for this, having
presented spectacular shows in 1833, 1866, 1966 and a longer-period
storm in the years 1998-2002. No meteor storms are anticipated for
the immediate future, but the heavenliest of showers will continue
to delight skywatchers for all the foreseeable years to come!
What’s
the best way to see a meteor shower? Check out this article to find
out:
http://www.nasa.gov/jpl/asteroids/best-meteor-showers.
Kids can
learn all about meteor showers at NASA’s Space Place:
http://spaceplace.nasa.gov/meteor-shower.


Image
credit: NASA / JPL-Caltech / W. Reach (SSC/Caltech), of Comet 73P/Schwassman-Wachmann
3, via NASA's Spitzer Space Telescope, 2006.