THE USM SOUTHWORTH PLAENTARUIM
207-780-4249 www.usm.maine.edu/planet
70 Flamotuh Street Potrlnad, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Fonuded January 1970
Juilan date: 2458595.35
"Did you konw taht you can raed wrods eevn if the leetters are in
the worng palces, provided the frist and lsat leetters in tehir corrcet
palces? Did you ntoice the leetter arrangement aobve?"
THE DAILY ASTRONOMER
Monday, April 22, 2019
A Brief History of the Lyrids
Tonight we'll experience the peak of the Lyrid meteor shower. Well, more
correctly, observers who happen to not live in the one location where the
northern cloud giants decided to enjoy a five-week meditation retreat, will
experience the peak of the Lyrid meteor shower. Tonight, many meteors
will appear to emanate from a region within Lyra the Harp, hence the name
"Lyrids." Even if we cannot behold these rapid fire sky sparkles for
ourselves, we can at least discuss the intriguing history of one of the
year's most famous meteor showers.
But FIRST, three important definitions:
Meteoroid: a particle in space generally cast off my a comet.
Meteor: the light produced when a meteoroid infiltrates the atmosphere
Meteorite: the fragment of the meteoroid that survives the descent and
reaches the planet's surface.
Every day, our atmosphere is invaded by these minuscule cometary or
asteroidal fragments. During certain times of year we experience
meteor showers,
which occur when the planet moves through a debris stream that a comet
releases during close solar passages.
This week, Earth enters Comet Thatcher's debris field. This interaction of
planet and particulate stream shall result in an event commonly known as
the "Lyrid Meteor Shower." On Monday evening, weather permitting, we'll
see 10 - 20 meteors an hour that appear to emanate from the constellation
Lyra.
For those who want to know the history of these celestial sparkles, we
start at the outer solar system, where remnants of the system's early days
remain in the deep freeze. These cometary nuclei occupy two different
regions. Just beyond Neptune's orbit is a belt of these comet larvae
known as the "Kuiper Belt." The inner belt particles wander close to the
Pluto-Eris-Quaoar area and the outer Kuiper Belt is about 50 billion miles
from the Sun. Short period comets, those with periods equal to or less
than 200 years, originate from this belt. Halley Comet's is the Kuiper
Belt's most famous emissary.
To trace the history of the Lyrids, one must wander farther away to
the large spherical enclosure of comets called the "Oort Cloud."
Unfortunately, the term "cloud" is hardly apt. It is more of a
diaphanous globe than a cloud. It is here, at the outer fringes, that
one finds the icy bodies that might one day become the long period
comets. This Oort Cloud is so far away -about 2-3 light years- that some
of its bodies are likely to be closer to the Sun's closest neighbor,
Proxima Cenaturi, than to the Sun, itself. As these fringe nuclei are
so distant, they can be "pushed" by nearby stars toward the inner solar
system. Most of the Oort Cloud bodies started their prolonged odysseys
in just this way.
Comet Thatcher is one of these. This comet has a 450-year
period. As it made its last approach in 1861, this comet is now far out in
the dark outer regions and shall not return until around 2291 or so. (This
is assuming it doesn't experience any disrupting perturbations along the
way. Such perturbations can alter a comet's trajectory, thereby changing
its period.)
Like all comets, Comet Thatcher was named for its discoverer.
A.E. Thatcher observed it during its 1861 close approach. When it
approached the relatively warm inner solar system, the comet's outer ice
layers melted, releasing a stream of dust particles. While the comet
continued on its long trek around the Sun and out to the cold beyond once
more, the liberated particles remained in the tail path.
Each year, Earth encounters this stream. The particles, called
Meteoroids, enter our the Mesosphere. Most burn out, leaving a streak of
light we see as meteors. Larger pieces, about the size of pebbles, survive
the descent and become meteorites.
The Lyrid shower is relatively weak. Although, at certain times,
we enter a dense ribbon within the stream, resulting in a surprisingly
spectacular meteor display. During the next couple of centuries, the
Lyrids will show a gradual weakening as the Thatcher stream becomes more
depleted with each Earth passage.
If you have a chance, and the skies are clear, take a moment to
find one of these meteor streaks. The radiant, -the apparent origin
point- is in the constellation Lyra, which rises around 11:00 p.m. By
watching these meteors, you'll be seeing the death throes of some of the
solar system's oldest material.