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A
comet is a celestial body that is trapped in orbit
around
something large. When close to the sun they
have
tails. The tail is material from the frozen comet
that
is released from the comet when the sun’s energy
heats
it. But the tail doesn’t follow the comet. It
always
points away from the sun – blown by the solar
wind.
Comet 67P/C-G’s tail will be 100,000 miles long
when
it is closest to the Sun next year.
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The comet, named 67P/Churyumov–Gerasimenko, or 67P/C-G for
short, has a period of 6.6 years. Every 2/3 of a decade or so, it travels in
from deep space to make a trip around the sun. For this particular trip, there was a probe
waiting on it, the Rosetta orbiter. After going into orbit around 67P/C-G,
Rosetta is now set to release Philae to drop down and land on the comet's
surface.
But how did Rosetta meet the comet? It took a lot of doing.
Comet 67P/C-G is traveling at a steady rate of 24,600 mph (39,589 kph). That
means it travels from NYC to Boston in 30 seconds. Let’s see the planned NYC-Boston maglev train match that!
When we send rockets into space to meet the International
space station, they only have to travel 17,000 mph (27,358 kph). The ability to
reach this higher speed, just to get into the orbit of the comet could be
achieved two ways.
One - you could put all the fuel on board the orbiter and thrust yourself to the proper speed. But that would require a space ship the size
of a football field. You just can’t get all that fuel into space for a price
anyone could pay. So that’s out.
Second - you could use the power of gravity. By launching
Rosetta out into space and then having it swoop by a planet, it can use the
gravity of the planet to pull it to a higher speed and then curve around the
planet and get shot out the other side. Every time you do this, you gain some
speed.
Rosetta achieved its amazing pace by three separate Earth
gravity assists and one Mars gravity assist. It has
traveled over 3.8 billion miles just to gain enough speed so it can sneak up
behind the comet. And how long did this take? Rosetta/Philae were launched in
2004! And my kids have a hard time planning for a paper due in two weeks.
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You
can see the gravity assists that Rosetta used to catch
up
with 67P. Also notice that Rosetta went into
hibernation
for three years. It’s instruments run on solar
power,
but it was 500 million miles from the sun. Out
there
you only get about 4% of the sunlight we get here on Earth.
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After all this chasing, now Philae is ready to separate from
Rosetta and land on 67P/C-G. This is where the luck comes in. Once Philae
leaves Rosetta, there is no controlling it’s path. The plan, based on thousands
of photographs taken of 67P/C-G’s surface by Rosetta while in orbit, is to land on
Philae somewhere flat and sunny.
Flat is obvious, the comet’s surface is mostly
irregular, with huge boulders and deep crevices; finding a good parking spot on 67P is
harder than the week before Christmas at the mall. Flat is also important so
Philae can get a good grip. Gravity is so low on 67P that there is a fear that
Philae will just bounce off the surface, even though it will be moving at a
pace similar to a slow walk.
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The
ESA had a naming contest for the landing spot for
Philae,
shown above. The winner – Aglilkia. Suggested by
150
people, Agilkia is an island in the Nile River where
several
temples were moved when the Nile Valley was
flooded
by the Aswan dams. The Island they were moved
from
– Philae. The numbers show the diameters of the
boulders
to give it some scale.
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As far as sunny is concerned, the sun is needed to charge
the instruments on Philae. If they land in the shade, she’s only going to
be functional for about 60 hours. With a good sunny spot, she might work for up
to 6 months.
The question you’re now asking is, “Work to do what?” Why
spend all this money and time to land on a comet? In response, I ask if you care
where your parents came from, or their parents. Go back far enough and you have
to wonder where life on Earth came from. Did life start here on its own or was
it brought here?
Philae may help answer these questions. Comets are rock,
ice, and who knows what else. Water is needed for all life that we know about,
so did comets bring water to Earth? Is there something alive in the ice, or are
the building blocks for life present on that comet? Philae has nine instruments
to help answer these questions.
If Philae finds organic molecules, then we better start
preparing a list of questions for our neighbors, because that finding will almost assure
us that they’re out there. And it may mean more -chirality in organic molecules
is important here on Earth.
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Amino
acids in Earth based life are all left-handed (called
L).
Their mirror image is the D-amino acid, but they don’t
work
the same way in proteins. D- amino acids would
make
a protein fold differently, and the way a protein
folds
determines its function. If there are
amino acids of
67P/C-G,
will they be L- or D-?
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If they are opposite handed, then Star Trek got it wrong with all those humanoid aliens, and we may be for some real surprises in the future.
A 2012 paper explains how one of the instruments on Philae
is designed just for chirality question. It will determine the handedness of any
organic molecules found on 67P/C-G. After all, it is important to find Grandma,
and 67P/C-G might be her house, or at least her hulking old sedan with the 35
gallon gas tank and steering wheel as big as a hubcap.
Comets are remnants from when the solar system was young;
they are where we were, and we need to know them in order to get a better idea
of where we should be going. I personally am very excited to find out if we are
alone in the universe – that would be so sad. Isn't that important enough for a 3.8 billion mile road trip?
Contributed by Mark E. Lasbury, MS, MSEd, PhD
As Many Exceptions As Rules
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