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Ed Lu's Latest Letter




 
 
 

Expedition 7: Home | EVA | Timelines | Experiments 
 
Earth From Space Images 

Dark Side of the Earth(For pictures see:)

http://www.station.nasa.gov/station/crew/exp7/luletters/lu_letterlatest.
html


Some of the most incredible sights you can see from up here are on the
dark side of the Earth, when the Sun doesn't dominate the sky. I like to
go down into the docking compartment and turn out all the lights and
watch the nighttime sky through the two portholes there. Just like at
home, if you are indoors looking out through the window when all the
lights are on inside, it is very hard to see the stars. Your eyes are
adjusted to the bright interior, and besides all you can usually see is
glare off the window. After turning off the lights, it takes a few
minutes for your eyes to adapt to the dark, and slowly the stars get
more and more distinct. These past couple of weeks the moon has been
close to a new moon, so without the light from the moon, the stars seem
even brighter. 

The view is something close to what you might see on a very dark
mountaintop on a very clear night. Only better. Our solar system is
located midway out from the center of the big disk of stars that is our
galaxy. When you look in the directions along the disk you see more
stars than if you look perpendicular to the disk. This is the Milky Way
- the line of stars, gas, and dust that cross the sky, and it is
incredibly distinct when viewed from here. We have a pair of image
stabilized binoculars (that work by suspending the prism in some sort of
fluid so it smoothes out the jitter), and through those I like to look
at the various nebula and star clusters you can see when looking towards
the center of our galaxy along the Milky Way. In truth, the view of all
these objects is better with a real telescope on the ground than with my
handheld binoculars, but there is something really cool about floating
in a spaceship looking at all the stars! 

It is fun to watch stars as they rise or set through the atmosphere as
we circle the Earth. They start to twinkle as the light rays bend while
passing through the uneven density of the atmosphere. Then, as they get
closer to the actual horizon, they start to look orange and then red
before blinking out. Sometimes they even turn green briefly. This is
just the same effect that makes sunsets look orange and red (if you
wonder why that is, it turns out that dust in the atmosphere scatters
blue light better, so when the blue color is taken out of something that
is white, it appears reddish). In fact, it is really just a star set -
the only difference with a regular sunset is that in this case the star
is much further away! Actually, astronomers sometimes use this technique
to study the atmospheres of other planets as they cross in front of
stars. 

The bright red dot of the planet Mars has been a great sight recently,
with Earth and Mars being very close now (relatively speaking). Here in
low Earth orbit, we aren't significantly closer than you are on the
ground to Mars, but without the atmosphere to look through it makes it
clearer and brighter. It is bright enough that even when we are on the
lit side of the Earth, and with all the lights on inside, it is clearly
visible against the black background of space. With our binoculars or
the high power lenses on the cameras you can see the disk of Mars, but
we don't have anything onboard with enough magnification to really see
the polar ice caps. 

Closer to home, whenever we pass south of Australia at nighttime we get
to see the green and orange curtains of the aurora. Since most of our
mission has been during the summertime in the northern hemisphere, we
haven't gotten to see much of the northern lights over Alaska since it
has been mostly in daylight. Lately though, as the seasons are changing
we are starting to get some nighttime views of the northern lights.
About two months ago I saw something interesting, and still unexplained,
when watching the aurora. We were south of Australia, and the sun had
just set. I was watching Mars rise up through the atmosphere as we flew
eastwards. The aurora was off to the right, and was fairly bright that
day. A couple minutes after Mars had cleared the upper part of the
atmosphere, I turned my attention to watching the aurora, when I saw a
flash of light amidst the auroral curtains. It was a small point, but it
was brighter than a typical star, maybe about the same as a 1st or 2nd
magnitude star. It lasted maybe a second or so. Then I saw another
flash, and then another - all together maybe five or six flashes over a
period of about a minute or two. I'd never heard of anyone describe such
flashes coming from the aurora, and in researching it further, I still
haven't! Perhaps we've discovered something new. But first we had to
rule out some other explanations. When the station crosses over the
day/night line, we are at an altitude where we are in sunlight for a few
minutes while the ground below is still dark. Small dust particles,
which are continually shed by the Station, scatter the sunlight and are
easily visible against the black background. You can see the same effect
when looking at a sunbeam shining in through a window - you can see all
the very tiny dust particles floating in the air. It turns out that some
of the tiny particles (mostly paint flecks) that come off the Station
are actually big enough so that they can look like a twinkling star as
they float away and rotate. So when looking out the window at these
times when we are in sunlight but the ground below is dark, you can
often see little bright specks slowly drifting away from the Station. We
were able to rule this out by finding out the exact time I saw the
flashes. Our navigation experts in Mission Control were able to work out
to the second when Mars rose above the horizon from our point of view,
and using that we were able to say that the sun had already set several
minutes earlier, so it is unlikely that I was seeing scattered sunlight
from dust particles. Next, we wanted to rule out lightning (which is
known to sometimes extend upwards - these are called sprites). I don't
remember seeing any nearby lightning storms, but just to be sure we
checked the weather maps and found that indeed the weather was clear. So
that leaves us with a mystery. I've been watching the aurora carefully
since then, but haven't seen this phenomenon again. 

Well, since I only have a month and a half left before we come home, I
better appreciate the view of the sky here while I still can!


This is a photo of the aurora. You can see parts of the Space Station
structure on the left.


This is the crescent moon just over the horizon - taken with a telephoto
lens. On this day there are rare high altitude clouds, known as
noctilucent clouds. These are the silver looking clouds you can see
above the orange color where most clouds are usually located. The dark
bands in the orange section are the tops of thunderstorms, which extend
up to about nine miles high. 
 



 
 





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