Ed Lu Letter from space #11

Dark Side of the Earth

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!

- Ed Lu -

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Jacques :-)

Editor: www.spacepatches.info

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