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70% of all catalogued objects are in low-Earth orbit (LEO), which extends to 2000 km (1200 miles) above the Earth's surface. To observe Earth, spacecraft must orbit at such a low altitude. The spatial density of objects increases at high latitudes. Note: The debris field shown in the image is an artist's impression based on actual data. However, the debris objects are shown at an exaggerated size to make them visible at the scale shown. European Space Agency
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Space Junk Danger
What You Should Know About 'Space Junk'
by
Carolyn Collins Petersen
In
the movie Gravity, a group of
astronauts finds out first-hand what it might be like for space explorers
to run into an orbiting bit of space debris.
The
results are not good, although at least one astronaut makes it through
safely.
Although
the movie has generated a lot of discussion among space experts about its
accuracy in some places, it highlights a growing problem that we don't often
think about here on Earth (and probably should) -- space junk returning home.
WHAT
GOES UP OFTEN COMES DOWN
There's a cloud of space debris
around Earth. Most of it eventually comes back to Earth, such as the object
WTF1190F, piece of hardware likely dating back to the Apollo mission days.
Its
return to Earth on November 13, 2015 can tell scientists a lot about what
happens as material plunges through our atmosphere (and "burns up" on
the way down).
It's particularly important for people in the space launch
business because there are nearly 20,000 pieces of space debris up there.
Most
of it ranges from such small objects as gloves and cameras to pieces
of rockets and artificial satellites.
There's
enough "stuff" up there to pose a real danger to such objects as Hubble Space Telescope and weather and communications
satellites, as well as those of us on Earth.
That's
the bad news. The good news, for us on Earth at least, the chances of something
hitting us on land are fairly small.
It's far more likely that a
piece of space debris will fall into the oceans, or at least into an
unhabitated part of a continent.
To keep launch vehicles and orbiting satellites from running
into these bits of space junk, organizations such as the North American
Aerospace Defense Command (NORAD) observes and maintains a list of known
objects orbiting Earth.
Before every launch (and as satellites orbit the globe), the positions of all
known debris must be known so that the launches and orbits can proceed without
damage.
THE ATMOSPHERE CAN BE A
DRAG (AND THAT'S GOOD!)
Pieces of junk in orbit can
and do get caught up in our planet's atmosphere, just as meteoroids do. That
slows them down, in a process called "atmospheric drag".
If
we're lucky, and a piece of orbital debris is small enough, it will likely
vaporize as it falls to Earth under the tug of our planet's gravity.
(This
is exactly what happens to meteoroids when they encounter our atmosphere
and the resulting flare of light we see as they vaporize is called a meteor.
Earth
regularly encounters streams of meteoroids, and when it does, we often see
meteor showers.)
But,
larger pieces of space junk can pose a threat to folks on Earth as
well as get in the way or orbiting stations and satellites.
Earth's atmosphere is not the same "size" all the
time.
For
example, scientists need to know how the density of the atmosphere changes over
time in the low-Earth orbit (LEO) zone.
That's
an area several hundred miles above the surface of our planet where most
orbiting materials (including satellites and the International Space Station)
exist.
THE SUN PLAYS A ROLE IN SPACE JUNK RE-ENTRY
Heating by the Sun helps
"swell" our atmosphere, and waves propagating from lower in the
atmosphere can also have an effect.
But,
there are other events that affect our atmosphere and could have the effect of
catapulting larger objects toward Earth's surface.
Occasional
solar storms cause the upper atmosphere to expand.
These
erratic solar storms (caused by coronal mass ejections) can zip from the Sun
toward Earth in less than two days, and they produce rapid changes in air
density.
Again, most space "junk" falling to Earth can and does
vaporize on the way down. But, larger pieces can land and cause damage on our
planet.
Imagine
being in the neighborhood if a large piece of a defunct satellite fell on your
house?
Or,
imagine what would happen if a large solar storm resulted in enough atmospheric
drag to pull a working satellite (or a space station) into a lower and
more potentially dangerous orbit?
It would not be good news for
the satellite operators or the astronauts working aboard the International Space Station.
The U.S. Air Force (which is involved with NORAD), and the U.S.
National Center for Atmospheric Research (NCAR),
the University of Colorado at Boulder, and the U.S. National Oceanic and
Atmospheric Administration Space Weather Prediction Center work together to forecast space
weather events and the effects they have on our atmosphere.
Understanding
those events will help us all in the long run by understanding the same effects
on the orbits of space junk.
Ultimately,
the junk trackers will be able to forecast more accurate orbits and
trajectories of space debris in near-Earth space.
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