The Upper Atmosphere Research Satellite (UARS) was launched in 1991 to study the ozone layer. It collected data that showed levels of ozone-depleting chemicals had stabilized by 2005, when UARS was decommissioned after exceeding its planned 3-year mission lifetime. As a satellite's orbit decays due to drag from solar wind and atmosphere, it is difficult to predict exactly where it will re-enter and debris will land, within a margin of about 4,280 miles, due to uncertainties from atmospheric effects.

1. 10/3/11

2. The UARS Satellite
• The Upper
Atmosphere
Research Satellite
(UARS) was
launched in 1991. Its
mission was to study
what at the time was
termed the “ozone
hole” in the upper
atmosphere.

3. The UARS Satellite
• To learn more about
the UARS Satellite,
go to the NASA
archive and search
for UARS.
Go to
http://www.nasa.gov/multimedia/vid
Search for: UARS.

4. The UARS Satellite
• As a result of the
data obtained from
the UARS, global
levels of CFCs have
reached a plateau
since the satellite’s
launch.
Launch of
UARS

5. The UARS Satellite
• The UARS was
decommissioned in
2005. Since it was
originally scheduled for a
three-year mission, it
outlived its mission by
over a decade.
• When decommissioned
a satellite is allowed to
continue its orbit, but
with no further
intervention.

6. The UARS Satellite
• We think of space travel
as movement through a
vacuum. If this were
really the case, then
given enough energy, a
satellite would continue
orbiting the Earth
indefinitely.

7. The UARS Satellite
• But a satellite, like any
other spacecraft, is
subject to solar wind,
which is made up of
subatomic particles and
photons. Like the wind
that can slow down an
airplane, solar wind acts
as a drag on the motion
of a satellite, slowing it
down.

8. The UARS Satellite
• As the satellite slows
down, its orbit around
the Earth shrinks. The is
an example of a
decaying orbit.

9. The UARS Satellite
• If solar wind was a drag
on the satellite, even
more dramatic changes
occur when the satellite
enters the Earth’s
atmosphere. Here, a
much more dense
collection of gases and
other particles not only
slow down the satellite
but eventually cause it to
break apart.

10. The UARS Satellite
• This is where it becomes
difficult to predict where
the satellite will
ultimately land. There
are two main reasons for
this. Let’s look at the
first.

11. The UARS Satellite
• There are a number of
detectors around the
Earth. As the satellite
passes each detector,
its expected path past
the next detector is
determined. When the
satellite is not
detected, that means
it descended
somewhere between
the two detectors. So,
there is a margin of
error introduced.

12. The UARS Satellite
• The second reason
that it becomes
difficult to predict the
satellite’s descent is
the Earth’s
atmosphere, which
changes the path of
the satellite. Imagine
throwing a paper
airplane past a fan.
You cannot accurately
predict where the
airplane will land
because of the
deflection from the
fan.

13. The UARS Satellite
• As a result, there is a
degree of uncertainty
about where exactly a
satellite’s begins its
descent. Let’s
estimate that this
uncertainty is 15
minutes in duration.
Let’s also assume that
the satellite is
traveling at a speed of
17,000 mph.

14. The UARS Satellite
• Now let’s assume that
the satellite is at an
altitude of 100 miles
when it begins its
descent. We can use
this freefall function to
determine the range
of where the the
satellite debris will
land. (All distance
units shown are in
feet.)

15. The UARS Satellite
• Solve this quadratic
equation for t to find
out the time the
satellite will be in the
air before it comes to
a crashing halt.

16. The UARS Satellite
• Use this value for t to
find the horizontal
displacement of the
satellite. One it enters
the atmosphere its
speed is no longer
17,000 mph. Assume
a horizontal speed of
600 mph.

17. The UARS Satellite
• So the total range of
where the satellite can
land has increased to
4,280 miles.