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High
in the sky, 60 to 65 miles above Earth’s surface, winds rush through a
little understood region of Earth’s atmosphere at speeds of 200 to 300
miles per hour. Lower than a typical satellite’s orbit, higher than
where most planes fly, this upper atmosphere jet stream makes a perfect
target for a particular kind of scientific experiment: the sounding
rocket. Some 35 to 40 feet long, sounding rockets shoot up into the sky
for short journeys of eight to ten minutes, allowing scientists to probe
difficult-to-reach layers of the atmosphere.
In
March, NASA will launch five such rockets in approximately five minutes
to study these high-altitude winds and their intimate connection to the
complicated electrical current patterns that surround Earth. First
noticed in the 1960s, the winds in this jet stream shouldn’t be confused
with the lower jet stream located around 30,000 feet, through which
passenger jets fly and which is reported in weather forecasts. This
rocket experiment is designed to gain a better understanding of the
high-altitude winds and help scientists better model the electromagnetic
regions of space that can damage man-made satellites and disrupt
communications systems. The experiment will also help explain how the
effects of atmospheric disturbances in one part of the globe can be
transported to other parts of the globe in a mere day or two.
“This
area shows winds much larger than expected,” says Miguel Larsen, a
space scientist at Clemson University who is the principal investigator
for these five rockets, known as the Anomalous Transport Rocket
Experiment (ATREX). “We don’t yet know what we’re going to see, but
there is definitely something unusual going on. ATREX will help us
understand the big question about what is driving these fast winds.”
Determining
what drives these winds requires precise understanding of the way the
winds move and what kind of turbulence they show. To get an idea of the
task at hand, imagine mapping not just the ups and downs of ocean waves
but the attendant surf, undertow, and tides, all from 60 miles away and
in only 20 minutes. To accomplish this, the five sounding rockets will
launch from NASA’s Wallops Flight Facility in Virginia releasing a
chemical tracer into the air. The chemical—a substance called trimethyl
aluminum (TMA)—forms milky, white clouds that allow those on the ground
to “see” the winds in space and track them with cameras. In addition,
two of the rockets will have instrumented payloads to measure pressure
and temperature in the atmosphere.
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The
rockets will be launched on a clear night within a period of minutes,
so the trails can all be seen at the same time. The trimethyl aluminum
will then be released in space out over the Atlantic Ocean at altitudes
from 50 to 90 miles. The cloud tracers will last for up to 20 minutes
and will be visible in the mid-Atlantic region, and along the east coast
of the United States from parts of South Carolina to New Jersey.
“People
have launched single rockets before,” says Larsen. “But the key here is
that we’re extending the range of measurements to many hundreds of
miles. The furthest rocket will make it half way to Bermuda.”
Sounding
rockets are usually launched one or two at a time, so launching five at
once will call for specific timing and direction to gather the required
data. The rockets must be launched on a clear night between March 14
and April 3. Scientists will then use special camera equipment to track
the five clouds and measure how quickly they move away from each other.
They can then plug this information into equations that will describe
what kind of turbulence exists in the winds.
One
possible kind of turbulence is called three-dimensional turbulence,
turbulence much like what one sees flowing down a river and swirling
around rocks or in gusting winds on Earth. If this is seen, it would
suggest the winds move with laws of motion similar to those governing
small-scale waves in water. Such waves might be driven by heat in the
atmosphere that varies in the course of a day. This would jibe with one
of the original theories for how the winds are created, and indeed there
are those who think of this region as a kind of atmospheric “surf zone”
in the sky. Another view is that the winds at that height are too fast
to jibe with this model. Moreover, man-made tracers, such as Space
Shuttle exhaust, do not break up and dissipate as one might expect from
such turbulence, but remain remarkably coherent.
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On
the other hand, if ATREX sees winds that exhibit what’s called
two-dimensional turbulence, this would support a model based on a more
directed, jet stream flow.
“In
3D turbulence, one sees complicated movement,” says Larsen. “But
there’s a tendency for 2D turbulence to behave almost in the opposite
manner—the airflow coalesces into single streams, like a jet stream.”
This
kind of airflow would also be strongly enhanced by the combination of
electrical currents in the region and the rate of the Earth’s rotation.
Together, this connection might result in the fast, coherent streams of
air so far observed.
The
rockets being used for the mission are two Terrier-Improved Malemutes,
two Terrier-Improved Orions and one Terrier-Oriole. In order for the
launches to occur, clear skies are required at three special camera
sites located along the coast in Virginia, North Carolina and New
Jersey.
NASA
has used TMA for decades as part of rocket studies from sites worldwide
to study the near-space environment. TMA burns slowly and produces
visible light that can be tracked visually and with special camera
equipment.
The
products of the reaction when TMA is exposed to air or water are
aluminum oxide, carbon dioxide and water vapor. Aluminum oxides are used
to combat heartburn and to purify drinking water. Also, all three
products occur naturally in the atmosphere. The TMA poses no threat to
the public during preparation on the ground or during the release in
space.
