Recovery and Climate Experiment (GRACE)
National Aeronautics and
Goddard Space Flight Center
GRACE background information
For the Classroom: Modeling GRACE
The primary gravity measurement is made by recording changes
in the speed and distance between the two GRACE satellites. The two satellites
fly in formation over the Earth and the precise speed of each satellite and
the distance between them is constantly communicated via a microwave K-band
ranging instrument. As the gravitational field changes beneath the satellites
– correlating to changes in mass of the surface beneath - the orbital motion
of each satellite is changed. This change in orbital motion causes the distance
between the satellites to expand or contract and will be measured using the
K-band instrument. From this, the fluctuations in the Earth’s gravitational
field can be determined.
5-20 magnets of varying strengths, cardboard or foam board,
magnetic want, spring scale, two ring stands, rod, 2 clamps, grid of graph
paper marked in 5cm squares, and masking tape.
up two ring stands. Attach each end of the rod to a ring stand so it is
suspended between the two ring stands. Attach the spring scale from the
rod so the scale slides freely on the rod. Attach the magnetic want to the
spring scale and adjust the height so that the scales and wand hang freely.
Record the starting weight in Newtons.
the grid of graph paper to the top of the card or foam board.
the desktop, between the ring stands, place 5 to 20 magnets of varying strengths,
randomly in a space slightly smaller than the size of the foam board.
the card or foam board on top of the magnets and tape all around the sides.
graph paper to each student the same size of the card or foam board.
slide the spring scale/magnetic wand over each column of the graph paper.
For each grid, have students record observed changes in the spring scale.
Move the ring stand so the scale/magnetic wand covers the next column of
the foam board until the entire board has been mapped.
was the starting weight in Newtrons? This represents the mass with standard
each grid reading that is different from the starting weight, subtract this
standard gravity reading. Divide this number by the starting weight. This
represents the percent change in gravity. Negative values will represent
less gravity than the standard. Positive values show an increase in gravity.
does the model’s gravity increase? Decrease?
Earth, what types of things might account for an increase or decrease in
GRACE will obtain a gravity field map by looking at how the
Earth’s mass varies from place to place on the surface as the twin satellites
pass over. Mass and gravity are positively correlated-that is to san an increase
in mass relates to an increase in the gravitational force exerted. Mass is
also related to the density and amount of materials located in any one place.
four equal sized containers, fill the first with rocks, second with water,
third with plastic bag of air, and fourth with sand.
each container and record the weight.
Which container weighed the most? The least?
Based on the demonstrations would you expect the Earth’s gravitational
pull to be constant?
Describe geographic features on earth that will cause GRACE
to detect changes in gravity
Discuss with students
may read in pounds or kilograms but weight in the metric system is correctly
recorded in Newtons.
difference between acceleration due to gravity and the gravitational force.
between gravity field and magnetic field
models are used in science.
For more information on the ESSP Program, visit http://essp.gsfc.nasa.gov/
For more information on GRACE, visit http://www.csr.utexas.edu/grace/