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The extreme feebleness of gravitational waves poses an enormous technical challenge for LISA. The waves themselves will move the spacecraft's sensors by a distance smaller than the size of an atom.
But space is hardly a pristine environment. A number of forces will affect the separation of the three spacecraft, such as the pressure of sunlight and the Sun's variable magnetic field. The gravity of the Earth and Moon will also disturb the spacecraft. These motions are much larger than those caused by gravitational waves. How can LISA weed out these motions to sense any gravitational waves that happen to be passing through?
That's the job of LISA's gravitational reference sensors (GRS), which are being developed by the European Space Agency. ESA is planning to test these sensors in an early 2010 mission known as LISA Pathfinder.
Each of the three LISA spacecraft will carry one GRS. These sensors work in tandem with the test mass—a metal cube that would fit snugly inside a Chinese restaurant take-out carton. LISA's job is to "sense" the relative motion of these test masses due to gravitational waves.
Each spacecraft's GRS will continuously monitor the distance between itself and the test mass to an amazing precision of 10-9 meter (1 nanometer)—about 1/10,000th the width of a human hair! The sensors will stabilize the spacecraft’s motion with respect to the test mass, which will enable LISA to sort out unwanted background motions in space.
If a GRS detects that a test mass has shifted relative to the spacecraft, it will command micronewton thrusters to compensate by pushing the spacecraft in the opposite direction. Together, LISA's gravitational reference sensors and >thrusters will be 10,000 times less "noisy" than any previous system that has flown in space. Thus the GRS technology will keep the test masses free from unwanted background forces, so that they will be ready to move in response to a gravitational wave.
This sophisticated technology will allow LISA to detect gravitational waves with wavelengths of millions and even billions of miles. Ground-based experiments have no hope of ever "hearing" such low-frequency rumbles because our planet is buzzing in a cacophony of ground movements from earthquakes, trucks, falling trees, and other sources of low-level tremors. These motions drown out low-frequency gravitational waves.
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