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The Laser Interferometer Space Antenna (LISA) has the most technically challenging goal of NASA's three proposed Beyond Einstein missions: directly detect subtle ripples in the fabric of space-time known as gravitational waves.
These waves are a direct prediction of Einstein's General Theory of Relativity. They are triggered by cataclysmic events that rock the fabric of space-time, such as the collision of two monster black holes. Gravitational waves are somewhat akin to undulating waves on the surface of a pond. But unlike water waves, gravitational waves stretch and compress space-time itself, causing the distance between two objects to alternately grow and shrink.
Near their source, these waves are so powerful they could tear a human body to shreds. But as they cross the immense depths of space to reach Earth, they weaken to nearly imperceptible levels.
Proving the existence of gravitational waves is itself an enormous technological challenge. Even Einstein thought they might be impossible to detect. Numerous experiments have failed to bag this elusive quarry. Gravitational waves have only been detected indirectly by radio astronomers, who have measured the orbital shrinkage of neutron stars in binary systems—a loss of orbital energy attributed to gravitational waves.
LISA’s lofty ambition is not just to detect gravitational waves, but to measure their strength and polarization. LISA will be unlike any space mission ever flown. NASA will launch three identical free-floating spacecraft into solar orbit. The three craft will be guided into a formation shaped like a nearly perfect equilateral triangle, with each craft being separated from the others by about 3 million miles (5 million kilometers).
Over LISA’s five-year planned mission, passing gravitational waves will alter the distances between the three craft by amounts as small as 10-12 meter (1 picometer), 100 times smaller than an atom! This might seem impossible, but LISA's method for detecting these waves, laser interferometry, offers such exceptionally high precision that the array will actually be able to measure these minuscule shifts.
Detecting these waves is simple in principle, but extremely difficult in practice. For LISA to succeed, scientists and engineers will have to push the envelope in three core technologies—laser interferometry, gravitational reference sensors, and micronewton thrusters—to weed out disturbances that mask the feeble signatures of gravitational waves.
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