Challenges | LISA Interferometry | Spacecraft Description | Instrument Description
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Artist's concept of LISA in space. A cover will protect the y-shaped science module centered inside each spacecraft.
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The Laser Interferometry Space Antenna (LISA) is the first dedicated space-based gravitational wave observatory. So far the only space searches for gravitational waves have been made using measurements of radio signals, not optimized for gravitational wave searches, from spacecraft on their way to other planets. LISA will use an advanced system of laser interferometry and the most delicate measuring instruments ever made to directly detect gravitational waves.
To accomplish its science objectives, the LISA mission depends on three core technologies: gravitational reference sensors, micronewton thrusters, and laser interferometry. These technologies will help to conteract the mission's biggest challenge——disturbances that mask gravitational waves. Because LISA detects gravitational waves by measuring the change in distance between freely floating test masses, sources of both external and internal disturbance need to be eliminated or damped down to extremely low levels——much lower than what is needed by other missions. By minimizing such disturbances, motions that would imitate or mask the effect of gravitational waves are less likely to occur.
Examples of external disturbances are the pressure from the light of the Sun and its very small variations, the variable solar magnetic field, and distortion of the LISA array by the gravitational effects of the Earth and Moon. To counteract the solar disturbances, the spacecraft structure will act as a shield to protect the test masses. And, LISA's orbit, 20 degrees behind the Earth's orbit of the Sun, will minimize the effects of the Earth's gravity.
Examples of internal disturbances are the interaction of the electrical field generated by the spacecraft computer acting on the test masses, effects from residual gas pressure near the test masses, and thermal radiation by the electrodes used to measure the spacecraft position. In order to minimize the effect of internal disturbances, the spacecraft must be controlled to follow the test masses with an accuracy of 10 nanometers, or about 1/100th of the wavelength of light! This spacecraft position-control operation, called "drag-free," is similar to the operation of low-Earth orbiting satellites that need to correct for the force due to the friction, or drag, of the Earth's atmosphere.
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Disturbances are conteracted by micronewton thrusters.
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To keep the test masses floating freely in space, the distance between the test masses and the surrounding spacecraft is constantly monitored by LISA's gravitational reference sensor. If this distance shifts, microthrusters fire to move the spacecraft back into position, away from the undisturbed test masses. Additionally, other specific design measures will counteract the possible disturbances. By minimizing external and internal disturbances, LISA will be able to detect passing gravitational waves in low-frequency bands not previously possible.
Another challenge is to do the interferometry at the required precision. The orbital motion of the three spacecraft causes the LISA armlengths to be unequal and time-varying. Laser interferometry measurements are more difficult to make if the distances between pairs of spacecraft are not nearly equal. LISA's advanced technologies will address this challenge and ensure the mission's success.
On to LISA Interferometry I
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