A high-tech mission
LISA will consist of three independent spacecraft in a triangular formation, each craft separated by 5 million kilometres from its companions. The spacecraft will orbit the Sun, maintaining their formation. They will continuously monitor their separations using lasers, in order to detect the tiny periodic squeezing and stretching of space caused by passing gravitational waves. Sensitive at frequencies from 0,1 mHz to 1 Hz (between one tenth of a thousandth and one cycles per second), just below the audio range, LISA will hear the whistling and humming of the Universe!
Over the next few years, scientists and engineers will refine and test the key technologies and complete the design of the LISA mission. This includes building and launching a small satellite – LISA Pathfinder in 2009 – to test the technologies in space. After successful tests, construction of the LISA spacecraft will begin. After their launch around 2015, it will take a year to bring them to their correct orbits around the Sun. The mission could make continuous observations for up to 10 years after that.
What LISA is actually monitoring is not the distance between its satellites, but between test masses floating freely inside each satellite. In order to ensure that the distances between the test masses are disturbed only by gravitational waves and not by other influences, such as the pressure of light from the Sun, LISA’s test masses have no direct connection with the LISA spacecraft that carries them. The spacecraft, in turn, have the job to prevent disturbances from affecting the test masses - so, in particular, they must keep their position relative to those test masses very accurately. For accurate position control, the spacecraft are equipped with special low-thrust jets called FEEPS (see image above). Whenever external forces act on a spacecraft, these jets are activated to counteract its effect and to ensure that the craft does not bump into the test masses. This is called drag-free control; it has been demonstrated in other missions, but never to the degree of isolation required by LISA. Making sure that the spacecraft does not influence the motion of the test masses is a major requirement. Even small effects, like the changing gravitational influence of parts of the spacecraft on the proof mass as the spacecraft expands and contracts due to small fluctuations in the light from the Sun, may have significant effects and must be controlled.
Making the connection: Lasers
LISA’s laser communication system is also sophisticated. Ground-based detectors simply shine laser light from one mirror to another, allowing it to reflect back and forth hundreds of times in order to build up the effect of the gravitational wave on the light. But the LISA spacecraft are too far apart to use reflections: the reflected light would be too dim to measure. Instead, each spacecraft carries lasers of its own: When light from one arrives at another, the on-board laser of the receiving spacecraft amplify the incoming light and return it, exactly as if it had been reflected, but much stronger. These “active mirrors” have never been used in spacecraft before. They have been demonstrated in the laboratory and are awaiting a test in space.
LISA lasers need to exhibit a high degree in stability, for instance regarding their frequency. The image above shows an oscillator of the type that will be used for such lasers.
Blazing the trail: Pathfinder
Given that LISA technology is so innovative, can we demonstrate that it will work as projected? The answer to that question is the LISA Pathfinder mission, an ESA mission with contributions by NASA. For this mission, a miniature version of one of LISA's arms will be recreated aboard a single satellite: two free-falling test masses and a high-precision set-up using laser light to monitor their relative motion.
Just as for LISA, the test masses will be cubes with a side-length of 4,6 centimeters, consisting of a special gold-platinum alloy. Surrounding these cubes will be gold-coated electrodes, which fulfill a double function: They can be used both to monitor the position and orientation of the test masses as well as to apply electric forces in order to keep them aligned. A laser interferometer reads out the relative displacement between the two test masses along the line joining their centers of mass. An additional interferometer provides an independent readout of the motion of one of the test masses relative to the spacecraft. As on LISA, the LISA Pathfinder spacecraft will follow one of the test masses by using extremely precise thrusters.
The technology tested on LISA Pathfinder is foreseen to be transfered to LISA with no change of design. At the same time, the mission is expected to provide hard data about the minute disturbances which will be experienced by the test masses. A number of dedicated sub-experiments aboard the satellite will monitor the strengths of different kinds of disturbance, and in the end, the scientists should have a realistic model for the noise which will limit their ability to detect gravitational wave signals, and define LISA's sensitivity as a gravitational wave detector.
LISA Pathfinder is now in its implementation phase. It just passed its "Preliminary Design Review," which marks the “freezing” of its main design features, and a large team of industries and institutions is working toward the launch, currently planned for October 2009. The entire experiment will last for six months, but the preliminary data that will clear the way for LISA is expected to be in hand within the first month or so after launch. If all systems perform as expected then, at this point, LISA will be ready to go!
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