LISA consists of three identical spacecraft whose positions mark the vertices of an equilateral triangle five million km on a side, in orbit around the Sun. LISA can be thought of as a giant Michelson interferometer in space. The spacecraft separation sets the range of GW frequencies LISA can observe (from 0.03 milliHertz to above 0.1 Hertz).

The center of the LISA triangle traces an Earth-like orbit in the ecliptic plane, one astronomical unit from the Sun, but 20 degrees behind Earth. The plane of the triangle is inclined at 60 degrees to the ecliptic. The natural free-fall orbits of the three spacecraft around the Sun maintain this triangular formation, with the triangle appearing to rotate about its center once per year.

The LISA spacecraft will be launched from a single medium-lift rocket and injected into an Earth-escape trajectory. The three spacecraft will then leave the rocket, and each will be guided by an individual propulsion module to its own independent orbit around our Sun. After reaching the final orbits, about 13 months after launch, the propulsion modules will separate. Each spacecraft's orbit will then evolve under gravitational forces alone, and remain stable for the mission duration goal of ten years.

Spacecraft

Each of the three Laser Interferometer Space Antenna (LISA) spacecraft is designed as a short cylinder, 2.8 x 0.76 meters. Solar panels, mounted on a solar shield, power the spacecraft. The LISA radio antennas and microNewton thrusters (see Technology), are mounted on the outer wall of the cylinder. The two antennas on each spacecraft have a diameter of 30 centimeters, and operate in the X and Ka bands; they communicate with NASA's Deep Space Network.

The LISA science instruments (identical on each spacecraft) are housed in two cylindrical thermal shields. The instruments consist of two optical assemblies and lasers, mounted on a disk-shaped radiator, and protected by thermal shielding. Each optical assembly contains a 40-cm telescope, used for transmission and reception of laser signals to another spacecraft; an optical bench that supports main optics (for laser injection, detection, and beam-shaping), and the housing of the test masses (see Technology).

The LISA sensitivity (the strength of the gravitational-wave signals to which LISA is sensitive, as a function of frequency) is limited at low frequencies by test-mass acceleration noise; at mid frequencies by laser shot noise and optical-path measurement errors; and at high frequencies by the fact that the gravitational wavelength becomes shorter than the LISA arm length, reducing the efficiency of the interferometric measurement.

LISA can determine a source's position in the sky using AM and FM modulation (much like radio transmissions). For sources above 1 milliHertz, LISA will observe the Doppler shifts of gravitational-wave frequencies as LISA orbits the Sun (which changes the relative velocity between LISA and the source). At lower frequencies, LISA will measure the amplitude modulations induced on the signals by the yearly rotation of the LISA triangle (which changes the angle between the LISA sensitive arms and the incoming waves). Both of these methods can provide sub-degree location accuracy for strong sources.