The mission goals of the LTP can be summarized as:
• demonstrating that a test-mass can be put in pure gravitational free-fall within one order of magnitude of the requirement for LISA. The one order of magnitude rule applies also to frequency, thus the flight test of the LTP on LPF is considered satisfactory if free-fall of one TM is demonstrated to within 3x10-14ms-2/ÖHz 1mHz, rising as f2 between 3mHz and 30mHz.
• demonstrating laser interferometry with a free-falling mirror (test mass of LTP) with displacement sensitivity meeting the LISA requirements over the LTP measurement bandwidth. Thus the flight test of LTP is considered satisfactory if the laser metrology resolution is demonstrated to within 9x10-12m/ÖHz between 3mHz and 30mHz, rising as 1/f2 down to 1mHz.
• assessing the lifetime and reliability of the micro-Newton thrusters , lasers and optics in a space environment.
The basic idea behind the LTP is that of squeezing one arm of LISA from 5x106km to a few centimetres and placing it on board a single S/C. Thereby the key elements are two nominally free flying test masses (TM), and a laser interferometer whose purpose is to read the distance between the TM’s (Figure top right).
The two tests masses are surrounded by their position sensing electrodes . This position sensing provides the information to a “drag-free” control loop that, via a series of micro-Newton thrusters, keeps the spacecraft centred with respect to some fiducial point. In LISA, as in LPF, each spacecraft hosts two test-masses. However these two test-masses belong to different interferometer arms. This has an important consequence for the logic of the spacecraft control. The baseline defined by the system level study for LISA, sees a control logic where the spacecraft is simultaneously centred on both test-masses. However the spacecraft follows each test-mass only along the axis defined by the incoming laser beam. The remaining axes have to be controlled by a capacitive suspension (or by some other controlled actuation scheme). On LPF however, in order to be able to measure differential acceleration, the sensitive axes of the two test-masses have to be aligned. This forces one to develop a capacitive suspension scheme that carries one or both test-masses along with the spacecraft, including along the measurement axis, while still not spoiling the meaningfulness of the test.In LISA, the proper distance between the two free-falling test masses at the end of the interferometer arms is measured via a three step process; by measuring the distance between one test mass and the optics bench (known as the local measurement), by measuring the distance between optics benches (separated by 5 million kilometers), and finally be measuring the distance between the other test mass and its optics bench. In LISA Pathfinder, the optical metrology system essentially makes two measurements; the separation of the test masses, and the position of one test mass with respect to the optics bench. The latter measurement is identical to the LISA local measurement interferometer, thereby providing an in-flight demonstration of precision laser metrology directly applicable to LISA.
In LISA and in LPF, charging by cosmic rays is a major source of disturbance, thereby each test-mass carries a non contacting charge measurement and neutralisation system based on UV photoelectron extraction . An in-flight test of this device is then obviously a key element of the overall LPF test.
Disturbance Reduction System
The DRS is a NASA provided payload to be flown on the LISA Pathfinder spacecraft. When first proposed, the DRS payload closely resembled the LTP, namely in that it consisted of two inertial sensors with associated interferometric readout, as well as the drag-free control laws and micro-Newton colloidal thrusters, although the technologies employed were different from the LTP. However, due to budgetary constraints, the DRS was descoped, and now consists of the micro-Newton colloidal thrusters , drag-free and attitude control system (DFACS), and a micro-processor . The DRS will now use the LTP inertial sensors as its drag-free sensors.
The primary goal of the DRS is to maintain the position of the spacecraft with respect to the proof mass to within 10nm/ÖHz over the frequency range of 1-30mHz.
Launch and Orbit
LISA Pathfinder is due to be launched in mid 2011 on-board a dedicated launcher. VEGA is presently the baseline vehicle, while Rockot is a back-up in the event that VEGA is not available. The spacecraft and propulsion module are injected into a low earth orbit (200 x 1600km), from which, after a series of apogee raising burns, will enter a transfer orbit towards the first Sun-Earth Lagrange point (L1). After separation from the propulsion module, the LISA Pathfinder spacecraft will be stabilised using the micro-Newton thrusters, entering a Lissajous orbit around L1 (500,000km by 800,000km orbit).
Following the initial on-orbit check-out and instrument calibration, the in-flight demonstration of the LISA technology will take place in the latter half of 2011. The nominal lifetime of the mission is 180 days, this includes the LTP operations, the DRS operations, and a period of joint operations when the LTP will control the DRS thrusters.