LISA Pathfinder

Paul McNamara, ESTEC-ESA Paul.McNamara-at-esa.int
LISA Pathfinder (formerly known as SMART-2), the second of the ESA Small Missions for Advanced Research in Technology, is a dedicated technology demonstrator for the joint ESA/NASA Laser Interferometer Space Antenna (LISA) mission.

The technologies required for LISA are many and extremely challenging. This coupled with the fact that some flight hardware cannot be tested on ground due to the earth induced noise, led to the LISA Pathfinder (LPF) mission being implemented to test the critical LISA technologies in a flight environment. The scientific objective of the LISA Pathfinder mission consists then of the first in-flight test of gravitational wave detection metrology.

LISA Pathfinder carries two payloads, the European provided LISA Technology Package (LTP) and the NASA provided Disturbance Reduction System - Precision Flight Control Validation (DRS PCFV), formerly known as the DRS.

Mission Goals

The mission goals of the LTP can be summarized as:

$\bullet$ 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

\begin{displaymath}
S_a^{1/2}(f) \leq 3\times 10^{-14}\left[ 1
+\left({f \over 3 {\rm mHz}}\right)^2\right] {\rm ms}^{-2} /\sqrt{\rm Hz}
\end{displaymath} (1)

over the frequency range, $f$, of 1 to 30 mHz.

$\bullet$ 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

\begin{displaymath}
S_{\delta x} ^{1/2}(f) = 9.1\times 10^{-12}\left[ 1
+\lef...
... \over 3 {\rm mHz}}\right)^{-2}\right] {\rm m} /\sqrt{\rm Hz}
\end{displaymath} (2)

over the frequency range, $f$, of 1 to 30 mHz.

$\bullet$ assessing the lifetime and reliability of the micro-Newton thrusters, lasers and optics in a space environment. LTP The basic idea behind the LTP is that of squeezing one arm of LISA from $5\times 10^6$ km to a few centimeters 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 1).

Figure: a) CAD drawing of the LISA Technology Package showing the two vacuum enclosures housing the test masses, and the optical bench interferometer (OBI), b) photograph of the EM of the OBI (vacuum enclosures replaced with end plates).
\begin{figure}
\centerline{\psfig{file=lisa1.eps,height=2in}}
\end{figure}
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 centered 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 centered 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 neutralization 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 - Precision Control Flight Validation

The DRS-PCFV 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 de-scoped, and now consists of the micro-Newton colloidal thrusters, drag-free and attitude control system (DFACS), and a micro-processor. The DRS-PCFV will now use the LTP inertial sensors as its drag-free sensors.

The primary goal of the DRS-PCFV is to maintain the position of the spacecraft with respect to the proof mass to within 10nm $/\sqrt{\rm
Hz}$ over the frequency range of 1-30mHz.

Launch and orbit

LISA Pathfinder is due to be launched in October 2009 on-board a dedicated launcher. Rockot is presently the baseline vehicle, while VEGA is considered the target vehicle that will be used if available. The spacecraft and propulsion module (Figure 2) are injected into a low earth orbit (200 x 900km), 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 stabilized using the micro-Newton thrusters, entering a Lissajous orbit around L1 (500,000km by 800,000km orbit).

Figure: The LISA Pathfinder spacecraft separating from its propulsion module.
\begin{figure}
\centerline{\psfig{file=lisa2.eps,height=4in}}
\end{figure}
Following the initial on-orbit check-out and instrument calibration, the in-flight demonstration of the LISA technology will take place in the first half of 2010. 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.

Status

LISA Pathfinder is currently in Implementation Phase. The contract with the prime industrial contractor, Astrium UK, was signed in May 2004. During the last year, all ITTs for spacecraft subcontractors have been issued.

In October 2004, the Science Program Council (SPC) approved the LTP Multi-Lateral Agreement, detailing the national agency responsibilities for the construction of the LTP. All subcontracts for the LTP have started.

The project has also successfully completed a number of significant agency level reviews over the last year, including the Technology Readiness Review, the LTP Preliminary Design Review, System Preliminary Design Review, and the Mission Preliminary Design Review. Also, all the LTP subsystems have undergone PDR within the last year.

With the deletion of the GRS from the DRS, it was recommended that the DRS undergo a joint ESA/NASA delta-Critical Design Review ($\delta$-CDR)/Risk Review. This was completed successfully in January 2006.

The first LTP subsystem flight hardware is due to be delivered to the LTP Architect (Astrium GmbH) during the third quarter 2006. The delivery of the assembled and tested LTP instrument to the prime contractor is scheduled for July 2008.


Jorge Pullin 2006-02-28