LPF Science Requirements
The main aim of the LISA Pathfinder mission is to demonstrate, in a space environment, that free-falling bodies follow geodesics in space-time by than two orders of magnitude better than any past, present, or planned mission (with the exception of eLISA itself).
In Einstein's General Theory of Relativity, gravity is not considered as an external force: instead gravity is the source of spacetime curvature. Therefore, in a universe devolved of mass (a flat spacetime), free-falling test masses will move in straight lines with uniform velocity (Newton's 1st Law). However, in the real (as described by General Relativity) Universe, the presence of mass, hence gravity/curvature, modifies Newton's 1st Law to state that in the absence of any external force, free-falling test masses move along geodesics. The concept that a particle falling under the influence of gravity alone follows a geodesic in space-time is at the very foundation of General Relativity; all experiments aimed at demonstrating a prediction of GR require the use of particles that are, to varying accuracies, in geodesic motion.
The difficulty of achieving high purity geodesic motion is that any parasitic forces compete with spacetime geometry to set masses into motion, perturbing them away from their geodesic lines. As gravity is by far the weakest of all fundamental interactions, achieving the required extremely low level of non-gravitational acceleration requires the understanding, reduction and control of the disturbances produced by a wide range of physical phenomena. The LISA Pathfinder experiment concept is to improve the uncertainty in the proof of geodesic motion. This is achieved by tracking, using pico-metre resolution laser interferometry, two test-masses nominally in free-fall, and by showing that their relative parasitic acceleration, at frequencies around 1mHz, is at least two orders of magnitude smaller than anything demonstrated or planned so far.
To reach its goals, LISA Pathfinder will have to achieve many firsts simultaneously. Its test masses will be the first large-mass high-purity metal test bodies flown freely in space at a distance of several millimetres from their immediate surroundings and with no mechanical contact to them. With its test mass to test mass and test mass to spacecraft interferometric motion readout, it will realise the first high precision laser interferometric tracking of orbiting bodies in space. And with its sub nano-g self-gravity suppression at both test masses locations, it will be the first high-quality orbiting gravitational laboratory for Fundamental Physics missions.
The main performance goal of LISA Pathfinder is to demonstrate immunity from relative accelerations of non-gravitational origin to
over the frequency bandwidth of 1 - 30mHz. This is the top-level science requirement of the mission. It should be noted that this requirement is a factor of ten less stringent than eLISA, both in frequency and performance. However, the current best estimate of the performance (estimated from ground testing results) that can be achieved from LISA Pathfinder is significantly better than the requirements.
In addition, LISA Pathfinder will demonstrate the ability of tracking free-floating test masses by laser interferometry with a resolution of
over a frequency bandwidth of 1 - 30mHz with a dynamic range on the order of a millimetre.
Here the main challenge is the low frequency at which this performance is required. Achieving low stray accelerations and low displacement noise becomes increasingly difficult at the lowest frequencies. Resolutions of better than 10-19m/√Hz are routinely achieved by the LIGO, VIRGO and GEO600 ground-based interferometers, but at frequencies above 100Hz. However, all effects of interest for space missions are comparatively slow processes as they involve the motion of large bodies.
It is important to stress that LISA Pathfinder is not a mission mainly aimed at demonstrating drag free control. Drag-free control is just one of the many tools used to achieve test mass geodesic motion. The main difference is that geodesic motion is the lack of relative acceleration between free test masses other than due to spacetime curvature, while drag free motion is the lack of acceleration of the spacecraft relative to a local inertial frame.
LISA Pathfinder is both a mission in General Relativity and in Precision Metrology, pushing these disciplines several orders of magnitude beyond their current state of the art. In doing so it opens new ground for an entire class of new missions in General Relativity, in Fundamental Physics at large, and in Earth Observation.
Also, it must be stated that the true objective of LISA Pathfinder is not to develop hardware, but to confirm the overall physical model of the forces that act on a test mass in interplanetary space. To fulfil this program, the mission will implement a full menu of measurements: at the end of this set of measurements, the residual acceleration noise model will be verified down to painstaking detail.