Launch: December 3, 2015
Scientific Goals of the LISA and LISA Pathfinder Missions
LISA Pathfinder serves as a technology demonstration mission (formerly SMART-2) for the L3 mission LISA (Laser Interferometer Space Antenna) in ESA's "Cosmic Vision 2015-2025" programme. LISA shall be launched presumably in 2034, and is intended to detect low frequency gravitational waves from space in the frequency range starting from below 0.1 Millihertz to about 0.1 Hertz.
LISA will thus be operated complementary to earthbound (interferometric) gravitational wave observatories, such as LIGO and Advanced LIGO (USA), GEO 600 (Germany/Great Britain), VIRGO (France/Italy), KAGRA (Japan), and their replacements, which will all be sensitive in the frequency range between about ten and 10,000 Hertz. A leading role in the development and the operation of GEO 600 is taken by the Max Plank Institute for Gravitational Physics/Albert Einstein Institute (Golm near Potsdam/Hannover, Germany), which has also been collaborating in a prominent role in the development of LISA Pathfinder and with LISA.
Cosmic sources of gravitational radiation which is expected to be detected by earthbound observatories and from space in the frequency ranges mentioned above are short-period binaries, close and collapsing systems of neutron stars and/or black holes, gamma ray bursts and super-novae, super massive black holes in the centers of galaxies, and a stochastic background of sources within and outside our Galaxy. For a long time the existence of gravitational waves could only be inferred from indirect observations of pulsars. Then finally in September and December 2015 the first observations with Advanced LIGO occurred (GW150914 and GW151226). In both cases the sources were identified as merging black holes with stellar masses.
Moreover, gravitational waves may be detected which can be interpreted as the relicts of different processes in the early universe, such as vibrating cosmic strings, phase transitions, and the big bang itself. Typical amplitudes, i.e. relative alterations in length, ΔL/L, of distances measured by interferometry, are of the order of 10 exp -19 to 10 exp -23, depending on the nature of the sources and the frequencies and durations of the signals emitted.
While, in particular, electromagnetic waves from the gamma to the radio-frequency range provide information on the surface conditions of astrophysical objects, the gravitational wave astronomy will allow drawing conclusions about the entire mass distributions of their sources and their temporal variations. Terrestrial as well as space borne gravitational wave observatories will open a new and very important window to the universe, and let expect novel and fundamental scientific discoveries on single objects as well as on our universe as a whole.
The LISA Mission
LISA will presumably consist of a configuration (cluster) of three satellites, placed at the corners of an equilateral triangle with a side length of approximately two and a half million kilometers, which will follow the Earth on its orbit around the sun at a distance of about 50 million kilometers (i.e. the center of gravity of the cluster will follow the Earth at a phase angle of about 20 degrees when viewed from the sun). Moreover, the entire configuration is inclined by 60 degrees with respect to the orbital plane of the Earth around the Sun (i.e. the ecliptic plane). The spacecraft carries two free-flying test masses each that will be kept free from external disturbances as far as possible. The mutual distances of the test masses from satellite to satellite will be measured by means of high-precision heterodyne laser-interferometry.
In this way, the extremely small distance variations between the test masses of two satellites can be detected which are caused by the passages of gravitational waves. The required measurement accuracy of the distances amounts to typically 1/100 of the diameter of a hydrogen atom (10 exp -12 meters) at a distance of two and a half million kilometers (for a broadband measurement in the frequency range from 1 to 10 Millihertz). The tiny orbital and attitude corrections which are necessary to keep each satellite centered on the test masses will be determined by a "Drag-free Attitude Control System" (DFACS) using the measurements of inertial sensors. The attitude measurements will be converted into correctional motions via Micro-Newton thrusters. Cold gas and colloid thrusters will be tested during the LISA Pathfinder mission.
LISA Pathfinder: Technology Goals
Owing to the combined disturbing effects that are to be controlled, in particular, the gravity of the Earth and its variations, the required freedom of the test masses from disturbing forces cannot entirely be verified on ground. Thus, LISA Pathfinder as the necessary precursor mission to LISA followed the goal to test the key technologies of the system in space. In particular, these technologies were:
During the tests performed with LISA Pathfinder the system performance shall converge to the specification of the LISA mission regarding the freedom from disturbances to within one order of magnitude. The maximum spectral energy density of the disturbing accelerations of the test masses shall be < 3 x 10 exp -15 m x s x exp -2 x Hz exp -1/2 in the frequency range from 0.1 Millihertz to 1 Millihertz. This goal is followed both by the LISA Technology Package (LTP) developed under the auspices of ESA, and by the Disturbance Reduction System (DRS) has been supplied by NASA (JPL) as a second and analogue payload to LISA Pathfinder. Both systems will be operated separately as well as in joined operation. In operation since March 2016 LISA Pathfinder already met the requirements of LISA at nearly all frequencies or even exceeded them.
After its launch LISA Pathfinder was at first injected into an elliptic transfer orbit. The aphelion of this orbit was then raised during several burning phases by means of a dedicated propulsion module to finally reach a halo orbit which is centered on the Lagrangian point L1 of the Sun-Earth system, about 1.5 million kilometers from the Earth. Immediately before turning into its final orbit, and the begin of the scientific (drag-free) operations under the smallest possible influence of disturbances, the payload module was separated from the propulsion module to exclude disturbances by the latter one. The halo orbit around L1 has been selected in order to fulfill the stringent requirements regarding the thermal stability of the payload (constant solar irradiation and temperature), and the small gravitational disturbances that are prevailing near the point of gravitational equilibrium between the Earth and the Sun.
The LISA Pathfinder Team
The LISA Technology Package has been developed under the auspices of ESA as the responsible space agency of the project, with important contributions from a number of national European space agencies and government administrations. These contributions came from research institutes and universities as well as space companies from Spain, Italy, France, Great Britain, the Netherlands, Switzerland and Germany. While the LISA Pathfinder spacecraft was built by the EADS Astrium Ltd. (Great Britain), the EADS Astrium GmbH (Friedrichshafen, Germany) as the "Industrial Architect" coordinated the development, the assembly and integration, and the tests of the entire LTP. As an important German contribution to the LTP the Albert-Einstein-Institut (AEI) led the development of the interferometer which represents the core of the scientific payload of LISA Pathfinder.
The German Contribution to the LISA Pathfinder Technology Package
In particular, the German contributions to the LTP payload, funded by DLR, comprise:
Mission and technical Parameters