Scientific Goals of the eLISA and LISA Pathfinder Missions
LISA Pathfinder will serve as a technology demonstration mission (formerly SMART-2) for the cornerstone mission eLISA (evolved Laser Interferometer Space Antenna). eLISA shall be launched presumably in 2028 or later, and is intended to detect low frequency gravitational waves from space in the frequency range starting from below 0.1 Millihertz to 0.1 Hertz.
eLISA 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), TAMA 300 (Japan), and their replacements, which will all perform their measurements 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 is also collaborating in a prominent role in the development of LISA Pathfinder and eLISA.
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. 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 eLISA Mission
The three spacecraft of eLISA form an equilateral triangle with sides of one million kilometers in length.
eLISA will consist of a configuration (cluster) of three satellites, one mother and two daughter spacecraft, placed at the corners of an equilateral triangle with a side length of approximately one 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 20 degrees when viewed from the sun). Moreover, the entire configuartion is inclined by 60 degrees with repsect to the orbital plane of the Earth around the Sun (i.e. the ecliptic plane). The mother spacecraft carries two and each of the daughter spacecraft carry one free-flying test masses that will be kept as far as possible free of external disturbances. 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 a 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 one 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 eLISA follows the goal to test the key technologies of the system in space. In particular, these technologies are:
During the tests performed with LISA Pathfinder the system performance shall converge to the specification of the eLISA 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 ms 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) which will be 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.
After its launch LISA Pathfinder will at first be injected into an elliptic transfer orbit. The aphelion of this orbit will then be risen 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 will be 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 is presently being 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 come 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 is built by the EADS Astrium Ltd. (Great Britain), the EADS Astrium GmbH (Friedrichshafen, Germany) as the “Industrial Architect“ is coordinating 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) is leading 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: