Artist's impression of the LISA Pathfinder spacecraft
|Mission duration||Nominal: 1 year (with sufficient Cold Gas for mission extension)|
|Manufacturer||Airbus Defence and Space|
|Launch mass||1,910 kilograms (4,210 lb)|
|BOL mass||480 kg (1,060 lb)|
|Dry mass||810 kg (1,790 lb)|
|Payload mass||125 kg (276 lb)|
|Dimensions||2.9 m × 2.1 m (9.5 ft × 6.9 ft)|
|Start of mission|
|Launch date||July 2015|
|Launch site||Kourou ELV|
|Reference system||Sun–Earth L1|
|Periapsis||500,000 km (310,000 mi)|
|Apoapsis||800,000 km (500,000 mi)|
|35 cm Laser interferometer and Disturbance Reduction System|
LISA Pathfinder, formerly Small Missions for Advanced Research in Technology-2 (SMART-2), is an ESA/NASA space probe to be launched in 2015. The mission will test technologies needed for the Laser Interferometer Space Antenna (LISA), an ESA gravitational wave observatory. Additionally LISA Pathfinder will probe Einstein´s geodesic motion at an unprecedented level.
LISA Pathfinder will contain one arm of LISA, shortened from 5 Gm (5 million km) to 35 cm. In particular, it will verify:
- Drag-free attitude control of a spacecraft with two proof masses,
- The feasibility of laser interferometry in the desired frequency band (which is not possible on the surface of Earth), and
- The reliability and longevity of the various components—capacitive sensors, microthrusters, lasers and optics.
The estimated cost for the mission is €400 million.
LISA Pathfinder will place two test masses in a nearly perfect gravitational free-fall, and will control and measure their relative motion with unprecedented accuracy. The test masses and their environment will be the quietest place in the solar system.
LISA Pathfinder is an ESA led mission. It involves European space companies and research institutes from France, Germany, Italy, The Netherlands, Spain, Switzerland and UK and the US space agency NASA.
LISA Pathfinder Science
The primary science objective of LISA Pathfinder is that of measuring deviations from geodesic motion. Much of the experimentation in gravitational physics requires measuring the relative acceleration between free-falling, geodesic reference test particles. For LISA, the test masses will be 2 kg cubes housed in separate spacecraft million km apart. In LISA Pathfinder, precise inter-test-mass tracking, by optical interferometry, will allow scientists to assess the relative acceleration of the two LISA test masses 38 cm apart in a single spacecraft. The science of LISA Pathfinder consists of measuring, and creating an experimentally-anchored physical model for all the spurious effects – including stray forces and optical measurement limits – that limit the ability to create, and measure, the perfect constellation of free-falling test particles that would be ideal for eLISA.
LISA Pathfinder is being built by Airbus Defence and Space of Stevenage, UK under contract to the European Space Agency. It will carry a European 'LISA Technology Package' comprising inertial sensors, interferometer and associated instrumentation as well as two drag-free control systems: a European one using Cold Gas Micro thrusters (similar to those used on GAIA), and a US-built 'Disturbance Reduction System' using slightly different sensors and colloid thrusters that use ionised droplets of a colloid accelerated in an electric field. The colloid thruster system was built by Busek and delivered to JPL for integration with the spacecraft. 
The LISA Technology Package (LTP) is being integrated by Airbus Defence and Space Germany, but the instruments and components are being supplied by contributing institutions across Europe. The noise rejection technical requirements on the interferometer are very stringent, which means that the physical response of the interferometer to changing environmental conditions, such as temperature, must be minimised.
On the planned LISA mission environmental factors will influence the measurements the interferometer takes. These environmental influences will include stray electromagnetic fields and temperature gradients, which could be caused by the sun heating the spacecraft unevenly, or even by warm instrumentation inside the spacecraft itself. As LISA Pathfinder's job is to test the technologies for LISA, it is designed to find out how such environmental influences change the behaviour of the interferometer and the other instrumentation. LISA Pathfinder will fly with an extensive instrument package which can measure temperature and magnetic fields at the test masses and at the optical bench. The spacecraft is even equipped to stimulate the interferometer and the instruments: it carries heating elements at the struts holding the test mass enclosures and at the electrodes surrounding the test masses which can warm the spacecraft's structure, causing the optical bench and the test mass housing to distort. This will enable scientists to see how the measurements change with varying temperature effects.
The spacecraft will first be launched into an elliptical LEO parking orbit. From there it will execute a short burn each time perigee is passed, slowly raising the apogee closer to the intended halo orbit around the Earth–Sun L1 point. The results and technology of the LISA Pathfinder mission will provide the basis for a space-borne gravitational-wave observatory which will be launched in 2034 as the third L-class mission in ESA's cosmic vision programme addressing the science theme of "The Gravitational Universe".
Possible Mission Extension
A possible mission extension could be to perform some measurements to confirm whether gravity is all general relativity says it is. By flying through the 'Saddle Point' where the Earth and Sun's gravity cancel out, the spacecraft might probe whether Einstein's theory still holds when gravitational accelerations are incredibly small. If it does, these gravitational lacunae will be the last resting place of other occasionally fashionable theories, such as Modified Newtonian Dynamics (MOND) and TeVeS.
- eLISA and LISA Pathfinder's Homepage
- Videos for LISA Pathfinder on eLISAscience.org
- LISA Pathfinder mission home at ESA
- LISA Pathfinder at eoPortal
- New Gravitational Wave Observatory mission home
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- New Scientist 2 March 2013 page 44