Lisa Pathfinder — Wikipedia

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Satellite LISA Pathfinder .

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LISA Pathfinder is an artificial satellite of the European space agency which should make it possible to validate the technologies which are retained for the project eLISA . The purpose of the latter is to make observations of gravitational waves using a cluster of 3 satellites implementing measurement techniques by laser interferometry. The role of LISA Pathfinder is to validate the technical devices to measure gravitational waves: capacitive accelerometers, micropropulseurs, precision laser interferometer, draft compensation software.

The payload includes two sub-assemblies: the LPT formed from two masses in free fall with a very high precision measurement system and the DRS Supplied by NASA, which is a micropropulsion system capable of compensating for all the forces working on the satellite.

LISA Pathfinder is launched the by a Vega launcher and places himself in orbit around the point of Lagrange L first in Before starting its mission with an initial duration of six months. The mission, which ends the , is a complete success. The system tested has managed to limit the relative acceleration of the control masses to ten millionths of billionth, two orders of magnitude better than the objectives set. This technical success opens the way to operational satellite eLISA whose development is confirmed at the end of the mission and whose launch is scheduled for 2034.

Clusters of colloidal micropropulsion engines responsible for compensating for the forces exercising on the satellite LISA Pathfinder . These engines are at the heart of the DRS experience ( Disturbance Reduction System ) provided by the NASA .

Gravitational waves [ modifier | Modifier and code ]

Gravitational waves are deformations of space-time predicted by the theory of general relativity which are propagated from a massive acceleration object. These deformations are low. Detectable waves are those produced by very large masses subject to very large accelerations. Thus, the sources of observable gravitational waves are mainly those of astrophysical systems involving massive and very dense objects such as neutron stars or black holes. Experiences are carried out on earth to measure gravitational waves but variations in the gravitational field are so reduced that they are masked by other forces (electromagnetic, seismic, etc.) present. Given these important disruptions, the first direct observation of gravitational waves GW150914 only takes place Par l’Astrusta Ligo.

A new window on the universe [ modifier | Modifier and code ]

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Gravitational waves move at the speed of light without being disturbed by matter because they little interact with it unlike electromagnetic waves. As such, they provide new information on massive objects such as black holes. Gravitational waves can also provide data on the beginnings of the universe (dark ages = 377,000 first years of the universe) inobservable with conventional instruments because no astrophysical process produces electromagnetic radiation during this period. By relying on the theoretical models of the universe and on electromagnetic observations made, scientists believe that the most interesting gravitational waves to observe are mainly concentrated in frequencies between 10 -4 and 10 −1 Hz : the observation of this frequency band makes it possible to test the theories relating to the evolution of the universe to a gap towards red of 20 (first stars formed after the Big Bang ), to verify the models relating to gravity in strong gravitational fields and to observe the beginnings of the universe implementing energies of the order of the TEV [ first ] .

The Elisa mission [ modifier | Modifier and code ]

The European Space Agency decides to develop a mission called eLISA operating this new observation technique. The mission is based on 3 satellites now their constant relative distances and forming a gigantic optical interferometer. The passage of a gravitational wave results in a modification of the distance between the satellites which is measured by interferometry. Each satellite contains two test masses which serve as a benchmark to measure the distance with the other two satellites. These masses are kept in free fall within the satellite thanks to a propulsion system responsible for compensating for all the other forces acting on the space machine. However, to be able to measure gravitational waves, it is necessary to obtain extreme precision concerning the measurement of the position of the test mass (of the peak order) and the trajectory followed by the satellite (of the order of a few nanometers). Also the space agency decides to develop technologies that are implemented by eLISA as part of a dedicated mission called LISA Pathfinder (formerly SMART-2 For Small Missions for Advanced Research in Technology ) Before carrying out the mission eLISA properly said. The launch of the latter, which is one of the projects proposed for the L3 mission of the program Cosmic Vision , can take place around 2034.

The first works on a satellite capable of studying gravitational waves date back to 1993 when the American and European researchers established the specifications of the Lisa space mission (renamed later eLISA ) with the support of the European Space Agency and NASA. The two agencies finance from 2000 to 2011 research on the technologies necessary to carry out this mission. The two agencies decide to develop a joint mission. In 1998, a technological demonstrator was proposed under the name ELITE ( European LIsa Technology Experiment ). This project is refined and proposed in 2000 in response to the call for proposals of ESA, Smart-2. It is a question of developing technologies used both by future missions LISA (free fall) and Darwin (Flight in formation of several satellites). The mission, which has two satellites, must win three useful charges: two prepare Lisa and are provided respectively by each agency, while the third prepares Darwin . The mission is approved in 2000 but after a study by industrialists, its ambitions are revised downwards: it has only one satellite and two useful charges: LISA Technology Package (LTP) provided by the THAT And Disturbance Reduction System (DRS) provided by the NASA . The DRS Initially includes inertial sensors and a laser measurement system that are abandoned for cost reasons thereafter. The launch of the satellite, renamed LISA Pathfinder , is scheduled for 2011 [ 2 ] .

Construction of satellite and equipment [ modifier | Modifier and code ]

Construction of the satellite LISA Pathfinder is carried out by around forty companies and research institutes distributed in 14 countries with the leader and integrator the English subsidiary of Airbus Defence & Space . The cost amounts to 430 million euros by excluding national participations and the contribution of the NASA [ 3 ] . Development difficulties reject the launch in 2015.

Operating diagram LISA Pathfinder : first Satellite, 2 Capacity measure, 3 Micropropulseurs, 4 Secondary test mass, 5 Primary test mass, 6 Laser, 7 Photodetector.

To manage to measure gravitational waves, the measurement system used must be influenced only by gravity and all other forces are dismissed. Thus the objective of the mission LISA Pathfinder is to demonstrate that two metallic cubes can be protected in the heart of a satellite of all internal and external forces. This requires that the satellite follows a trajectory with an accuracy of the order of a few nanometers and that the measurement system makes it possible to detect the position of the metal cubes with an accuracy of the peak order. The two cubes are in free fall within the satellite without contact with the rest of the satellite and their respective distance is measured using a laser interferometer. Under these conditions, the cubes in free fall are influenced only by gravity and their respective displacement only reflects the variations of these including gravitational waves [ 4 ] .

To be able to maintain the cubes in free fall, you must first compensate for the external forces which modify the trajectory of the satellite. The main strength is the radiation pressure exerted by solar photons on the body of the satellite. The internal forces are of magnetic origin, electric but also linked to the gravity generated by the satellite itself. To reduce magnetic forces that can be exercised on the cubes, all magnetic materials are excluded from the construction of the satellite. The magnets used by lasers and propellants are very small and are positioned outside the external satellite speaker. The material used to make the cubes, a platinum and gold alloy, is chosen because their magnetic susceptibility is reduced. To reduce the gravitational attraction of the satellite on the cubes, the satellite is built so that the forces of gravity generated by its components balance at the level of the cubes. The position of a component located a meter from the cubes must be determined to the nearest millimeter while the components closest to the cubes must be determined with an accuracy of the order of the 10 millionth of a meter. Small masselots are placed after assembling the satellite to meet these constraints [ 5 ] .

LISA Pathfinder must validate the compensated drag piloting system, the expected performance of which is 10 −14 ms −2  Hz −1/2 and which cannot be tested on earth due to gravity. The satellite must [ 6 ] :

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  • demonstrate that a test mass can be placed in free fall;
  • Validate the operation of the laser interferometer with a free fall mirror;
  • Check reliability over the duration of micropropulsers, lasers and optics in the space environment.

Internal and external view of the satellite LISA Pathfinder With and without its solar panel. Position of the two main equipment: first DRS , 2 LPT.

Le lpt ( LISA Technology Package ).

LISA Pathfinder has a mass of 1,900 kg . On the one hand, it includes the scientific satellite proper, the mass of which is 420 kg on the other hand a propulsion module with 1,100 kg liquid ergols responsible for placing LISA Pathfinder on his work orbit near the point of Lagrange L first . The upper side of the scientific satellite is covered with 36 rows of 24 photovoltaic cells Triple Junction Gallium type representing an area of ​​2.4 m 2 . This panel provides 650 watts at the end of the mission. The propulsion module is 1.9 meters high for a diameter of 2.1 meters. Its mass is 1,420 kg When the ergols are loaded in its four tanks. The main propellant has a modular thrust ranging from 340 to 440 newtons and burns a hypergolic mixture of UDMH and nitrogen peroxide [ 7 ] .

Payload [ modifier | Modifier and code ]

The satellite has two experiences [ 8 ] :

  • LTP ( LISA Technology Package ) contains two test masses of 1.96 kg made in an alloy of gold and platinum and having the shape of cubes of 46 mm Aside: these must serve both as a mirror for the interferometer and an inertial repository for the position control system. THE LTP is a reduced model of the interferometer eLISA : While the distance between mirrors will be 1 million kilometers for Lisa, it is 35 cm on LISA Pathfinder ;
  • Le DRS ( Disturbance Reduction System ) is a position control system developed by the NASA , responsible for compensating for all forces other than gravity exerted on the satellite (such as radiation pressure). He uses ionic micropropulseurs with continuous push (Feep for Field Emission Electric Propulsion ) to maintain the satellite centered around a point of reference in free fall.

The satellite LISA Pathfinder separates from its propulsion module.

LISA Pathfinder is launched the by the European launcher Vega drawn from the launch base of Kourou [ 9 ] and initially placed on an elliptical low terrestrial orbit of 200 × 1 620 km with an inclination of 5.3 ° [ ten ] , [ 11 ] .

Transit, posting and recipe [ modifier | Modifier and code ]

Using its own engines, which provide a total delta-v 3.1 km/s , LISA Pathfinder Gradually increases the altitude of its peak on fifteen times. After 3 weeks, he escapes the attraction of the earth. The satellite then headed for the point of Lagrange L first of the sun-earth system, 1.5 million kilometers from the earth that it reaches the . He then begins to circulate on an orbit in Lissajous (500,000 × 800,000 km ) Around this point of Lagrange. The rotation of the satellite is canceled and the propulsion module, which can disrupt the measures, is dropped. This phase lasts a fortnight. Once the satellite is stabilized, the operation of the equipment DRS And LTP is checked for ten days.

Realization of technological tests [ modifier | Modifier and code ]

LISA Pathfinder In February begins its mission for a scheduled period of six months by browsing an orbit of Lissajous around the first : these operations include two phases of 90 days each: the first is devoted to the use of LTP and the second to DRS . Each month two to three days are neutralized to make trajectory corrections. The data is transmitted to the terrestrial station of Cerebros in Spain (parabolic antenna 35 meters in diameter) as part of daily sessions of 8 hours per day with a speed of 52.3 kilobits per second. The functioning of the satellite is controlled by the European Center for Space Operations (ESOC) in Darmstadt, Germany, while scientific operations are controlled by the European Space Astronomical Center (ESAC) at Villafranca de la Cañada, Spain.

LPT experience

The 15 and , the two Masses of the LPT experience baptized Jake And Elwood (in reference to The Blues Brothers ) are released from the two “maintenance fingers” [ twelfth ] who have kept them in place since the launch. An electrostatic field created by electrodes located at 4 mm Faces of the cubes prevents them from contacting the walls. During the following weeks, the electrostatic field is gradually reduced to its cancellation and the masses are free to float. The laser interferometer must then measure the distance separating the two masses with an accuracy of 0.01 nanometer and identify the sources of possible measurement errors (presence of a force exercising on the masses in the axis of the measurement) [ 13 ] .

DRS experience and mission extension

From the end of 2016 begins the DRS experience ( Disturbance Reduction System ) developed by the NASA . The objective is to test the colloidal micropropulsion engines which constitute an alternative technology allowing to compensate for the forces exerted on the satellite LISA Pathfinder [ 14 ] . After two weeks devoted to calibration, micropropulsers are tested until the end of November. An extension of the mission Lisa Pathfinder of six months is decided by the European Space Agency to better define the operation of the instrumentation for gravitational waves which are emitted with much lower or higher frequencies (1–60 mHz ) that those tested so far. These new objectives are fulfilled in [ 15 ] .

The mission LISA Pathfinder is a complete success. From the first days of the test phase, the accuracy obtained for frequencies between 60 millihertz and 1 hertz reached the objectives set and the optimizations made thereafter made it possible to exceed them Factor 100 . At frequencies below 1 mHz , the device is disturbed by the gas molecules ejected by the test masses. This technical success opens the way to operational satellite eLISA whose development is confirmed at the end of the mission and whose launch is scheduled for 2034 [ 16 ] , [ 17 ] .

The mission LISA Pathfinder ended the . In accordance with the rules for managing space debris, the satellite uses its propulsion to leave its orbit around the point of Lagrange L first . It is placed on a stable heliocentric orbit closer to the sun than that of the earth. On this orbit the spacecraft has 0.2% chance of hitting the earth or the moon in the next 100 years [ 16 ] .

Notes and references [ modifier | Modifier and code ]

  1. ↑ (in) ‘ eLISA Science Goals A New Window in Astronomy » , THAT (consulted the ) .
  2. ↑ (in) Introduction to LISA Pathfinder, p. 2.
  3. ↑ (in) LISA Pathfinder media kit, p. 5.
  4. ↑ (in) LISA Pathfinder media kit, p. 3.
  5. ↑ (in) LISA Pathfinder media kit, p. 6-7.
  6. ↑ (in) ‘ LISA Pathfinder : Objectives » , THAT (consulted the ) .
  7. ↑ (in) Patric Blau, ‘ LISA Pathfinder Spacecraft Overview » , on Spaceflight101 (consulted the ) .
  8. ↑ (in) ‘ LISA Pathfinder : Instruments » , THAT (consulted the ) .
  9. ↑ (in) ‘ ESA PR 47-2015: LISA Pathfinder en route to gravitational wave demonstration » , THAT, .
  10. ↑ (in) ‘ LISA Pathfinder : Orbit/Navigation » , THAT (consulted the ) .
  11. ↑ (in) Introduction to LISA Pathfinder, p. 9-11.
  12. ↑ Twelve fixing clamps maintained each mass to avoid the influence of vibrations during the orbit.
  13. ↑ (in) Jean François Mouriaux , ‘ Lisa Pathfinder releases her stallion masses » , Air et Cosmos , n O 2488, , p. 42 .
  14. ↑ (in) ‘ LISA Pathfinder completes first operations phase » , THAT, .
  15. ↑ (in) ‘ LISA Pathfinder’s pioneering mission continues » , THAT, .
  16. ↑ a et b (in) Patric Blau, ‘ LISA Pathfinder Spacecraft ends Communications with Earth after superb Mission Success » , on Spaceflight101 , .
  17. ↑ (in) ‘ LISA Pathfinder to conclude trailblazing mission » , THAT, .

Bibliography [ modifier | Modifier and code ]

  • (in) Paul McNamara and Giuseppe Rough , Introduction to LISA Pathfinder , THAT, , 42 p. ( read online )

    Presentation of the mission LISA Pathfinder 2009.

  • (in) Paul McNamara and Giuseppe Rough , LISA Pathfinder media kit , THAT, , 24 p. ( read online )

    Presentation of the mission LISA Pathfinder For the media.

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