ESA’s potential future mission, LISA, will detect and observe gravitational waves that are emitted during the most powerful events in the universe. LISA will detect gravitational radiation from astronomical sources, observing galaxies far back in time and testing the fundamental theories of gravitation.
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The Laser Interferometer Space Antenna (LISA) is a cooperative mission with NASA, designed to detect 'ripples' in space-time.
As predicted by Einstein’s general theory of relativity, the ripples are created during events in which very massive objects undergo strong acceleration. Examples of such events are massive black holes swallowing neutron stars or the collision of two massive black holes. Such ripples are called gravitational waves and LISA will be the first mission to detect them from space.
LISA’s three spacecraft will form an equilateral triangle with an arm’s length of about 5 million km. Each spacecraft houses two free-floating cubes made of a gold-platinum alloy inside the spacecraft, shielded from adverse effects of being in interplanetary space. The distance between the cubes in different spacecraft is monitored using highly accurate laser-based techniques. In this manner, it is possible to detect minute changes caused by passing gravitational waves.
Gravitational waves are an integral part of Einstein's theory of general relativity. When a massive body is accelerated in the proper manner, it radiates gravitational waves.
The difficulty is that, even for very massive bodies, such as black holes or neutron stars, gravitational waves are very weak and their effects are small. To detect gravitational waves, increasing the size of the detectors and going to a very quiet place is key. This is why scientists need LISA – a space-based detector, 5 million km in size.
Physicists have come to two fundamental conclusions about gravitational waves:
- The most predictable and most powerful sources of gravitational waves mostly emit their radiation at very low frequencies, below 10 miliHertz, or less than one oscillation every 100 seconds
- The presence of Earth's gravitational field and noise. both man-made and natural makes it difficult to observe the low frequency signals caused by gravitational waves. This makes detectors in space necessary, far away from Earth's gravitational disturbance and the noise on ground
One of the most certain sources of gravitational waves in our galaxy are binary stars, pairs of stars held together by their mutual gravitational pull. LISA's observations of these systems will be very interesting both for fundamental physics and for astrophysics.
Its design is such that it can measure the amplitude, direction, and polarisation of gravitational waves simultaneously. Comparing measurements with theory will be a very powerful test of general relatvity. With the help of polarisation measurements, the inclination of the orbit of the binary system can be determined – a crucial factor missing from many optical observations of these systems, necessary to infer the mass of stars in the binary pair.
There are thousands of binary systems whose individual components can be detected in this manner, including some already identified from optical and X-ray observations. Candidate sources include X-ray binaries, neutron-star binaries, black-hole binaries, and helium cataclysmic variables.
The most powerful sources of gravitational waves are the mergers of super-massive black holes in distant galaxies. When such events occur, their signal is about 10 million times stronger than the expected level of noise in a space-based detector. Observation of signals involving massive black holes will test general relativity and particularly black-hole theory to unprecedented accuracy. It will provide astronomers with information that cannot be obtained in another way.
Massive blackholes were proposed in theory to explain the presence and behaviour of powerful quasars and active galactic nuclei, although there were no direct observations. Nowadays, the observational evidence is compelling.
Data collected by the ESA/NASA Hubble Space Telescope shows that the galaxy M87 contains a central black hole, about 3000 million times more massive than the Sun. Andromeda, or M31, is also believed to contain a black hole of 30 million solar masses. Many more observations of black holes in nearby galaxies have been made and so astronomers expect a lot of signals for LISA to detect them.
However, to detect signals of passing gravitational waves, all other sources of noise must be eliminated or damped down to an acceptable level. The spacecraft must be able to compensate for constant low-level buffeting by the solar wind and correct for minute orbital changes introduced by solar radiation pressure. This is one of the most challenging technical aspects.
Each of the three LISA spacecraft will carry two telescopes with associated lasers and optical systems. Pointing in directions separated by 60 degrees, the telescopes in each spacecraft will communicate with the other two spacecraft, located at the other two corners of an equilateral triangle. The task of aiming laser beams from one small spacecraft to another across 5 million kilometres of space is quite complex.
Additionally LISA has to deal with other forces that will alter the separation of the spacecrafts – for example, solar radiation pressure. The spacecraft must sense the extraneous forces and counteract them with highly sensitive electric thrusters.
Central to each optical system, is a cube of side length 4cm, made of gold-platinum alloy. This 'test mass' will float freely in the weightless conditions of space. Acting as a reflector for the laser beams, the cube will provide the reference for measuring the distance between spacecraft.
According to the current concept, the three identical LISA spacecraft will be launched together on a single Atlas V launcher. They will then independently reach their final orbits around the Sun using their propulsion modules that will be jettisoned prior to starting the scientific operations. The three spacecraft will be located at the vertices of a triangle, with an arm’s length of 5 million kilometres. The orbits will be similar to that of the Earth, but will trail our planet by approximately 50 million kilometres.
It will take one year for the three spacecraft to reach their final position and to start the actual mission. The LISA triangle will face the Sun, at an angle of 60 degrees to the plane of Earth's orbit, revolving with Earth around the Sun.
These heliocentric orbits for the three spacecraft were chosen so that the triangular formation is maintained throughout the year, with the triangle appearing to rotate about the centre of the formation once per year. The relative movement of the three spacecraft will help to detect the direction of each source and to reveal the nature of the gravitational waves.
The distance between the spacecraft determines the frequency range in which LISA can make observations; it has been carefully chosen to allow observation of most of the interesting sources of gravitational radiation, namely massive black holes and binary stars.
Early designs for laser-interferometer gravitational-wave detectors in space were first suggested in 1976. Three drag-free spacecraft were to be placed in space 'as far apart as practical' and their relative motion determined by a laser interferometer. This concept was worked out in greater detail and tentatively named Laser Antenna for Gravitational-radiation Observation in Space (LAGOS).
In May 1993, LISA was proposed by a team of United States and European scientists as a joint ESA/ NASA mission. It was chosen by ESA instead for an Assessment Study as a possible ESA-only mission. The ESA-led study indicated that LISA was too expensive for a single agency to develop it.
A six-spacecraft version of LISA was suggested in October 1993, as a candidate for a Cornerstone mission under ESA’s former Horizon 2000 Plus programme. The mission was not approved due to high expected costs and size.
To reduce mission costs, the science team studied an alternative configuration using only three spacecraft. Each spacecraft would replace a pair of spacecraft at the vertices of a triangle, with two instruments in each spacecraft.
The three spacecraft would maintain all scientific capabilities of the six-spacecraft mission and would include redundancy such that no single failure would compromise the mission. In the case of the failure of one instrument, the mission would degrade into a two-arm interferometer, rather than the preferred three-arm mission, but would still provide much of the expected science return.
LISA is a collaborative ESA-NASA project.
An initial agreement between ESA and NASA on roles and responsibilities for the Mission Formulation phase was finalized in August 2004. Under this plan, NASA will provide the three spacecraft, the launch vehicle, operations, the use of the Deep Space Network and elements of the payload. ESA will provide the complete payload and the three propulsion modules. The current agreement will be updated at a later stage to formalize ESA and NASA roles and responsibilities for the implementation phase.