LISA Gravity Waves

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Laser Interferometry Space Antenna (LISA)

LISA and the Gravitational Waves.

Contact S. Aston

The LISA (Laser Interferometry Space Antenna) mission will open up a new

window on the universe by enabling the observation of low frequency

gravitational waves. It can be expected to have an impact as significant as the

development of infra-red or X-ray astronomy. LISA is a joint ESA-NASAmission that involves having three spacecraft flying approximately 5 million

kilometres apart in an equilateral triangle formation. Together, they act as a

Michelson interferometer to measure the distortion of space caused by passing

gravitational waves.

The mission goal of LISA is to detect and observe gravitational waves from

massive black holes and galactic binaries with periods in the range of a few

seconds to a few hours (i.e. 10-4

to 10-1

Hz). Ground based interferometers,

such as LIGO, find this range inaccessible due to the background of local

gravitational noise arising from atmospheric effects and seismic activity.

Ground-based interferometers are also physically limited in length to a few

kilometres, restricting their coverage to events such as supernova core

collapses and binary neutron star mergers.

LISA Pathfinder (SMART-2)

The entire LISA system is a device for measurement of changes in the lengths
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of the interferometer arms by observation of the phase of laser light passed

between the spacecraft. The Phase Measurement System can therefore be

considered as one of the keys to achieving the required mission performance.

A contract to assess the feasibility of producing a phase measurement system

has been undertaken by the University of Birmingham in collaboration withindustrial partners SEA. The objectives of the contract are focussed on

understanding the Phase Measurement System for LISA by means of software

tools, identifying the critical components needed to achieve the performance

requirements, and justifying the technical solutions proposed by a combination

of laboratory testing and preparations for in-orbit testing on SMART-2 (The

Small Missions for Advanced Research in Technology). SMART-2 will test

the technology needed to develop the ambitious ESA mission LISA.

The current status of the project is that the prototype SMART-2 Phase

Measurement System has been delivered to the University of Glasgow, forfurther optical bench testing. The images available below show the fully

populated phasemeter electronics board and some early results from testing.

SMART-2 PMS Electronics Board.

Early Testing Results.

Birmingham Gravitation Group, University of Birmingham 2004, all rights

reserved

Phasemeter for LISA Pathfinder

LISA is a program jointly funded by ESA and

NASA. This is a mission to launch three satellites,

working cooperatively as an interferometer with

huge 5 Million Kilometre arms. LISA is made up oftwo sub missions, LISA Pathfinder and LISA. As
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LISA is such an enormous and technologically

difficult project LISA Pathfinder will launch in 2008

and will be the technology precursor to test

whether such LISA will be possible.

Lisa Pathfinder Orbit

LISA Pathfinder - The Basics

Making up LISA Pathfinder is an optical bench,

containing all the components to make up a small

interferometer, two chambers for test masses,

thrusters to enable the test masses to "free fall"

and a device known as the phasemeter.

An interferometer works by splitting a ray of light

between two paths, assuming both paths are

equal when the two rays are brought back

together they will be in phase and no interference

pattern will be seen. When one of these paths

changes length (for example by a gravitational

wave) the light recombining is not in phase and an

interference pattern is seen.

The phasemeter takes this interference signal and

uses it to calculate the distance the path length

has changed. In the case of LISA pathfinder the

path length will have changed by an amount equal

to the movement of the test mass inside of its

chamber.
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Lisa Pathfinder Structure

The Phasemeter

Recently the phasemeter has undergone its PDR

(Preliminary Design Review), this means that the

prototype is considered suitable and work can

begin on the engineering model. This is then

reviewed and if successful will be made into the

flight model.

Phasemeter prototype electronics

This is therefore a very important time for the

phase meter, and much testing is needed. The

prototype model is currently being modified to

deal with more channels of data, and also is

undergoing a change in the clock frequency.


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Two pieces of software (working together) are

needed to test the phasemeter, these are the Data

AcQuisition Software (DAQS) and also the Data

Processing Software (DPS). DAQS is the software

that interfaces to the phasemeter via a standard

known as military bus. It then takes phase and

housekeeping data and processes these. Phase

data is stored directly in files, and the DPS is used

to read and analyse this. Housekeeping data is

acquired every second. This is also stored in a file

but will also be displayed on the screen in a series

of text fields.

This housekeeping data is extremely important, it

displays temperatures, voltages, currents of the

various circuits inside of the phasemeter.

Temperature is an important consideration as it

leads to a method of determining faulty parts of

the phasemeter. Voltage and current data can be

used for a similar purpose.

Space missions are governed by many budgets,

the housekeeping data is important as it means it

is possible to view the power consumption of the

phasemeter in all of its different modes and to

make sure it does not exceed its budget.

The data processing software loads data from the

stored files, analyses this and produces graphs.

The graphs are used to test the noise floor of the

phasemeter. All analogue circuits contain noise, in

standard home made circuits this is rarely a

consideration. However, when considering LISA

Pathfinder needs to measure a phase of as low as

10^-4 Rads/sqrt(Hz) noise becomes an important

factor.


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Noise floor of prototype phasemeter

A seemingly simple part of the phasemeter

construction is the enclosure used to house the

circuitry. This is not a simple task! The

phasemeter enclosure has been designed in such a

way so that the whole circuit is shielded by a

metal box all around it. The main source of noise

is the power supply, it is therefore housed in a

sub-box in the bottom of the main enclosure.

Another problem encountered is during launch.

Every part of the spacecraft will undergo massive

vibrations, if incorrectly designed the box could

have natural resonances at frequencies that could

damage the equipment. Under recent vibration

tests the phasemeter enclosure has been found to

resonate at a safe frequency, above the minimum

recommended value.


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Phasemeter enclosure

University of Birmingham E-mail: webmaster[at]star.sr.bham.ac.uk

Mission

The Laser Interferometer Space Antenna (LISA) is a cooperativemission with NASA, designed to detect 'ripples' in space-time.

As predicted by Einsteins general theory of relativity, the ripplesare created during events in which very massive objects undergostrong acceleration. Examples of such events are massive blackholes 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.

LISAs three spacecraft will form an equilateral triangle with anarms 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 usinghighly accurate laser-based techniques. In this manner, it is possible to detect minute changes causedby passing gravitational waves.

Gravitational waves are an integral part of Einstein's theory of general relativity. When a massive bodyis 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 thedetectors and going to a very quiet place is key. This is why scientists need LISA a space-baseddetector, 5 million km in size.

LISA (Laser Interferometer SpaceAntenna) line drawing
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What's special?

Physicists have come to two fundamental conclusions about gravitational waves:

The most predictable and most powerful sources of gravitational waves mostly emit theirradiation 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 itdifficult to observe the low frequency signals caused by gravitational waves. This makesdetectors in space necessary, far away from Earth's gravitational disturbance and the noise onground

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 interestingboth 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 bedetermined a crucial factor missing from many optical observations of these systems, necessary toinfer 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-raybinaries, 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 theexpected 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 provideastronomers 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 observationalevidence 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 tocontain a black hole of 30 million solar masses. Many more observations of black holes in nearbygalaxies 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 radiationpressure. This is one of the most challenging technical aspects.

Spacecraft

Each of the three LISA spacecraft will carry two telescopes with

associated lasers and optical systems. Pointing in directionsseparated by 60 degrees, the telescopes in each spacecraft willcommunicate with the other two spacecraft, located at the othertwo corners of an equilateral triangle. The task of aiming laserbeams from one small spacecraft to another across 5 millionkilometres of space is quite complex.

Additionally LISA has to deal with other forces that will alter the separation of the spacecrafts forexample, solar radiation pressure. The spacecraft must sense the extraneous forces and counteractthem with highly sensitive electric thrusters.

LISA payload
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Central to each optical system, is a cube of side length 4cm, made of gold-platinum alloy. This 'testmass' 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.

Journey

According to the current concept, the three identical LISAspacecraft 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 tostarting the scientific operations. The three spacecraft will be

located at the vertices of a triangle, with an arms length of 5million 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 finalposition and to start the actual mission. The LISA triangle will facethe 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 sothat the triangular formation is maintained throughout the year,

with the triangle appearing to rotate about the centre of theformation once per year. The relative movement of the threespacecraft will help to detect the direction of each source and toreveal the nature of the gravitational waves.

The distance between the spacecraft determines the frequencyrange in which LISA can make observations; it has been carefully chosen to allow observation of most ofthe interesting sources of gravitational radiation, namely massive black holes and binary stars.

History

Early designs for laser-interferometer gravitational-wave detectors in space were first suggested in1976. Three drag-free spacecraft were to be placed in space 'as far apart as practical' and their relativemotion determined by a laser interferometer. This concept was worked out in greater detail andtentatively 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 ESAs former Horizon 2000 Plus programme. The mission was not approved due to highexpected 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 twoinstruments in each spacecraft.

The three spacecraft would maintain all scientific capabilities of the six-spacecraft mission and wouldinclude 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 preferredthree-arm mission, but would still provide much of the expected science return.

Partnership

LISA is a collaborative ESA-NASA project.

Schematic of LISA's Orbit

LISA's yearly orbit
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An initial agreement between ESA and NASA on roles and responsibilities for the Mission Formulationphase 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 providethe complete payload and the three propulsion modules. The current agreement will be updated at alater stage to formalize ESA and NASA roles and responsibilities for the implementation phase.

Last update: 21 August 2009

LISA Gravity Waves

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