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Author: Joffrey Rieger

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LPF Technology

LISA Pathfinder (LPF) mission was to test in space those key technologies that are specially developed for the LISA mission. In LPF, one laser arm of LISA was effectively reduced from a length of 1 million kilometres down to around 38 cm and the experiment installed into a single spacecraft.

LPF demonstrated the fundamental technologies needed to build a gravitational wave observatory in space.

The LISA Technology Package LTP
The LTP (LISA Technology Package) was a payload developed by European institutes and industry. It contained the two identical test masses: 46 mm cubes made of gold-platinum, each suspended in its own vacuum vessel, capacitive sensors to monitor the relative position of the test masses with respect to the satellite, laser interferometry to determine the relative positions and attitudes of the two test masses, and the drag-free control system to adjust the relative alignment of the satellite and test masses through a mixture of Micro-Newton (cold gas) thrusters and capacitive actuation. The cubes served both as mirrors for the laser interferometer and as inertial references for the drag-free control system of the spacecraft, exactly as will be used for LISA.

Drag-free Control
The LPF drag-free control system onboard the spacecraft monitored the micro motions of the test masses. When one of the test-masses moved away from its null position, a signal was sent to the control system which activated the micro-propulsion thrusters to enable the spacecraft to remain centred on this test mass. The second test mass was made to follow the first by control forces applied via the capacitive actuator. It was the very first time that ESA operates a spacecraft with such high precision micro-Newton thrusters as the only propulsion mechanism.

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It takes a large and varied team to make a successful space mission

LISA Pathfinder was an international effort under European leadership. Scientific institutes, ESA and industry  worked closely together:
  • Science Working Team: The LISA Pathfinder Science Team provided ESA with advice on all aspects of Science and Technology related to the mission. The team was chaired by the LPF Project Scientist, and composed of seven members from ESA member states, and representatives for the LISA project, both in Europe and the US.
  • ESA commissioned the European aerospace company Astrium Ltd (an EADS company) at Stevenage, Great Britain to be the Prime Contractor for the mission and to build the LISA Pathfinder spacecraft, i.e., the payload carrier.
  • The LISA Technology Package (LTP) was built by a consortium of European space companies from France, Germany, Great Britain, Italy, Netherlands, Spain and Switzerland led by the industrial partner EADS Gmbh (an EADS company), Friedrichshafen, Germany.
  • The second payload package, the Disturbance Reduction System (DRS), was developed in the United States by JPL (Jet Propulsion Laboratory) under the leadership of NASA. It provided a system of Micro-Newton thrusters complementary to the on-board LTP technology with a dedicated control system.
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LISA Pathfinder operations

Preparing for LPF operations meant developing the analysis, building the tools and training the team.

Given the relatively short duration planned for mission operations (90 days), the data analysis team had to be extremely well prepared. This meant that not only did the in-orbit experiments have to be defined well in advance, but the analysis routines and the associated tools also had to be prepared and tested.

Using the LTPDA Toolbox framework, analysis pipelines had been being developed to analyse the extensive menu of possible experiments which formed the mission time-line during operations. These pipelines, and the component tools, were then tested on simulated data sets which were generated using a variety of LPF simulators. From linear state-space models to non-linear simulators running the flight software, the data analysis team leveraged all available simulators to test the tools and procedures on as wide a range of data sets as possible.

In addition to the development and testing of the experiment procedures and associated analysis pipelines, the data analysis team had to prepare itself for the intense period of operations, during which the data needed to be analysed essentially in real-time so that the subsequent investigations could be first properly selected, and second appropriately configured. The output of each investigation also yielded information with which the operations team configured the spacecraft and the LTP to best achieve the mission goals.

Part of this preparation involved exposing the team to mission-like scenarios in which simulated data sets were delivered to the team as if they had just been downloaded from the spacecraft. The teams on duty then analysed the data, drew conclusions, fully documented the activity, and worked with the operations engineers to ensure the correct configuration of the mission timeline at all times. These operations training simulations typically lasted for about 1 week, with the team being split into sub-teams to take shifts at ESA’s Astronomy Center (ESAC) near Madrid. An additional set of teams was deployed to the Complementary Data Center located at the APC in Paris.

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LISA Pathfinder Science

LISA Pathfinder probed Einstein’s geodesic motion at an unprecedented level, giving us a first glimpse at what will be achieved with LISA.

Much of the experimentation in gravitational physics requires measuring the relative acceleration between free-falling, geodesic reference test particles. In lunar laser ranging experiments, the test particles are the Earth and the Moon. In Earth-based gravitational wave measurements, the test particles are the pendulum-suspended mirrors of a Michelson interferometer, which are effectively free-falling, on time scales much shorter than the pendulum period, along the line of sight connecting them. For LISA, the test masses are 2 kg cubes housed in separate spacecraft million km apart.

In LISA Pathfinder, precise inter-test-mass tracking, by optical interferometry, allowed us to assess the relative acceleration of the two LISA test masses 38 cm apart in a single spacecraft. The science of LISA Pathfinder consisted of measuring, and creating a experimentally-anchored physical model for all the spurious effects – including stray forces and optical measurement limits – that limit our ability to create, and measure, the perfect constellation of free-falling test particles that would be ideal for LISA.

A number of characterisation investigations had been designed to tease out the contribution of the various disturbances present in this complex system. These investigations formed the core of the mission timeline, with their exact ordering being driven by the observed performance of the system.

https://youtu.be/3fE8r1zdGQI?si=4JETmkDnT-9T4r9u
LISA Pathfinder operations: New science
Exploring the science behind the measurements
LISA Pathfinder is an ESA technology test mission for the LISA mission. LISA Pathfinder is testing most LISA technologies in space and will validate a complete model of all physical noise that can be extrapolated to the full LISA mission.
© Max Planck Institute for Gravitational Physics / Milde Marketing Science Communication

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The LISA Pathfinder mission

LISA Pathfinder paved the way for the LISA mission by testing in space the very concept of gravitational wave detection.

The quietest place in the solar system

LPF launched successfully on 3rd December 2015 at 4:04 GMT. After a picture perfect start, a journey to its destination some 1.5 million kilometers from Earth towards the Sun, and a successful release of the test masses, LISA Pathfinder began its job as a space laboratory on 1 March.

https://youtu.be/1j0YGl8oXbw?si=YT5WWy836CluhKAw
The Path to LISA in 90 seconds
The LISA Pathfinder Mission Teaser: Follow LISA Pathfinder’s story from the assembly of its heart, the optical bench through testing to the assembly of the spacecraft and getting it ready for launch. Meet some of the key people who are involved and learn how the mission operates.
© Max Planck Institute for Gravitational Physics / Milde Marketing Science Communication

LISA Pathfinder (LPF) placed two test masses in a nearly perfect gravitational free-fall, and controls and measures their relative motion with unprecedented accuracy. This was achieved through innovative technologies comprising inertial sensors, an optical metrology system, a drag-free control system and micro-Newton thruster system. The test-masses and their environment were at that time the quietest place in the solar system.

With LISA Pathfinder, scientists have created the quietest place known to humankind. Its performance exceeded all  expectations by far. Observing the most perfect free fall ever created was only made possible by reducing and eliminating all other sources of disturbance.

The first results, presented in June 2016, show that the two test masses at the heart of the spacecraft are falling freely through space under the influence of gravity alone. They are unperturbed by other external forces, to a precision more than five times better than originally required. The results show that the test masses are almost motionless with respect to each other, with a relative acceleration lower than 1 part in ten millionths of a billionth of Earth’s gravitational acceleration. This corresponds to the weight of a virus on Earth.

All these technologies are not only essential for LISA, they also lie at the heart of any future space-based test of Einstein’s General Relativity. The successful demonstration of the mission’s key technologies thus opens the door to the development of a large space observatory capable of detecting gravitational waves emanating from a wide range of exotic objects in the Universe.

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Merging intermediate-mass black holes

Intermediate to massive black holes with masses in the interval between 10 000 and 10 000 000 solar masses will be detected by LISA, to explore for the first time the low-mass end of the massive Black hole population, at cosmic times as early as z ~ 10.

The Gravitational Universe will make it possible to survey the vast majority of all coalescing massive black hole binaries throughout the whole universe. This will expose an unseen population of objects which will potentially carry precious information about the black hole population as a whole. It will provide both the widest and deepest survey of the sky ever, since gravitational wave detectors are non-directional in nature, and operate as non-pointed and weakly directional full-sky monitors. The range of black hole redshifts and masses that will be explored is complementary to the space explored by electromagnetic observations.

LISA will detect all binary black hole mergers even when the black holes are not active. With this unbiased and complete survey, it will be possible to investigate the link between the growing seed population with the rich population of active supermassive black holes evolving during cosmic dawn and high noon, probing the light end of the mass function at the largest redshifts.

Black hole coalescence events will illuminate the physical processes of black hole formation and feeding. While the mass distribution carries information about the seeds, the spin distribution charts the properties of the accretion flows, whether they are chaotic or coherent. Gravitational wave observations alone will be able to distinguish between the different massive Black hole formation and evolution scenarios.

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Gravitational-wave background

LISA will be able to detect a gravitational-wave background emitted at the beginning of the universe. This will help us to probe the universe just afte the Big Bang.

Image of cosmic microwave background from WMAP data.
Image of cosmic microwave background from WMAP data. Credit: NASA / WMAP Science Team

Gravitational waves are the next messengers to probe the very early Universe, after the cosmic microwave background. LISA is sensitive to gravitational waves that may have been produced when the early Universe plasma cooled below a billion billion Kelvin at roughly a billionth-billionth of a second after the big bang. Such energies (of hundreds of TeV) are inaccessible to current laboratory experiments, but may be within reach of far future particle colliders.

LISA will be able to gather information on the state of the Universe at much earlier epochs than those directly probed by any other cosmological observation, thereby helping us on our quest to understand the earliest moments of the Universe.

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Exotic sources

LISA can help us probe the unknown and test for new physics, such as the existence of extra dimensions or cosmic super-strings.

With LISA, we will continue our search for the (currently) unknown, in the hope of finding new physics. Amongst other things, we will be able to search for the existence of extra dimensions, cosmic superstrings predicted by string theory, and investigate the nature of dark matter.

The LISA frequency band corresponds to horizon scales in the early Universe at energies of the order of the TeV, where new physics, such as extra dimensions, is expected to become visible. One of the effects of the new physics are primordial first order phase transitions, proceeding through the explosive growth of broken phase bubbles, and leading to efficient gravitational wave production.

LISA will also provide a unique probe on cosmic (super-)strings, relics of the very early Universe at even higher energy scales, which are predicted in several theories unifying the fundamental interactions of nature, including string theory. Their detection would offer direct evidence for phenomena occurring at energies up to 1016 GeV and which are not observable with conventional astronomy.

Through gravitational wave detection, LISA is therefore capable of probing energy ranges much beyond the reach of present particle accelerators and gather informations on the state of the Universe at much earlier epochs than those directly probed by any other cosmological observation. Gravitational waves represent the next messenger to probe the very early Universe, after the use of the Cosmic Microwave Background.

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White dwarf binary systems

LISA will be able to detect gravitational waves from white dwarf binaries when they are still far apart.

The waves from these systems will enter the frequency band of current and future generation ground-based gravitational-wave detectors as they get closer to merger and can then be observed on Earth. Using both LISA and the ground-based observatories, we will thus be able to observe these binary systems in the gravitational-wave spectrum across a great part of their existence.

In a binary star system known as J0806, two dense white dwarf stars orbit each other once every 321 seconds.
In a binary star system known as J0806, two dense white dwarf stars orbit each other once every 321 seconds. © NASA/Tod Strohmayer (GSFC)/Dana Berry (Chandra X-Ray Observatory)

White dwarf binaries have been detected by electromagnetic observations and are known to exist in plenty across our galaxy.

LISA guarantees the detection of gravitational waves by monitoring the most promising of these known nearby galactic binary systems, which we sometimes call “verification binaries”. Gravitational waves from neutron star binaries will allow us to explore extremely dense matter systems, stellar evolution and high-energy astrophysics. LISA will detect these systems before they enter the ground-based band, allowing us to constrain their

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Extreme-mass-ratio inspirals

The inspiral of a stellar-mass black hole and supermassive black hole. Their gravitational waves will allow us to test the theory of general relativity.

When a stellar-mass black hole orbits a supermassive black hole it undergoes hundreds of thousands of orbital cycles. The smaller black hole acts as a probe, mapping out the geometry of the surrounding spacetime. This information will be encoded into the gravitational waves emitted during the inspiral of the system. With the detection of these waves we will be able to test Einstein’s theory of general relativity, by examining the force of gravity in its strongest regime within the cosmos.