The LISA space mission has been given the green light. In this crucial step, known in technical jargon as "adoption", ESA authorises the construction of the satellites and on-board instruments, with the important contribution of ASI, the Italian Space Agency. LISA, which stands for Laser Interferometer Space Antenna, will open a new window on the universe: the aim is to build a space observatory to detect gravitational waves from various cosmic sources. At the heart of ESA's Cosmic Vision scientific programme, which includes this mission, is the role of the University of Milan-Bicocca and the team led by Professor Monica Colpi of the "Giuseppe Occhialini" Department of Physics, who has held leading positions in various research groups at ESA and in the LISA Consortium, an international consortium of scientists that defined the scientific objectives of LISA and designed the mission.
LISA isn't a single spacecraft, but a trio of satellites orbiting the Sun, arranged at the vertices of an equilateral triangle. Each side of the triangle will be 2.5 million kilometres long (more than six times the distance from the Earth to the Moon), and the spacecraft will exchange laser beams over this distance. LISA is scheduled for launch in 2035 aboard an Ariane 6 rocket.
But what are the gravitational waves that LISA will observe? A century ago, Albert Einstein showed in his theory of general relativity that very massive celestial bodies, when accelerated, perturb the fabric of space-time, producing tiny ripples known as gravitational waves that travel through the universe at the speed of light. Thanks to modern technological developments, we are now able to detect the passage of these waves, which are among the most elusive in the Universe, in order to trace the nature of their sources.
LISA will detect gravitational waves from the most distant regions of the Universe, caused by the collision of massive black holes at the centres of galaxies, millions of times heavier than our Sun. This will allow scientists to discover the origin of these objects, reconstruct their history and the role they play in the evolution of galaxies. The mission will also be ready to listen to the gravitational "whisper" of the birth of our Universe, providing a window into the first moments after the Big Bang. In addition, LISA will help researchers accurately measure the expansion rate of the Universe, using gravity rather than light as a messenger, and compare the result with measurements made by other techniques and missions (such as Euclid). LISA will also observe a large number of sources in our Galaxy, including binary systems of white dwarfs and neutron stars: an unprecedented opportunity to study the final stages of stellar evolution. By measuring their position and distance, LISA will build a map of the structure of the Milky Way, observing beyond the dark curtain of the Galactic centre. Together with ESA's Gaia mission, we will learn how our Galaxy, the environment in which we live, was formed.
"The first design of LISA dates back to the 1970s: it has been a long journey which, after ups and downs, has brought us today to the 'Acceptance', which is the decisive step towards the construction of LISA," explains Monica Colpi. "The success of the LISA Pathfinder mission and the discovery of gravitational waves emitted by colliding black holes by the LIGO-Virgo-KAGRA ground-based interferometers have been crucial. With LISA, we will capture the oscillations of space-time caused by the merging of giant black holes. Here at the University of Milan-Bicocca, we are trying to understand how and when these collisions occur in the Universe and how LISA will observe them".
So how will gravitational waves be observed? LISA will use pairs of cubes made of a gold-platinum alloy - called "test masses" (each slightly smaller than a Rubik's cube) - which will float in "free fall" at the centre of each satellite, equipped with special shielding against external disturbances. Gravitational waves will cause tiny changes in the distance between the test masses of two satellites, and the mission will track these changes using laser interferometry.
This technique involves sending laser beams from one satellite to another in the constellation. By comparing the recorded signals, we will measure changes in the distances between the test masses to within a billionth of a millimetre. The satellites must be designed so that nothing but the geometry of space-time can disturb the movement of the masses, which will therefore be in near-perfect free fall. The mission's satellites will follow in the footsteps of LISA Pathfinder, which demonstrated that it is possible to maintain test masses in free fall with an impressive degree of precision.
The same propulsion system used on ESA's Gaia and Euclid missions will ensure that each satellite maintains the required position and orientation with great accuracy.
To illustrate the complexity of the operation, Riccardo Buscicchio, a researcher at Milano-Bicocca working on the analysis of data produced by LISA, uses a musical metaphor: "Ground-based detectors today receive isolated signals, one after the other, much like listening to short concerts of solo violin. The typical timbre of the instrument allows us to identify it, even in the presence of 'noise'. LISA's satellites will instead be listening to an extremely loud concert, performed by instruments that are out of tune and out of sync for the entire duration of the space mission. Nevertheless, the orchestra will be made up of millions of strings, woods, brass and percussion. Buscicchio concludes: "My work at the University of Milan-Bicocca is to transcribe the scores of the concert, starting from a single high-fidelity recording and extracting as many instruments as possible, including those whose existence we do not yet know".
"Now that LISA has been 'adopted' by ESA, its realisation requires a significant contribution from the entire international scientific community," adds Alberto Sesana, astrophysicist and professor at the department working on the project. "In Italy, this effort is becoming increasingly concrete, with a long-standing collaboration between the University of Milan-Bicocca and other Italian universities".
Selected as a flagship mission of ESA's Cosmic Vision 2015-2025 programme, LISA will be part of ESA's fleet of "cosmic observers" to address two profound questions: What are the fundamental physical laws that describe the Universe? How did the Universe form and what is it made of? In this adventure, LISA will work in conjunction with NewAthena, another ESA mission currently in the study phase. NewAthena will be the largest X-ray observatory ever built in space and is scheduled for launch in 2037.
ESA is leading the LISA mission and will provide satellites, launchers, mission support and data acquisition. The ultra-stable lasers, 30 cm diameter telescopes to collect the laser light, and ultraviolet light sources to neutralise the electrostatic charge on the test masses will be provided by NASA. Other key components include: test masses shielded from external forces, provided by ASI Italy with contributions from Switzerland; the picometre-precision interferometric signal measurement system (one trillionth of a metre), provided by Germany, the UK, France, the Netherlands, Belgium, Poland and the Czech Republic; the Science Diagnostics Subsystem (an arsenal of sensors on board satellites), provided by Spain.