#AESASpazio: L.I.S.A. or: How We Moved from Staring at the Stars to Feeling the Fabric of Reality
- Skrijelj Nicolas
- 22 mag
- Tempo di lettura: 4 min
Thousands of years have passed since the Babylonians compiled the first known astronomical catalogues. Then, just two centuries ago, William Herschel's discovery of infrared radiation, followed by Johan Wilhelm Ritter’s identification of ultraviolet radiation in the early 1800s, revealed the vastness of the electromagnetic spectrum. This exploration ultimately led to the elegance of Maxwell's equations, which unified electricity, magnetism, and light.
In 1931, the birth of radio astronomy by Karl Jansky marked another leap in our ability to study the universe, as scientists and engineers developed telescopes capable of observing the cosmos through the full electromagnetic spectrum. However, a not so quiet theory proposed a radically different approach to studying not only the universe but reality itself.
In 1916, with the publication of Einstein’s General Relativity, gravity ceased to be viewed merely as a force. Instead, it was the curvature of the space-time fabric, shaped by mass and energy.
GRAVITATIONAL WAVES
Einstein’s field equations, formulated in 1915, describe how the geometry of spacetime is influenced by the distribution of energy and mass. These are nonlinear partial differential equations, extremely difficult to solve in their general form. By applying weak-field approximation (weak gravitational field and velocities far below speed light), physicists were able to linearize the equations and derive wave solutions, which suggested the existence of perturbations in the fabric of spacetime.
To this day, the generally accepted description of gravitational wave mechanics is Einstein’s tensorial formulation of General Relativity. The perturbation is caused by inhomogeneities in the mass distribution of a body, representing deviations from perfect spherical symmetry, when the body is subjected to intense mass redistribution.
Gravitational waves (GW) propagate at the speed of light and induce periodic distortions in spacetime, oriented perpendicularly to the direction of propagation. The effects are contraction and expansion along two orthogonal axes. Technically, gravitational waves possess two polarization states, known as plus (+) and cross (×), each corresponding to a distinct deformation pattern in the transverse plane.
GWs amplitude is measured by the ratio between the change of length and the original length, the most intense GW measured, GW150914 revealed by LIGO in 2015, has a strain of 10^-21, that means a distortion thousands times smaller than the diameter of a proton over the 4km of the measured length. The frequency of GW150914 started from 35Hz since the start of the signal to 250Hz at the end.
In 2016 LIGO (Laser Interferometer Gravitational-waves Observatory) and VIRGO collaboration announced that in 2015 the first gravitational wave has been revealed, confirming Einstein’s hypothesis.
Interferometers measure small variations in distance using the principle of interference between light waves. A laser beam is split into two identical paths typically along perpendicular arms. When the beams are reflected and recombined, any difference in path length causes constructive or destructive interference, altering the resulting intensity pattern. By looking at the interference it is possible to reveal the time travelled by the two beams, consequently it’s possible to trace back the modified distances.
L.I.S.A.: LASER INTERFEROMETER SPACE ANTENNA
After the announcement in 2016 of the first direct detection of gravitational waves, ESA selected LISA in 2017 as the L3 mission of the Cosmic Vision program, part of ESA’s 2015–2025 science cycle.
Since ground-based detectors cannot measure gravitational waves below 10 Hz due to seismic and atmospheric noise, the LISA project proposed placing an observatory directly in space, but it’s easier said than done.
LISA will study gravitational waves that are produced by merging stellar mass, black holes, supermassive black holes and white dwarfs. It will also detect waves produced by extremely compact objects, such as neutron stars and small black holes falling into a supermassive black hole.
The observatory will consist of three spacecraft arranged at the vertices of an equilateral triangle. The system will orbit the Sun in a heliocentric trajectory, trailing the Earth by approximately 20 degrees.To detect gravitational waves in the frequency range between 0.1 mHz and 0.1 Hz, the spacecraft must maintain an arm length of about 2.5 million kilometers between each laser interferometer.
But orbital positioning isn’t the only issue. LISA is designed to detect changes in the length of the order of magnitude of 10^-11 so there must be strict requirements for thermal management and error corrections in the attitude of the spacecrafts, due to solar radiation, electrostatic forces and residual charges.
The mission lifetime is expected to be 4 years, with the possibility to extend it to 10 years.
But before even starting with the development and spending of billions of euros and dollars, scientists and engineers needed to be sure if the theoretical calculations are accurate. This is why L.I.S.A PATHFINDER was launched.
L.I.S.A. PATHFINDER
LISA Pathfinder was a technology demonstrator mission designed to validate key systems for future space gravitational wave observatories. Launched in 2015, it operated around the Sun-Earth L1 Lagrange point. The spacecraft effectively miniaturized one arm of the LISA interferometer, from 2.5 million kilometers down to just a few tens of centimeters.
According to Einstein’s General Relativity, free-falling masses follow geodesics in curved spacetime. The mission's primary goal was to minimize and characterize non-gravitational disturbances acting on two test masses made of a gold-platinum alloy, which constituted the main scientific payload. The objective was to precisely measure the relative motion between the masses with picometer-level accuracy, under near-perfect free-fall conditions.
The mission was a success demonstrating the feasibility of the L.I.S.A. mission.
ENGINEERING AT ITS FINEST
This project is groundbreaking in engineering and experimental physics; it’s an ambitious project that involves several cutting-edge technologies:
INTERFEROMETRIC MEASUREMENT SYSTEM: Each spacecraft exchanges laser beams with the others to measure changes in distance with picometer precision.
DRAGFREE AND ATTITUDE CONTROL SYSTEM: This is probably the most important sub-system on board. It must control the spacecraft dynamics to maintain the requirements on the strictly precise pointing and relative velocities caused by non-gravitational perturbations.
LISA exemplifies what can be achieved when the international community comes together around the same objective. Beyond advancing our understanding of the universe’s most violent and enigmatic phenomena, LISA’s innovations in sensing, drag-free control, and data analysis will enhance drastically Earth-based applications such as satellite navigation, telecommunications, and environmental monitoring. This mission proves that international cooperation is the key to solving the most complex challenges.
A CURA DI
Skrijelj Nicolas
FONTI
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