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Prototype Gravitational Wave Spacecraft Sets New Free Fall Record
Hypothesized by Albert Einstein a century ago, gravitational waves are oscillations in the fabric of spacetime, moving at the speed of light and caused by the acceleration of massive objects.
They can be generated, for example, by supernovas, neutron star binaries spiralling around each other, and pairs of merging black holes.
Even from these powerful objects, however, the fluctuations in spacetime are tiny by the time they arrive at Earth – smaller than 1 part in 100 billion billion.
At the heart of the experiment is a two-kilogram cube of a high-purity gold and platinum alloy, called a test mass. The cube is nestled inside the shell-like LISA Pathfinder spacecraft, and has been in orbit since February 2016. The researchers found the test mass could be sufficiently stable and isolated from outside forces to fly in space and detect a whole new range of violent events that create gravitational waves.
The LISA Pathfinder spacecraft is equipped with electrodes adjacent to each side of the test mass cube to detect the relative position and orientation of the test mass with respect to the spacecraft. An array of tiny thrusters on the outside of the spacecraft compensates for forces that could affect the test mass orbit, chiefly including the pressure from the solar photon flux.
The mission is a crucial test of systems that will be incorporated in three spacecraft that will comprise the Laser Interferometer Space Antenna (LISA) gravitational wave observatory scheduled to launch in 2034. The LISA observatory will follow a heliocentric orbit trailing fifty million kilometers behind the Earth. Each LISA spacecraft will contain two test masses like the one currently in the LISA Pathfinder spacecraft. The LISA Pathfinder mission’s success is a crucial step in developing the LISA observatory.
Researchers report that the system reduces acceleration noise between the test masses to less than 0.54 x 10-15 g/(Hz)^½ over a frequency range of 0.7 mHz to 20 mHz. The noise in this range is five times lower than the LISA Pathfinder design threshold, and within a factor of 1.25 of the LISA observatory requirements. Above 60 mHz, acceleration noise is two orders of magnitude better than design requirements. According to the researchers, the measured performance of the Pathfinder mission systems would allow gravitational wave observations close to the original plan for the LISA Observatory.
Results from only two months of science operations show that the two cubes at the heart of the spacecraft are falling freely through space under the influence of gravity alone, unperturbed by other external forces, to a precision more than five times better than originally required.
In a paper published today in Physical Review Letters, the LISA Pathfinder team 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 gravity
This experiment saw the characteristic signal of two black holes, each with some 30 times the mass of the Sun, spiralling towards one another in the final 0.3 seconds before they coalesced to form a single, more massive object.
The signals seen by LIGO have a frequency of around 100 Hz, but gravitational waves span a much broader spectrum. In particular, lower-frequency oscillations are produced by even more exotic events such as the mergers of supermassive black holes.
With masses of millions to billions of times that of the Sun, these giant black holes sit at the centres of massive galaxies. When two galaxies collide, these black holes eventually coalesce, releasing vast amounts of energy in the form of gravitational waves throughout the merger process, and peaking in the last few minutes.
James Riordon
American Physical Society
ESA
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