Gravitational wave detector LISA officially adopted by ESA as mission

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The Laser Interferometer Space Antenna (LISA) opens up a new part of the gravitational wave spectrum as the first space-based detector. It is now officially adopted by the European Space Agency (ESA) as one of its large class missions. Launch is scheduled for the mid-2030s. The Netherlands plays a large role in the development of hardware and software.

LISA consists of three spacecraft orbiting the Sun, trailing behind Earth. By continuously measuring their mutual distances using laser beams, they detect gravitational waves from various sources the Universe. Those are ripples in the fabric of space created by exotic cosmic events-such as colliding black holes or the Big Bang-comparable to an oil tanker creating waves in the ocean. As the first ever space-based gravitational wave detector, LISA becomes ESA’s third L-class mission, making Europe the world leader in this field.

Unlike ground-based detectors, LISA is able to stretch her arms to 2.5 million kilometers, making her susceptible to longer wavelengths from larger objects. It allows us to ’listen’ for the first time to for example the Big Bang or baby black holes from the early Universe. Or to the chaotic orbits of stars as they are being engulfed in the complex geometry around a supermassive black hole, as a practical exam for Einstein’s general theory of relativity.

’It is wonderful to see LISA progress to this final phase,’ says Gijs Nelemans (SRON/Radboud University). ’The first plans for such a completely new type of mission are more than 30 years old.’ Elena Maria Rossi (Leiden University) adds: ’We are entering an entirely new field of science. Ground-based detectors already let us listen to the nearby Universe. LISA gives us hearing at much lower frequencies so we can hear the largest and farthest events in the cosmos for the first time.’

Dutch contribution

Dutch scientists have been involved in LISA for a long time. A Dutch LISA consortium consisting of SRON, Nikhef, Radboud, Leiden University, UvA, Utrecht University, TNO, Maastricht University and RUG, has received an NWO Roadmap grant in 2023, led by SRON, to work on the Dutch hardware contribution to LISA. They are building the photodiodes (LISA’s ’eyes’), software, pointing mechanism and corresponding readout electronics. These are all designed at the edge of what is currently possible, and beyond.

Each laser needs to be pointed at a moving lens 2.5 million kilometers away, which takes light eight seconds to reach. For comparison: if a dime falls down from the Eiffel Tower, a laser beam from The Netherlands must aim exactly at the spot where that dime will be in eight seconds.

For the photodiodes things are not much easier. They must detect the laser beams of originally 1 Watt, like a table lamp. But during their journey the beams diverge, delivering only 250 picoWatt to the photodiode-less than a billionth of the power of said lamp.

The development of the software is complex even for a space telescope. Especially in the LISA mission a synergy is required between engineers, scientists and data science professionals. The software áhas to distinguish between the multitude of gravitational waves from all possible directions that continuously vibrate the spacecraft at different frequencies and amplitudes.

’The Dutch contribution is a striking example of collaboration and innovation within the space sector in The Netherlands,’ says Michael Wise, SRON director. ’Not only do people from multiple disciplines work together, this is also a huge collaborative effort across scientific institutes and companies. Together we are pushing the boundaries of what is technologically possible. At SRON we are proud to be leading this effort.’

Mission Adoption

In an ESA mission lifetime cycle, the Mission Adoption Review is the formal end of the study phase, in which the requirements for the mission are determined and the technology readiness is reviewed. Following LISA’s successful review, ESA’s Science Programme Committee (SPC) approved the transition from the study phase to the implementation phase. This step is called ’Mission Adoption’. The following implementation phase lasts until launch in the mid-2030s.