Laser Interferometer Space Antenna’s Cosmic Quest

Last month, work began on the world’s first space-based gravitational wave detector. The European Space Agency (ESA) and partner aerospace companies are developing orbiting detector spacecraft to observe gravitational waves from some of the universe’s most massive objects.

The Laser Interferometer Space Antenna satellite constellation (LISA) will be three satellites orbiting the sun in a triangular formation, each separated from the other by 2.5 million kilometers, trailing 60 million km behind Earth. Each of LISA’s three spacecraft, expected to launch in 2035, will shoot laser beams at the other two to measure distances between the satellites down to the distance of a picometer (1 trillionth of a meter).

Detecting interference patterns in the laser light, LISA is expected to discover whole new ranges of gravitational waves, the mysterious ripples in spacetime triggered by collisions of extremely massive objects such as black holes and dense stars.

The nearly $2 billion mission, led by the ESA, will be a next-generation observatory, joining the ranks of the current workhorse of gravitational wave physics, the Nobel Prize–winning Laser Interferometer Gravitational-Wave Observatory (LIGO), in Livingston, La., and Hanford, Wash. Astrophysicists hope LISA will fill major gaps in their understanding of the evolution of supermassive black holes, the gargantuan monsters that reside at the center of galaxies such as our Milky Way.

“Ground-based detectors can measure gravitational waves with frequencies from about 20 hertz to up to a few kilohertz,” says Guido Mueller, a professor of precision interferometry at the Max Planck Institute for Gravitational Wave Physics in Germany and a member of the LISA science consortium. “In the future, we might be able to push toward 1 hertz on the ground, but below that, it will be virtually impossible because of the sources of noise present on Earth.”

An orbiting detector like LISA will be able to capture those slower oscillating gravitational waves, thus opening entirely new possibilities in black hole research.

The frequency of gravitational waves is determined by the energy of the collision that gives rise to them. LIGO does a good job picking up waves generated by collisions of smaller black holes—up to a hundred times as massive as our sun. However, it can’t detect collisions of galactic supermassive black holes with the mass of millions of suns. Mergers of these monsters produce slow millihertz waves, according to Mueller. And those can only be detected from space.

How LISA Will See the Universe’s Biggest Black Holes

In addition to the perfect silence of space, LISA will also benefit from the vast distance between the three spacecraft of the interferometer, enabling it to detect much fainter signals than the Earth-based LIGO.

“In space we can have a much larger interferometer,” said Frank Steier, lead system engineer for the LISA Spacecraft, at OHB Systems, the lead contractor building the LISA satellites. “On the ground we are limited to a distance of a few kilometers, but in space we can have the instrument operate across distances of millions of kilometers.”

At the heart of each LISA spacecraft are two free-floating golden cubes, about the size of a Rubik’s cube. These cubes function as test masses as well as mirrors, reflecting the laser light shot from the other two spacecraft. When a gravitational wave passes the spacecraft, the golden cubes’ positions change by up to a nanometer. With a slight expansion or compression in spacetime, the travel time of the laser light shortens or expands. By studying tiny variations in the interference patterns from the combined laser signals, researchers can study both the gravitational waves and the nature of the supermassive bodies that created them.

ESA tested the basics of the LISA system in a precursor mission called LISA Pathfinder, which orbited Earth between 2015 and 2016. Still, many technical challenges remain to be solved. NASA’s Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission is currently using a simpler interferometry system to study Earth’s gravitational anomalies using two satellites orbiting Earth 220 km apart. But LISA’s “long-arm” interferometer, spanning 2.5 million km, is a complete novelty, said Mueller.

  European Space Agency director of science Carole Mundell [left] shakes hands with Chiara Pedersoli [right], CEO of Bremen, Germany’s OHB Systems—lead contractor building ESA’s LISA gravitational wave detector.M. Polo/ESA

The optical bench recombining the laser beams will be much larger and more complex than what’s on LISA Pathfinder and Grace FO missions, according to Mueller. LISA’s phase measurement system (a.k.a. its phasemeter) must be able to measure the phase evolution of the laser signals and the time variations in its travel time with unprecedented accuracy.

To keep the golden cubes in perfect free fall, engineers are developing a Drag-Free Attitude Control System (DFACS), which will keep the hexagonal, 800-kilogram satellites perfectly centered around the cubes. The system will detect the most minuscule changes to the spacecraft’s attitude and nudge it back.

“We have to make sure that we manage the gravity of the satellite in a way that it doesn’t disturb the measurements,” said Steier. “That’s something you only need for a mission like LISA.”

Black Hole Ringdowns and Other Observational Targets

In addition to capturing gravitational waves oscillating at very slow frequencies, LISA will be able to record much longer progressions of gravitational waves than LIGO. It will thus provide deeper views into the cataclysmic events that gave rise to them.

From the moment two black holes enter each other’s gravitational field, the spacetime around them begins to quiver. Although the two bodies merge within a split second, the new black hole continues to emit gravitational waves as it settles down. This is called a “ringdown.” The LISA detector will be able to detect gravitational waves produced at various stages of black hole mergers in the distant universe—including approach, merger, and ringdown. LISA is also being designed to detect gravitational waves triggered by collisions of small black holes and neutron stars in the Milky Way galaxy.

“These black hole mergers take millions of years. But on Earth we can only detect a few seconds at best. Then it’s gone,” Steier said. “With LISA, we will have a much greater sensitivity and will be able to receive much longer signals. It will not be just a ping but a long continuous signal, which will allow us to observe the evolution of the merging system for months or even years.”

Science and engineering teams from 10 European countries have begun developing the range of hardware elements needed to make the mission work—including the precision optics, avionics, and golden test masses, noted above. However, non-engineering challenges may also lie ahead. The LISA mission is a collaboration with NASA, but the Trump administration has expressed intentions of cutting its contribution to LISA. NASA was supposed to develop twin telescopes that will be placed on board each LISA spacecraft to transmit and receive the powerful laser beams.

Mueller said he hopes Europe would be able to cover the shortfall if those cuts were to make it through the U.S. Senate.

“If they drop off now, it’s much better than if they drop off five or six years from now,” Mueller said. “The earlier, the better, as we can actually start our own program developments and not be hit with a large schedule penalty.”