Yesterday, the European Space Agency said goodbye to one of its spacecraft that has helped pave the way for a new method of studying the Universe: the LISA Pathfinder, a car-sized probe that has been testing out technology needed to detect ripples in the fabric of space-time — called gravitational waves — from space.
LISA Pathfinder completed its mission in June with resounding success. Now, the ESA has shut down the vehicle and sent it on a course far away from Earth — that way the spacecraft doesn’t become another piece of space junk that interferes with future missions.
LISA Pathfinder was always meant to be the opening act for a future mission called the Laser Interferometer Space Antenna, or LISA. Meant to launch sometime in the 2030s, LISA will consist of three spacecraft that will live in orbit around the Sun and form the first space-based observatory for detecting gravitational waves.
So far, gravitational waves have only been detected a few times from Earth, but a space-based detector like LISA could potentially pick up even more. That’s because detecting these space-time ripples is an incredibly delicate process, one that involves measuring tiny changes in the positions of objects spread across vast distances.
Earth has only so much room that can be used for detection, and it’s a pretty active place, filled with vibrations and forces that can mess with these tiny measurements. Space has lot less of this “noise” than Earth does. It also has a lot more room to spread out for detection, making it far more ideal.
Still, creating a space-based gravitational wave detector is an incredibly complicated feat of engineering, involving new technologies that have never been used in space before. Once LISA is in space, there will be no way to fix it if something goes wrong. That’s why ESA launched the LISA Pathfinder mission at the end of 2015: to test out crucial tech that will be used on LISA, as well as to better understand what the spacecraft will experience in the space environment. ESA engineers can now use what they’ve learned from LISA Pathfinder to create the best of version of LISA possible.
LISA is actually meant to work a bit like the Laser Interferometer Gravitational-Wave Observatories, or LIGOs, in Washington and Louisiana, which made the very first gravitational wave detection ever in 2015. These facilities are specifically designed to pick up waves coming from massive objects that are rapidly moving in the distant Universe. In reality, everything that moves causes ripples in space-time — from a person walking down the street to an orbiting planet. But those types of waves are way too weak to detect here on Earth. Supermassive objects moving incredibly fast, like black holes spinning around each other, generate much bigger waves that we can measure, but by the time these waves reach Earth, they’ve diminished in size quite a bit, which means we need special instrumentation to pick them up: the LIGOs.
Each LIGO is shaped like a giant “L.” Each line of the L is a 2.5-mile-long tunnel, with mirrors suspended at each end. The mirrors measure when a gravitational wave passes: a passing wave will make each mirror move slightly, just one ten-thousandth the size of a proton. The mirrors’ movement is measured by a laser; the length of time it takes the laser to bounce reveals whether a wave has passed.
LISA takes the same concept of LIGO and scales it up — a lot. In space, the three LISA spacecraft will position themselves 1.5 million miles apart from one another in a triangle. Laser beams will connect each vehicle to the other two. Instead of bouncing off of mirrors, however, the lasers will hit gold-platinum cubes that are essentially floating independently inside the spacecraft. When a wave passes by, the lasers can measure the shifts in space-time around the cube, which will appear to move.
In order to work, those cubes have to be in a state of constant free fall; they cannot touch the spacecraft shielding them and must be unperturbed by outside forces, like solar radiation or tiny particles. So the cubes will live in a little cavity inside the spacecraft, which will be constantly adjusting their position with super small thrusters so the cube and spacecraft don’t touch. These little engines are incredibly teeny, producing just 30 microNewtons of thrust, which is about the weight of a mosquito, says Thorpe. A thruster firing at full strength would “be like one mosquito landing on the side of the spacecraft and moving it slightly,” he says.
LISA Pathfinder demonstrated that it could keep two golden cubes in a constant state of free fall. The mission also allowed scientists to create a model of how much noise the spacecraft can handle out in space and still do its job. That will be critical when it comes to building the LISA spacecraft over the next decade, says Thorpe. If any changes have to be made to LISA’s design, the engineers will know exactly what changes to the spacecraft are acceptable and which ones will prevent the vehicle from working properly.
But moving forward with LISA meant putting LISA Pathfinder to rest. By 2PM ET yesterday, the mission team had sent all the final commands to the spacecraft, turning off all its instruments and its radio antenna. LISA Pathfinder is situated a million miles from Earth, but there’s still the possibility that radio waves from the spacecraft could interfere with other satellite communications and radio telescopes. So ESA fired up the vehicle’s thrusters in April and sent it into an orbit around the Sun. That way there’s an incredibly small chance it will come near the Moon and Earth in the next 100 years.
Thanks to LISA Pathfinder, ESA is feeling very confident about LISA, which could help pick up whole new types of waves. Because of the vast distance it can cover, LISA may be able to pick up massive objects that are moving much more slowly than what LIGO can see. Potentially, LISA could see the slow mergers of supermassive black holes at the centers of galaxies or super-dense stars that are slowly spirally in toward each other. When these objects combine, they start to spin around each other more and more rapidly before finally merging in an explosive force. There’s even the possibility that LISA will be able to see a black hole merger before LIGO can see it.
“We’ll be able to tell what day and what part of the sky a merger will occur,” says Thorpe. “It changes the paradigm… We’ll be able to give advanced notice and approximate areas of where to go look.”