LISA’s gravitational wave adventure set for 2030s

Gravitational Waves

Gravitational waves are the current cutting edge of space research. These ripples in space and time allow us to ‘hear’ the universe, and by detecting more of them, we hope to better understand the large scale structure and violent interactions of our universe. Gravitational waves were originally predicted by Albert Einstein in 1916, but it was a full century before the first gravitational wave was detected, back in September 2015 by the ground-based detector called LIGO. This first detection is thought to have been created by the catastrophic collision of two massive black holes more than a billion years ago.

Around the same time as the first detection, the European Space Agency (ESA) laid out their ‘Cosmic Vision’, which is their space exploration plan for the next decade. One of the goals of this vision was to further understand gravitational waves. The spacecraft nominated to take up this mantle is called LISA, or the Laser Interferometer Space Antenna. The LISA mission is one of ESA’s large satellite missions planned for the early 2030s, however in order to ensure a successful mission, a test of the technology was needed. For such a test, ESA launched LISA Pathfinder in 2015.

Read more: What are gravitational waves?

LISA Pathfinder

LISA Pathfinder consists of a single spacecraft, with two free floating cubes inside that represent the final LISA mission. It was launched to a special point in space between the Earth and the Sun called the L1 Lagrange Point where the gravitational pull from each is balanced.

Once there, LISA Pathfinder monitored the floating cubes to prove that maintaining an accurate separation would be possible. This is vitally important for the upcoming LISA mission and something that had not attempted before. The two cubes had to be held in such a way that they remained precisely 40 centimetres away from each other for the entire mission.

This is a massively scaled-down from the full version of the mission. When LISA launches in the 2030s, three separate spacecraft fly 2.5 million kilometres apart! The reason for this huge separation is to increase LISA’s ability to detect less powerful, but more plentiful gravitational waves.

How will LISA work?

The reason the spacecraft need to be held in these very precise positions is due to the how gravitational waves are detected. As the three LISA spacecraft orbit each other, they will beam a laser between each other forming a triangle. If there is currently a gravitational wave passing between these three spacecraft it will distort space such that the distance travelled by each laser is different, causing them to be out of sync. These measurements can then show the exact nature of the wave detected. This technique was originally devised in the 19^th century and was originally used to measure the speed of light. More recently we have used it in ground-based gravitational wave experiments such as LIGO. Down on Earth, we can only manage detector lengths of a few kilometres. The move to space will hugely increase this distance, and as a result, the sensitivity of the detector.

Success of LISA Pathfinder

For the last 18 months LISA Pathfinder has been out conducting these tests. And the results have shown that the technology has far exceeded all expectations. It not only reached the required level of precision for the test mission, but towards the end, was approached the performance expected from the full LISA mission. With such a successful set of results, ESA recently announced that the LISA Pathfinder mission would conclude at the end of the month and that they would press ahead with plans for the full mission. The date for LISA’s launch has been set as 2034 following several other large ESA missions.