The European Space Agency has successfully tested out a technology needed to build vehicles that can detect gravitational waves from space. The work was done by the LISA Pathfinder mission — a team trying to prove that a wave-detecting spacecraft can actually work. The mission team said the technology they tested exceeded their "most optimistic expectations."
The technology they tested exceeded their "most optimistic expectations"
Detecting gravitational waves — ripples in space-time that result from objects moving through the Universe — is a very difficult process that involves measuring tiny variations in the positions of objects. Those measurements can get mucked up easily on Earth, which is a noisy place filled with lots of vibrations and movements that can throw off results. That’s why scientists want to move gravitational wave detection to space, where there are far less forces to control for. But creating the right kind of setup is still very complicated.
To test out the tech needed for a space-based observatory, the LISA Pathfinder mission created a smaller test spacecraft, which launched into space in December 2015. The main goal was to see if two objects inside the spacecraft could remain in a constant state of free fall. To remain constantly in free fall means the objects must stay completely motionless; they can't even touch the spacecraft. The ESA scientists demonstrated that they were able to create this free fall state, as well as greatly reduce all outside forces on the objects inside the spacecraft, according to a new study published in Physical Review Letters.
Okay, but why does perpetual free fall matter to anyone except Tom Petty? Well, it all has to do with how the space-based wave detectors would be set up. Eventually, ESA wants to launch three vehicles that would position themselves in a triangle formation in space; each probe would be hundreds of thousands of miles apart from the other two. The spacecraft would then bounce lasers off of objects housed inside the other two probes. Those objects would be an identical pair of 1.8-inch cubes made of a gold-platinum alloy. The tiny movements of these cubes relative to the spacecraft could indicate whether or not a gravitational wave has passed by.
These cubes need to remain in perpetual free fall for the system to work
These cubes need to remain in perpetual free fall for the system to work. That way, their positions are only affected by gravitational waves. This is why the cubes can’t touch the spacecraft carrying them; the vehicles will have to float around the cubes. To do this, each spacecraft will be equipped with micro-thrusters, or tiny little engines that can help adjust the probe's positions to make sure the cubes remain untouched. The spacecraft also have the added responsibility of shielding the cubes from outside forces; tiny things like gas molecules or even cosmic rays can influence the position of the gold cubes, but the probes have different tools they can use to lessen those forces.
It all makes for a very complex spacecraft, so the LISA Pathfinder mission was set up to see if such a vehicle could even be built. But after analyzing the test spacecraft in the lab, the technology worked even better than expected. The team was able to reduce outside forces on the cubes more so than they needed to. "The measurements have exceeded our most optimistic expectations," said Paul McNamara, a scientist with LISA Pathfinder, in a statement. "We reached the level of precision originally required for LISA Pathfinder within the first day, and so we spent the following weeks improving the results a factor of five."
A space-based detector could cover a much wider area
A space-based observatory could help us make even more observations of gravitational waves. So far, we’ve only tried picking them up from Earth, which involves detecting waves from huge, cataclysmic events up to a billion light years away. Massive objects moving at super high speeds, such as merging black holes or collapsing neutron stars, generate enormous, high-frequency waves that can be detected from Earth. But these waves weaken a lot by the time they reach our planet, which makes picking them up really difficult. That's why it's taken nearly a century for scientists to measure gravitational waves directly. It wasn't until February of this year that scientists at LIGO — or the Laser Interferometer Gravitational-Wave Observatory — announced that they had detected gravitational waves for the first time.
But a space-based detector could cover a much wider area of space than LIGO can. That means it could pick up waves coming from slower moving objects, something that Earth-based wave detection can’t really do.