Science Of...

To Detect Gravitational Waves, ESA Is Putting Spacecraft In Orbit Around The Freakin' Sun

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Humans have sent spacecraft to some really impressive places—the moon, Mars, Pluto, even beyond the reaches of our solar system. One place we've never gone? The sun. It's 1.3 million times bigger than our planet and reaches temperatures above 27 million degrees in its core. Bottom line? It's daunting. But the European Space Agency isn't scared. They want to detect gravitational waves, and they figure sending three spacecraft to orbit the sun is the best way to do that.

Do The Wave

What are gravitational waves, exactly? They're a phenomenon predicted by Albert Einstein's general theory of relativity: any accelerating objects massive enough (like a pair of neutron stars or black holes orbiting one another) should wreak such havoc on spacetime that they send out "waves" of distorted space around them, like ripples around a penny tossed in a fountain. On September 14, 2015, scientists made the first direct observation of gravitational waves. It was, to put it mildly, a pretty big deal.

Scientists made their huge discovery using a pair of Earth-based detectors known as the Laser Gravitational Wave Observatory (LIGO). Each one splits a laser beam down two arms of an L-shaped observatory with mirrors at each end. Under normal circumstances, those lasers would travel the same distance to the mirrors and the same distance back, canceling each other out when they arrived. But if a gravitational wave popped in to distort spacetime, the distances would briefly mismatch, and a light detector would tell researchers what happened. The fact that there are two of these observatories helps researchers pinpoint where the wave came from—it's akin to the way you have better depth perception with both eyes open than you do with just one.

LISA Pathfinder in front of the space chamber at IABG.

You Might As Well Be Orbitin' The Sun

But you can only do so much on Earth. When you're looking for the tiniest changes in the fabric of spacetime, earthquakes and other disturbances can certainly gum up the works. There's also the matter of having enough land. LIGO's "arms" were a respectable 4 kilometers (2.5 miles) long, but for a detector like this, the longer the better. That's why the ESA is looking to space for its next detector. Their Laser Interferometer Space Antenna (LISA) will consist of three detectors orbiting the sun in a triangle, separated by a whopping 1 million kilometers (more than 621,000 miles) and connected by three arms of a laser interferometer.

Why the sun? For one thing, it's big enough for the laser distances they're aiming for. That extra distance means that LISA is much more sensitive, and is able to pick up lower frequency, longer duration waves—the kind you'd get from space objects with more mass and in wider orbits. The sun is also far enough away from other objects that their gravity won't interfere.

The key to LISA, however, is inside of each craft: two identical cube-shaped, gold-platinum masses sit extra-still in a state of free-fall. The presence of a gravitational wave would change the distance between these masses, and register the wave's existence. ESA has already performed a successful test flight of one LISA spacecraft, which contained the two test masses and a smaller version of one laser arm. With that, there may be nothing stopping them. Next stop, the sun!

Golden Cubes and Gravitational Waves: ESA's LISA Mission

What Are Gravitational Waves?

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