The problem with gravity waves is that having finally detected them after a century of trying, scientists now have to figure out what to do with them. To this end, ESA is soliciting proposals from European scientist for its eLISA L3 space mission slated to launch in 2034. Part of ESA's Cosmic Vision plan, the eLISA invitation is based on recommendations from the Gravitational Observatory Advisory Team convened in 2014, which called for a multi-satellite mission using free-falling tests masses linked over millions of kilometers as a means of detecting gravity waves.

Gravity waves were long theorized by Einstein's General Theory of Relativity, but weren't detected directly until September 2015 by the ground-based Laser Interferometer Gravitational-Wave Observatory and Virgo collaborations. They are important not only because they prove a key part of Relativity, but also because gravity waves provide a unique insight into the nature of the universe.

Unlike all other radiations, there's no negative version of gravity and nothing can block it, because it's an actual bending of the space-time continuum. This means gravity waves can travel from anywhere to anywhere and through anything without interference, so astronomers regard them as the cosmic equivalent of X-ray vision – yet they put real X-rays to shame. Using gravity waves, scientists will see farther in space and back in time than ever before.

Unfortunately, gravity waves are very difficult to detect because generating waves of a high enough amplitude requires energetic events on a cosmic scale. For example, the 2015 signal was generated by two black holes, each 30 times the mass of the Sun, colliding about 1.3 billion light-years away, and the second signal recorded last December required the collision of two objects seven and 14 times the mass of the Sun.

Part of the problem is that Earthbound detectors are limited in what sort of gravity waves they can detect. They're restricted to about 100 Hz, but a space observatory using multiple satellites linked by lasers over vast distances can pick up lower frequency waves in the range of 1 Hz down to 0.1 mHz. According to ESA, by using these lower frequencies, it should be possible to focus on more exotic cosmic objects, like supermassive black holes a billion times more massive than the Sun found at the center of giant galaxies, or events like a neutron star spiraling into a supermassive black hole.

To do this, ESA plans to build on the success of its Laser Interferometric Space Antenna (LISA) Pathfinder mission, which launched in December 2015. This demonstrator mission uses two electrostatically-suspended 4.5 cm (1.7-in) gold-platinum test masses that float without connection to the LISA science module and are constantly measured by a laser interferometer to within one billionth of a millimeter. A micro-Newton microthruster system then recenters the masses by shifting the spacecraft, so it maintains its station with great precision.

The space agency says that in its first two months of operation, LISA has demonstrated it is possible to eliminate outside interference on the test masses and maintain the level of precision that gravity wave detection requires.

The lessons from LISA will eventually be used in 2034's eLISA. This is ESA's "L3 mission" (L for Large) and will follow the 2028 launch of ESA's L2 mission, which will be an advanced X-ray observatory. It will use a constellation of three spacecraft, which will fly in formation to form a high precision Michelson interferometer floating in outer space with a baseline of one million km (620,000 miles).

However, ESA says that to make eLISA possible more work needs to be done, especially in the area of building more powerful lasers and highly stable telescopes to link the unmanned probes across millions of kilometers.

ESA will be accepting letters of intent until 15 November and full proposals until January 16, 2017. A preliminary study will follow soon after.

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