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The European Space Agency (ESA) is planning to launch two incredibly sophisticated gravity wave testing satellites in 2036 — but they’re a long, long way out. So before launching the Laser Interferometer Space Antenna (LISA) and the gravitational wave observatory, they’ll have to launch a more subtle experiment to find the path forward. The name, logically enough, is the LISA Pathfinder, and it will help scientists calibrate their ability to test the fine variations in the universe’s most mysterious force.

Scientists often ask people to imagine spacetime as a taught sheet of rubber, like the surface of a trampoline, and gravity as the conical depressions we make in this sheet when we place a mass anywhere on it. These three-dimensional trampoline wells in physical space stand in for four-dimensional gravity wells in spacetime, and they illustrate the basic logic of gravity: a bigger mass leads to a deeper, wider gravity well. And multiple wells can overlap to create complex fields of peaks and valleys.

In the real cosmos, masses are never still with respect to one another, but constantly hurtling through space, so this uneven gravitational field doesn’t stay static. As masses move around, their moving gravity creates complex “gravity waves,” or rolling changes in the dispersion of peaks and valleys in the overall field. Where the peaks and valleys fall in our solar system dictates how the planets orbit the sun, and the moons their planets. Where the peaks and valleys fall in the Milky Way dictates how stars orbit the galactic center.

The 2036 mission will put two masses millions of kilometers apart, to measure the differences in their gravitational surroundings and get a better picture of how gravity varies throughout space. The LISA Pathfinder will replicate this experiment on the extreme small-scale, with a pair of two-kilogram masses held just 15 inches apart.

There will be extremely small variations in the gravitational force measurable on such small objects placed so physically close to one another.  Each mass is measured to have precisely 2 kilograms of a gold-platinum alloy that remains perfectly uniform through launch and the extreme environment of space. The LISA Pathfinder itself is designed to enter a gravitationally perfect free-fall orbit, so there is absolutely zero motion in the spacecraft, relative to unsecured objects floating within it.

The LISAPathfinder spacecraft separates from its propulsion module.

The goal is to see no relative motion in the masses — they should finish the 90-day test run at precisely 15 inches from one another, just as when they started. If they don’t, then NASA can’t run perfect freefall experiments quite as well as they’d believed — and that’s a big problem for the proposed 2036 mission. Pathfinder will attempt to validate the experimental setup for the ESA’s enormous proposition to gravity-map the universe.

Gravity maps are useful for all sorts of reasons, but these days their main virtue is probably their ability to offer insights into the distribution and behavior of dark matter. Dark matter is thought to interact with regular matter solely through the force of gravity, which means comparing the predicted and actual effects of gravity in a region is currently the only way of assessing how much dark matter is around. Missions like the ESA’s 2036 mapping project could shed light on some of the most fundamental parts of our universe’s history.

The LISA Pathfinder is set to launch in late 2015.