When: 2012
Who: ESA, NASA
What: A test satellite that hunts gravity waves
Where: Orbiting the Sun at 1 AU
Why: To test if non-motion technology works
How: Lasers, optics, and free-falling gravity sensors

OK, you know gravity exists, because you are not floating out of your chair as you read this. I don't have to convince you that gravity is holding you down! But if I asked you exactly how gravity is doing that, or even if you read our Gravity and Microgravity tutorial, you might struggle to answer me.

Would you be surprised to learn that even physicists aren't completely sure they know how to answer that question? Sure, Einstein introduced the idea of the space time continuum. He described a sheet-like fabric that warps in the presence of mass sitting on it, forcing objects near to that mass to be pulled down into its warp.

But no-one has actually measured that continuum, nor seen the threads of the fabric -- gravitational waves -- that are supposedly out there. That's why engineers are building a spacecraft that will go out into space and try to feel the thread count of the universe.

Gravity Waves
Measuring gravitational waves is not going to be easy. But space scientists are sure that the universe is really as violent as it looks. Like pebbles dropped into a puddle, changes in an object's mass can cause ripples across the fabric of space. Einstein said these ripples can be felt by any other masses in the ripples' path.

Astronomers therefore believe a ship could sense these tiny gravity waves from exploding stars and merging black holes -- even if these events happen halfway across the galaxy.

Surfing the Waves
Here's the rub: gravity is nearsighted. Its effects dwindle across space. Waves from merging black holes that reached us from across the Galaxy should be itsy teensy wee things.

Any spaceship measuring such tiny effects would need to be free of its own wiggles and jiggles and burps, else it would ruin its own data.

And space is not entirely empty; debris from comets, asteroids and old spacecraft litter the regions around Earth. The stream of atomic particles flowing out of the Sun is enough to bang around any instrument sitting in space.

How to create a spacecraft that can fly perfectly still in a field of space debris?

Little Cubes that Could
Enter the Laser Interferometer Space Antenna, or LISA for short. It is a free-floating array of three little ships forming a perfect triangle in space.

Inside each of the three ships are two vacuum tubes. Placed in each tube is a little gold cube, about the size of a shot glass. One of the cubes is held in place by a strong field of electricity. The other is allowed to float freely. Both are shielded from heat and cold.

These little cubes are the gravity sensors called "test masses". As their name implies, they have mass, so they will feel a gravity wave rippling by. But the trick is to make sure they don't feel anything else.

To keep their little test masses from feeling any jolts or bumps, each of the ships has lasers mounted to it. The lasers beam out to the other ships, like lifelines hanging them together. Except these lines also are part of a smart system that is measuring the distances between them.

The lasers shoot right into the vaccum tubes where the perfect cubes float freely. The polish on the cubes is like a mirror that can reflect back the beam. Like radar, the timing and angle of the return beam tells the onboard computers about the distance and tilt of the other ships. Tiny jets around the ships fire to keep the ships flying in the correct position and location.

In this way, the ships can always keep the correct triangle shape. In terms of being a gravity telescope, the ships are making sure their "lens" is not deforming out of shape. If the ships can keep themselves in formation, then any vibrations that the little cubes feel is due only to gravity waves and not the ships knocking them about. Cool.
 
Says Mike Cruise of the University of Birmingham, "LISA is expected to provide the best chance of success in the search for the exciting, low frequency gravity waves. However, the mission is one of the most complex, technological challenges ever undertaken."

Although the technology to keep spaceships in proper orbits is not new, using them to keep a space antenna in shape is. This is a very ambitious idea, and a true feat of control engineering. So, before the antenna launches, NASA and ESA want to test the idea first.

Testing the Theory
A single ship will fly with all of the new tech onboard. It is called LISA Pathfinder, and it is currently in the UK being kitted out.

LISA Pathfinder will be a model of one of the ships. Inside, two cubes will nestle. The test ship will do a full run-through of a mission, with the cube sensors, little course adjustments and the lot.

"When LISA is operating in orbit, we expect to observe the universe through the new window offered by gravity waves," said Cruise. "In addition to neutron stars and massive black holes, we may be able to detect the echoes of the Big Bang from gravity waves emitted tiny fractions of a second after the event that started our universe on its current evolution."

Careers
If you are interested in the career and or educational opportunities this mission will create in the UK, here is a list of participants: Glasgow University, Imperial College London, University of Birmingham and commercially at Astrium Ltd., and at SciSys Ltd.